PLASMA-NITRIDED ALPHA-BETA-TI ALLOY - LAYER CHARACTERIZATION AND MECHANICAL-PROPERTIES MODIFICATION

Beyond continuous efforts to develop advanced processing methods or ne w directions in surface modification, the foundations for assessment o f appropriate surface layers still remain very challenging. In this co ntext, Ti-6Al-4V alpha-beta alloy was investigated mainly after plasma nitriding by nitrogen or by a nitrogen mixture with hydrogen and/or a rgon. The current study objectives consist in gradually developing som e aspects of the microstructure and property relationship. As such, th e study centred on the characterization of refined layers as well as c onfronting critical questions of how layers and interfacial microstruc ture might affect the near-surface mechanical properties (i.e. microha rdness, fatigue resistance and erosion). In particular, the effects on fatigue behaviour are emphasized by utilizing single edge notched spe cimens and fatigue stepdown techniques. It is found that two distinct sublayers, comprising delta-TiN and delta-TiN + epsilon-Ti2N phases, w ere formed with alloying elements in a segregated zone, followed by a solid solution of N in the Ti. Here, the far field affected zone exten ded up to about 20 mum. It was observed that the formation of the uppe rmost sublayer (delta-TiN phase) with a composition including H, NH, a nd N, as well as Ti depleted of Al and V, has a strong effect on the l ayer properties. A microhardness value as high as 29.4 GPa (3000 kgf m m-2) was obtained with significant improvements in the erosion resista nce and fatigue life. It was found that in some controlled plasma nitr iding conditions the fatigue life for crack initiation increased by mo re than a factor of 3. Accordingly, the cyclic crack initiation behavi our is described, revealing substantial influences due to crack tip fi eld 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 elabo rated.