Volume Effects of Alloying: A Thermodynamic Perspective

This study presents a thermodynamic analysis of volume change (Δ°V m ) accompanying alloy formation. Utilizing the thermodynamic identity connecting volume change with Gibbs energy (Δ°G m ) of mixing, Δ°V m ( x ) = (∂Δ°G m /∂p) T,x and closed-form analytical approximations are obtained for both enthalpy (Δ°V m ) h and entropy (Δ°V m ) s -based contributions to Δ°V m . It is shown that both positive and negative volume change could be supported in a single alloy system in different composition regimes, depending on whether the effect of pressure on enthalpy is dominating over entropy effects of alloying or otherwise. Further, it is also established that entropy contribution to volume change is primarily influenced by volume expansion mismatch between solid solution and compositionally averaged end members. The thermodynamic analysis is supplemented by a phenomenological treatment of enthalpy contribution to volume change arising out of different microscopic contributions to mixing enthalpy. Possible thermodynamic justification of empirical polynomial representation of composition induced volume variation is also presented. An illustrative case study on volume effects accompanying the formation of bcc solid solution phase in U 1− x Zr x system is presented.