Synthesis of NiTi Alloy Powder by the Reaction of NiO-TiH2 Mixing Powders

Abstract

<p>The synthesis of NiTi alloy powders by hydrogen reduction and dehydrogenation process of NiO and TiH<sub>2</sub> powder mixtures is investigated. Mixtures of NiO and TiH<sub>2</sub> powders are prepared by simple mixing for 1 h or ball milling for 24 h. Simple-mixed mixture shows that fine NiO particles are homogeneously coated on the surface of TiH<sub>2</sub> powders, whereas ball milled one exhibits the morphology with mixing of fine NiO and TiH<sub>2</sub> particles. Thermogravimetric analysis in hydrogen atmosphere reveals that the NiO and TiH<sub>2</sub> phase are changed to metallic Ni and Ti in the temperature range of 260 to 290°C and 553 to 639°C, respectively. In the simple-mixed powders by heat-up to 700°C, agglomerates with solid particles and solidified liquid phase are observed, and the size of agglomerates is increased at 1000°C. From the XRD analysis, the presence of liquid phase is explained by the formation and melting of NiTi<sub>2</sub> intermetallic compound due to an exothermic reaction between Ni and Ti. The simple-mixed powders, heated to 1000°C, lead to the formation of NiTi phase but additional Ni-, Ti-rich and Ti-oxide phases. In contrast, the microstructure of ball-milled powders is characterized by the neck-grown particles, forming Ni3Ti, Ti-oxide and unreacted Ni phase.</p>