Study of martensitic transformation in TiNiHfZr high temperature shape memory alloy using in situ neutron diffraction

The thermoelastic martensitic transformation in the polycrystalline Ti29.7Ni50.3Hf10Zr10 alloy was studied using high-resolution and in situ neutron diffraction in the temperature range of 25–250 °C. The thermal expansion tensors of the B19′ martensite and austenite were determined. B19′ martensite is characterized by a strong anisotropy of the thermal expansion coefficients. The analysis of the diffraction peak integrated intensities suggests that nearly 10% of austenite, which is partly disordered, does not take part in martensitic transformation. The volume effect of the transformation in the Ti29.7Ni50.3Hf10Zr10 alloy was 0.79%. The simultaneous addition of Hf and Zr atoms into the TiNi matrix leads to the deterioration of the crystallographic compatibility between martensite and austenite, which may be responsible for the large thermal hysteresis and volume changes upon transformation.