Phase diagrams in the NiMnGa system under compression

Abstract Some thermodynamic aspects of transformation behavior of two shape-memory and superelastic L2 1 ordered single-crystalline NiMnGa alloys exhibiting martensitic transformation in the stress-free state at M s = 287 K T c (alloy 1) and M s = 428 K > T c (alloy 2), where T c is Curie temperature, was studied in compression (hydrostatic and/or uniaxial along [110] axis) tests. The uniaxial compression experimental results for alloy 1 which exhibits multiphase stress-induced transformation behavior as well as literature data were summarized in non-equilibrium σ-T phase diagrams. The phenomenological treatment of the equilibrium high hydrostatic pressure versus transformation temperature data for alloys 1 and 2, i.e. p-T diagrams, was found to be useful in the framework of both the Clausius-Clapeyron relationship and Landau-type theory.