Assessment of entropy differences from critical stress versus temperature martensitic transformation data in Cu-based shape-memory alloys

Abstract

The entropy differences per unit volume (ΔStrans) between the close-packed phases in a martensitic transformation (MT) in Cu-based shape-memory alloys are obtained from mechanical tests by measuring, as a function of temperature (T), the critical resolved stress (τ). Specifically, Δ Strans values are obtained from the slope of τ versus T plots by invoking a relation which is straightforwardly derived from the classical Clausius–Clapeyron equation, viz., dτdT=-ΔStransγ, where γ is the transformation shear strain. Motivated by the significant scatter of the so obtained Δ Strans values, the thermodynamic bases of such evaluation procedure have been revised, by accounting for the nucleation step of a martensite plate. The interface, elastic strain, and chemical contributions to the Gibbs energy of nucleation have been considered. A new expression of the type dτdT=Ω-ΔStransγ is obtained, where the Ω term involves the elastic properties and their temperature dependence. The new τ- T- Δ Strans relation is used to assess the Δ Strans values corresponding to the 2H/18R and 18R/6R MTs in Cu–Al–Ni and Cu–Zn–Al alloys. The ΔStrans values obtained by the present approach fall on a scatter band centered around the zero value.