Synthesis and characterization of nanocrystalline NiTi intermetallic by mechanical alloying

Mechanical alloying (MA) has been used to produce NiTi intermetallic with nanocrystalline structure from the elemental powders. The product was characterized using X-ray diffraction, scanning electron microscopy and microhardness measurements. The results showed that disordered B2-NiTi phase can be obtained with grain size of 25 nm, particle size of 15 mu m, lattice distortion and high microhardness, of 1.2% and 922 HV, respectively. The mechanism of NiTi formation was investigated. In the early stages of MA, a composite lamellar structure of components is formed with the dissolution of Ti in Ni at the same time. The resulting solid solution finally leads to the formation of nanocrystalline disordered B2-NiTi phase. Annealing of the milled powder at 1173 K leads to grain growth, decrease of microhardness and transformation of disordered structure to ordered NiTi, with long-range order of 0.94. A small amount of NiTi2 and Ni3Ti phases was also detected. (C) 2007 Elsevier B.V. All rights reserved.