Near-spherical micron-porous NiTi alloys with high performances fabricated via metal injection molding

Abstract

In this study, NiTi alloys with different porosity were prepared using metal injection molding combined with the powder space-holder method (MIM-PSH). The microstructures, transformation temperatures, compression and its recovery properties were investigated. The porous structure consisted of near-spherical pores (40.8 and 44.9 μm). The pore size was tied to the size of space-holder but not porosity, which was beneficial to controllability of pore diameter. The matrix was dominated by B2 austenite, with small amounts of B19′ martensite and Ti2Ni inclusions. Ni4Ti3 precipitates were also observed, not only in micrometer scale, but also as nanodomains. Highly broadened R peaks were observed in the differential scanning calorimetry thermograms, which were attributed to extensive, non-uniform, multi-scale precipitation. Overall, pores at this scale shown no influence on the transformation behaviors. Due to the low final oxygen content (0.19 % and 0.23 %) and homogeneous near-spherical pores, high compressive properties had been exhibited. The compressive recovery was almost complete at 3 %, 6 % and 8 % pre-strains. Irreversible strain increased with increasing pre-strain. The average volume variation of the pores under different deformations was analyzed in detail. In the future, MIM-PSH should be used to further enhance the customized fabrication of porous NiTi alloys.