Mechanically activated reactive forging synthesis (MARFOS) of NiTi

The production of NiTi alloys through mechanically activated reactive forging synthesis (MARFOS) of elemental powder mixtures with equiatomic composition has been investigated for the first time. The short duration of mechanical activation (4 h) successfully produced powder mixtures capable of being hot deformed at relatively low temperatures. As a consequence, a bulk product constituted by intermetallic and metallic nanocrystalline phases was effectively obtained at 700 °C. An additional heat treatment at 950 °C/24 h removed almost all the undesired phases and maintained the nanocrystalline structure, resulting in an Ni-rich NiTi matrix containing some Ti2Ni/Ti4Ni2Ox and having a micro-hardness of 708 HV0.3. The formation of such microstructure may be attributed to the presence of oxygen and nitrogen impurities in the materials. After ageing at 500 °C/48 h, some degree of compositional rectification was achieved in the NiTi matrix due to the formation of Ni4Ti3 precipitates.