Sintering studies on Ni–Ti–Fe elemental powder mixtures using differential scanning calorimetry

Abstract

Shape memory alloys based on the Ni–Ti system derive their versatile properties from the non-stoichiometric intermetallic compound NiTi. The transformation temperature associated with its shape memory effect is dependent on composition, which can be controlled by minor additions of a third element like iron. This work considers the formation of various intermetallic compounds and the evolution of phases during the sintering of ternary Ni–Ti–Fe powder compacts. Elemental powder mixtures of nickel, titanium and iron were prepared by adding 0–20 at.% Fe to equiatomic Ni–Ti. The powders were compacted into discs and sintered by heating to 1200 °C in a differential scanning calorimeter. In separate experiments, heating was interrupted to identify the phases present in the partially sintered samples at various temperatures. The microstructures of the sintered samples were characterized using scanning electron microscopy. The distribution of nickel, titanium and iron in the samples was studied with EDS mapping and the phases present were identified using XRD. In the equiatomic Ni–Ti powder compact, NiTi₂, NiTi and Ni₃Ti were formed in the solid state (< 942 °C) through diffusion. At 942 °C a strong reaction between the remaining titanium and NiTi₂ takes place, leading to the formation of a liquid. At 1120 °C, NiTi and Ni₃Ti combine to form a liquid. These reactions are affected by the addition of iron to the powder mixture. The results show that at 20 at.% iron in the ternary compact, the first reaction occurred at 999 °C, instead of 942 °C for the binary composition and iron did not form any compound with nickel or titanium. Instead, the iron could replace nickel in NiTi₂ and in NiTi, forming (Fe, Ni)Ti₂ and (Fe,Ni)Ti. This leads to more Ni₃Ti formation and explains why the reaction at 1120 °C is more prominent at high iron contents. A linear dependence on the iron content in the sample was also observed for the onset temperatures for two split exothermic peaks in the DSC curves. The results also suggest that the temperatures associated with the β-Ti + (Fe, Ni)Ti₂ → L and (Fe, Ni)Ti₂ → (Fe,Ni)Ti + L reactions depend on the ratio of iron to nickel in (Fe, Ni)Ti₂.