Ni-50Ti shape memory alloy was synthesized by mechanical alloying of the elemental powders mixture under an argon gas atmosphere. The structural and microstructural properties of the alloyed powders were evaluated by X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. Moreover, the Vickers microhardness of the powders was estimated at different milling times. According to the results, by milling progression, the amount of the amorphous phase increased considerably and after sufficient milling time (48 h), the mechanocrystallization of the amorphous phase into the more stable crystalline phases (i.e. B2 and B19′) occurred. It was found that the particles size and microhardness were significantly affected by the formation of the amorphous, B2, and B19′ phases. It was also deduced that, by appropriate heating and cooling cycles, the B2 and thermally-induced B19′ phases can be created. Furthermore, it was inferred that the formation of undesirable intermetallic phases (particularly NiTi 2 ) during the heating cycle was considerably reduced by milling time evolution.
The microstructure and phase evolution of the mechanically alloyed FeCoNiMnV high entropy alloy during spark plasma sintering are examined. The milled powders are sintered at different temperatures ranging between 500 C and 1000 C. A multiphase structure is produced during the sintering process. The produced alloy experiences different types of decompositions and phase formations as the sintering temperature is raised. During sintering at different temperatures, the body centered cubic and face centered cubic (FCC) phases are transformed to new FCC and tetragonal phases. According to the results, V does not show the tendency to dissolve into the structure and the tetragonal phases, mainly composed of V and Mn, tending to be formed in addition to the FCC solid solution. Sintering at 900 C results in the grain size of about 220 nm, attaining the highest hardness of about 450 HV. The sintered alloy at 1000 C shows the highest relative density (about 98%) with an average grain size of about 580 nm. The phase formation is predicted and evaluated by different criteria, and the consistency between the theoretical and experimental results is explained.
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