The role of volume expansion in the formation of metallic glasses

Amorphisation and melting processes are generally regarded as topological order–disorder phase transitions characterised by basically different thermodynamic and kinetic features. Experimental and theoretical findings suggest, however, the existence of intimate connections between them. In particular, it seems that both the static and dynamic atomic displacements from the equilibrium positions characteristic of the ideal crystalline lattice play a fundamental role in driving the necessary instabilities for the loss of crystalline order. If the melting of a pure element and the amorphisation of a binary mixture with solute atoms larger than the matrix atoms are considered, the static and dynamic atomic displacements are simply related to the volume expansion. In this study, the volume expansions associated with the occurrence of amorphisation and melting are compared. The critical volume required in a binary system for the metallic glass to form is found to be approximately coincident with the volume reached by one of the two metals at the melting temperature. Theoretical estimates of the critical volume are obtained through the application of the criterion of Egami and Waseda and compared with experimental values. Significant connections are established between the so-called Bornʹs shear instability and the loss of crystalline order.