Mechanically alloyed and hot pressed Ni–49.7Ti alloy showing martensitic transformation

Abstract

Abstract The Ni–49.7Ti (at.%) alloy was prepared by mechanical alloying (MA) and different steps of milling were investigated. An almost fully amorphous alloy was obtained after 32 h of milling. After consolidation by uniaxial hot die pressing and annealing at 1000 °C for 28 h, a complicated multiphase structure composed of Ni3Ti, NiTi2 and NiTi phases was formed. The β-NiTi phase undergoes reversible thermoelastic martensitic transformation (MT) with Ms temperature as high as 172 °C. This phenomenon was explained by Fe and Cr solute content introducing during milling and formation of the 4H martensite structure. The structure observations performed using scanning electron microscopy allowed to conclude that the average chemical composition of individual grains was Ti—49.7, Ni—47.76, Fe—2.54 (all in at.%). The second phase between the rounded β-NiTi grains was identified as the Ni3Ti phase and additionally, in some other places of the sample the Ni3Ti phase was also found. Transmission electron microscopy (TEM) of sintered samples confirmed the presence of 4H martensite within fine grains of the β-NiTi phase.