Coherent phase equilibria: Effect of composition-dependent elastic strain

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Recent investigations on coherent equilibria have shed much light on the nature of phase stability. For example, equilibrium phase concentrations are shown to depend on the alloy composition, and two-phase coexistence can terminate at a critical coherency strain, termed a Williams point. This work, however, presents that when lattice parameter, hence coherency strain, is taken to be a function of phase composition, Williams point can disappear, i.e. the two-phase termination occurs only at an infinite elastic strain. In particular, if lattice parameter depends linearly on phase composition following Vegard's law, the equilibrium phase compositions are found to be independent of alloy composition, similar to the incoherent equilibrium case, but the chemical potentials vary with alloy composition, underlying breakdown of the usual Gibbs phase rule. When the analysis is extended to a binary regular solution obeying Vegard's law, complete nature of both coherent solvus and coherent spinodal is revealed in accordance with Cahn's prediction made a generation ago. © 1994.

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Acta Metallurgica Et Materialia