Small-depth nanoindentation studies of an additively manufactured titanium alloy: Anisotropic nanomechanical properties and correlation with microscopic mechanical behaviour

Nanoindentation has been widely used to measure local mechanical properties of heterogeneous materials. The nanohardness (H), typically measured at indentation depths more than 200 nm, cannot be well correlated with the global mechanical behaviour of tested materials. Here, we study the H of the alpha phase in an additively manufactured titanium alloy using a nanoindentation mapping method with small indentation depths (< 200 nm). The results show that the alpha grains with various crystal orientations exhibit two groups of H values, different from a wide range of H values measured by conventional large-depth nanoindentation. The two groups of H values are attributed to a contribution of elasticity in the measurement of the H values using the small-depth nanoindentation. Furthermore, we correlate the H with the microscopic mechanical behaviour of alpha grains: With respect to the tensile direction, hard alpha grains are plastically deformed by shear banding and soft alpha grains are deformed by dislocation slip.