MatSci244 Notes - Chapters 12, 10, 18, 13, 14, 15, 16, 17 PDF

Chapter 12 – Phase transformations Two solid curves = one representing time required at each temperature for the transformation to start, the other representing the time requi...

Chapter 12 – Phase transformations Two solid curves = one representing time required at each temperature for the transformation to start, the other representing the time required for the conclusion of the transformation. Basic concepts Dashed curve = 50% transformation ▪ Phase transformation – A change in the nr and/or character of the completion phases that constitute the microstructure of an alloy. ▪ Pearlite ▪ Simple diffusion dependent – no change in nr/composition of Microstructural product of cooling phases present(eg solidification, recrystallization) austenite. Coarse / fine. ▪ Diffusion dependent – some alteration in nr/composition of phases present (eg eutectoid reaction) ▪ Diffusionless – a metastable phase is produced (eg martensitic) Kinetics of phase transformations ▪ Transformations aren’t instantaneous = nucleation -> growth ▪ Nucleation - initial stage of phase transformations evidenced by the formation of small particles of the new phase that are capable of growing. ▪ Homogenous – new nuclei form uniformly throughout parent phase ▪ Bainite ▪ Heterogeneous – nuclei form preferentially at structural inhomogeneities = austenitic transformation product ▪ Growth – the increase in size of a particle of a new phase found in some steels and cast irons. ▪ Kinetics – study of reaction rates and factors that influence them Forms at T’s between pearlite and ▪ The fraction of reaction that has occurred is measured as a martensite transformations occur. function of time while the temperature is maintained constant Microstructure = alpha ferrite with a fine ▪ Data are plotted as the fraction of dispersion of cementite. No proeutectoid phase transformed material versus the forms with bainite. Pearlitic and bainitic transformations are logarithm of time competitive (once either has formed, transformation to the other is not ▪ 𝑦 = 1 − exp −𝑘𝑡 𝑛 possible without reheating to form austenite. ▪ y-fraction of transformation, t-time, ▪ Spheroidite k,n- time-independent constants for reaction = consisting of sphere-like cementite 1 ▪ 𝑟𝑎𝑡𝑒 = ; 𝑡0.5- time for transformation particles within an alpha-ferrite matrix. 𝑡0.5 to be halfway complete Produced by elevated temperature heat Metastable vs equilibrium treatment of pearlite/bainite/austenite. ▪ When phase transformations are induced by temperature changes, Relatively soft. equilibrium conditions are only maintained if heating/cooling is carried ▪ Martensite out extremely slowly = Metastable iron phase ▪ For other-than-equilibrium cooling/heating, transformations are shifted supersaturated in carbon that is the to lower/higher temperatures product of a diffusionless -> degree depends on rate of temperature change transformation from austenite. Competitive with pearlite and bainite. Microstructural and property changes in iron-carbon alloys Occurs when quenching rate is rapid ▪ Isothermal transformation diagrams enough to prevent carbon diffusion. = plot of temperature vs logarithm of time for a steel of definite FCC austenite -> composition. Used to determine when transformations begin and end for body centred tetragonal. an isothermal heat treatment of a previously austenitized alloy Continuous cooling transformation diagrams ▪ Bainite ▪ Isothermal heat treatments = impractical as alloys must be rapidly Finer structure -> harder and stronger than pearlitic steels. Desirable cooled to and maintained at a temperature above eutectoid combination of strength and ductility. ▪ Isothermal transformation diagrams only valid at constant ▪ Martensite temperature conditions Hardest and strongest microstructure, most brittle – negligible ductility. ▪ Continuous cooling – time required for reaction to begin/end is delayed Hardness depends on carbon content. Hardness and strength results of -> diagrams are used to indicate when transformations occur as interstitial carbon atoms hindering dislocation motion and relatively few initially austenized material is continuously cooled at a specified rate. slip systems in BCT crystal structure. ▪ The transformation starts after a Tempered martensite time period corresponding to the ▪ Ductility and toughness of martensite is enhanced by tempering intersection of the cooling curve ▪ Tempering = heating martensitic steel to T below eutectoid for specified with the beginning reaction curve time period to relieve any internal stresses. and concludes upon crossing the ▪ Single phase BCT martensite (supersaturated in carbon) -> stable ferrite completion transformation curve. and cementite phases = similar to spheroidite only cementite particles are much smaller ▪ There exists a critical quenching rate which represents the minimum rate of quenching that produces a totally martensitic structure. Mechanical behaviour of iron-carbon alloys ▪ Pearlite Cementite is harder but more brittle than ferrite-> more cementite yields harder, stronger material. Ductility and toughness decreases with more C. Fine pearlite is harder+stronger than coarse pearlite-> more phase boundaries Strong+rigid cementite phase boundaries severely restricts deformation of ferrite … the more cementite, the stronger More phase boundaries = restricts dislocation motion ▪ Spheroidite Less hardness&strength than pearlite microstructure -> less boundary area per unit volume, plastic deformation not as constrained, relatively soft and weak, but extremley ductile and notably tough (crack encounters very little cementite when it propagates through ductile ferrite matrix)  Chapter 10 – Failure Maximum stress occurring at crack tip: 𝑎 𝜎𝑚 = 2𝜎0 ( )1/2 Fracture 𝜌𝑡 𝜎0 -nominal applied stress, a-length ▪ = separation of a body into 2+ pieces in response to a static, applied of surface crack, 𝜌𝑡 -curvature stress at temperatures low relative to melting temperature radius at tip ▪ Ductile fracture = fracture attended by gross plastic deformation Stress concentration factor ▪ Brittle fracture = occurs by rapid crack propagation without appreciable 𝜎 𝑎 𝐾𝑡 = 𝑚 = 2( )1/2 macroscopic deformation 𝜎0 𝜌𝑡 ▪ Fracture process -> crack formation -> propagation Critical stress for crack propagation in a brittle material ▪ Ductile fracture is preferable to brittle fracture because 2𝐸𝛾 ▪ Plastic deformation warns that failure is imminent 𝜎𝑐 = ( 𝑠 )1/2 𝜋𝑎 ▪ More strain energy is required to induce ductile fracture (ductile 𝛾𝑠 -specific surface energy, E- modulus of elasticity materials are generally tougher) ▪ Fracture toughness Ductile fracture = material’s resistance to brittle fracture when a crack is present ▪ Ductile fracture surfaces = distinct features 𝐾𝑐 = 𝑌𝜎𝑐 𝜋𝑎 ▪ Highly ductile materials neck to point of fracture Y-dimensionless parameter (1 / 1.1 if edge), critical strength and crack length ▪ Stages of ductile fracture (in response to axial tensile stress) Plane strain = for tensile loading, there is zero strain perpendicular to stress a) Necking begins axis and direction of crack propagation b) Small cavities (microvoids) form Plain strain fracture toughness c) Microvoids enlarge and coalesce to form an elliptical crack 𝐾𝐼𝑐 = 𝑌𝜎 𝜋𝑎 d) Fracture ensues by rapid propagation of a crack around - decreases with increasing strain rate and decreasing temperature outer perimeter of neck by shear deformation - increases with reduction in grain size e) Final shear fracture occurs at 45° relative to tensile axis ▪ Modes of crack surface displacement Brittle fracture ▪ a) opening/tensile b) sliding c) tearing ▪ Rapid crack propagation – crack motion nearly