HEAT TREATMENT EFFECTS ON STRUCTURE AND PROPERTIES OF SYNTHESIZED NANOCRYSTALLINE NiTi INTERMETALLIC BY MECHANICAL ALLOYING

Abstract

NiTi intermetallic with nanocrystalline structure was produced by mechanical alloying of the elemental powders and the effect of subsequent heat treatment was investigated. The products were characterized using X-ray diffraction and microhardness measurements. The results showed that after 60 h of mechanical alloying, disordered B 2- NiTi phase can be obtained as a metastable phase at room temperature with grain size of 25 nm, lattice strain and high microhardness of 1.2% and 922 HV, respectively. After heat treatment, disordered structure transformed to ordered NiTi and a small amount of NiTi 2 and Ni 3 Ti phases. The long range order parameters of nanocrystalline NiTi were obtained to be 0.63 and 0.94 after annealing for 30 and 60 mins respectively. After 30 mins of heat treatment, 81.5% of Ni (or Ti ) atoms were on the right sites and after 60 mins it increased to 97%. Grain growth and decrease of microhardness and lattice strain were also observed after heat treatment. After 60 mins of heat treatment the grain size, lattice strain and microhardness of NiTi were 95, 0.09 and 624 HV respectively.