DM308 Lecture 3 - Production Techniques 2 PDF

DM308 Production Techniques 2 Lecture 3 – Microstructures Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Crystal structure and material properties In Lecture 2 we learned that the cr...

DM308 Production Techniques 2 Lecture 3 – Microstructures Dr. Vassili Vorontsov Department of Design, Manufacture and Engineering management, Faculty of Engineering, University of Strathclyde [email protected] Crystal structure and material properties In Lecture 2 we learned that the crystal structure of solid materials is important in determining their properties. Crystal structure is the arrangement of atoms/molecules in a material that obeys translational symmetry. (Amorphous = no symmetry) E.g. the different forms (allotropes) of solid carbon have markedly different properties. Diamond is hard, transparent and an electrical insulator, graphite is soft, opaque and conducts electricity. Crystal defects and material properties Disruptions in the crystal symmetry are known as crystalline defects. Defects have a strong effect on the properties of crystalline materials. E.g. Let us consider the mineral corundum – a crystalline form of aluminium oxide (alumina) Al2O3 Man-made sapphire crystal (No impurities) Blue natural sapphire crystal (Substitutional iron and titanium impurities) Natural ruby crystals (Substitutional chromium impurities) But this is not the whole story Most materials used by engineers are not single crystals, but rather polycrystalline. E.g. An alumina ceramic, such as the crucible below, will consist of numerous crystals of (a-Al2O3) corundum crystals. Bulk structure Microstructure 1mm Crystal structure What is a microstructure? There is no exact definition. One definition is that microstructure is the very fine, micrometre-scale structure of a material, not visible to the naked eye, that is revealed at magnifications greater than 25X. A microstructure is the arrangement of crystals, phases and defects within a material. The latter is a more flexible definition because some of the structural features can be observed with the naked eye. At the same time, these features may be smaller than a micrometre. Thus, thus the term microstructure may also encompass the nanostructure of the material. The microstructure defines the properties of a material in conjunction with the crystal structure and chemical composition. First we have to understand phases Phase = a region of space throughout which all physical properties of a material are essentially uniform. E.g. Steam, Water and Ice are Gaseous, Liquid and Solid phases of the chemical substance H2O. Gas Liquid Solid Transitions between phases Phase diagrams can help us understand transitions between phases of a substance/material at different conditions. 105 Pa Question Substances A and B are completely miscible in one another. (i.e. they mix/dissolve entirely in one another) Substance A has a melting point of 1000°C. Substance B has a melting 800°C. What will be the melting point of a 50:50 mixture/alloy of A and B? You may apply the rule of mixtures !!! But this is not what really happens. Temperature (°C) 1000 Liquid 900 800 Solid 0 100% A 50 Composition (wt.% B) 100% B There is no defined melting point Over a certain temperature range liquid coexists with solid. Liquid Temperature (°C) 1000 Liqui d +Soli d 800 Solid N.B. The solid formed is called a Solid Solution of A and B 0 100% A 50 Composition (wt.% B) 100% B There is no defined melting point Liquidus and solidus temperatures are observed instead of a fixed melting point. Liquid Temperature (°C) Tliquidus Tsolidus Freezing range Liqui d Liqu idus +Soli d line Solidu s line a Solid Solid solution of A and B 0 100% A 50% Composition (wt.% B) 100% B Cooling curves of pure A and B A when pure A and B are cooled from the melt to their freezing/melting points they enter a constant temperature (isothermal) state known as a thermal arrest as liquid transforms to the solid phase a. N.B. By convention solid phases are often assigned Greek letters. Pure A 1000 L a a Time Temperature (°C) Temperature (°C) L Pure B L 800 L a a Time Formation of new bonds on solidification releases energy, which is known as the latent heat of fusion. This released heat is responsible for the thermal arrest. The higher melting point of A suggests that bonds between A atoms are stronger than those between B atoms. Cooling of an alloy of A and B No thermal arrest is observed when an alloy of mutually soluble A and B solidifies to form the solid solution a. Pure A and B Alloy of A and B 1000 800 L L L a a a a Time Temperature (°C) Temperature (°C) L L Tliquidus L a Tsolidus a Time Solidification of the alloy results in the formation of A-A, B-B as well as A-B bonds the proportions of which changes until all liquid is transformed into solid. Consider the types of solid solution There are two principal types of solid solution: Substitutional Interstitial Solute atoms substitute into the host crystal lattice Solute atoms occupy the (interstitial) spaces between host lattice atoms Similarly-sized atoms Smaller solute atoms Microstructure formation 100% liquid L Temperature (°C) Liquid first solid crystals form S L+a last liquid between solid crystals S S S solid crystals grow S S S S S S S S S S S S S S S a S S 100% solid crystals 0 100% A S S S S 50% Composition (wt.