Gas-phase surface alloying under “kinetic control”: A novel approach to improving the surface properties of titanium alloys

Abstract

Abstract A novel process, “nitridation under kinetic control of the nitrogen activity”, has been developed for diffusing substantial amounts (≍10at.%) of interstitially dissolved nitrogen into the surface of Ti alloys (Ti–6Al–4V). By operating with a gas phase providing a very small, controlled nitrogen activity, this process generates a homogeneous Ti–N solid solution, free of detrimental titanium nitride precipitates, in which the nitrogen concentration smoothly decreases from the surface towards the interior. The process is conformal (applicable to workpieces of arbitrary shape) and provides a substantial (about twofold) increase in surface hardness. The hardened surface layer appears to possess adequate ductility for many structural applications. The concept of “surface alloying under kinetic control” is very general and may also serve to generate well-controlled surface concentration profiles of carbon or oxygen in Ti-base and other structural alloys.