Plasma Heat Treatment of Titanium and Titanium Alloy Ti-6Al-4V: Effect of Gas Composition and Temperature

Abstract

The results of investigations into the thermochemical heat treatment of titanium and the titanium alloy Ti-6Al-4V in a glow discharge are reported. The effect of the important process parameters, gas composition and temperature, on the microstructure, microhardness, and mechanism of layer formation has been studied by means of X-ray diffraction and electron microscopy. The microhardness (Knoop) profiles have been measured and related to various heat treatment conditions. It has been found that the microstructure and the microhardness profiles depend strongly on gas composition. The presence of hydrogen in the gas mixture led to hydride formation in pure titanium, while the microstructure of the titanium alloy Ti-6Al-4V was not affected by hydrogen. It was shown that plasma heat treatment in a gas mixture containing carbon resulted in the formation of titanium carbide. At high carbon contents, compound layer formation can be retarded by a deposit of soot on the surface. It has been observed that, in addition to the established methods of nitrogen mass transfer, i.e. implantation of reactive species and reactive sputtering, chemisorption of nitrogen at the surface plays a significant role in layer formation if pure nitrogen gas is used for the plasma heat treatment; however, the addition of carbon to the gas mixture causes variations in the layer formation mechanism.