Volume expansion and ultimate mechanical stability of crystalline phases in amorphisation and melting processes

Crystalline phases can undergo two different topological order–disorder transitions when the temperature is raised. The first one, usually referred to as melting transition, involves the heterogeneous nucleation of disorder at defects and is described by equilibrium thermodynamics. The second one, usually pre-empted by thermodynamic melting, involves the homogeneous loss of crystalline order as a consequence of a shear instability. In this work, the connections among homogeneous melting, amorphisation and shear instability are investigated. Two different mechanisms of volume change are considered, involving the gradual heating of a pure crystalline phase on the one hand, and the dissolution of solute atoms into a crystalline matrix on the other. In the latter case, the volume changes as a result of the elastic response of the matrix. Theoretical estimates of the elastic volume expansion are evaluated and compared with the experimental data of thermal expansion at melting. It is shown that the critical volume expansion required for a metallic glass to form is approximately coincident with the thermal volume expansion at melting. Molecular dynamics (MD) simulations are also employed to relate the loss of crystalline order to shear instability.