Hydrogen solubility and diffusion in the shape-memory alloy NiTi

The solubility and the diffusion coefficient of hydrogen were determined in the austenitic (cubic) high-temperature phase of the shape-memory alloy NiTi. The measurements were made between 500 and 950 degrees C, with hydrogen gas pressures of up to 1.3 bar, and with hydrogen-to-NiTi ratios of up to 0.027. The hydrogen was in interstitial solid solution. The absorption process is exothermic. The enthalpy of solution per hydrogen atom is -183+or-40 meV. The diffusion coefficient can be described by an Arrhenius relation with an activation energy E=480+or-50 meV and a pre-exponential factor D₀=(4.7_-2.5^+3.9)*10^-3 cm²s^-1. NiTi hydride was synthesised in experiments with a higher hydrogen gas pressure (p=150 bar, T=500 K). Because of its brittleness, the hydride is readily pulverised by mechanical methods. This yields, after extraction of the hydrogen, NiTi powder which is difficult to prepare by other experimental techniques.