Effect of gas nitriding treatment on cavitation erosion behavior of commercially pure Ti and Ti−6Al−4V alloy

In this study, the microstructure, hardness depth profile and cavitation erosion behavior of commercially pure Ti (CP Ti) and Ti−6Al−4V alloys nitrided at 973, 1123 and 1273 K for 16 h were examined. The results revealed that the cavitation erosion behavior of both CP Ti and Ti−6Al−4V alloy was considerably improved by gas nitriding process. The CP Ti nitrided at 973 K has a 2.65-fold increase in cavitation erosion resistance (Re) as compared with untreated specimens, whereas the CP Ti nitrided at 1273 K has the lowest Re. In contrast, the Ti−6Al−4V alloy nitrided at 1273 K exhibited the highest Re while the specimen nitrided at 973 K has the lowest resistance. These results were mainly due to the different properties of the nitrogen diffused zone (NDZ) for CP Ti and alloy. For CP Ti, the NDZ prevented the propagation of microcracks to interior and retarded the erosion due to its homogeneous structure with high hardness, crack-free and good metallurgical bonding. However, the well-organized microstructure was damaged by high temperature (1273 K) treatment and induced the decrease of Re. For Ti−6Al−4V alloy, the NDZ consisted of hard α-Ti grains and soft β-Ti grains resulted in a selective attack for cavitation damage and accelerated the erosion process. But high temperature (1273 K) treatment induced the transformation of β-Ti to α-Ti due to solid solution of nitrogen, which promoted the formation of nitrogen-enriched α-grains (α-Ti(N)) layer on the top of NDZ and sequentially increased the Re up to 6.72 folds. In summary, the significant improvement of cavitation erosion behavior after gas nitriding treatment was mainly attributed to the formation of a unique and homogenous α-Ti(N) layer on the top of the NDZ.

► Gas nitriding enhanced cavitation erosion resistance of CP Ti and Ti–6Al–4V alloy.
► The nitrogen diffused zone played a key role on cavitation erosion behavior.
► Improvement in Re of Ti and Ti–6Al–4V is due to the formation of α-Ti(N) layer.