Assessment of Entropy Differences from Critical Stress Versus Temperature Martensitic Transformation Data in Cu-Based Shape-Memory Alloys

The entropy differences per unit volume (Δ S trans ) 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 S trans 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., d τ d T = - Δ S trans γ , where γ is the transformation shear strain. Motivated by the significant scatter of the so obtained Δ S trans 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 d τ d T = Ω - Δ S trans γ is obtained, where the Ω term involves the elastic properties and their temperature dependence. The new τ - T - Δ S trans relation is used to assess the Δ S trans values corresponding to the 2H/18R and 18R/6R MTs in Cu–Al–Ni and Cu–Zn–Al alloys. The Δ S trans values obtained by the present approach fall on a scatter band centered around the zero value.