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Phase equilibrium diagrams. Overview Flipbook PDF
MCEN 5024. Fall 2003. Page 1 of 9. Phase equilibrium diagrams. Overview Phase diagrams assist in the interpretation of m
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MCEN 5024. Fall 2003.
Phase equilibrium diagrams. Overview Phase diagrams assist in the interpretation of microstructure of metals. Traditional sectioning, mounting, polishing and etching techniques are often used to determine the microstructure including around regions of failure (e.g. welds, inclusions, fracture sites) which can then be subsequently related to the physical (typically mechanical) properties. This forms the basis of structure – property relationships. Thus, phase equilibrium diagrams ‘define the region of stability’ of the phases that can occur in an alloy system under the conditions of constant pressure. Equilibrium diagrams are presented in the form of temperature versus composition and represent the interrelationship between phases, temperature and composition only under equilibrium conditions. A metal quenched from a higher temperature to a lower temperature may contain phases that are non-equilibrium phases i.e. may normally only exist at higher temperatures, not at the lower temperature. Given sufficient time at this lower temperature, a non-equilibrium phase may convert via diffusion to its equilibrium state at this lower temperature. When we are considering the effects of time and temperature on microstructure then we need to use time – temperature transformation (TTT) continuous cooling transformation (CCT) diagrams. Isomorphous alloy systems. The simplest phase diagram to consider is the isomorphous system which is a two-component, so called binary, alloy system. Only one type of crystal structure is observed. A classic example is that of copper – nickel shown below:
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MCEN 5024. Fall 2003.
These two elements combine to form a single liquid and a single solid phase. With reference to the above diagram, we define two lines: the liquidus and the solidus. Above the liquidus line the region of stability for the liquid phase. The region below the solidus line is the region of stability for the solid phase. The region between the liquidus and the solidus is where both liquid and solid phases co-exist. In the case of the copper-nickel system, this is a substitutional alloy in that there is a direct substitution of one type of atom for another so that solute atoms come into the lattice to take up positions typically occupied by solvent atoms. The atomic diameter of copper is 2.551 Å and that of nickel is 2.487 Å. The difference is negligible (2%) and hence only leads to a slight distortion of the lattice Page 2 of 9.
MCEN 5024. Fall 2003.
thus they are truly soluble and whatever the composition, they form a FCC structure. Solid-solubility of one metal in another only occurs if the diameters of the metals differ by less than 15% (Hume Rothery rule). This is directly related to the strains put on the solvent lattice by the solute atoms. If we replace nickel with silver (also FCC), which is similar chemically to copper, we find that the solubility of silver in copper or copper in silver is less than one percent. The atomic diameter of silver is 2.884 Å, which is about 13% larger than that of the copper atom. Other considerations for the formation of an alloy relate to their electro-potential. A more electronegative element will combine with an electropositive element ionically through sharing of valence electrons. This is not considered an alloy in the sense discussed here. On the other hand, two elements lying close to one another in the periodic table tend to act in a similar manner chemically leads to metallic rather than ionic bonding. This will however only occur if both metals have the same valence and crystallize in the same lattice form. An alternative to the formation of a substitutional alloy is that instead of displacing a solvent atom, the solute atom resides in the interstices between the atoms. This is termed an interstitial alloy. Interstitial solid solutions only occur if the solute atom has an apparent diameter smaller than 0.59 that of the solvent. The four most important interstitial solute atoms are carbon, nitrogen, oxygen and hydrogen. Interstitial solute atoms dissolve much more readily in transition metals (Fe, Vn, W, Ti, Cr, Th, Zr, Mn, U, Ni, Mo) than in other metals primarily due to their incomplete valence electron configuration. Interstitial atoms can diffuse easily through the lattice, hopping from one interstitial position to another and therefore can have a large impact on the properties of the solvent than might be expected at first sight. One of the major interstitial alloys is that of carbon in iron where the iron – carbon phase diagram represents one of the most important classes of alloys known. These two elements form a eutectic system.
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The copper nickel phase diagram represents an alloy system in which the free energy–composition curves for a given temperature intersect at only one composition.
Other systems exhibit equivalent curves but intersect at two compositions forming a minima or maxima at a given composition / temperature.
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In this case, both the liquidus and solidus curves are tangential to each other and to an isothermal line at the point of intersection.
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Such points are termed incongruent points whereby the freezing of the alloy at such a point is equivalent to that of the freezing of a pure metal. The resulting solid however is not a pure component but a solid solution of the two metals. There are a number of alloys that show minima in the liquidus / solidus curves. An example is the gold-nickel system:
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Eutectic systems. In eutectic systems (e.g. copper – silver), there is a composition at which the alloy freezes at a lower temperature than all of the other compositions. Thus, under slow (pseudo – equilibrium) cooling conditions, this composition would freeze at a single temperature, however, instead of forming a single phase as above, it forms two different solid phases. Hence, at the eutectic temperature (an invariant point), the liquid freezes to form two solid phases. The eutectic point in the copper – silver system occurs at 28.1% copper and 779.4oC.
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If we compare this to the gold – nickel system, we see obvious similarities between the two. We note however that in the case of the gold – nickel system, the alloy with composition at the minima first solidifies as a single solid homogenous solution, which then upon further cooling breaks down into two solid phases as it passes through the miscibility gap. (Note: this is related to the relative sizes of the gold and nickel atoms such that a lower lattice strain is achieved when the gold and nickel atoms arrange themselves in alternating layers when compared to isolated clusters of gold and nickel atoms. A still greater decrease in the strain energy is achieved if segregation occurs such that two distinct crystal phases occur, one gold rich and the other nickel rich). Again, going back to the copper – silver phase diagram, we not that there is no miscibility gap such that the liquid is able to transform directly into a two-phase mixture. In order for a system to exhibit a miscibility gap, there must be a tendency for the atoms of the same kind to segregate in the solid state. A miscibility gap as shown by the gold – nickel system can only occur if the component metals are very similar chemically and crystallize in the same lattice form since the two components need to be capable of dissolving in each other at high temperatures. In a eutectic system, the two components do not need to have the same crystal structure nor do they have to be chemically similar.
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The peritectic transformation. Besides the eutectic reaction, another three-phase reaction between a liquid and a solid to form a new and different phase occurs at the peritectic point. As an example we can study the iron – nickel phase diagram. In this system, the atomic diameters are almost identical (Fe, 2.476, Ni, 2.486) and both are from group VIII of the periodic table and hence chemically similar. Both crystallize in the FCC form, Ni is FCC at all temperatures but the stable form of Fe is BCC above 1390oC and below 910oC.
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