perpendicular to applied tensile stress axis = yields relatively flat fracture surface ▪ Brittle fracture forms distinctive patterns: ▪ V-shaped “chevron’ markings (form near centre of fracture and point backward to crack initiation site) ▪ Fan-like pattern (Lines/ridges that radiate from origin of crack) ▪ Design using fracture mechanics ▪ Very hard, fine-grained materials yield no discernible fracture pattern Maximum allowable flaw size ▪ For most brittle crystalline materials -> crack propagation responds to 1 𝐾 𝑎𝑐 = ( 𝐼𝑐 )1/2 successive & repeated breaking of atomic bonds along specific 𝜋 𝑌𝜎 crystallographic grains = Transgranular fracture Fracture toughness testing ▪ Intergranular fracture = Crack propagation occurs along grain Impact testing boundaries Charpy and Izod tests are used to measure impact energy (notch toughness) Principles of fracture mechanics Impact energy = measure of energy absorbed during fracture of a standard ▪ = used to determine stress level at which pre-existing cracks of known specimen when subjected to very rapid loading size will propagate and lead to fracture ▪ Ductile-to-brittle transition ▪ Stress concentration Some low strength steel (BCC) alloys, transition form Microscopic flaws/cracks always exist in material = detriment to fracture Ductile to brittle with a decrease in temperature strength -> applied stress is amplified at tip of crack (this temperature range is determined during Flaws = stress raiser impact tests) ▪ Ductile fracture - failure surface appears fibrous or dull ▪ The higher the magnitude of stress, the smaller the nr of cycles the ▪ Brittle fracture – granular (shiny) texture sample is capable of sustaining before fracture ▪ Fatigue limit = Maximum stress amplitude level below which a material can endure a seemingly infinite nr of stress cycles and not fail. ▪ Fatigue life = Total nr of stress cycles that cause a fatigue failure at some specified stress level ▪ Fatigue strength = Max stress level that a material can sustain without failure for some specified nr of cycles. ▪ Some alloys have a limiting stress level, most do not have a fatigue limit ▪ Considerable scatter exists in data -> fatigue life specified in terms of probability Fatigue Crack initiation and propagation = form of failure that occurs in structures subjected to dynamic and ▪ Fatigue failure in three steps: fluctuating stresses. Occurs after a lengthy period of repeated stress or 1. Crack initiation – small crack forms at point of high stress concentration strain cycling 2. Crack propagation – crack advances incrementally with each stress cycle Brittle-like failure even in normally ductile materials 3. Final failure – occurs very rapidly once crack has reached some critical size ▪ Crack initiate/nucleate on surface of component at stress concentration Cyclic stresses (eg scratches, sharp fillets, keyways, threads, dents). Microscopic surface ▪ 3 different fluctuating stress-time modes (stress cycles): discontinuities resulting from dislocation slip steps may also act as Reversed Repeated Random stress raiser and crack initiation sites ▪ Fracture surface during crack propagation (both appear as concentric ridges that expand away from crack initiation site) Beachmarks and striations do not appear on region over which rapid failure occurs: ▪ Beachmarks – macroscopic, each band represents a period time over which crack growth has occurred. (one benchmark consists of thousands of striations) Max or min stress; mean stress; ▪ Striations – microscopic, each represents the advance stress range; stress ratio = min/max distance of a crack front during a single stress cycle. Striation width depends on, and increases with, increasing S-N curve stress range. ▪ Test apparatus is designed to ▪ During the propagation of fatigue cracks and on a closely duplicate service stress microscopic scale, there is very localized plastic conditions deformation at crack tips, even though the maximum ▪ Rotating-bending fatigue tests = applied stress to which the object is exposed in each recording nr of cycles to failure at stress cycle lies below the yield strength of the metal. series of stresses ▪ This applied stress is amplified at crack tips to the degree ▪ Data plotted as stress S vs that local stress levels exceed the yield strength. logarithm of nr of cycles to failure N Factors that affect fatigue life 3. Tertiary creep – acceleration of creep rate and ultimate failure = rupture ▪ Mean stress – Increasing the mean stress level leads to a decrease in resulting from microstructural/metallurgical changes (eg formation of fatigue life internal cracks and voids, a neck may form) ▪ Design factors – Any notch or geometrical discontinuity can act as a ▪ Steady state creep rate stress raiser. The sharper the discontinuity, the more severe the stress 𝜖𝑠ሶ = 𝐾1 𝜎 𝑛 K,n material constants concentration. Reduce probability of fatigue failure by avoiding sharp 𝜖𝑠ሶ = 𝐾2 𝜎 𝑛 exp − 𝑄𝑐 Qc, creep activation energy, R 𝑅𝑇 corners and using large radii fillets. gas constant, T abs temperature ▪ Surface treatments – Surface markings (grooves and scratches) limit ▪ Correlations exist between creep activation energy and diffusion fatigue life. Improving the surface finish by polishing improves fatigue activation energy life. Imposing residual compressive stresses within a thin outer surface layer also increases fatigue performance Alloys for high temperature use -> can be accomplished by shot peening (projecting small, hard ▪ Melting temperature, elastic modulus, grain size influence creep particles onto surface at high velocities characteristics -> higher melting T, greater E, larger grain size -> case hardening – enhance fatigue life and surface hardness. A C = better a material’s resistance to creep or N rich outer layer is introduced by atomic diffusion from the ▪ Smaller grain size permit more grain boundary sliding resulting in larger gaseous phase creep rates Environmental effects ▪ Stainless steels & super alloys (because of solid solution strengthening) ▪ Thermal fatigue = Cyclic stresses are introduced by changes in are especially resistant. temperature. ▪ Directional solidification makes a material more resistant to creep as it ▪ Thermal stress-> 𝜎 = 𝛼𝑙 𝐸∆𝑇 produces highly elongated grains or single crystal components Coefficient of thermal expansion, elastic modulus, temperature change ▪ Thermal stresses do not arise if there is no mechanical restraint ▪ Reduce thermal fatigue by allowing unhindered dimensional changes with temperature variations ▪ Corrosion fatigue = failure resulting from simultaneous action of cyclic stress and chemical attack ▪ Small pits may form as a result of chemical reactions -> may serve as points of stress concentrations & crack nucleation sites Creep ▪ = time dependent permanent deformation that occurs under stress (normally only at elevated temperatures > 0,4*melting temperature) ▪ Undesirable as it limits the life time of a part ▪ Constant stress tests are employed to investigate mechanisms of creep 1. Primary/transient creep – continuously decreasing creep rate (material experiences increase in creep resistance / strain hardening) 2. Secondary/steady-state creep – plot becomes linear-> rate is constant (Balance between strain hardening and recovery)  Chapter 18 – Corrosion & Degradation Passivity ▪ = loss of chemical reactivity -> under particular environmental ▪ Corrosion = deteriorative loss of a metal as a result of dissolution conditions some metals and alloys become extremely inert, often due to environmental reactions the formation of a protective film ▪ Oxidation = formation of non-metallic scale or film ▪ Displayed by Cr, Ni, Ti and their alloys ▪ Degradation = deteriorative processes that occur with polymers ▪ Stainless steel (iron alloyed with at least 11% Cr) is highly corrosion resistant in variety of atmospheres Corrosion of metals ▪ Passive behaviour results from formation of a highly adherent and very ▪ Corrosion – destructive, unintentional electrochemical attack on a metal thin oxide film on the metal surface which serves as a protective barrier to further corrosion Electrochemical considerations ▪ If damaged, the protective film normally re-forms rapidly ▪ Electrochemical = chemical reaction by electron transfer ▪ Change in character of environment van cause metal to revert to active ▪ Oxidation – Metal atoms lose electrons in an state -> subsequent damage to pre-existing passive film could result in oxidation reaction substantial increase in corrosion rate ▪ The metal becomes a positively charged ion as it loses valence electrons Environmental effects ▪ Takes place at anode -> anodic reaction ▪ Increasing fluid velocity -> enhances corrosion rate due to erosive effects ▪ Reduction – electrons are transferred to other ▪ Increasing temperature -> increases rate of chemical reactions chemical specie (possibly also metal) - takes place ▪ Increasing concentration of corrosion -> increases corrosion rate at cathode ▪ For materials capable of passivation -> raising corrosion content ▪ Electrochemical reaction = consists of at least one reduction and one may result in an active-to-passive transition -> considerable oxidation reaction = half-reactions reduction in corrosion Electrode potentials ▪ Cold working / plastically deforming a metal -> metal is more ▪ Not all materials oxidise with the same level of ease susceptible to corrosion ▪ Galvanic couple – two metals electrically connected in a liquid electrolyte in which one metal becomes an anode and corrodes while the other acts Forms of corrosion as an cathode ▪ Uniform attack ▪ An electric potential exists between two cell halves (measure with Occurs in equivalent intensity over the entire exposed surface and voltmeter) often leaves behind a scale or deposit. Oxidation and reduction ▪ Standard half cell – pure metal electrode, 1 M solution of its ions at 25℃ reactions occur randomly over surface ▪ Standard emf series Examples – rusting of steel and iron; tarnishing of silverware ▪ Half cell voltages measured relative to standard hydrogen ▪ Galvanic corrosion reference half call = occurs when two metals/alloys are electrically coupled while ▪ Oxidation – negative voltage // reduction – positive voltage exposed to an electrolyte. The less noble/ more reactive metal ▪ Standard electrode potential 𝑉 0 experiences corrosion, while the more inert metal is protected from 0 0 ▪ Cell potential ∆𝑉 0 = 𝑉𝑟𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛 − 𝑉𝑜𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛 corrosion. The metal lower on emf / galvanic series corrodes. ▪ Spontaneous reaction if ∆𝑉 > 0 0 Reducing effects: ▪ Influence of temperature and concentration - choose metals close together on galvanic series ▪ ∆𝑉 0 = 𝑉20 − 𝑉10 − 𝑅𝑇 𝑙𝑛 [𝑀1𝑛+ ] - avoid unfavourable anode-to-cathode ration (a smaller 𝑛𝔉 [𝑀2𝑛+ ] anode area corrodes more quickly) ▪ R- gas constant, T- abs temperature, n nr of participating electrons, Faraday constant - electrically insulate dissimilar metals from each other (96500C/mol), M1, M2, molar ion concentrations - electrically connect a third, anodic metal = form of cathodic ▪ Galvanic series protection ▪ Relative reactivities of metals and commercial alloys in seawater ▪ Crevice corrosion ▪ Erosion-corrosion Occurs as a result of concentration differences of ions / dissolved Combined attack of chemical attack and abrasion / wear as a result gases in the electrolyte solution and between regions of the same of fluid motion. Especially harmful to alloys that passivate as the metal piece. Example – crevices, recesses where solution becomes abrasive action erode away the protective film continually, maybe stagnant and there is a localised depletion of oxygen. preventing the film from reforming, exposing bare metal where Can be prevented by: severe corrosion will then take place. - using welded in stead of riveted or Bubbles & suspended particulate solids -> more erosive solution. bolted joints Occurs at sudden turbulence and directional changes. - using non-absorbing gaskets Reduce erosion-corrosion by: - removing accumulated deposit - change design to eliminate fluid turbulence and frequently impingement effects - designing containment vessels to - use materials that are inherently resist erosion avoid stagnant areas - remove particles and bubbles from solution ▪ Pitting ▪ Stress corrosion = very localised corrosion attack which forms small holes/pits that Results from combined action of an applied tensile stress and a penetrate downward from the top of a horizontal surface. corrosive environment. Some materials are practically inert and only Often goes undetected with very little material loss become susceptible to corrosion when a stress is applied. Similar mechanism to crevice corrosion, growing downward Small cracks form then propagate perpendicular to stress – failure due to gravity. May be initiated by small surface defect. will eventually occur -> failure is like that of a brittle material even if Prevention: material is intrinsically ductile. Can occur at relatively low stress - polish surfaces levels or as a result of residual stresses in material. - alloy stainless steels with Mo Reduce stress corrosion: - lower magnitude of or eliminate stress ▪ Intergranular corrosion - anneal out any residual stresses Occurs preferentially along grain boundaries for some alloys in specific ▪ Hydrogen embrittlement environments -> results in specimen = loss or reduction of ductility and tensile strength of a metal alloy disintegrating along grain boundaries. (often steel) as a result of diffusion of atomic hydrogen into the Especially stainless steels -> material. The interstitial hydrogen atoms cause cracking which can heat treatment allows for formation of Cr23C6 precipitate particles lead to catastrophic brittle fracture. which leaves Cr-depleted zones next to grain boundaries, now Cathodic protection can lead to initiation / enhancement of HE susceptible to corrosion. (Also = weld decay) Pickling of steels in sulfuric acid, electroplating, welding and heat Prevention in stainless steels: treatments = situations where HE can occur - High temperature heat treatment to redissolve the Reducing the likelihood of hydrogen embrittlement: chromium carbide - Reduce tensile strength via heat treatment - lower C content below 0.03% - Remove source of hydrogen - alloy with Nb or Ti which has greater tendency to form - Bake metal at elevated T to drive out dissolved H carbides than Cr (Cr remains in solid solution) - Substituting a more embrittlement resistant alloy ▪ Selective leaching Corrosion environments Found in solid solutions and occurs when one constituent is ▪ Atmosphere, aqueous solutions, soils, acids, bases, inorganic solvents, preferentially removed as a result of corrosion processes. molten salts, liquid metals. Corrosion prevention ▪ Materials should be judiciously selected specific to the type of corrosive environment, although cost is often a factor. ▪ Changing character of environment can influence corrosion eg – Increasingly inert decreasing fluid temperature &/ velocity, changing concentration of (cathodic) species in solution ▪ Inhibitors = substances that can decrease corrosiveness if used in Standard emf Series relatively small amounts -> some react with and eliminate a chemically active specie in solution -> others interfere with oxidation/reduction reactions at the corroding surface ▪ Physical barriers such as films and coats with high level of surface adhesion. -> Virtually nonreactive and resistant to mechanical damage Cathodic protection ▪ = corrosion prevention by which electrons are supplied to the structure to be protected from an external source such as another, more reactive metal or a dc power supply. Increasingly active ▪ Galvanic couple (anodic) Metal to be protected is electrically connected to more reactive metal = sacrificial anode Galvanising – steel is coated with zinc which is very anodic and catholically protects the steel in case of surface damage. Zinc corrodes very slowly because of large anode-to-cathode surface area ratio ▪ External power source Negative terminal of power source is connected to structure to be protected, other terminal to inert anode (often graphite), which is buried in high conductivity backfill material, in soil. Current can travel through the soil, completing the circuit.  