Syntheses of the Ni<sub>3</sub>Ti, NiTi, and NiTi<sub>2</sub> Compounds by Mechanical Alloying

Abstract

The aim of this work is to prepare the Ni3Ti, NiTi, and NiTi2 compounds by mechanical alloying from elemental Ni-25at.%Ti, Ni-50at.%Ti, and Ni-66.6at.%Ti powder mixtures. The milling process was carried out in a planetary ball mill under argon atmosphere using a rotary speed of 200rpm, stainless steel balls (10 and 19 mm diameters) and vials (225mL), and a ball-to-powder weight ratio of 10:1. Following, the milled powders were heat treated at 900oC for 1h in order to attain the equilibrium microstructures. The milled powders were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and microanalysis via EDS. Similar ball milling behavior of Ni-Ti powders was noted in this work, e.g., a pronounced cold-welding between ductile powders occurred during the initial milling times. The Ni3Ti, NiTi, and NiTi2 compounds were synthesized after milling for 30h. Atomic disordering of the NiTi and NiTi2 compounds was achieved, and amorphous structures were then formed in Ni-50Ti e Ni-66.6Ti powders milled for 60h and 210h, respectively. Homogeneous matrixes constituted by the Ni3Ti, NiTi, and NiTi2 phases were formed in Ni-Ti powders after heat treatments at 900oC for 1h. Iron contamination lower than 2 at-% was measured by EDS analysis in heat-treated Ni-Ti alloys.