Glow-discharge and furnace treatments of Ti-6Al-4V

The poor tribological properties of titanium and titanium alloys can be improved by using diffusion treatments, which take advantage of the high reactivity of titanium with respect of carbon, nitrogen or oxygen to produce high hardness surface layers well bonded to the tough matrix. The present paper reports a comparison of the effects of glow-discharge and furnace processes, performed using air as treatment atmosphere, on the microstructure and mechanical properties of Ti-6Al-4V alloy samples. Both treatment types, performed at 973 and 1173 K for 0.5, 2 and 4 h, produce modified surface layers, consisting of an outer compound layer and an inner diffusion layer. The compound layer consists mainly of TiO2, small amounts of TiNxOy and, in the 1173-K treated samples, small amounts of Al2O3 concentrated at the outer part of the scale. In the diffusion layer, a solid solution of interstitial atoms in α-Ti is present; moreover, at the compound layer–diffusion layer interface a peculiar microstructure, consisting of oxygen-rich Ti3Al phase and α-Ti solid solution, is observed on samples treated at 1173 K. A higher efficiency in surface hardening is shown by the glow-discharge treatment in respect of the furnace process: by using the same treatment temperature and time, glow-discharge treated samples have thicker hardened layers with higher hardness values than the furnace treated ones, and this effect is more marked for 973-K treated samples, while it is reduced for 1173-K treated samples. Moreover, the a, c lattice parameters of the α-Ti solid solution, evaluated by means of the Rietveld method from X-ray diffraction patterns, are also influenced by the treatment type.