Chapter 13 – Metals: Properties & Applications Ferritic stainless steels – composed mainly of alpha ferrite (BCC) Ferritic & austenitic -> hardened by cold work (not heat treatable) Ferrous alloys Austenitic stainless steels – most corrosion resistant because of high Cr & ▪ = iron is the prime constituent. Ni, but not magnetic ▪ Very widespread because -> Applications: Abundant quantities of iron-containing compounds in earth’s crust At elevated temperatures in severe environments, gas turbines, Relatively economical extraction, refinement, alloying and fabrication steam boilers, furnaces, aircraft, missiles, nuclear-power- techniques exist gathering units Extremely versatile ▪ Chief limitations = Relatively high densities Cast Irons Low electrical conductivity ▪ Carbon content above 2.14 wt% -> usually 3.0 < wt% C < 4.5 Susceptibility to corrosion in common environments ▪ Melting temperature is considerably lower than for steels -> easily melted and amenable to casting Steels -> casting is most convenient as some are very brittle ▪ = iron-carbon alloys containing appreciable amounts of other alloying elements ▪ Cementite can be made to disassociate into BCC iron and graphite & less than 1.0 wt% carbon -> graphite formation is promoted by presence of> 1 wt% Si and slower ▪ Low carbon steels cooling rates during solidification < 0.25 wt% C -> carbon exists as graphite for most cast irons and its microstructure and Unresponsive to heat treatments to form martensite mechanical behaviour are dependent on composition and heat treatment. Strengthening is accomplished by cold work ▪ Gray iron Microstructure = ferrite end pearlite 2.5 < wt% C < 4.0 & 1.0 < wt% Si < 3.0 -> relatively soft, weak, outstanding ductility and toughness, machinable, Graphite exists as flakes in alpha-ferrite or pearlite matrix weldable & least expensive to produce Comparatively weak and brittle in tension -> graphite flakes are sharp and Applications: pointed and serve as stress point concentrations Automobile body components, structural shapes, sheets Strength and ductility are high under compressive loads High strength, low alloy – more resistant to corrosion, higher tensile Very effective in damping vibrational energy strengths than plain carbon steels, ductile, formable, machinable High resistance to wear Alloying elements – Cu, V, Ni, Mo up to 10 wt% High fluidity of molten state -> intricately shaped pieces can be cast ▪ Medium carbon steels Among least expensive of metallic materials 0.25 < wt% C < 0.60 Lower Si content / increasing cooling rate -> graphite flakes in pearlite May be heat treated by austenitizing, quenching then tempering to improve matrix (complete disassociation of cementite is prevented) mechanical properties ▪ Ductile (Nodular) Iron Applications: Small amount of Mg &/ Ce is added before casting procedures Railway tracks and wheels, gears, crankshafts, machine parts, Graphite then forms as nodules/sphere-like particles in pearlite/ferrite components requiring -> high strength, wear resistance, toughness matrix (depending on heat treatment) Cr, Ni, Mo improve capacity to be heat treated Castings are stronger and more ductile than gray cast iron ▪ High carbon steels Applications: 0.60 < wt% C < 1.4 Valves, pump bodies, crankshafts, gears, automotive and Hardest, strongest, least ductile of carbon steels machine components Usually hardened and tempered = wear resistant & capable of holding a ▪ White iron sharp cutting edge >1 wt% Si & rapid cooling rates -> carbon exists as cementite Tool & die steels = high carbon steels with Cr, V, W, Mo Fracture surfaces have white appearance -> used to cut, form, shape metal Thick sections -> only surface layer of white iron – gray iron forms in Applications: knives, razors, hacksaw blades, springs, wire interior regions where it cools more slowly ▪ Stainless steels Lots of cementite = extremely hard by very brittle (unmachinable) Highly corrosion resistant especially in ambient atmosphere Limited to applications requiring a very hard and wear resistant surface, At least 11 wt% Cr (also Additions of Ni & Mo) without a high degree of utility -> rolling mills Martensitic stainless steels – martensite is prime microconstituent ▪ Malleable iron ▪ Aluminium White iron is heated between 800 & 900 C for long causing cementite to Al & alloys -> relatively low density, high electrical and thermal decompose into graphite in the form of clusters in a pearlite or ferrite matrix conductivities, corrosion resistant, high ductility = easily formed Similar microstructure to nodular iron – high strength and appreciable FCC structure so ductility is retained at low temperatures ductility and malleability Low melting temperature = chief limitation Applications: Strength can be enhanced by cold working and alloying (however this Rods, transmission gears, flanges, pipe fittings, valve parts for decreases corrosion resistance) heavy duty industries Principle alloying elements: Cu, Mg, Mn, Zn ▪ Compacted graphite iron Some alloys are heat treatable 3.1 < wt% C < 4.0 & 1.7 < % Si < 3.0 ▪ Magnesium Graphite in worm-like shapes and some nodules -> produced by adding Mg, Lowest density of structural metals -> used where light weight is an import Ce and other in specific quantities consideration (like aircraft components) Pearlite or ferrite matrix Relatively soft and has HCP structure High strength and ductility Difficult to deform at room temperature -> fabrication usually by casting or Pearlitic matrices have higher strength and lower ductility hot working Higher thermal conductivity, better resistance to thermal shock, lower Mg & alloys are chemically unstable and especially susceptible to corrosion oxidation at elevated temperatures in marine environments (better in normal atmosphere) Applications: Either cast or wrought, some are heat treatable Diesel engine blocks, exhaust manifolds, gearbox housings, Mg & alloys have replaced engineering plastics as they are stiffer, more brake discs, flywheels recyclable and less costly to produce Applications: handheld devices, computer & communications equipment, Nonferrous alloys luggage ▪ Classified according to base metal or specific characteristic ▪ Titanium ▪ Cast alloys -> to brittle to be shaped/formed by appreciable deformation Relatively low density, high melting point, extremely strong, highly ductile ▪ Wrought alloys -> Amenable to mechanical deformation and easily formed HCP (alpha phase) -> High T applications, not heat treatable BCC (beta phase) -> Highly forgeable, high fracture toughness V, Nb, Mo decreases T at which this transformation takes place High chemical reactivity at elevated temperatures Used in airplane structures, space vehicles, surgical implants ▪ Refractory metals