On Cyclical Phase Transformations in Driven Alloy Systems

Abstract

Cyclical phase transformations occurring in driven materials syntheses such as ball milling are described in terms of a free energy minimization process of participant phases. The oscillatory flow behavior of metals with low stacking fault energies during hot working is taken as a prototype in which a ductile crystalline phase sustains undulation in its free energy, due to the alternate succession of work-hardening and work-softening mechanisms. A time-dependent, oscillatory free energy function is then obtained by solving a delay differential equation (DDE), which accounts for a time lag due to diffusion. To understand cyclical transitions on an atomistic scale, work is extended to molecular dynamics simulations. Under shear deformation, a two-dimensional nanocrystal shows cyclical transitions between an equilibrium rhombus and a nonequilibrium square phase. Three-dimensional simulations show crystalline-to-glass transitions at high strain rates, but very high shear strain rates are found to lead to a latticelike network structure in the plane perpendicular to the shear direction, with strings of atoms parallel to the shear direction.

Amorphization and nanocrystallization of silicon under shock compression

“…In addition to experimental studies, several thermodynamic and kinetic approaches have been implemented to study silicon amorphization mechanisms. [20][21][22][23] Demkowitz and Argon [24] performed MD simulations and predicted various amorphous silicon phases whose density depends largely on the cooling rate. Levitas [25] developed a kinetic and thermodynamic theory for strain-induced phase transitions, including amorphization, indicating that superposition of plastic work leads to a significant reduction in pressure required for strain induced chemical changes.…”