MATERIALS SCIENCE AND ENGINEERING

SURFACE NITRIDATION OF TI-BASE ALLOYS

Ti-base alloys are technologically important because of their high specific strength and resistance against corrosion in oxidizing environments. However, in many applications (aerospace engineering, biomedicine, sports, jewelry, architecture) the performance of such alloys is severely limited by their notoriously poor surface hardness and scratch resistance, very high friction coefficients and rapid wear, as well as poor corrosion resistance in reducing media. A powerful approach to counter these deficiencies is to harden the surface of Ti-alloy parts – preferentially after they have been formed into their final shape. Such "case" hardening is most effectively achieved by inward diffusion of interstitial solutes, for example nitrogen. However, titanium forms very stable compounds with all relevant interstitial solutes. Consequently, conventional methods to case-harden Ti-base alloys with interstitial solutes inevitably produce precipitates (oxides, carbides, or nitrides), which severely degrade the wear-, fatigue-, and corrosion resistance. Pioneering experiments by our group have resulted in a new concept enabling inward diffusion of nitrogen into titanium based alloys from a gas phase without precipitating detrimental nitride particles. This new concept is denoted as "nitridation under kinetic control" [1,2] and has been successfully implemented in two different processing schemes, yielding precipitation-free cases with nitrogen concentrations of up to 20 at\% and a tremendous increase in surface hardness (Ti–6Al–4V: a factor of 2, commercially pure Ti: a factor of 8). To our knowledge, this is the first time that highly concentrated homogeneous solid solutions of interstitial nitrogen in Ti-base alloys can be fabricated in a controlled manner.

Ongoing research aims to study the surface, microstructure, and fundamental properties of these new and unusual materials. Of particular interest are the effects of nitridation on (i) the surface (roughness, friction, wear), (ii) the microstructure (dislocation/twin density, lattice expansion and residual stress, proportions and spatial distribution of phases in duplex alloys, texture), (iii) plasticity and corresponding micromechanisms (hardness, yield stress, strain to failure, slip, twinning), and (iv) corrosion resistance (in oxidizing and reducing environments). Moreover, we plan to model the diffusion kinetics of nitrogen and the effect of alloy composition on the thermodynamic stability of the nitrogen solid solution compared to nitride phases.

The scientific merit of this work is to provide initial insight into the structure and properties of unusual materials that have not been available before. Obtaining a fundamental understanding of the impact of high nitrogen concentrations on the thermodynamics, microstructure, mechanical, and electrochemical properties is key for exploring the full potential of a new concept for improving the performance of a very important class of structural alloys. Moreover, the results will enable probing the potential applicability of the new concept to other important alloy families.

Fig. 1. X-ray diffractogram of Ti–6Al–4V before and after surface nitridation under kinetic control. The peak shifts are direct evidence of ≅10 at% nitrogen in solid solution. Peaks indicating the presence of Ti₂N or TiN are not observed; the nitrogen is entirely retained in homogeneous solid solution.