In situ observation and neutron diffraction of NiTi powder sintering

This study investigated NiTi powder sintering behaviour from elemental powder mixtures of Ni/Ti and Ni/TiH2 using in situ neutron diffraction and in situ scanning electron microscopy. The sintered porous alloys have open porosities ranging from 2.7% to 36.0%. In comparison to the Ni/Ti compact, dehydrogenation occurring in the Ni/TiH2 compact leads to less densification yet higher chemical homogenization only after high-temperature sintering. For the first time, direct evidence of the eutectoid phase transformation of NiTi at 620 °C is reported by in situ neutron diffraction. A comparative study of cyclic stress-strain behaviours of the porous NiTi alloys made from Ni/Ti and Ni/TiH2 compacts indicate that the samples sintered from the Ni/TiH2 compact exhibited a much higher porosity, larger pore size, lower fracture strength, lower close-to-overall porosity ratio and lower Young's modulus. Instead of enhanced densification by the use of TiH2 as reported in the literature, this study shows an adverse effect of TiH2 on powder densification in NiTi.