"the Study of Phase Transformation of Nanocrystalline NiTi Shape Memory Alloy Prepared Via Mechanical Alloying"

Nickel and Titanium elemental powders with a nominal composition Ni-50at.%Ti were mechanical alloyed in a planetary high-energy ball mill in different milling conditions (5, 10, 20, 40 and 60 h). The investigation revealed that increasing milling time leads to a reduction in crystallite size, and after 60 h of milling, the Ti dissolved in Ni lattice and NiTi (B2) phase was obtained. With milling time, morphology of pre-alloyed powders changed from lamella to globular. Annealing of as-milled powders at 1173 K for 900 s led to formation of nanocrystalline NiTi (B19′), grain growth and release of internal strain. The results indicated that this technique is a powerful and high productive process for preparing NiTi intermetallic compound with nanocrystalline structure and appropriate morphology. Differential scanning calorimetery (DSC) result indicated that martensitic transformation occurred in a single-stage manner, and thermal hysteresis and transformation temperatures were found to decrease.