Transformation behavior of sintered porous NiTi alloys

TiH2 powder is added as a reactant and pore-forming agent to produce porous NiTi shape-memory alloys (SMAs). The transformation behavior of porous NiT i alloys is investigated because it is relevant to the engineering and medi cal applications of SMAs. It is found that the transformation behavior of p orous NiTi alloys is different from that of cast NiTi alloys. It is demonst rated for the first time, by in situ X-ray diffraction (XRD), that there is no R-phase transformation in porous NiTi alloys, and a broadened, two-peak phenomenon observed with a differential scanning calorimeter (DSC) is not associated with R-phase transformation. The characteristic transformation t emperatures of porous NiTi alloys are independent of sintering temperature, sintering time, TiH2 content, and the heating/cooling rate during thermal cycling between +123 and +423 K. Further, the latent heats of transformatio n are associated with the TiH2 content and the sintering conditions.