Refractory = having extremely high melting temperatures ->Ni, Mo, W, Ta Copper Extremely strong interatomic bonding – accounting for high melting points, Unalloyed copper = too soft and ductile to machine properly & near high strengths and hardness unlimited capacity to be cold worked Mo alloys -> extrusion dies, space vehicle structures Highly resistant to corrosion W alloys -> light filaments, x ray tubes, welding electrodes Copper alloys cannot be hardened/strengthened by heat treatments Ta -> immune to chemical attack in any environment below 150 C Mechanically properties can be improved by cold working &/ solid solution ▪ Superalloys alloying Used where severe oxidizing environments and high temperatures need to Brasses -> zinc is the predominant alloying element be withstood -> Jewellery, cartridge casings, radiators, Predominantly Fe-Ni, Ni and Co alloyed with refractory metals instruments, electronic packaging, coins Turbine applications, nuclear reactors, petrochemical equipment Bronzes -> Several other elements: Sn, Al, Si, Ni ▪ Noble Metals -> Somewhat stronger than bronzes Expensive & characteristically soft, ductile and oxidation resistant Beryllium coppers are heat treatable but costly – jet aircraft gear bearings Ag, Au, Pt, Pd, Ru, Rh, Ir, Os and bushings, spring, surgical & dental instruments Au & Ag may be strengthened by alloying with Cu Pt -> laboratory equipment & thermocouples  Chapter 14 – Ceramics: Properties & Applications Stress-strain behaviour ▪ Flexural strength Ceramic Phase Diagrams Transverse bending test is used to ▪ Experimentally determined phase diagrams -> similar to those of metals measure flexural strength ▪ Useful in assessing high temperature performance = stress at fracture from a bend test ▪ Elastic behaviour Mechanical properties Linear relationship between stress and strain Brittle fracture of ceramics Slope = modulus of elasticity ▪ Crystalline & noncrystalline ceramics fracture before any plastic Mechanisms of plastic deformation deformation takes place in response to applied tensile load ▪ Crystalline ceramics ▪ Brittle fracture process = formation and propagation of cracks Occurs by dislocation motion but slip is difficult & materials are perpendicular to applied load hard and brittle because ->Covalent bonds are extremely strong ▪ Crack growth is either trans- or intergranular ->Limited nr of slip systems ->Dislocation structures are complex ▪ Plane strain fracture toughness ▪ Noncrystalline ceramics 𝐾𝐼𝑐 = 𝑌𝜎 𝜋𝑎 There is no regular atomic structure for noncrystalline ceramics so Y-dimensionless parameter, applied stress and crack length plastic deformation cannot occur by dislocation motion ▪ Static fatigue/delayed fracture -> slow propagation of cracks if load is Deform by viscous flow -> in response to an applied shear stress, static – sensitive to environment, esp moisture atoms/ions slide past each other by the breaking and reforming of ▪ Fractography of ceramics interatomic bonds (no prescribed manner or direction) = examining path of crack propagation & fracture surface Viscosity = measure of noncrystalline material’s resistance to deformation After nucleation and during propagation, the crack accelerates until some 𝜂= 𝐹/𝐴 𝑑𝑣/𝑑𝑦 critical velocity is achieved at which point the crack may branch } this shear force, area, change in velocity dv with distance dy process is successively repeated. Glass has high viscosity at ambient temperatures Rate of crack acceleration and degree of branching increases with Rising temperature diminishes interatomic bonding, facilitation increasing stress levels motion of atoms/ions, which decreases material’s viscosity Typical crack configurations: Mechanical considerations ▪ Influence of porosity Ceramics first in powder form -> compressed -> heat treated Most porosity eliminated in heat treatment Bending Impact / point loading Residual porosity negatively impacts elastic properties & strength Pores reduce area to which force is applied = larger stress Act as stress concentrators Magnitude of E decreases with volume fraction porosity P: 𝐸 = 𝐸0 (1 − 1.9𝑃 + 0.9𝑃2 ) Flexural stress decreases with volume fraction porosity: 𝜎𝑓𝑠 = 𝜎0 exp(−𝑛𝑃) Torsion Internal pressure 𝜎0 , n – experimentally determined constants ▪ Hardness Crack surface formed during initial acceleration is smooth and flat; upon Measured using Vickers and Knoop techniques. Hardness decreases branching, the crack interface becomes rougher with increasing load (or indentation size). Higher fracture stress level -> smaller mirror radius ▪ Creep 1 𝜎𝑓 ∝ 0.5 Ceramics experience creep deformation when exposed to (usually 𝑟𝑚 compressive) stresses at elevated temperatures Types & Applications ▪ Silica refractories Silica is prime ingredient. Good high temperature load bearing capacity. Small portion of brick exists as liquid. Alumina content should be kept minimal. ▪ Special refractories Relatively high purity oxide materials -> can be produced with very little porosity. Relatively expensive. Abrasives Glasses ▪ = used to wear, grind or cut away other, softer material ▪ Containers, lenses, fiberglass ▪ Requisites -> Hardness or wear resistance, high fracture toughness, ▪ = noncrystalline silicates (SiO2) containing other oxides (Ca,Na,K,Al) some refractoriness ▪ Optical transparency & relative ease of fabrication ▪ Diamonds, silicon carbide, tungsten carbide, aluminium oxide, silica sand Glass-ceramics ▪ Grinding wheels – abrasive particles are bonded to wheel by a glassy ▪ fine-grained polycrystalline material (glass having undergone ceramic or resin – should have some porosity & air/coolant flow crystallization by high T heat treatment) ▪ Coated abrasives – Abrasive powder coated on paper or cloth ▪ Relatively high mechanical strengths, low coefficients of thermal ▪ Grinding, lapping, polishing wheels often employ loose abrasive grains expansion, good high temperature capabilities, good dielectric Cements properties, good biological capability, optically transparent or opaque ▪ = a substance that binds particulate aggregates into a cohesive structure ▪ Ovenware, tableware, range tops, circuit board substrates, architectural by chemical reaction. -> When mixed with water, a paste forms that cladding, heat exchangers, regenerators subsequently sets and hardens Clay products ▪ Portland cement – clay and lime bearing minerals are ground and mixed ▪ Inexpensive raw material (clay) is found abundantly intimately, then heated. Calcination occurs. Product is ground into a very ▪ Mixed with water = plastic mass -> very amenable to shaping fine powder and small amount of gypsum is added to retard the setting ->formed piece is dried to remove moisture process. Setting and hardening results from relatively complicated ->fired to improve mechanical strength hydration reactions when water is added. ▪ Structural clay products – building bricks, tiles, sewer pipes ▪ Cement does not harden by drying, but hydration in which water actually ▪ Whitewares – porcelain, pottery, tableware, china, plumbing fixtures participates in a chemical binding reaction Refractories Carbons ▪ = able to withstand high temperatures without melting or decomposing ▪ Diamond & remaining chemically unreactive and inert in severe environments Chemically inert, hardest known material, lowest sliding coefficient ▪ Bricks -> furnace linings for metal refining, glass manufacturing, of friction, high thermal conductivity, optically transparent (in visible metallurgical heat treatment, power generation and infrared regions of spectrum). HPHT techniques are used to ▪ Porosity must be controlled -> strength, load bearing capacity, corrosion synthesize diamonds. resistance increases with porosity reduction