Stress-dependent subsurface structural transformations of gradient nanograin Ti–6Al–4V alloy and its impact on wear behavior

Gradient nanograin (GNG) metals possess outstanding resistance to sliding friction and wear. The tribolayer plays a major role in the wear performance of metals. However, the current understanding of the tribolayer in GNG metals is still limited. In particular, some GNG metals show low wear resistance under high contact stress. Here, we investigate the wear behavior of coarse grain (CG) and GNG Ti-6Al-4V alloys and its impact on the wear resistance by sliding contact experiments in combination with finite element modeling. We have found wear-induced the oxidation tribolayer with amorphous structures in GNG Ti-6Al-4V alloy for the first and its impact on wear behavior. It is found that the wear damage mainly occurs in the oxidation tribolayer with amorphous structures, which can generate micro-cracks in Ti-6Al-4V alloy. The wear behavior, including wear resistance, wear micro-cracks, and subsurface structural transformations, is stress-dependent, which can be divided into two distinct regimes-elastic and plastic. Coarse grains (CGs) can form the oxidation tribolayer with amorphous structures in elastic and plastic regimes. Compared to CGs, gradient nanograins (GNGs) can suppress the formation of the oxidation tribolayer with amorphous structures and have higher wear resistance in elastic regime, while GNGs form the oxidation tribolayer with amorphous structures more quickly and have lower wear resistance in plastic regime. This work forges the links between stress-dependent subsurface structural transformations and wear resistance for GNGs, and may provide guidance for the wear application of GNG metals.