Alloys: Phase Diagrams

• Most engineering metallic materials are not pure metals of one kind, but alloys that contain atoms of more than one metal or of a metal and a nonmetal. In an alloy, the combination of atoms of the basic metal A, the solvent, and of the added metal B, the solute, may be a solid solution or it may be a compound, and in a metal the grains representing different solid solutions or compounds may be mixed together.

• Solid solutions are of two kinds:– In a substitutional solution, the atoms of the solute replace some

atoms of the solvent in its crystal cells. Such a solution is formed under the following conditions:

• The atoms of the two metals do not differ in diameter by more than 15%.

• Their space lattices are similar.

• They are close to another in the electromotoric series.

– An interstitial solid solution is formed if the atoms of the solute occupy positions in between the atoms of the solvent in its cell. Obviously this is only possible if the solute atoms are much smaller than those of the solvent. Even then the solubility is limited. The most common example is that of steel, where the C atoms are located in the Fe cells.

– Solid solubility increases with temperature as it does with liquid solutions. Correspondingly, solute atoms precipitate on cooling (by diffusion) out of the atoms of the solvent.

– Substitutional solid solutions in which there is a long-range order of the two metallic atoms and the composition is well defined in simple ratios of atom fractions are considered as intermetallic compounds. They have a sharp melting point and other characteristics of distinct chemical species. Very often, such compounds have higher melting points than either of the elements, and they care hard and brittle.

– The lever rule can be used to determine the weight proportions of the two components at 1300°C as follows:

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%33.33100Ni) (63 solid %

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• Eutectic alloy equilibrium diagrams– Just as a limited amount of salt can be dissolved in water, there are

many instances where a limited amount of the alloying ingredient can be absorbed by the solid mixture.

– The elements of a binary alloy may be completely soluble in the liquid state but only partially soluble in the solid state.

– When the alloying ingredient is present in amounts above the maximum solubility percentage, the alloying ingredient precipitates out. In aqueous solutions, the precipitate falls to the bottom of the container.

– In metallic alloys, the precipitate remains suspended as pure crystals dispersed throughout the primary metal.

– In chemistry, a mixture is different from a solution. Salt in water forms a solution. Sugar crystals mixed with salt crystals form a mixture. A mixture of two solid ingredients with no solubility is known as a eutectic alloy. Since the two ingredients do not mix, reducing the temperature below the eutectic line results in crystals of both pure ingredients forming.

– In another basic type of a phase diagram, the two components A and B are fully mutually soluble in the liquid phase but completely insoluble in the solid state.

– The melting temperatures of the alloys decrease from both ends, and there is a particular composition called the eutectic which has the lowest melting temperature.

• The Fe-C Phase Diagram– The phase diagrams of actual alloys differ from the idealized

forms because there is always some partial solid solubility available.

– C < 0.1% low carbon steels

• cans, automobiles body panels, sheet metal parts (UTS = 350MPa, = 35%)

– C ~ 0.2% structural beams

• UTS = 420MPa, = 30%

– C ~ 0.3% - o.45% medium carbon steel

• machine parts

– C ~ 0.7% railroad rails

• UTS = 800MPa, = 10%

• Grain size– fine grained steel is stronger and tougher than steel with coarse

grains.

– is soft and ductile

• perlite => + Fe3C

• => FCC

• Fe3C => an orthorhombic structure

– C < 0.8% => hypoeutectic

– C > 0.8% => hypereutectic

(perlite) 93.2% C) γ(0.8%

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(perlite) wt%29 C) (0.8% eutectoid

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• Recovery, recrystallization, and grain growth– the strain hardened metal is stronger and harder but it is less ductile

– recovery and recrystallization may revert these properties back to the precold-worked state

– Recovery:

• Some of the stored internal strain energy is relieved, as a result of enhanced atomic diffusion at the elevated temperature.

– recrystallization:

• Can be used with all metals to relieve stresses induced during cold working. It involves heating the material in a furnace to a specific temperature (recrystallization temperature) and holding it there for a long time.

