Materials – Structure, Behavior and Properties PDF

MATERIALS – STRUCTURE, BEHAVIOR AND PROPERTIES T0026T - PRODUCTION, AUTOMATION AND MATERIALS Kumar Babu Surreddi  Materials ??...

MATERIALS – STRUCTURE, BEHAVIOR AND PROPERTIES T0026T - PRODUCTION, AUTOMATION AND MATERIALS Kumar Babu Surreddi  Materials ?? Science-driven approach 10-14 m 10-2 m Design-driven approach Ansys Granta EduPack  support both approaches L U L E Å T E K N I S K A U N I V E R S I T E T 2  Properties Michael Ashby et al, Materials engineering science, processing and design L U L E Å T E K N I S K A U N I V E R S I T E T 3  Properties Michael Ashby et al, Materials engineering science, processing and design L U L E Å T E K N I S K A U N I V E R S I T E T 4  Material groups L U L E Å T E K N I S K A U N I V E R S I T E T 5  Materials Different materials used in the car L U L E Å T E K N I S K A U N I V E R S I T E T 6  Materials Metals from liquid state Source: Courtesy of NASA. Turbine blades for jet engines, manufactured by three different methods: left: conventionally cast; center: directionally solidified, with columnar grains as can be seen from the vertical streaks, and right: single crystal. Schematic illustration of the stages during the solidification of molten metal; each small square represents a unit cell. (a) Nucleation of crystals at random sites in the molten metal; note that the crystallographic orientation of each site is different. (b) and (c) Growth of crystals as solidification continues. (d) Solidified metal, showing individual grains and grain boundaries; note the different angles at which neighboring grains meet each other. L U L E Å T E K N I S K A U N I V E R S I T E T 7  An outline of the engineering materials L U L E Å T E K N I S K A U N I V E R S I T E T 8  An outline of the behavior and the manufacturing properties of materials L U L E Å T E K N I S K A U N I V E R S I T E T 9  The Structure of Metals Types of Atomic Bonds The Crystal Structure of Metals Deformation and Strength of Single Crystals Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working L U L E Å T E K N I S K A U N I V E R S I T E T 10  The Structure of Metals Types of Atomic Bonds Atoms can transfer or share electrons; in doing so, multiple atoms The Crystal Structure of Metals combine to form molecules. Molecules are held together by Deformation and Strength of Single Crystals attractive forces called bonds, which act through electron interaction. Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working Type of chemical bonds Ionic bonds Covalent bonds Metallic bonds Van der Waals forces Hydrogen bond https://www.chemistrylearner.com/chemical-bonds L U L E Å T E K N I S K A U N I V E R S I T E T 11  The Structure of Metals When metals solidify from a molten state, the atoms arrange themselves Types of Atomic Bonds into various orderly configurations, called crystals. The Crystal Structure of Metals This atomic arrangement is called crystal structure or crystalline Deformation and Strength of Single Crystals structure. Grains and Grain Boundaries The smallest group of atoms showing the characteristic lattice structure of a particular metal is known as a unit cell. Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working bcc The following are the three basic atomic arrangements in metals: Body-centered cubic (bcc): alpha iron, chromium, molybdenum, tantalum, tungsten, and vanadium. fcc Face-centered cubic (fcc): gamma iron, aluminum, copper, nickel, lead, silver, gold, and platinum. Hexagonal close-packed (hcp): beryllium, cadmium, hcp cobalt, magnesium, alpha titanium, zinc, and zirconium. (a) hard-ball model, (b) unit cell, and (c) single crystal with many unit cells. L U L E Å T E K N I S K A U N I V E R S I T E T 12  The Structure of Metals Elastic deformation: it returns to its original shape when the Types of Atomic Bonds force is removed. The Crystal Structure of Metals plastic deformation or permanent deformation: it does not Deformation and Strength of Single Crystals return to its original shape when the force is removed. Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working Permanent deformation of a single crystal under a tensile load; the highlighted grid of atoms emphasizes the movement that occurs within the lattice. (a) Deformation by slip. The b/a ratio influences the magnitude of the shear stress required to cause slip. (b) Deformation by twinning, involving the generation of a “twin” around a line of symmetry subjected to shear. Note that the tensile load results in a shear stress in the plane illustrated. Anisotropy: When crystal exhibits different properties when tested in different directions. L U L E Å T E K N I S K A U N I V E R S I T E T 13  The Structure of Metals Types of Atomic Bonds Elastic deformation: it returns to its original shape when the The Crystal Structure of Metals force is removed. Deformation and Strength of Single Crystals plastic deformation or permanent deformation: it does not Grains and Grain Boundaries return to its original shape when the force is removed. Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Tensile testing Cold, Warm, and Hot Working The equipment measures the ratio between applied force (load cell) and resulting strain (extensometer) : Stress – Strain diagram L U L E Å T E K N I S K A U N I V E R S I T E T 14  The Structure of Metals Types of Atomic Bonds The Crystal Structure of Metals Deformation and Strength of Single Crystals Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working Solidified metal, showing individual grains and Crystal defects › Dislocations › grain boundaries stresses in the matrix! Dislocations moving in a coarse- If the path to the grain boundary is grained material are not slowed down short, the dislocations are slowed as quickly as in the fine-grained down quickly → higher yield strength material → lower yield strength Smaller grain size → more barriers to dislocation movement Hall-Petch equation σy =σ0 +kyd -1/2 σy = yield strength d = average grain diameter σ0, ky = material constants Wiley - VMSE (drbuc2jl8158i.cloudfront.net) L U L E Å T E K N I S K A U N I V E R S I T E T 15  The Structure of Metals Types of Atomic Bonds The Crystal Structure of Metals Deformation and Strength of Single Crystals Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working (a) Schematic illustration of a crack in sheet metal that has been subjected to bulging (caused, for example, by pushing a steel ball against the sheet). Note the orientation of the crack with respect to the rolling direction of the sheet; this sheet is anisotropic. Plastic deformation of idealized (equiaxed) grains in a (b) Aluminum sheet with a crack (vertical dark line at the center) specimen subjected to compression (such as occurs in the developed in a bulge test; the rolling direction of the sheet was vertical. forging or rolling of metals): (a) before deformation and Source: Courtesy of J.S. Kallend, Illinois Institute of Technology. (b) after deformation. Note the alignment of grain boundaries along a horizontal direction, an effect known as preferred orientation. L U L E Å T E K N I S K A U N I V E R S I T E T 16  The Structure of Metals Types of Atomic Bonds The Crystal Structure of Metals Deformation and Strength of Single Crystals Grains and Grain Boundaries Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Cold, Warm, and Hot Working Schematic illustration of the effects of recovery, recrystallization, and grain growth on mechanical properties and on the shape and size of grains; note the formation of small new grains during recrystallization. Source: After G. Sachs. L U L E Å T E K N I S K A U N I V E R S I T E T 17  The Structure of Metals Types of Atomic Bonds Cold working refers to plastic deformation carried out at The Crystal Structure of Metals room temperature. Deformation and Strength of Single Crystals When deformation occurs above the recrystallization Grains and Grain Boundaries temperature, it is called hot working. Plastic Deformation of Polycrystalline Metals Recovery, Recrystallization, and Grain Growth Warm working is carried out at intermediate temperatures; Cold, Warm, and Hot Working thus, warm working is a compromise between cold and hot working. Homologous Temperature Ranges for Various Processes L U L E Å T E K N I S K A U N I V E R S I T E T 18 L U L E Å T E K N I S K A U N I V E R S I T E T 19  An outline of the Mechanical Behavior, Testing, and Manufacturing Properties of Materials Tension Compression Torsion Bending (Flexure) Hardness Fatigue Creep Impact Failure and Fracture of Materials Residual Stresses L U L E Å T E K N I S K A U N I V E R S I T E T 20  Relative Mechanical Properties of Various Materials L U L E Å T E K N I S K A U N I V E R S I T E T 21  Tension testing FIGURE 2.1 (a) A standard tensile-test specimen before and after pulling, showing original and final gage lengths. (b) Stages in specimen behavior in a tension test. L U L E Å T E K N I S K A U N I V E R S I T E T 22  Stress-strain Curves Engineering stress Engineering strain Yield stress Tensile strength or Ultimate Tensile Strength Fracture stress Hooke’s law Modulus of elasticity or Young’s modulus Poisson’s ratio Ductility ‒ total elongation FIGURE 2.2 A typical stress–strain curve obtained from a tension test, showing