Mechanically activated reactive extrusion synthesis (MARES) of NiTi

Mechanically activated reactive extrusion synthesis (MARES), as an innovation in powder sintering, was used for the first time to produce NiTi alloys. Equimolar powder mixtures of elemental Ni and Ti were firstly mechanically activated for 4 h and then extruded at relatively low temperatures (between 300 and 600 °C). No intermetallic phase formation was obtained after mechanical alloying. Instead, crystalline phases (Ni and Ti) and an amorphous phase were observed. The end product was constituted of agglomerated micrometer sized particles containing inhomogeneous areas of Ti and Ni. Differential thermal analysis of the as-milled powders showed two small exothermic peaks at about 460 and 540 °C, corresponding to the structure relaxation of the amorphous phase and to the synthesis of Ni–Ti intermetallics. The outcome of MARES trials was sensitive to the extrusion temperature, ram speed and to the total heating time prior to the application of load. The microstructure of the extruded material consisted of NiTi, NiTi2 and Ni3Ti besides Ni and Ti. MARES results are very encouraging for the formation of Ni–Ti intermetallics through a controlled synthesis reaction.