Drill bits and saws, dies for wire drawing, abrasives ▪ Fireclay refractories ▪ Graphite = alumina (Al2O3) and silica mixtures. Upgrading alumina content Properties depend on crystallographic directions. Weak interplanar increases service temperature. bonds -> good lubricative properties. Soft and flaky. Opaque with Mainly furnace construction – confining hot atmospheres & black-silver colour. Lubricants, pencils, battery electrodes, friction insulating structural components materials, heating elements, high temperature insulations Strength not as crucial ▪ Carbon fibers Dimensional accuracy and stability is carefully controlled Small diameter, high strength fibers -> reinforcements in polymer ▪ Basic refractories matrix composites. Carbon = in the form of graphene layers. Rich in periclase (MgO). Used in steel-making open hearth furnaces. Graphitic or turbostatic. Presence of silica is deleterious to high T performance  Chapter 15 – Polymers: Properties & Applications ▪ Viscoelastic creep = time dependent deformation at constant stress level. May be significant Mechanical behaviour of polymers at room temperature and low stress levels Stress strain behaviour Creep modulus-> ▪ Mechanical characteristics are highly Susceptibly to creep decreases (Ec(t) increases) as degree of crystallinity sensitive to deformation rate, temperature increases and chemical nature of the environment. Fracture of polymers ▪ Three types of stress-strain behaviour: Thermosetting plastics A -> Brittle polymer – fractures while ▪ Brittle fracture -> Cracks form at localized stress concentrations -> deforming elastically stress is amplified at crack tips, leading to propagation and fracture B -> Plastic polymer – initial deformation Thermoplastics is elastic, followed by yielding and ▪ Ductile or brittle (some materials undergo ductile to brittle transition) region of plastic deformation ▪ Factors that favour brittle fracture: C -> Elastomer – large recoverable strains - Reduction in temperature - Presence of a sharp notch at low stress levels - Increase in specimen thickness - Increase in strain rate ▪ Tensile strength for polymers correspond to stress at which fracture - Any modification that raises the glass transition temperature occurs – may be less or greater than yield stress ▪ Glassy thermoplastics – brittle below glass transition Ts, become more ▪ Increasing temperature (or decreasing the strain rate/rate of ductile in vicinity of Tg and experience plastic yielding prior to fracture deformation) -> decreases elastic modulus, reduces tensile strength and ▪ Crazing often precedes fracture -> regions of enhances ductility very localised plastic deformation lead to the Macroscopic formation of small and interconnected microvoids. ▪ For semicrystalline polymers -> at upper Fibrillar bridges form between microvoids. yield point a small neck forms, chains in neck If tensile load is sufficient, the bridges elongate become orientated parallel to elongation and break, causing microvoids to grow and direction, leading to localised strengthening, coalesce, which eventually becomes a crack. resistance to continuing deformation, ▪ Craze growth supports prior to cracking absorbs fracture energy and specimen elongation proceeds by propagation effectively increases fracture toughness of neck region, again accompanied by chain orientation Miscellaneous mechanical characteristics Viscoelastic deformation ▪ Impact strength ▪ An amorphous polymer behaves like glass at low temperatures, like a Semicrystalline and amorphous polymers are brittle at low rubbery solid at intermediate temperatures and like a viscous liquid at temperatures, with relatively low impact strengths high temperatures Ductile to brittle transition occurs over very narrow T range and ▪ Viscoelasticity = deformation exhibiting impact strength decreases gradually with higher temperatures mechanical characteristics of viscous flow ▪ Fatigue and elastic deformation Polymers also experience fatigue under cyclic loading at stresses levels relatively low to yield strength. Only some polymers have a ▪ Totally elastic -> deformation is instantaneous fatigue limit. ▪ Totally viscous -> deformation is delayed / time dependent Cycling polymers at high frequencies &/ large stresses can cause ▪ Viscoelastic behaviour -> localized heating -> consequently failure may be due to softening instantaneous strain is followed by rather than result of typical fatigue processes a viscous, time-dependent strain Tear strength and hardness ▪ Viscoelastic relaxation modulus Tear strength = energy required to tear apart a cut specimen of standard geometry. Hardness = resistance to scratching. Polymers are softer than metals and ceramics. Mechanisms of Deformation & Strengthening Deformation of elastomers Deformation of semicrystalline materials ▪ Elastomers have the ability to undergo large deformations then spring ▪ Semicrystalline = spherulite structure (numerous elastically back to their original form chain folded lamellae radiating outward from centre; ▪ Unstressed -> elastomer is amorphous and composed of highly twisted lamellae are surrounded by regions of amorphous material and coiled, crosslinked chains. ▪ Mechanism of elastic deformation ▪ Elastic deformation is the uncoiling & straightening of these chains 1. Onset -> chain molecules in amorphous regions elongate in the ▪ Upon release of stress, chains spring back and specimen reverts to direction of the applied tensile stress original shape 2. Continued deformation -> amorphous chains become aligned + ▪ Driving force = entropy – elastomer becomes warmer when stretched there is bending and stretching of the covalent bonds within the – elastic modulus increases with rising T crystallites, leading to a slight, reversible increase in ▪ Criteria for polymer to be elastomeric: lamellar crystallite thickness ▪ Not crystallize easily ▪ Chain bind rotations must be relatively free ▪ Onset of plastic deformation must be delayed (crosslinking chains prevents them form sliding past each other) ▪ Elastomer must be above glass transition temperature ▪ Vulcanization 1 2 3 4 5 = nonreversible reaction involving sulphur in which crosslinks are ▪ Mechanism of plastic deformation formed between molecular chains in rubber materials 3. Adjacent lamellae chains slide past each other and the -> enhances elastic modulus, tensile strength, resistance to so the chain folds become more aligned with the tensile axis degradation – proportional to density of crosslinks 4. Crystalline blocks separate from the lamellae, attached by tie ▪ Because they are crosslinked, elastomers are thermosetting chains 5. Finally, all the blocks and chains become orientated in the Crystallization, Melting and Glass Transition direction of the tensile axis Crystallization ▪ Processes are reversible -> if the deformed material is heated to a ▪ = process by which, upon cooling, an ordered solid phase is produced temperature close to its melting point, material will crystallise from a liquid melt having a highly random molecular structure back to spherulite structure ▪ Occurs by nucleation and growth processes Factors influencing mechanical properties of semicrystalline polymers ▪ Nuclei form -> small regions of tangled molecules become ordered and ▪ Molecular weight aligned = chain-folded lamellae -> tensile strength increases with molecular weight ▪ The lamellae remain the same thickness but grow in laterally ▪ Degree of crystallinity ▪ Fraction crystallized is a function of time according to the Avrami -> extensive secondary exists between adjacent chain segments equation, as 𝑦 = 1 − exp(−𝑘𝑡 𝑛 ) which require more energy to break -> tensile modulus & strength ▪ 100% crystallinity is