Plasma-nitrided α-β Ti alloy: layer characterization and mechanical properties modification

Beyond continuous efforts to develop advanced processing methods or new directions in surface modification, the foundation for assessment of appropriate surface layers still remain very challenging. In this context, Ti6Al4V α-β alloy was investigated mainly after plasma nitriding by nitrogen or by a nitrogen mixture with hydrogen and/or argon. The current study objectives consist in gradually developing some aspects of the microstructure and property relationship. As such, the study centred on the characterization of refined layers as well as confronting critical questions of how layers and interfacial microstructure might affect the near-surface mechanical properties (i.e. microhardness, fatigue resistance and crosion). In particular, the effects on fatigue behaviour are emphasized by utilizing single edge notched specimens and fatigue stepdown techniques. It is found that two distinct sublayers, comprising σ-Tin and σ-Tin + ε-Ti₂N phases, were formed with alloying elements in a segregated zone, followed by a solid solution of N in the Ti. Here, the far field affected zone extended up to about 20μm. It was observed that the formation of the uppermost sublayer (σ-TiN phase) with a composition including H, NH, and N, as well as Ti depleted of Al ar. 'V, has a strong effect on the layer properties. A microhardness value as high as 29.4 GPa (3000 kgf mm^-2) was obtained with significant improvements in the erosion resistance and fatigue life. It was found that in some controlled plasma nitriding conditions the fatigue life for crack initiation increased by more than a factor of 3. Accordingly, the cyclic crack initiation behaviour is described, revealing substantial influences due to crack tip field perturbations, or fracture resistance modifications. Finally, the role of extrinsic crack tip shielding effects as related to closure or to the local effective driving force for microcracking onset is elaborated.