Micro-cantilever testing of microstructural effects on plastic behavior of Ti–6Al–4V alloy

Abstract

To evaluate the effects of microstructural factors on the plastic behavior of Ti–6Al–4V alloys, micro-cantilevers featuring several grains were machined using a focused ion beam. The equiaxed structure (a mixture of equiaxed α phase and intergranular β phase) and bimodal structure (a mixture of an equiaxed α phase and lamellar phase) were examined to verify the microstructural effects. To determine the slip plane and direction, the crystal structure was evaluated using electron backscattered diffraction, following bending tests of the microcantilever. In addition, the elastic modulus and hardness of each grain were determined via nanoindentation testing. In the equiaxed microstructure, the β-phase deformation often occurred first; then, slips were activated inside the equiaxed α phase. It is concluded that the β phase deforms more easily than the α phase because the elastic modulus and hardness of the former are lower, as indicated by the nanoindentation testing. In the bimodal microstructure, β-lath deformation occurred along the acicular direction in the lamellar-phase region, and slips occurred inside the α grain. The deformation of the lamellar structure was associated with β-phase deformation and affected by the high local stress at the grain boundary and phase interface, owing to the difference in grain orientation within the lamellar structure. The results of this study suggest that the plastic behavior of Ti–6Al–4V alloys depends not only on the slip system of the α grains but also on the deformability of the β phase and the misorientation of adjacent grains in polycrystalline Ti–6Al–4V alloy.