various features. ‒ reduction of area applied load, P original gage length, lo True stress and true strain instantaneous length of the specimen, l original cross-sectional area, Ao L U L E Å T E K N I S K A U N I V E R S I T E T 23  Mechanical Properties of Metallic Materials at Room Temperature L U L E Å T E K N I S K A U N I V E R S I T E T 24  Mechanical Properties of Non-Metallic Materials at Room Temperature L U L E Å T E K N I S K A U N I V E R S I T E T 25  Construction of Stress–strain Curves FIGURE 2.4 (a) Load–elongation curve in tension testing of a stainless steel specimen. (b) Engineering stress–engineering strain curve, drawn from the data in Fig. 2.4a. (c) True stress–true strain curve, drawn from the data in Fig. 2.4b. Note that this curve has a positive slope, indicating that the material is becoming stronger as it is strained. (d) True stress–true strain curve plotted on log–log paper and based on the corrected curve in Fig. 2.4c. The correction is due to the triaxial state of stress that exists in the necked region of the specimen. The true stress–true strain curve in Fig. 2.4c can be represented by the equation 𝜎𝜎 = 𝐾𝐾𝜀𝜀 𝑛𝑛 where K is the strength coefficient and n is the strain-hardening or work-hardening exponent. L U L E Å T E K N I S K A U N I V E R S I T E T 26  Tension FIGURE 2.5 True stress–true strain curves in tension at room temperature for various metals; the curves start at a finite level of stress. The elastic regions have too steep a slope to be shown in this figure; thus, each curve starts at the yield stress, Y, of the material. L U L E Å T E K N I S K A U N I V E R S I T E T 27  Effect of temperature on Mechanical Properties Increasing the temperature generally has the following effects on stress–strain curves a) The ductility and toughness increase b) The yield strength and modulus of elasticity decrease Temperature also affects the strain-hardening exponent, n, of most metals, in that it increases with increasing temperature. FIGURE 2.6 Effect of temperature on mechanical properties of a carbon steel; most materials display similar temperature sensitivity for elastic modulus, yield strength, ultimate strength, and ductility. L U L E Å T E K N I S K A U N I V E R S I T E T 28  Effects of Rate of Deformation and Strain Rate FIGURE 2.7 The effect of strain rate on the ultimate tensile strength for aluminum. Note that, as the temperature increases, the slopes of the curves increase; thus, strength becomes more and more sensitive to strain rate as temperature increases. Source: J.H. Hollomon. L U L E Å T E K N I S K A U N I V E R S I T E T 29  Compression FIGURE 2.9 Disk test on a brittle material, showing the direction of loading and the fracture path. FIGURE 2.8 Barreling in compressing a round solid cylindrical specimen (7075-O aluminum) between flat dies. Barreling is caused by friction at the die–specimen interfaces, which retards the free flow of the material. L U L E Å T E K N I S K A U N I V E R S I T E T 30  Torsion The shear stress, τ, can be calculated from the formula 𝑇𝑇 𝜏𝜏 = 2𝜋𝜋𝑟𝑟 2 𝑡𝑡 where T is the torque applied, r is the average radius of the tube, and t is the thickness of the tube at its narrow section. The shear strain, γ, can be calculated from the formula 𝑟𝑟𝑟𝑟 𝛾𝛾 = 𝑙𝑙 where l is the length of the tube section and 𝜙𝜙 the angle of twist in radians. FIGURE 2.10 A typical torsion-test specimen; it is mounted between the two heads of a The ratio of the shear stress to the shear strain in the elastic range is testing machine and twisted; note the shear known as the shear modulus or modulus of rigidity, G. deformation of an element in the reduced The shear modulus, or modulus of rigidity, G section of the specimen. 𝑬𝑬 𝑮𝑮 = 𝟐𝟐(𝟏𝟏 + 𝝂𝝂) L U L E Å T E K N I S K A U N I V E R S I T E T 31  Bending (Flexure) FIGURE 2.11 Two bend-test methods for brittle materials: (a) three-point bending and (b) four-point bending. The areas over the beams represent the bending-moment diagrams, described in texts on the mechanics of solids. Note the region of constant maximum bending moment in (b); by contrast, the maximum bending moment occurs only at the center of the specimen in (a). L U L E Å T E K N I S K A U N I V E R S I T E T 32  Hardness Hardness is generally defined as resistance to permanent indentation. Hardness, however, is not a fundamental property. It gives a general indication of the strength of the material and of its resistance to scratching and wear. Hardness Tests Brinell Test Rockwell Test Vickers Test Knoop Test Leeb Test Mohs Hardness Shore Test and Durometer FIGURE 2.12 A selection of hardness Hot Hardness testers. (a) A Micro