Antisite Defects and Nonequilibrium Phase Transition in Intermetallics
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tisite Defects and Nonequilibrium Phase Transitions in Intermetallics G. Martin and P. Bellon Introduction Among the many point defects in crystalline solids, antisite defects play a key rôle in the stability of intermetallics. Such defects are either thermal equilibrium defects or are introduced by some external forcing. There are, indeed, many examples where intermetallics or compound semiconductors are "driven," Le., sustained in nonequilibrium configurations by external forcing. Good examples include the following: • Intermetallics in alloys under irradiation, like FeZr2 in Zircalloy used as a cladding material in pressurized water nuclear reactors, or any of the compounds produced by ion implantation or ion beam mixing (atoms are continuously forced to change lattice sites because of replacement collsions). See, for example, Référence 1. • Intermetallics in superalloys under cyclic fatigue (y' précipitâtes in persistent slip bands undergo sustained shearing, and sometimes redissolves).2 • Intermetallics during high-energy bail milling, a promising technique to stabilize nonequilibrium phases.3 • Ordered compounds when formed by vapor phase déposition.4 In such compounds, atoms are forced to leave their optimum local surroundings by nuclear collisions, shearing, fracturing, and welding respectively, or land on a surface at a random position, while thermal jumps tend to restore some degree of local atomic order. For simplicity, we call such compounds "driven Systems" or "driven intermetallics." Indeed, such Systems are driven away from the usual thermodynamic equilibrium by a permanent dynamical forcing (nuclear collisions, plastic shear, etc.). Under such circumstances, a rather
MRS BULLETIN/DECEMBER1991
involved question arises. Assuming the driven alloy may achieve several steadystate configurations (e.g., long-range ordered structures with distinct symmetries, or an ordered compound versus disordered solid solution, or crystalline versus amorphous), is there a way to predict the respective stability of the various a priori possible structures, much in the same way as thermodynamics, in principle, allows predicting the most stable configuration of an alloy? The relevance of this question is not purely académie since answering it would reveal the control parameters of the state of the alloy under dynamical
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{Q Figure 1. Exploration of the configuration space (schematic) under thermal conditions (dotted Une) and driven conditions (solid Une); the various régions of the configuration space are not explored with the same frequency under both sets of conditions.
During the last few years, definite progress has been achieved in the theory of the stability of such "driven intermetallics." The second section gives the principle of this theory that relies on a very simple idea. In an "undriven System," which obeys thermodynamics, Gibbs hypothesis (equiprobability of ail microstates of equal energy) permits one to express the probability of a macrostate at fixed température as the exponential of a free energy fu
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