In-situ high-energy X-ray diffuse-scattering study of the phase transition in a Ni₂MnGa ferromagnetic shape-memory crystal

The full information on the changes in many crystallographic aspects, including the structural and microstructural characterizations, during the phase transformation is essential for understanding the phase transition and "memory" behavior in the ferromagnetic shape-memory alloys. In the present article, the defects-related microstructural features connected to the premartensitic and martensitic transition of a Ni₂MnGa single crystal under a uniaxial pressure of 50 MPa applied along the [110] crystallographic direction were studied by the in-situ high-energy X-ray diffuse-scattering experiments. The analysis of the characteristics of diffuse-scattering patterns around different sharp Bragg spots suggests that the influences of some defect clusters on the pressure-induced phase-transition sequences of Ni₂MnGa are significant. Our experiments show that an intermediate phase is produced during the premartensitic transition in the Ni₂MnGa single crystal, which is favorable for the nucleation of a martensitic phase. The compression stress along the [110] direction of the Heusler phase can promote the premartensitic and martensitic transition of the Ni₂MnGa single crystal. © The Minerals, Metals & Materials Society and ASM International 2008.