Mechanical and microstructural characteristics of as-sintered and solutionized porous 60NiTi

Abstract

60NiTi is a Ni-rich NiTi alloy containing ∼55 at.% (60 wt%) Ni and ∼45 at.% (40 wt%) Ti. This paper covers the use of Ni and Ti blends to fabricate porous 60NiTi parts by conventional press-and-sinter method. In this study, critical processing parameters such as heating rate and sintering holding time were altered to process porous parts with different open porosity levels. In the next step, as-sintered samples were heated at ∼1050 °C and then quenched (solutionized) to obtain hard and homogenized 60NiTi parts. Effects of these processing factors on the microstructure and mechanical properties of porous 60NiTi parts were investigated. It was found that processing the parts under a faster heating rate or a shorter sintering holding time resulted in as-sintered and solutionized samples with lower relative density and a higher amount of open porosity volume. However, a lower hardness and compressive strength was achieved as compared to the situations where a slower heating rate or a longer sintering holding time was applied. It was observed that the effect of heating rate is more pronounced compared to the sintering holding time. Final solutionizing treatment resulted in solution of soft Ni-rich phases such as Ni3Ti and Ni3Ti2 in the parent NiTi phase. This consequently resulted in a drop of austenite-martensite phase transformation to well below room temperature. This final heat treatment was beneficial in terms of increasing the amount of open porosity. Solutionized parts exhibited higher hardness, approximately the same compressive strength but more embrittlement compared to as-sintered specimens.