Continuous multi-cycle nanoindentation behavior of a gradient nanostructured metastable β titanium alloy fabricated by rotationally accelerated shot peening

Abstract A novel gradient nanostructured structure fabricated by rotationally accelerated shot peening (RASP) has been proven useful in the enhancement of strength and ductility. In this study, a series of nanoindentation tests using continuous multi-cycle (CMC) mode are first used to characterize the nanoindentation behavior of the RASP-processed metastable β titanium alloy with a gradient nanostructured structure. The experimental results indicate that indentation depth and grain size paly a dominant role in the nanoindentation behavior. Inverse indentation size effect (ISE) on the hardness occurs at shallow indentation depth ( 225 nm), which is dominated by the known strain gradient mechanism. The hardness, reduced module and back stress decrease from the treated surface with a nanostructured grain size of 20 nm to the center zone with a coarse grain size of 216 μm. For the RASP-processed sample, the fine grain zone exhibits a more pronounced inverse and normal ISEs than these of the coarse grain zone. In addition, the Bauschinger effect and back stress strengthening mechanism are also firstly used to explain the variation of gradient micro-mechanical properties.