STRUCTURAL EVOLUTIONS OF NI-TI SYSTEMS CAUSED BY MECHANICAL ALLOYING IN DIFFERENT ATMOSPHERES

An investigation of the effects of gaseous elements on the structural evolutions of Ni-Ti systems caused by mechanical alloying (MA) is reported in this paper. X-ray diffraction was used to investigate the characteristics of the as-milled powders. For the MA of Ni60Ti40 (atomic ratio) the following results were obtained. Under Ar atmosphere, the crystalline phase directly transformed into the amorphous phase. Under N2 or H-2, atmosphere, the results showed that small amounts of nitrides (Ti2N and Ni3N) or hydride (TiH1.923) occurred and eventually the crystalline phase directly transformed into the amorphous phase. when the milling process was performed in an O2 atmosphere, intermetallic compounds and oxides appeared. A self-sustaining reaction and/or a combustion reaction occurred in this process. The powder mixture of Ni40Ti60 was also milled in an O2 atmosphere. The result showed that the combustion reaction induced by oxidation of Ti and/or Ni occurred after milling for 10 min. The agglomerates, i.e. the main combustion reaction products, with a size of 3-10 mm, have been synthesized in this process. The mechanically alloyed products are composed of intermetallic compounds such as Ti2Ni and TiNi and oxides of Ni and Ti. These intermetallic compounds were formed by combustion reaction during the MA process.