Vickers hardness tester, (b) Rockwell hardness tester and (c) Leeb tester. Source: (a) and (b) Courtesy of Buehler and (c) Courtesy of Wilson® Instruments. L U L E Å T E K N I S K A U N I V E R S I T E T 33  Hardness FIGURE 2.13 General characteristics of hardness-testing methods and formulas for calculating hardness. L U L E Å T E K N I S K A U N I V E R S I T E T 34  Hardness FIGURE 2.14 Indentation geometry in Brinell hardness testing: (a) annealed metal, (b) work-hardened metal, and (c) deformation of mild steel under a spherical indenter. Note that the depth of the permanently deformed zone is about one order of magnitude larger than the depth of indentation; for a hardness test to be valid, this zone should be fully developed in the material. Source: After M.C. Shaw and C.T. Yang. L U L E Å T E K N I S K A U N I V E R S I T E T 35  Hardness FIGURE 2.15 Chart for converting various hardness scales; note the limited range of most of the scales. Because of the many factors involved, these conversions are approximate. L U L E Å T E K N I S K A U N I V E R S I T E T 36  Fatigue FIGURE 2.17 Ratio of endurance limit FIGURE 2.16 (a) Typical S–N curves for two metals; note that, unlike steel, to tensile strength for various metals, aluminum does not have an endurance limit. (b) S–N curves for some polymers. as a function of tensile strength. L U L E Å T E K N I S K A U N I V E R S I T E T 37  Creep Creep is the permanent elongation of a material under a static load maintained for a period of time. FIGURE 2.18 Schematic illustration of a typical creep curve; the linear segment of the curve (secondary) is used in designing components for a specific creep life. L U L E Å T E K N I S K A U N I V E R S I T E T 38  Impact FIGURE 2.19 Impact test specimens. (a) Izod; (b) Charpy. L U L E Å T E K N I S K A U N I V E R S I T E T 39  Failure and Fracture of Materials FIGURE 2.20 Schematic illustration of types of failures in materials: (a) necking and fracture of ductile materials, (b) buckling of ductile materials under a compressive load, (c) fracture of brittle FIGURE 2.21 Schematic illustration of the types of materials in compression, and (d) cracking on the barreled surface fracture in tension: (a) brittle fracture in polycrystalline of ductile materials in compression. metals, (b) shear fracture in ductile single crystals—see also Fig. 1.6a, (c) ductile cup-and-cone fracture in polycrystalline metals, and (d) complete ductile fracture in polycrystalline metals, with 100% reduction of area. L U L E Å T E K N I S K A U N I V E R S I T E T 40  Ductile Fracture FIGURE 2.23 Sequence of events in the necking and fracture of a tensile-test specimen: (a) early stage of necking; (b) small voids begin to form within the necked region; (c) voids coalesce, producing an internal crack; (d) the rest of FIGURE 2.22 Surface of ductile fracture the cross-section begins to fail at the periphery, by shearing; and (e) the final inlow-carbon steel, showing dimples. fracture, known as a cup- (top fracture surface) and cone- (bottom surface) Fracture is usually initiated at impurities, fracture, surfaces. inclusions, or pre-existing voids (microporosity) in the metal. Source: Courtesy of K.-H. Habig and D. Klaffke. L U L E Å T E K N I S K A U N I V E R S I T E T 41  Ductile Fracture – Effects of Inclusions FIGURE 2.24 Schematic illustration of the deformation of soft and hard inclusions and of their effect on void formation in plastic deformation. Note that, because they do not conform to the overall deformation of the ductile matrix, hard inclusions can cause internal voids. L U L E Å T E K N I S K A U N I V E R S I T E T 42  Transition temperature FIGURE 2.25 Schematic illustration of transition temperature in metals. L U L E Å T E K N I S K A U N I V E R S I T E T 43  Brittle Fracture FIGURE 2.26 Fracture surface of steel that has failed in FIGURE 2.27 Intergranular fracture, at two different a brittle manner; the fracture path is transgranular magnifications; grains and grain boundaries are clearly visible in (through the grains). Magnification: 200x. this micrograph. The fracture path is along the grain Source: Courtesy of B.J. Schulze and S.L. Meiley and boundaries. Magnification: left, 100x; right, 500x. Packer Engineering Associates, Inc. Source: Courtesy of B.J. Schulze and S.L. Meiley and Packer Engineering Associates, Inc. L U L E Å T E K N I S K A U N I V E R S I T E T 44  Fatigue Fracture FIGURE 2.28 Typical fatigue-fracture surface on FIGURE 2.29 Reductions in the fatigue strength of cast metals, showing beach marks. Magnification: left, 500x; steels subjected to various surface-finishing operations. Note