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 repo rted in this paper. X-ray diffraction was used to investigate the char acteristics of the as-milled powders. For the MA of Ni60Ti40 (atomic r atio) the following results were obtained. Under Ar atmosphere, the cr ystalline phase directly transformed into the amorphous phase. Under N 2 or H-2, atmosphere, the results showed that small amounts of nitride s (Ti2N and Ni3N) or hydride (TiH1.923) occurred and eventually the cr ystalline phase directly transformed into the amorphous phase. when th e milling process was performed in an O2 atmosphere, intermetallic com pounds and oxides appeared. A self-sustaining reaction and/or a combus tion reaction occurred in this process. The powder mixture of Ni40Ti60 was also milled in an O2 atmosphere. The result showed that the combu stion reaction induced by oxidation of Ti and/or Ni occurred after mil ling for 10 min. The agglomerates, i.e. the main combustion reaction p roducts, with a size of 3-10 mm, have been synthesized in this process . The mechanically alloyed products are composed of intermetallic comp ounds such as Ti2Ni and TiNi and oxides of Ni and Ti. These intermetal lic compounds were formed by combustion reaction during the MA process .