The synthesis of amorphous NiTi alloy powders by mechanical alloying

Crystalline powders of Ni and Ti were mechanically alloyed by high-energy ball milling in an inert atmosphere at T < 240 K. The alloyed NixTi1-x powders were characterized by scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry. For 0.28 < x < 0.72, the powder is amorphous. This composition range is in good agreement with that deduced from a free energy diagram for the crystalline terminal solutions and the amorphous phase. Outside this regime, the powder is a two-phase mixture of an amorphous phase and the crystalline terminal solution of the major element. After ball milling, the solubility limit of Ti in FCC Ni is approximately 28%, which is significantly larger than that in annealed Ni. Atomic pair distribution functions, G(r), were calculated from the X-ray patterns for x = 0.32, 0.4, 0.6, and 0.7. For x = 0.4, the G(r) of the mechanically alloyed powder is almost identical with that of rapidly quenched amorphous Ni40Ti60. For Ni33Ti67, the crystallization temperature, the crystallization enthalpy, and the apparent activation energy for crystallization are 712 K, 0.74 kcal/g at., and 65.8 kcal/g at., respectively. These values are within 10% of those measured by Buschow for rapidly quenched Ni32Ti68. The amorphization by mechanical alloying is attributed to mechanical mixing and to a solid state interdiffusion reaction taking at or near clean boundaries between polycrystalline Ni and Ti. © 1985.