Thermodynamic parallels between solid-state amorphization and melting
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A thermodynamics-based description, in the form of an extended phase diagram, of melting and solid-state amorphization is proposed which brings out the parallels between these two phenomena and suggests that their underlying causes are apparently the same. Through molecular dynamics simulations we demonstrate that every crystal, in principle, can undergo two different types of melting transitions with characteristic features that are also observed in radiation- and hydrogenation-induced amorphization experiments on ordered alloys. The first type, defined in terms of free energies, is shown to involve the heterogeneous nucleation of the liquid or amorphous phase at extended lattice defects (such as grain boundaries, free surfaces, voids, or dislocations) and subsequent thermally-activated propagation of solid-liquid/amorphous interfaces through the crystal. The second type, arising from a mechanical instability limit described by Born, is homogeneous and does not require thermally-activated atom mobility. It is suggested that the role of chemical and structural disordering, a prerequisite for irradiation- but not hydrogenation-induced solid-state amorphization, is merely to drive the crystal lattice to a critical combination of volume and temperature at which the amorphous phase can form either heterogeneously or homogeneously.
I. INTRODUCTION
In 1962 Bloch1 amorphized the intermetallic compound U6Fe by fast-fission fragments, thus demonstrating that crystalline materials can be made amorphous by displacement-producing irradiation. The crystallineto-amorphous (C-A) phase transformation is currently receiving renewed attention due to new experimental evidence that amorphous alloys can be produced by a variety of irradiation-, chemically-, and mechanicallydriven processes.2 Given that the transformation can be induced by many different mechanisms, the question naturally arises as to what is the underlying nature of the transition that is common to all these processes. In the same context it can be asked what is the connection between melting and amorphization since both phenomena are concerned with the transition from an ordered to a disordered phase. That amorphization is analogous to melting in certain respects has been recognized recently by a number of workers.2"6 In particular, Cahn and Johnson have pointed out parallels which exist in the processes involving the heterogeneous nucleation of disorder,3 and Okamoto et al.6 have discussed the similarity in the volume dependence of the shear modulus during irradiationinduced amorphization and heating-induced melting.7 In the present work we suggest that the analogy between a'Permanent
address: Nuclear Engineering Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
286
http://journals.cambridge.org
melting and amorphization may be taken further. By focusing on the role of the two thermodynamic state variables of temperature and volume in the destruction of crystalline order, we propose a unifying description which appears to be a n
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