% B) 100% B Tie lines give phase compositions Liquid Temperature (°C) 1000 L+a 800 a Composition of first solid to freeze 0 100% A Composition of alloy 50% Composition (wt.% B) Composition of last liquid to freeze 100% B Question For a 50:50 alloy of A and B, what are the proportions of the two phases (liquid and solid) at 900°C? Lever Rule If an alloy/material contains two phases, the amount of each phase can be calculated using the Lever Rule. The composition of the alloy, Calloy , is the fulcrum of the lever. The compositions of the phases, C1 and C2 , are the two opposing ends of the lever. Calloy C1 C2 Thus we have: fraction of Phase 1: fraction of Phase 2: Plugging in the numbers From the phase diagram we have: Calloy = 50% Csolid = 34% Cliquid = 67% Calloy = 50% Csolid = 34% Cliquid = 67% Thus we have: fraction of Solid = (67 – 50) / (67 – 34) = 17 / 33 = 0.515 = 51.5% fraction of Liquid = (50 – 34) / (67 – 34) = 16 / 33 = 0.4848... = 48.5% E.g. Cu-Ni system Temperature (°C) Liquid a (Cu) Composition (wt% Ni) (Ni) Question What if A and B are insoluble in one another? Phase diagram with a miscibility gap N.B. A miscibility gap can exist for liquid and solid solutions alike. in fB lim it o l ity So lu b B A and B partially soluble in Miscibility gap fA ilit y it o im Temperature (°C) il lub So A A and B completely soluble A and B completely insoluble below this Temperature 0 100% A Composition (wt.% B) 100% B Consider partial solubility A solution a can decompose into enriched solutions a1 and a2 on cooling. Temperature (°C) a 0 100% A a1 + a 2 A-rich solution a1 B-rich solution Composition (wt.% B) a2 100% B The lever rule still applies Temperature (°C) a a1 + a 2 T Ca1(T) 0 100% A a1 Calloy Composition (wt.% B) Ca2(T) a2 100% B E.g. Cu-Ni system At lower temperatures the Cu-Ni phase diagram exhibits a miscibility gap. Liquid L+a a a1+ a2 Cu Ni Question So when do we observe solid solubility in alloys? Solid solubility rules Chemical affinity factor — the solvent and solute elements must not react chemically to form intermediate compounds. Relative valency factor — the solute and solvent elements must have the same valency. Electronegativity factor — the solute and solvent elements should have similar electronegativity. Also known as Hume-Rothery rules, after metallurgist William Hume-Rothery Relative size factor — the atomic radii of the solvent and solute elements must be less than 15%. Lattice type factor — the solute and solvent elements must have the same crystal structure. N.B. Exceptions exist to the above rules. If the rules aren’t obeyed E.g. The metals Ni, Cu, Pt, Ag and Au satisfy all the solid solution rules except one: they have different atomic radii. Increasing difference in atomic radii between A and B e.g. Cu-Ni e.g. Ni-Pt e.g. Au-Ni e.g. Cu-Ag Failure to satisfy the Hume-Rothery rules results in the appearance of miscibility gaps and congruent or eutectic melting points (red dots) when the alloy melts/freezes at a single temperature. Since most binary combinations of elements are unlikely to satisfy all the Hume-Rothery rules, the fourth type of phase diagram (Eutectic System) ends up being quite common and is therefore important. The eutectic system a Liq uid us Liquid L+a L+b b Eutectic point 0 100% A a+b Eutectic composition Solid solub limit ility Temperature (°C) us lid So Eutectic temperature Eutectic means “easy-melting” in Greek. A eutectic system has a eutectic (melting) point that is lower than those of the pure components. Composition (wt.% B) 100% B The eutectic alloy cooling curve Solidification of an alloy with the eutectic composition exhibits a thermal arrest due to the eutectic invariant reaction. L Teutectic L a+b aa + b Time Solidifying a eutectic alloy A eutectic alloy solidifies into a two-phase lamellar (layered) structure of immiscible solid solutions a and b, rich in A and B respectively. 100% liquid L Temperature (°C) Liquid a 0 100% A L+a L+b b L→ a + b a+b Lamellae of a and b Composition (wt.% B) 100% B Lamellar eutectic growth As A-rich a phase solidifies excess B diffuses a short distance laterally and incorporated into the B-rich b phase. Similarly A atoms are rejected ahead of b and diffuse into adjacent a lamellae. b B A B A B A B A a b a b Amounts of a and b in the eutectic As with all two-phase regions the lever rule is used to calculate the phase fractions: Temperature (°C) Liquid a L+a L+b b a+b Ca(T) 0 100% A Ceutectic Cb(T) Composition (wt.% B) 100% B Off-eutectic solidification Non-eutectic compositions highlighted by the regions below will follow one of three possible solidification patterns to produce different microstructures. Temperature (°C) Liquid a 0 100% A L+a hypo-eutectic compositions L+b b hyper-eutectic compositions a+b Composition (wt.% B) 100% B Solidification within solubility limit Solidification within the solubility limit, gives the same single-phase microstructure as that observed with complete solid solubility. 100% liquid L first a crystals form a crystals a a grow a a a last liquid between a crystals 100% primary a grains a a a a a a a a a Temperature (°C) Liquid a L+a a+b a a a a a 0 100% A Composition (wt.% B) Solidification + Precipitation If a composition goes through the solid solubility limit, a second phase precipitates out at the grain boundaries, as primary phase grains reject solute. 