right, 1000x. Source: Courtesy of B.J. Schulze and S.L. that the reduction becomes greater as the surface roughness Meiley and Packer Engineering Associates, Inc. and the strength of the steel increase. Source: M.R. Mitchell. L U L E Å T E K N I S K A U N I V E R S I T E T 45  Residual Stresses Residual stresses are stresses that remain within a part after it has been shaped and all the external forces (applied through tools and dies) are removed. May develop when workpieces are subjected to plastic deformation that is not uniform throughout the part. Can be developed by temperature gradients within the part, such as occur during cooling of castings or a hot forgings. Tensile residual stresses  generally undesirable  lower the fatigue life and fracture strength of the parts made. Compressive residual stresses on a surface  desirable  increase the fatigue life of components  shot peening or surface rolling Figure 2.31: Distortion of parts with residual stresses after cutting or slitting: (a) flat sheet or plate; (b) solid round rod; (c) thin-walled tubing or pipe. L U L E Å T E K N I S K A U N I V E R S I T E T 46 L U L E Å T E K N I S K A U N I V E R S I T E T 47  Physical Properties of Materials The density of a material is its mass per unit volume 𝑚𝑚 Density, ρ = (g/cc or kg/m3) 𝑉𝑉 Specific gravity is the material’s density relative to that of water 𝜌𝜌 Specific gravity = (no units) 𝜌𝜌𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 Important in weight saving and fuel economy  aircraft, aerospace, automotive structures, sports equipment, and for various other products strength-to-weight ratio (specific strength) and stiffness-to- weight ratio (specific stiffness) of materials Important for high-speed equipment, such as magnesium in printing and textile machinery, low density ceramics for components in high-speed automated machinery and machine tools Important where weight is desirable - counterweights for various mechanisms (using lead or steel), flywheels, ballasts on ships and aircraft, and weights on golf clubs (using high-density materials such as tungsten) L U L E Å T E K N I S K A U N I V E R S I T E T 48  Physical Properties of Materials The idea of a material property chart: Young’s modulus E is plotted against the density ρ on log scales. Each material class occupies a characteristic field. L U L E Å T E K N I S K A U N I V E R S I T E T 49  Physical Properties of Materials L U L E Å T E K N I S K A U N I V E R S I T E T 50  Physical Properties of Materials The thermal conductivity λ measures the flux of heat driven by a temperature gradient dT/dX. The linear-thermal expansion coefficient α The heat capacity—the energy to raise the measures the change in length, per unit temperature of 1 kg of material by 1°C. length, when the sample is heated. L U L E Å T E K N I S K A U N I V E R S I T E T 51  Physical Properties of Materials Melting point Pure metals have a definite value Alloys have a range (depending upon their composition) Plastics have lowest useful temperature range Ceramics, graphite, and refractory-metal alloys have the highest useful range effect on manufacturing operations (annealing, heat treatment, hot working) important in the selection of tool and die materials casting electrical-discharge machining process Specific strength (tensile strength/density) for a Specific heat variety of materials as a function of temperature. Note the useful temperature Energy required to raise the temperature of a unit mass by one degree (J/kgK) range for these materials and the high values for composite materials. MMC= metal-matrix composite; FRP= fiber-reinforced plastic. Excessive temperature rise in a work piece can: decrease product quality (affecting surface finish and dimensional accuracy) cause excessive tool wear and die wear result in undesirable metallurgical changes in the material L U L E Å T E K N I S K A U N I V E R S I T E T 52  Physical Properties of Materials Thermal conductivity Rate at which heat flows through a material Metals have high values; Alloying elements can have a significant effect on the thermal conductivity of alloys Plastics and ceramics have poor conductivity High thermal conductivity is desirable: cooling fins, cutting tools, die casting molds Low thermal conductivity is desirable: furnace lining, insulation, handles for pots, coffee cups and handles for pots and pans Thermal expansion Inversely proportional to melting point; Alloying elements have a relatively minor effect on the thermal expansion of metals. The tendency of matter to change in shape, volume, and area in response to a change in temperature important in forging