TiNi intermetallic compounds, produced by a combination of mechanical alloying and direct current sintering, were hydrogenated at several hydrogen pressures at 473 K for 4 h to investigate their hydrogen desorption and martensitic transformation behaviors. The microstructures and the hydrogen desorption properties were dependent on levels of hydrogen concentrations. The crystal lattice of TiNi (B2) phase expanded and TiNi hydride formed with an increase in hydrogen pressure. Two stage martensitic and reverse transformations were observed at low hydrogen concentration. An enthalpy of martensitic transformation was lower than that of reverse transformation. TiNi (martensite) phase was confirmed at intermediate hydrogen concentration, suggesting that hydrogen-induced martensitic transformation occurred during hydrogenation. No martensitic and reverse transformations occurred at high hydrogen concentration because of the hydride formation.
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