Experimental investigation on niti sma development with the effects of aging process

Abstract

The formulation of nanocrystalline nickel titanium shape memory alloy (NiTi-SMA) has potential effects in mechanical stimulation and medical implantology. The present research elucidates the effect of milling time on the productand#8223;s structural characteristics, chemical composition, and microhardness for NiTi synthesized by mechanical alloying. Increasing the milling time leads to the formation of a nanocrystalline NiTi intermetallic at a higher rate. The formation of nanocrystalline materials is directed through cold fusion, fracturing, and the development of a steady state, which were influenced by the accumulation of strain energy. In the morphological study, uninterrupted cold diffusion and fracturing were visualized using Transmission Electron Microscopy (TEM). Analysis of particle size reveals that the mean particle size is reduced to ~93 and#956;m after 20 h of milling. The mechanical strength is enhanced by the formation of a nanocrystalline intermetallic phase at longer milling time, which is confirmed by the results of vickers hardness analysis. Further the effects of aging temperature and time on microstructure and phase transformation behaviors of the NiTi shape memory alloy are also investigated along with the formation of precipitates. NiTi shape memory alloy is synthesized using powder metallurgy route followed by solution annealing at 850oC for 1 h. The thermal aging process has been executed on NiTi alloy at the temperature range of 450oC, 550oC, 650oC and 750oC for 1 h, 3 h and 5 h respectively. It is found that the enhancement of transformation temperatures by the formation of secondary phase precipitates Ni during the transformation of cubic structure (B2) to monoclinic structure (B19and#8223;). The lenticular structured Ni4Ti3 precipitate influences mainly the phase transformation by changing the Ni composition in NiTi matrix. Significant changes are observed in the austenite and martensite finish temperatures newline