• Induces the formation and growth of strain-free grains within the grains already formed.

• The resulting microstructure is essentially the microstructure that existed before any cold working and is softer and more ductile than the cold-worked microstructure.

• Recrystallization– Even after recovery is complete, the grains are still in a relatively

high strain energy state. Recrystallization is the formation of a new set of strain free and equiaxed grains that have low dislocation densities and are characteristic of precold-worked condition. The driving force to produce this new grain structure is the difference in internal energy between the strained and unstrained material. The new grains form as very small nuclei and grow until the completely replace the parent material, processes that involve short-range diffusion.

– The recrystallization process is more sensitive to temperature than it is to exposure time. Recrystallization will occur naturally over a wide range of temperatures; however, the reaction rates increase at higher temperatures.

• Grain growth– After recrystallization is complete, the strain-free grains will

continue to grow if the metal specimen is left at the elevated temperature - a phenomenon called grain growth. Grain growth does not need to be preceded by recovery and recrystallization; it may occur in all polycrystalline materials, metals, and ceramics.

– An energy is associated with grain boundaries, as explained in Section 4.5. As grains increase in size, the total boundary area decreases, yielding an attendant reduction in the total energy; this is the driving force for grain growth.

– Grain growth occurs by the migration of grain boundaries. Obviously, not all grains can enlarge, but some grow at the expense of others that shrink. Thus, the average grain size increases with time, and at any particular instant there will exist a range of grain sizes. Boundary motion is just the short range diffusion of atoms from one side of the boundary to the other. The directions of boundary movement and atomic motion are opposite to each other, as shown in Figure 9.6.

• Cold working versus hot working– As a material is worked and the dislocations move, plastic strain

builds up. If the strain occurs at a high enough temperature (above recrystallization temp.), there will be sufficient thermal energy to anneal out the lattice distortions.

– Forming operations above the recrystallization temperature are known as hot working, since annealing occurs simultaneously with the plastic forming. The material remains ductile.

– If the plastic forming takes place at a low temperature, there will be insufficient thermal energy to anneal out the dislocations. The material will become progressively stronger, harder, and more brittle until it eventually fails. This is known as cold working.

• Microstructural and property changes in iron-carbon alloys– Some of the basic kinetic principles of solid state transformations

are now extended and applied specifically to iron - carbon alloys in terms of the relationships between heat treatment, the development of microstructure, and mechanical properties. This system has been chosen because it is familiar and because a wide variety of microstructures and mechanical properties are possible for iron-carbon (or steel) alloys.

– Isothermal transformation diagrams

• consider again the iron-iron carbide eutectoid reaction,

• which is fundamental to the development of microstructure in steel alloys. Upon cooling, austenite, having an intermediate carbon concentration, transforms to a ferrite phase, having a much lower carbon content, and also to cementite, which a much higher concentration. For carbon atoms to selectively segregate in the cementite phase, diffusion is necessary.

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– Temperature plays an important role in the rate of the austenite-to-pearlite transformation. The temperature dependence for an iron-caarbon alloy of eutectoid composition is indicated in Figure 9.9, which plots S-shaped curves of the percentage transformation versus the logarithm of time at three different temperatures. For each curve, data were collected after rapidly cooling off a specimen composed of 100% austenite to the temperature indicated; that temperature was maintained constant throughout the course of the reaction.

• Steel Hardening Processes:– Hard steel resists plastic deformation. Steel is hard if it has a

homogeneous, austenitic structure with coarse grains. Some steels (with little carbon) are difficult to harden.

– The basic hardening processes consist of heating to approximately 50 degrees C above the A3 critical line, allowing austenite to form, and then quenching rapidly. Hardened steel consists primarily of martensite and bainite.

– Martensite is a supersaturated solution of carbon in alpha iron.

– Bainite is not hard as martensite, but it does have good impact strength and fairly high hardness.

