Effect of Reaction Atmosphere and Heating Rate During Reactive Sintering of Ni-Ti Intermetallics

The effect of reaction atmosphere and heating rate during reactive sintering of nickel and titanium powder blend (Ni: 52 at.%, particle size <150 μm, >99.99% purity, Ti: 48 at.%, particle size <44 μm, >99.5% purity) were studied. The powder mixtures were uniaxially compressed under pressure of 637 MPa and sintered at temperatures 500, 650, 900 and 1100 °C in air, vacuum, argon and nitrogen atmospheres. Two heating rates were applied: 200 and >300 K.min-1. The results show that a significant effect of reaction atmosphere in terms of the quantity of the Ti2Ni phase applies especially at very high heating rate. In terms of phase morphology, oxygen causes refinement of the structure, regardless of heating rate. Mechanism and kinetics of the reactions leading to Ni-Ti phases were determined by DTA and thermogravimetry. It was found that the Ti2Ni phase forms already at 500 °C. At 650 °C this phase undergoes a reaction with nickel to NiTi and Ni3Ti.