processes (hot work piece placed on a cold die) may lead to cracking/warping/loosening of components low expansion alloys (iron nickel alloys) Invar (64% Fe–36% Ni) & Kovar (54% Fe–28% Ni–18% Co)  2×10−6 to 9×10−6 per °C Coefficient of thermal expansion (α) the ratio of the fractional change in size of a material to its change in temperature represented by the symbol α (alpha) for solids and β (beta) for liquids SI unit inverse kelvin (K−1 or 1/K) or non SI unit inverse degree Celsius (°C−1 or 1/°C) ∆𝑙𝑙 = 𝑙𝑙0 𝜶𝜶∆𝑇𝑇 Significant effects in assemblies  electronic and computer components, glass-to-metal seals, struts on jet engines, coatings on cutting tools, and moving parts in machinery that require certain clearances for proper functioning Shrink fits utilize thermal expansion and contraction; a shrink fit is a part, often a sleeve or a hub, that is to be installed over a shaft. Thermal stresses (due to temperature gradients) - a combination of high thermal conductivity and low thermal expansion is desirable to reduce thermal stresses L U L E Å T E K N I S K A U N I V E R S I T E T 53  Electrical Properties Electrical resistivity, ρe, is measured as the potential gradient, V/L, divided by the current density, i/A. Dielectric constant: a measure of the ability of an insulator to polarize. Dielectric loss, important in dielectric heating L U L E Å T E K N I S K A U N I V E R S I T E T 54  Electrical Properties Dielectric Strength  a measure of the electrical strength of an insulator. An electrically insulating material’s dielectric strength is the largest electric field to which it can be subjected without degrading or losing its insulating properties. This property is defined as the voltage required per unit distance for electrical breakdown and has the units of V/m. Conductors  Materials with high electrical conductivity, such as metals, are generally referred to as conductors. Electrical resistivity is the inverse of electrical conductivity; materials with high electrical resistivity are referred to as dielectrics or insulators. Superconductors  Superconductivity is the phenomenon of near-zero electrical resistivity that occurs in some metals and alloys below a critical temperature, often near absolute zero (0K,– 273°C). The highest temperature (-123°C) at which superconductivity has been exhibited to date is with an alloy of thallium, barium, calcium, copper, and oxygen;* The main application of superconductors is for high-power magnets, magnetic resonance imaging (MRI), magnetic levitation (maglev) trains, efficient power transmission lines, and components for extremely fast computers L U L E Å T E K N I S K A U N I V E R S I T E T 55  Electrical Properties The piezoelectric effect The pyroelectric effect Piezoelectric Effect is the ability of certain materials Pyroelectricity is the ability of certain to generate an electric charge in response to applied materials to generate a temporary voltage mechanical stress. when they are heated or cooled. Natural piezoelectric substrates, such as quartz single Gallium nitride (GaN), cesium nitrate (CsNO3), polyvinyl crystals; piezoelectric ceramics, such as lithium niobate, fluorides, derivatives of phenylpyridine, and cobalt gallium arsenide, zinc oxide, aluminium nitride and lead phthalocyanine. Lithium tantalate (LiTaO3) is a crystal zirconate-titanate (PZT) exhibiting both piezoelectric and pyroelectric properties polymer-film piezoelectrics, such as polyvinylidene fluoride (PVDF). L U L E Å T E K N I S K A U N I V E R S I T E T 56  Electrical Properties Semiconductors  crystalline solids intermediate in electrical conductivity between a conductor and an insulator extremely sensitive to temperature and the presence and type of minute impurities (called dopants) This property is utilized in semiconductor (solid-state) devices, used extensively in miniaturized electronic circuitry. Ferromagnetism  high permeability and permanent magnetization that are due to the alignment of iron, nickel, and cobalt atoms. Applications  electric motors, electric generators, electric transformers, and microwave devices Ferrimagnetism  a permanent and large magnetization exhibited by some ceramic materials, such as cubic ferrites. Piezoelectric Effect  Two basic behaviors - (a) When subjected to an electric current, these materials undergo a reversible change in shape, by as much as 4% and (b) when deformed by an external force, the materials emit a small electric current. Ex. PZT (lead zirconate titanate), barium titanate, and lithium niobate Applications - transducers, sensors, force or pressure transducers, inkjet printers, strain gages, sonar detectors, and