not possible increases with degree of crystallinity, & it becomes more brittle Melting ▪ Predeformation by drawing ▪ = ordered structure of aligned molecular chains -> viscous liquid with -> drawing (~ strain hardening in metals) causes material to stiffen highly random molecular structure and strengthen – properties are enhanced in direction of applied ▪ Occurs at melting temperature tensile stress. The orientated molecular structure is only retained if ▪ Melting behaviour depends on crystallization temperature and rate of the material is cooled quickly heating (increase in rate elevates melting temperature ▪ Heat-treating Glass transition -> increases % crystallinity. Higher annealing temperatures result in ▪ = gradual transformation from a liquid into a rubbery material then increased tensile modulus, increased yield strength, reduced a rigid solid ( due to reduction in motion of large molecular chain ductility. Predrawn fibers lose strain-induced crystallinity. segments with decreasing temperature) Melting and glass transition temperatures ▪ Films ▪ Define limits for numerous applications and influence fabrication Very thin – used for packaging Influences on melting and glass transition temperatures Low density, high flexibility, high tensile and tear strengths, ▪ Melting temperature resistant to chemical attack, low permeability -> Chain stiffness (ease of rotation) – double bonds & bulky side ▪ Foams groups lowers chain flexibility which increase melting temperatures Contain high volume percentage of pores and trapped gas bubbles -> Molecular weight (chain length) Cushions, packaging, insulation -> Degree of branching – introduces defects into crystalline material, Blowing agent is incorporated into batch of material, that when lowering melting temperature heated, decomposes with the liberation of a gas. Gas bubbles are ▪ Glass transition temperature generated through fluid mass and remain in the solid upon cooling -> Same as for melting temperature Advanced polymeric materials ▪ Ultra high molecular weight polyethylene Polymer Types ▪ Liquid crystal polymers Plastics ▪ Thermoplastic elastomers ▪ = have some structural rigidity under load, some are very rigid and brittle, while others are flexible, may have any degree of crystallinity and be either thermoplastic or thermosetting ▪ Optical transparency -> polystyrene & poly(methyl methacrylate) ▪ Low friction coefficient -> Fluorocarbons (even at high temperatures) Elastomers ▪ SBR – used in automobile tyres, reinforced with carbon black ▪ NBR – highly resistant to degradation and swelling ▪ Silicon rubbers – backbone of alternating Si and O with hydrocarbon branches – high flexibility at low temperatures, resistant to weathering and lubricating oils, vulcanize at room temperature Fibers ▪ Long filaments with high length to diameter ratio ▪ Composite reinforcements or textiles ▪ Must have high tensile strength, high elastic modulus, high abrasion resistance, highly crystalline, chemical stability in extensive environments, relatively non-flammable and amenable to drying Applications ▪ Coatings Functions – to protect item form the environment, to improve item’s appearance, to provide electrical insulation ▪ Adhesives Used to bind together the surfaces of two solid materials. Mechanical or chemical Light weight, can bind dissimilar materials and thin component, better fatigue resistance, low manufacturing costs Drawback – service temperature limitation (strength decreases with rising temperature Chapter 16 – Composites: Properties& Applications Carbon black = small, spherical particles of carbon When added to vulcanized rubber, tensile strength, Intro toughness and resistance is enhanced. Need arose for material with specific and unusual properties Concrete Composite = multiphase material exhibiting a significant Both matrix and dispersed phases = ceramic materials proportion of properties of both constituent phases such that a = consisting of aggregate particles (gravel/sand) bound together in a better combination of properties is realized. solid body by some binding medium (cement) Matrix phase = continuous phase surrounding the other Portland cement concrete – sand, gravel, water Dispersed phase = phase surrounded by continuous phase Reinforced concrete – strengthened by steel rods, wires/bars Dispersed phase geometry refers to: Dispersion-strengthened composites shape, size, distribution, orientation = Strengthened and hardened by several volume percent uniformly of particles of dispersed phase dispersed particles of a very hard and inert material. Strengthening is retained at elevated temperatures Particle-Reinforced Composites Particulate phase is harder and stiffer than Fibre-Reinforced Composites matrix phase Dispersed phase is in the form of fibre Large-particle composites Design goals = high strength/stiffness on a weight basis Subclassification of particle-reinforced Have exceptionally high specific strengths and low density materials ‘large’ = particle-matrix interactions are not treated on Influence of fibre-length atomic/molecular level – continuum mechanics is used. Mech properties depend on fibre properties AND degree of load Polymers with fillers = large-particle composites transmittance to fibre - ! Load transmittance ends at fibre ends: Concrete Critical fibre length: For effective reinforcement, particles -> small + evenly distributed lc- critical length Rule of mixtures: d- fibre diameter Predict that elastic modulus should fall between upper bound: mu*f- tensile stress Tc-fibre-matrix bond strength As fibre length l increases, fibre reinforcement becomes more effective Continuous fibres = fibre length longer than critical length And lower bound: Discontinuous/short fibres = fibre length shorter than critical length E – elastic modulus; V - volume fraction; c – composite; m – matrix; p - particulate phase Cermets = ceramic-metal composites If fibre lengths are significantly shorter than the critical length, there Used extensively as cutting tools (extremely hard particles is virtually no stress transference and little reinforcement = of a refractory ceramic) particulate composites Toughness enhanced by ductile metal matrix Influence of fibre orientation and concentration Discontinuous, randomly orientated fibre composites 2 extremes of fibre orientation Rule of mixtures is used with K (a fibre efficiency parameter) Summary - parallel alignment of longitudinal axis of fibres Fibrous composites = anisotropic -> max strength + reinforcement achieved along - totally random alignment alignment (longitudinal direction) Continuous and aligned fibre composites In transverse direction, fibre reinforcement = virtually nonexistent + fracture Tensile stress – strain behaviour – longitudinal loading occurs at relatively low stresses f- fibre The Fibre Phase m- matrix Small diameter fibre is much stronger than bulk material -> decreased Initially, matrix & fibres probability of critical surface flaw that leads to fracture on small volume deform elastically (linear). Whiskers = thin single crystals with large length to diameter ratios Matrix yields – deforms Exceptionally strong, high crystalline perfection, very expensive plastically. Fibre continues Fibres = polycrystalline or amorphous with very small diameters deforming elastically, while Wires = relatively large diameters, often radial steel reinforcement Tensile strength fibre >> The Matrix Phase Yield strength matrix Metal, polymer or ceramic – generally metals and polymers -> ductility – binds fibres together and transmits, distributes external stress to fibres – protects fibres from surface damage (! leads to fractures) Elastic behaviour – longitudinal loading – prevents cracks