Self-Propagating High-Temperature Synthesis Compaction of Titanium Nickelide: Effect of Oxygen and Hydrogen Impurities on the Structure and Properties of the Alloys

Abstract

Titanium nickelide alloys have been prepared by self-propagating high-temperature synthesis compaction using equiatomic mixtures of nickel and titanium powders. The alloys were synthesized in a “sand” press die with the use of a “chemical furnace” and in a rigid press die. In the latter process, reaction mixtures were first subjected to mechanical activation (MA), which made it possible to carry out exothermic synthesis and consolidation of the synthesis products without preheating. No inert atmosphere was used in the syntheses. We obtained titanium nickelide samples 70 mm in diameter and 8 mm in thickness. The percentage of the NiTi phase has been shown to depend on the combustion temperature of the Ni + Ti powder mixture and the percentages of oxygen and hydrogen in the starting titanium powder. The largest percentage of the NiTi phase (85 vol %) was reached at a combustion temperature of 1400°C with the use of titanium containing 0.55 wt % oxygen and 0.14 wt % hydrogen. An increase in the oxygen content of the Ni + Ti powder mixture to 2.3 wt % as a result of MA leads to an increase in the content of the Ti₂Ni phase in the alloy to 53 vol %. As the hydrogen content of titanium increases to 0.6 wt %, the combustion temperature and speed decrease and free Ni remains in the alloy. The alloys with the highest content of the NiTi phase have the lowest microhardness: HV = 6.2 GPa. As the percentage of the Ti₂Ni, Ni₃Ti, and Ni₄Ti₃ phases in the alloys increases, their hardness rises to HV = 11.1 GPa.