Phase transformation temperatures for NiTi alloys prepared by powder metallurgical processes

The phase transformation temperatures (PTTs) of NiTi samples prepared by hot isostatic pressing (HIP) and metal injection moulding (MIM) from two prealloyed powders with a different Ni content were determined and compared to the PTTs of the starting powders and samples sintered from loose powders. The impurity content of the samples is influenced by the processing. Deviations of up to 50 K were found between the measured PTTs and values given in literature for samples with the same nominal Ni content. To explain these differences, preferentially the formation of Ti-consuming oxides was assumed to change the Ni content of the transforming matrix. Based on the oxygen content of the samples determined by chemical analysis, the Ni content was recalculated and the measured PTTs were correlated to the nominal and the calculated Ni content. It has been shown that the oxygen content itself is not sufficient to explain the influence of the manufacturing route on the PTTs of powder metallurgical (PM) NiTi samples.