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

PELEGRINA, J. L.; CONDÓ, A. M.; FERNÁNDEZ GUILLERMET, A.

Shape Memory and Superelasticity

Springer

Año: 2019 vol. 5 p. 136 - 146

2199-384X

The entropy differences per unit volume (DeltaS_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 (tau). Specifically, DeltaS_trans values are obtained from the slope of tau versus T plots by invoking a relation which is straightforwardly derived from the classical Clausius-Clapeyron equation, viz., d tau/ dT=- DeltaS_trans/gamma ; where gamma is the transformation shear strain. Motivated by the significant scatter of the so obtained DeltaS_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 tau/ dT=Omega - DeltaS_trans/gamma is obtained, where the Omega term involves the elastic properties and their temperature dependence. The new tau-T-DeltaS_trans relation is used to assess the DeltaS_trans values corresponding to the 2H/ 18R and 18R/6R MTs in Cu-Al-Ni and Cu-Zn-Al alloys.