microphones. L U L E Å T E K N I S K A U N I V E R S I T E T 57  Electrical, Magnetic, and Optical Properties Magnetostriction  The phenomenon of expansion or contraction of a material when it is subjected to a magnetic field. Materials such as pure nickel and some iron–nickel alloys exhibit this behavior. Magnetostriction is the principle behind ultrasonic machining equipment. Magneto-rheostatic and Electro-rheostatic Effects  When subjected to magnetic or electric fields, some fluids undergo a major and reversible change in their viscosity within a fraction of a second, turning from a liquid to an almost solid state. For example, magneto-rheostatic behavior is attained by mixing very fine iron filings with oil. Called smart fluids  developed for such applications as vibration dampeners, engine mounts, prosthetic devices, and clutches. Optical Properties  Transmittance, reflectance, absorptance and emittance… color and opacity are particularly relevant to polymers and glasses L U L E Å T E K N I S K A U N I V E R S I T E T 58  Corrosion Resistance Corrosion - the deterioration of metals and ceramics Degradation – the deterioration in plastics Resistance to corrosion depends on the composition of the material and on the particular environment Corrosive media  chemicals (acids, alkalis, and salts) and the environment (oxygen, moisture, pollution, and acid rain), including water (fresh or salt water). High corrosion resistance materials: non-ferrous metals, stainless steel and non-metallic materials Poor corrosion resistance materials: steel and cast iron Pitting: localized corrosion Intergranular corrosion: along grain boundaries of metals Two dissimilar metals may form a galvanic cell, that is, two electrodes in an electrolyte in a corrosive environment that includes moisture and cause galvanic corrosion. Two-phase alloys are more susceptible to galvanic corrosion (because of the physical separation of the two different metals involved) than are single-phase alloys or pure metals; as a result, heat treatment can have a significant beneficial influence on corrosion resistance. L U L E Å T E K N I S K A U N I V E R S I T E T 59  Corrosion Resistance Stress-corrosion cracking is an example of the effect of a corrosive environment on the integrity of a product that, as manufactured, had residual stresses. Likewise, cold-worked metals are likely to have residual stresses, hence they are more susceptible to corrosion than are hot worked or annealed metals. Tool and die materials also can be susceptible to chemical attack by lubricants and by coolants; the chemical reaction alters their surface finish and adversely influences the metalworking operation. Corrosion resistance of aluminum, titanium, and stainless steel: Aluminum develops a thin (a few atomic layers), strong, and adherent hard-oxide film (Al2O3) that better protects the surface from further environmental corrosion. Titanium develops a film of titanium oxide (TiO2). Stainless steels develop a protective chromium oxide film on their surfaces. These processes are known as passivation. When the protective film is scratched and exposes the metal underneath, a new oxide film begins to form. L U L E Å T E K N I S K A U N I V E R S I T E T 60  Case study – Selection of Materials for Coins There are six general criteria in the selection of materials for coins 1. The subjective factors, such as the appearance of the coin, its color, weight, and its ring (the sound made when striking). 2. The intended life of the coin is also a consideration; this duration will reflect resistance to corrosion and to wear while the coin in circulation. 3. The manufacturing of the coin includes factors such as the formability of the candidate coin materials, the life of the dies used in the coining operation, and the capability of the materials and processes to resist counterfeiting. 4. Another consideration is the suitability for use in coin-operated devices, such as vending machines and turnstiles. 5. Health issues must be considered. 6. A final consideration is the cost of raw materials and processing, and whether there is a sufficient supply of the coin materials. L U L E Å T E K N I S K A U N I V E R S I T E T 61  Suggested reading Kalpakjian, S., Schmid, S. R., Sekar, K. S. V. (2022). Manufacturing Engineering and Technology. United Kingdom: Pearson. ISBN: 9781292422299, 9781292422244, 1292422246 Chapter 1 The structure of Metals Chapter 2 Mechanical Behavior, Testing, and Manufacturing Properties of Materials Chapter 3 Physical Properties of Materials L U L E Å T E K N I S K A U N I V E R S I T E T 62 L U L E Å T E K N I S K A U N I V E R S I T E T 63