100% liquid L first a crystals form a crystals a a grow a a a last liquid between a crystals a a a a a a a Temperature (°C) Liquid a L+a a+b a a a 100% primary a grains a a a a a a a a a b precipitates out at grain boundaries a 0 100% A Composition (wt.% B) Applying the lever rule The fractions of the a and b phases at a given temperature can be calculated using the lever rule as before. Temperature (°C) Liquid a L+a L+b b a+b Ca(T) 0 100% A Cb(T) Calloy Composition (wt.% B) 100% B Hypo/hyper-eutectic solidification The primary phase forms first and grow until the eutectic temperature is reached, then the remaining liquid transforms to a two-phase eutectic structure. 100% liquid L first a crystals form a crystals grow a a a a remaining liquid is transformed into eutectic a and b surrounding the primary a a a a a a a a a a Temperature (°C) Liquid a L+a a+b a 0 100% A Composition (wt.% B) Fractions primary a and eutectic First we look at the L+a two-phase region to determine the fraction of primary a that forms at the end of solidification. Remaining liquid freezes into a eutectic. Temperature (°C) Liquid a L+a L+b b a+b Ca(T) 0 100% A Calloy Ceutectic Composition (wt.% B) 100% B Amounts of a and b in the eutectic The remaining liquid has a eutectic composition, so the proportions of eutectic a and b phases are the same as in a purely eutectic alloy. Temperature (°C) Liquid a L+a L+b b a+b Ca(T) 0 100% A Calloy Ceutectic Cb(T) Composition (wt.% B) 100% B Overall fractions of a and b The overall fraction of a in the hypo-eutectic alloy will be the sum of the fraction of the primary a and the proportion of the eutectic fraction that is a. The overall fraction of b in the hypo-eutectic alloy will be the proportion of the eutectic fraction that is b. OR if the total fraction of a is known: E.g. Pb-Sn system The lead-tin system was used extensively for solders in electrical connections, but has been largely phased out due to toxicity of lead. It is a good example of a binary eutectic alloy system. The low melting point and large freezing ranges made for good solder alloys. E.g. Al-Si system Aluminium-silicon alloys are widely used for producing intricate and lightweight castings. Stem has very good fluidity, low viscosity and good strengthening effect of Si on Al. (N.B. No solubility of Al in Si) L + Si L+a a a + Si E.g. Al-Si microstructures (a) – primary a (light blobs) surrounded by eutectic a+Si lamellae (b) – eutectic a+Si lamellae (c) – primary Si (dark blobs) surrounded by eutectic a+Si lamellae (a) hypo-eutectic (b) eutectic (c) hyper-eutectic Zirconia phase transformations Zirconia can have one of three crystallographic structures depending on the temperature. The resulting volume changes can lead to processing problems when manufacturing precision components. It also complicates the use of zirconia at high temperatures. Stabilised zirconia Zirconia can be blended with other oxides to stabilise a particular crystallographic structure. Yttria (Y2O3) is often added to stabilise zirconia in its cubic form. This form is known as yttriastabilised zirconia or YSZ. It is essentially a ceramic alloy. Zr:O ratio in ZrO2 is 1:2 Y:O ratio in Y2O3 is 1:1.5 Adding yttria therefore creates oxygen vacancies in the YSZ. At high temperatures YSZ can conduct oxygen vacancies (=electric current). YSZ is thus used for fuel cells and oxygen sensors. (See lecture 2) Intermetallic eutectics, e.g. Mg-Cu If there is chemical affinity between A and B, they will form chemical compounds AxBy with their own unique melting points. This also results in additional eutectic reactions. L+MgCu2 L+b b L+Mg2Cu L+a L+MgCu2 L+Mg2Cu Mg2Cu + MgCu2 a + Mg2Cu a MgCu2 + b Mg2Cu is known as a “line compound”, because it must have a fixed composition and does not allow any solubility of Mg or Cu. MgCu2 is not a line compound and allows has some limited solid solubility for Mg and Cu. Limitations of phase diagrams Phase diagrams only describe the equilibrium state of an alloy system. This means that they are true only for slow cooling and slow heating. This gives enough time for diffusion of the alloying elements, which is needed for the described phase transformations to be completed. E.g. if an alloy is quenched (rapidly cooled from a high temperature to a low temperature) then there is not enough time to complete the necessary diffusion. As a result, non-equilibrium phases may form. These phases are metastable, which means that they will eventually transform to stable equilibrium phases either over time or when heated. Phase diagrams also do not describe diffusionless phase transformations. E.g. such as those observed in shape memory alloys. Summary The microstructure is very important for determining the properties of a material. (In conjunction with crystal structure). A phase is a region of space across the physical properties of a material are essentially uniform. Equilibrium phase diagrams are a useful tool for understanding phase transitions across different temperatures and compositions as well as the microstructures that result. Pure substances/elements and eutectics have defined melting points, while solid-solutions tend to have freezing ranges. Hume-Rothery rules must be satisfied for complete solid solubility. If the above are not obeyed, the following may occur: miscibility gap, eutectic reactions, intermetallic compounds. Phase diagrams only describe materials in thermodynamic equilibria.