NITI: AN IN-SITU INVESTIGATION OF SINTERING FROM ELEMENTAL POWDERS

Abstract

Ni-Ti powder mixtures mixed to 49 at% Ni : 51 at% Ti were made from high purity Ti – fine Ni (HP mixture), low purity Ti – fine Ni (LP mixture) high purity coarse (RA) Ti – fine Ni (RA mixture) and high purity Ti – coarse Ni (CNi mixture). They were analyzed during sintering using in-situ differential scanning calorimetry, in-situ neutron diffraction and microscopy. The temperature traces from the neutron diffraction experiment were compared to DSC traces for temperature verification. A detailed description of the procedure for the Rietveld refinements of neutron diffraction patterns obtained in-situ during the reactive sintering is presented. The determination of accurate Uiso values and lattice parameters from separate experiments was found to be critical to the successful refinement of minority phases. The phase fractions determined from the two sintering runs on the LP mixture were found to be in excellent agreement, with an uncertainty estimated at +/- 0.016 wt%. Reaction between the Ni and Ti is not significant until the oxide layer surrounding the Ti particles dissolves at 600 °C. In all but the CNi mixture Ti2Ni(O) forms from 600 to 700 °C. Ni3Ti grows in this temperature range solely in the LP mixture. The α-Ti to β-Ti(Ni) phase transformation begins at the eutectoid temperature (765 °C) and ends as late as 880 °C in the CNi mixture. It is thought the LP mixture undergoes a higher rate of reaction due to the influence of impurities (O, Fe and Ni), with an earlier and a higher total Ni3Ti formation than the HP mixture during solid state sintering. The RA and CNi mixtures undergo TE-SHS commencing at or above 942 °C. Both DSC and in-situ neutron diffraction showed slower diffusion in the CNi mixture than in the RA mixture, resulting in a larger TE-SHS peak and specimens that slumped. During TE-SHS elemental Ti and Ni disappeared, with a corresponding surge in the fractions of all three intermetallic phases in the RA mixture, while in the CNi mixture Ni3Ti disappeared. From 1100 to 1200 °C, the microstructure in all four mixtures became a stable mixture of NiTi with a small fraction of Ti2Ni(O). Refinement of the O concentration in Ti2Ni(O) during sintering provided an understanding of the persistence of Ti2Ni(O). During the initial stages of sintering, Ti2Ni(O) with little or no O is formed. As the amount of this phase increases, its absolute O content also increases. In the later stages of sintering the absolute O content in Ti2Ni(O) remains the same even while Ti2Ni(O) decreases in phases fraction (consumed by NiTi), thus increasing the concentration of O in Ti2Ni(O). Oxygen raises the melting point of Ti2Ni(O) such that it is in equilibrium with NiTi at 1200 °C, despite a melting temperature of 984 °C for Ti2Ni. Also presented are lattice parameter values for all the phases in the Ni-Ti system as determined by Rietveld refinement on the in-situ neutron diffraction runs, including values for Ti2Ni, Ni3Ti and NiTi up 1200 °C. For the first time direct experimental evidence of the β-Ti(Ni)→Ti2Ni + α-Ti eutectoid reaction is presented.