Stress and temperature dependence of the structure of the martensite and X-phase in Ni₂MnGa

Abstract

Stress and temperature dependence of the structure of the X-phase in Ni₂MnGa has been investigated by neutron diffraction measurements to clarify the structural relationship among the parent (P-), intermediate (I-), martensite (M-) and X-phases. The satellite position and intensity of the X-phase differ from those of the I-phase under compressive stress, but approach those of the I-phase with increasing temperature and decreasing stress. In other words, the structure changes discontinuously with I → X transformation under compressive stress, but continuously under zero stress. On the other hand, the X → P transformation is continuous, regardless of stress or temperature. These results suggest the existence of a multicritical point for successive P → X → I transformation.

Keywords:

• neutron diffraction
• crystal structure
• martensitic transformation
• shape memory alloys
• incommensurate structure
• ferromagnetic composites
• Ni–Mn–Ga alloy

Acknowledgements

This study was supported by Priority Assistance for the Formation of Worldwide Renowned Centers of Research, The Global COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The work was carried out under the Common-Use Facility Program of JAEA.