from propagating from fibre to fibre Elastic modulus for continuous, aligned fibre in longitudinal direction: Strong bonding forces are crucial to prevent fibre pullout or Polymer-matrix composites matrix = polymer resin || reinforcement = fibres Ratio of load carried by fibre ------------------------→ Greatest diversity of applications, and in greatest quantity Elastic behaviour – transverse loading Good room-temperature properties, ease of fabrication, lower cost Isostress state = stress to which fibre&matrix is exposed is equal Fibre material: glass / carbon / aramid Modulus of elasticity in transverse direction: Metal-matrix composites Longitudinal tensile strength Matrix = ductile metal Often corresponds to point at Higher service temperatures than base metals which fibres fracture Reinforcement improves stiffness, strength, resistance, conductivity If fibres fracture, load is transferred fully to matrix Matrix = super alloys and alloys of Al, Mg, Ti, Cu Composite tensile strength: particulates| discontinuous/continuous fibres| whiskers sigma ’ m = stress in matrix at fibre failure Processing -> 1) Consolidation/synthesis, 2) shaping Transverse tensile strength Materials are low density (good for aerospace applications) Reinforcement of fibres is negative at this stage Ceramic-matrix composites Influenced by matrix/fibre properties, matrix-fibre bond strength, Ideal for high temperature and severe temperature applications presence of voids Fracture toughness of ceramics are improved by reinforcement Improve transverse tensile strength by improving matrix Carbon-carbon composites Discontinuous and aligned fibres New and expensive -> very complex processing Common in commercial market despite lower reinforcement efficiency High tensile strengths retained at high temps, low susceptibility to Longitudinal strength (fibre length > critical length: thermal shock, large fracture toughness and high creep resistance sigma*f – fracture strength of fibre Hybrid composites sigma‘m – stress in matrix when comp fails 2 or more different fibres in a single matrix –most commonly: glass and Longitudinal strength (fibre length > critical length: carbon fibres in a polymeric resin Stronger, tougher, higher impact resistance, lower cost processing  Chapter 17 – Fabrication & Processing ▪ Investment casting Pattern is made of wax or plastic with a low melting T. Fluid slurry is Fabrication of Metals poured over it and sets to become the mold. Mold is heated and (Normally preceded by refining, alloying or heat treatment processes) pattern burns out, leaving mold cavity of desired shape ▪ Lost foam casting Expendable pattern is a foam. Sand is packed around pattern to form mold. Molten metal is poured into mold to replace foam, which vaporises. ▪ Continuous casting The refined metal is cast into a long continuous strand, solidifies in Forming operations water-cooled die. Used later in subsequent forming processes/ ▪ = shape of metal is changed by plastic deformation (induced by external force or stress, magnitude of which exceeds material’s yield strength) Miscellaneous techniques ▪ Hot working – deformation is achieved at a temperature above that at ▪ Powder metallurgy which recrystallization occurs Powdered metal is compacted in desired shape then sintered to ▪ Cold working – below recrystallization T; produces increase in strength, produce a denser, stronger piece. Suitable for metals of low decrease in ductility, better surface finish ductilities or very high melting temperatures ▪ Forging ▪ Welding Mechanically working / deforming a single metal piece of unusually Two or more metal pieces are joined metallurgically hot metal -> accomplished by the application of successive blows or Arc welding, gas welding, brazing, soldering, laser beam welding continuous squeezing ▪ Rolling Piece of metal is passed between two rolls. The compressive stresses Thermal Processing of Metals exerted by the rolls results in a reduction of thickness Annealing processes ▪ Extrusion ▪ = heat treatment in which material is exposed to an elevated A bar of metal is forced through a die orifice by a ram. The extruded temperature for an extended period of time and then slowly cooled piece has the desired shape and reduced cross-sectional area ▪ Carried out to -> relieve stresses -> increase softness, ductility and ▪ Drawing toughness -> produce a specific microstructure Metal is pulled through a die having a tapered bore. Reduction in ▪ Time is important -> if rate of temperature change is too great, internal cross section and increase in length. stresses may be induced that lead to warping or cracking ▪ Annealing may be accelerated at higher temperatures because diffusion is involved Casting ▪ Process annealing ▪ = molten metal is poured into a mold cavity of the desired shape, upon Used to negate the effects of cold work -> to soften and increase solidification, metal assumes shape of the mold but experiences some ductility of a previously strain-hardened metal shrinkage Used in processes that require extensive plastic deformation. ▪ Used when -> finished shape is large or complicated Recovery and recrystallization are allowed to occur, but heat -> alloy is low in ductility treatment is usually terminated before grain growth as fine grains ▪ Sand casting are sometimes preferable Ordinary sand is mold material. Gating system is incorporated to ▪ Stress relief expediate metal flow and minimise defects At relatively low temperature ▪ Die casting Used to eliminate residual stresses that developed as a result of Liquid metal is forced into a mold under pressure at a high velocity machining or grinding, nonuniform cooling, or a phase and allowed to solidify with the pressure maintained. transformation where the two phases have different densities. Rapid casting rates are possible & dies are reusable ▪ Annealing of ferrous alloys ▪ Precipitation heat treating – the -> Normalising supersaturated alpha solid solution used to refine the irregular grain shapes produced by plastic is heated to an intermediate deformation - at 55 degrees C above upper critical T temperature within two phase region Metal is allowed to turn into austenite then cooled in air where diffusion rates become -> Full anneal appreciable. The beta precipitate to prepare low/medium carbon steels for extensive plastic phase begins to form as finely deformation. Heated to 50 degrees above A1, then furnace dispersed particles cooled results in coarse pearlite that is relatively soft&ductile -> Spheroidizing Ceramics heat-treated for cementite particles to coalesce into spheres ! Impractical to cast – melting temps too high so metal can be more conveniently deformed. Happens at Brittleness of ceramics prelude deformation temperatures close to eutectoid. Prior cold work increases Fabrication & processing of glass and glass-ceramics spheroidizing reaction rate Glass properties Heat treatment of steels Upon cooling, glass becomes more and more viscous -> no definite temp ▪ -> in order to produce a high content of martensite for best combination at which glass becomes solid mechanical characteristics For crystalline materials – discontinuous decrease of volume at melting ▪ Hardenability temperature || For glassy materials – volume decreases continuously = measure of the depth to which a ferrous alloy may be hardened with temperature reduction by the formation of martensite upon quenching from a T above the glass transition temperature – below this point, material is considered upper critical temperature as glass -> Jominy End-Quench Test: A specimen is austenitized, removed Viscosity vs temperature from furnace, attached to fixture, and the lower end is quenched by a jet of water