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A three step process involving, Ni powder size, pressed compact composition and sintering profile was used to develop an optimized pressureless sintering route to produce NiTi from pure Ni and Ti powders. A mixture consisting of 10.7 μm Ni and 32 μm Ti resulted in the strongest martensite to austenite (M/A) transformation. However, homogenization under longer sintering times, lead to a weaker and lower temperature M/A transformation. This behaviour was attributed to the changing composition of the NiTi phase during sintering, as well as the powder mixtures oxide content. Adjusting the compact composition to 49 at.% Ni overcame this problem. Slow heating from 500 to 1000 °C during sintering produced a high density NiTi (i.e., 98%) which exhibited a M/A transformation behaviour comparable to cast NiTi. This pressureless sintered material also possessed tensile properties and a shape memory effect comparable to HIPped pre-alloyed powders. |
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