Rietveld refinement on in-situ neutron diffraction during sintering of NiTi from elemental powders

Abstract

Rietveld refinement of neutron diffraction patterns obtained in-situ during the reactive sintering of Ni-Ti (49 at% Ni: 51 at% Ti) elemental powder compacts for two sintering profiles was performed. The atomic displacement parameter (Uiso) for each phase that occurs during reactive sintering (α-Ti, β-Ti(Ni), Ti2Ni(O), NiTi, Ni3Ti and Ni) were obtained from separate in-situ experiments. The phase fractions determined from the two sintering runs were found to be in excellent agreement, with an uncertainty estimated at ± 1.6 wt%. Intermetallic phases do not begin to form until 600 °C. Initially, Ti2Ni(O) grows faster than NiTi and Ni3Ti. By 780 °C, NiTi becomes the dominant phase, growing at the expense of all the other phases. Homogenization occurs at 1100 °C and the microstructure consists of 93 wt% NiTi and 7 wt% Ti2Ni(O). Experimental evidence along with the refinement of the O concentration in Ti2Ni(O) during sintering was used to explain the persistence of Ti2Ni(O) in the microstructure. The oxygen raises the melting point of the phase such that it is in equilibrium with NiTi at 1200 °C, despite Ti2Ni having a melting temperature of 984 °C.