Structure of the martensitic-type hydrides β-Ti0.64Zr0.36NiD and γ-Ti0.64Zr0.36NiD2.5 (S.G. Cmcm). For the β phase, site H1 is fully occupied. In the γ phase, site H3 is fully occupied and site H2 is half filled (50%).
TiNi alloy is polymorphic. It exhibits a martensitic transition near room temperature between a cubic CsCl-type (austenite) and a monoclinic TiNi-type (martensite) structure. This transformation is at the origin of its shape memory effect. In addition, TiNi has good hydrogen storage properties under normal conditions of pressure and temperature. In austenic form, TiNi absorbs 1.4 H/f.u. under atmospheric pressure. No plateau is observed in the pressure composition isotherm curve (PCI) showing that hydrogen is absorbed in solid solution . Hydrogenation properties of the martensite form are unknown. By adjusting a partial substitution of Ti by Zr and by using different elaboration techniques (melt spinning and induction melting), we were able to stabilize the two different crystallographic forms at room temperature for the same composition: Ti0.64Zr0.36Ni. The austenitic phase exhibits hydrogenation properties close to those of cubic TiNi. The martensitic phase absorbs up to 2.8 H/f.u. and shows a pressure plateau ranging between 1 and 2.5 H/u.f.. This plateau indicates the presence of two hydride phases in equilibrium. These results lead to the conclusion that hydrogenation properties of TiNi-type alloy are strongly influenced by the structural polymorphism  of the parent alloy. (more...)