Superelastic-superplastic powder-metallurgy NiTi shape memory alloy made by meso-scale particles

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This is the first report for the powder metallurgy of titanium-nickel shape memory alloy having both shape memory and mechanical strength comparable to the alloy made by casting and wire-drawing. The fatigue properties and mechanism of the fracture were also investigated.The process for the powder metallurgy is as follows ; Titanium-Nickel alloy powder was produced by Plasma Rotation Electrode Process. The method was used to suppress the impurity contamination during atomizing process. The powder with an average diameter of 0.287mm was hot-pressed by Hot Isosotatic Pressing, and a compact having a relative density of 99.7% was obtained. The martensitic transformation temperature was identical to that of the ingot used for making the powder.Monotonic tensile test of the compact showed the shape memory strain and also the superelastic strain of 7% : the stress-strain behavior was almost identical to the cast alloy. However, it fractured by the deformation above 7%, which is much smaller than in cast alloys.In the case of cyclic tensile deformation, the initiation and propergation of crack of the compact happenned by smaller tensile strain. The fracture occurred by the cleavage of the powder surfaces. By in-situ observation of the surface relief of tensile specimens, it is found that the stress-induced martensitic transformation develops by an inhomogenerous manner, which resembles the propagation of Ruders Band. The crack nucleated within the bands. Therefore, the fatigue life of the compact did not depend explicitely on the amplitude of tensile deformation.