– Neither martensite nor bainite are equilibrium substances - they are not found on the iron-carbon equilibrium diagram but are formed during the quenching operation.

– The allotropy of iron combined with the large difference in C solubility in beta and alpha irons is thus the basis of hardening of steel. The process is accomplished by first hearing the steel into an austenitic range, holding it there long enough to obtain austenite grains with uniform composition and then quenching the part for fast cooling. If the cooling rate is not fast enough, the martensite will not form fully or not at all. This is best illustrated by means of a diagram constructed from observations of isothermal transformations. Such a diagram is called a time-temperature transformation (TTT) diagram. This diagram, for an eutectoid carbon steel, is shown in Fig. 2.16. It is obtained by carrying out experiments like the one indicated by the line A - E. The austenitized material is first quenched into a salt bath of temperature T, line A-B. Then it is held at this temperature for a certain time, say, until point C, and then quenched to room temperature. Next time it is held at T until point D, next time until E, and so on. The microstructures so obtained are studied.

– A more convenient way of representing both the time and temperature dependence of this transformation is in the bottom portion of Figure 9.10. Here, the vertical and horizontal axes are, respectively, temperature and the logarithm of time. Two solid curves are plotted; one represents the time required at each temperature for the initiation or start of the transformation; the other is for the transformation conclusion. The dashed curve corresponds to 50% of transformation completion. These curves were generated from a series of plots of the percentage transformation versus the logarithm of time taken over a range of temperatures. The S-shaped curve in the upper portion of Figure 9.10, illustrates how the data transfer is made.

– An actual isothermal heat treatment curve (ABCD) is superimposed on the isothermal transformation diagram for a eutectoid iron-carbon alloy in Figure 9.11. Very rapid cooling austenite to a temperature is indicated by the near vertical line AB, and the isothermal treatment at this temperature is represented by the horizontal segment BCD. Of course, time increases from left to right along this line. The transformation of austenite to pearlite begins at the intersection, point C (after approximately 3.5 sec), and has reached completion by about 15 sec, corresponding to point D. Figure 9.11 also shows schematic microstructures at various times during the progression of the reaction.

– The hardness obtainable in quenched steels depends on the amount of carbon. Steels with less than 0.3% cannot be successfully hardened. For steels with 0.3% to 0.6% carbon, the maximum obtainable hardness increases from about 48 Rc to about 66 Rc.

– The steel quenched through the martensite transformation is very hard and brittle and, because of the brittleness, it cannot in most instances be used in this state. It is, therefore, subjected to another heat-treatment process called tempering. This is done by heating the part to some temperature below 723C, holding it at this temperature for a certain period of time, and quenching it again. During tempering the martensitic tetragonal, highly distorted structure reverts to regular BCC and allows the carbon atoms to migrate and form some very fine iron carbide crystals. Hardness decreases but ductility improves.

– The changes in hardness and ductility depend on the temperature and time of tempering. The temperature has a much larger effect than time. Therefore, in practical terms the temperature is given assuming that the time will be of the order of one half to several hours. There are several typical temperature ranges of tempering:

• 100-200C Upon reaching 200C the martensite is fully decomposed. Structural changes are obtained on a very fine scale and cannot be detected under a microscope. Hardness almost does not change at all; some ductility is obtained. This is called fine-tempered martensite.

• 200-300C Structure is still tempered martensite, but hardness decreases to 55-58 Rc. However, in this range, due to precipitation of oxides and nitrides a decrease of toughness may occur. This is called blue brittleness because it occurs at temperatures that leave a blue oxide of film on the steel.

• 360-723C The higher the temperature in this range, the coarser the cementite particles. At or above 650C these particles can easily be resolved under an optical microscope. Because of their spherical shape, the structure is called spheroidite. The decrease of hardness and increase of ductility are almost linearly proportional to tempering temperature. The range of hardness extends from about 60 Rc for martensite down to about 20 Rc for shperoidite.