Comparative mesoscale residual stress evaluation in a plastically deformed Ti-6Al-4V alloy bar by Ga+ and Xe+ FIB-DIC micro ring-core milling

Abstract

A large number of recent studies, both of non-destructive (synchrotron and laboratory microfocus X-ray diffraction) and destructive nature (e.g., FIB-DIC micro ring-core ion milling) report success in probing residual stresses at the 0.5–10 μm micro- to nano- scale to accompany conventional approaches at the macro-scale (the contour method, blind hole drilling) capable to provide data at the 0.5–10 mm resolution. However, the mesoscale range between 50 and 500 μm (0.05-0.5 mm) still presents a challenge both in terms of practical implementation, data processing and interpretation. One way to increase the size of the probe in FIB-DIC analysis is to use Xe ion beam in place of Ga. This allows scaling up the gauge volume (probe) size from 5–20 µm (Ga-ion FIB) to 50–80 µm (Xe-ion FIB) to obtain information about Type I + II residual stress. In this article, we present a comparative study of residual stress evaluation in an alpha–beta Ti-6Al-4V alloy bar after 3-point plastic bending. Electron Backscatter Diffraction microstructure mapping revealed almost texture-free polycrystal with near-equiaxed grains of mean diameter 5–10 μm. Nanoindentation was employed to obtain a correlation between hardness and residual stress. The results obtained allowed the consideration of statistical representativeness and fidelity of FIB-DIC techniques in inhomogeneous anisotropic polycrystals and offer recommendations for future technique development towards convergence with other methods.