Effect of Cu and Hf additions on NiTi martensitic transformation

The effect of the combination of two different additions on NiTi shape memory alloy is not straightforward. In order to investigate the effect of Hf and Cu additions on the martensitic transformation, a set of five alloys has been prepared with the following compositions: Ni49.8−xCuxTi40.2Hf10 with x = 0, 6, 10, 15 and 20 at.%. The transformation temperatures and the microstructures obtained have been analyzed by differential scanning calorimetry and transmission electron microscopy. While the transformation hystereses are smaller with the addition of Cu like in the ternary Ti50Ni50−xCux alloys, the martensitic transformation temperatures decrease rapidly. The structure of the B19′ martensite has also been modified with a decrease of the unit cell monoclinicity. The twinning modes are not modified, but the martensite plates are longer and contain fewer defects in comparison with the ternary NiTi(Hf) alloy. Internal friction analysis has shown that the addition of Cu to NiTiHf10 improves the damping properties.