Porous NiTi alloys produced by press-and-sinter from Ni/Ti and Ni/TiH2 mixtures

This work presents a comparative study of using two different titanium powders to synthesise porous NiTi alloys by a conventional press-and-sinter process. Two batches of powder mixture were prepared and compared: Ni/Ti-a mixture of titanium made from a hydrogenation-dehydrogenation process (HDH-Ti) and nickel, and Ni/TiH2-a mixture of TiH2 and nickel. The powder mixtures were pressed under three different compaction pressures and subsequently sintered in vacuum at three different temperatures (i.e., 1000, 1100 and 1200 °C) for 2 h. The results obtained from microstructure observations show that the porous alloys have open porosities from 10.2% to 33.8% and the largest pore size ranges from 3.5 to 27.4 µm. The predominant phase identified in all the produced porous NiTi alloys is B2 NiTi phase with the presence of other minor phases. Particularly, in the Ni/Ti compacts sintered at 1000 °C some Ti particles were left unreacted. The static and cyclic stress-strain behaviours of the porous NiTi alloys made from Ni/Ti and Ni/TiH2 compacts were also investigated. In comparison with the Ni/Ti sintered samples, the samples sintered from Ni/TiH2 mixture exhibit a higher porosity, smaller pore size, higher fracture strength, smaller cyclic residual strain, higher recovery pseudoelastic strain and lower secant modulus.