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

摘要

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