Thermodynamic parallels between solid-state amorphization and melting

Abstract

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.

Intrinsic response of crystals to pure dilatation

“…[4] It can be seen in Fig. 13 that the critical strains observed in the present work delineate the extension of the mechanical stability curve to temperatures below Tt.…”

“…It has been coz.jectured that in crossing this stability curve the lattice will become disordered, thus providing a simple thermodynamic connection between melting and solid-state amorphization. [4] What we have found is that in crossing such a curve the lattice does become mechanically unstable as manifested by sudden jumps in the hydrostatic pressure and the potential energy; however, the atomic configuration into which it evolves depends on the temperature. …”

“…Following the observation that structural disordering in all cases is accompanied by a volume expansion [2], it has been suggested that role of local density variations in SSA may be analogous to that in the melting transition. [4] The implication is that at temperatures below the triple point, essentially the melting point at zero pressure Tm, a sudden volume expansion (on the time scale short compared to that required for equilibrium sublimation) may bring about structural disordering.…”

Radiation-induced crystalline-to-amorphous transition in intermetallic compounds of the CuTi alloy system

“…The fi_ st of these results was somewhat surprising, since earlier computer simulat__ons c f the melting process in Cu [30] showed that the volume dependence of the shear mo_luli C44 and C' during homogeneous expansion at constant temperature was virtually identical to that associated with thermal expansion at constant pre., sure, which implies that the volume dependence of the elastic constant doe_ not depend on how the expansion is produced. However, it is now .…”

Modeling mechanical alloying: Advances and challenges

“…[19][20][21][22] The melting criteria are often involved (e.g., the Lindemann criterion for the critical thermal displacement of atoms 2o , 22 and the Egami-Waseda criterion for the critical elastic strain caused by the atomic-size mismatch 23 ).…”