Characterization of deformation anisotropies in an α-Ti alloy by nanoindentation and electron microscopy

The crystallographic dependence of the mechanical responses of an α-Ti–7 wt.% Al alloy was measured by nanoindentation using spherical and Berkovich indenters. Both elastic moduli and hardness responses of indents on the (0 0 0 1), (1¯100) and (1¯21¯0) planes were quantified. The dislocation structures resulting from indentation were characterized by electron microscopy. While scanning electron microscopy techniques were used for the observation of surface slip structures, site-specific focused-ion-beam thin foil preparation and scanning transmission electron microscopy techniques were employed for the imaging of sub-surface dislocation structures. Elastic modulus, hardness and load at pop-in were found to vary with crystallographic orientation. Indentation-induced plasticity was found to occur by multiple slip/twin mechanisms and to be dependent on crystal orientation, although 〈a〉 slip on (0 0 0 1) planes was found to be common to all orientations. The observed dislocation structures are rationalized on the basis of theoretical predictions based on the anisotropic elastic contact analysis and resolved shear stress calculations.