Microscopic mechanism of plastic deformation in a polycrystalline Co–Cr–Mo alloy with a single hcp phase

A Co–Cr–Mo alloy with a single ε (hexagonal close-packed, hcp) phase exhibits excellent tensile properties with a 0.2% proof stress of 630 MPa, an ultimate tensile stress of 1072 MPa and an elongation to fracture of 38.3%. The dominant deformation modes are basal 〈a〉 slip and prismatic 〈a〉 slip, and the apparent respective critical resolved shear stresses at room temperature are calculated to be 184 and 211 MPa. This simultaneous activation of both 〈a〉 slips can be explained in terms of the lattice constant ratio c/a of 1.610. There is a tendency for the geometrically necessary dislocations (GNDs) to accumulate at grain boundaries, and the magnitude of this GND accumulation at a particular boundary is dependent on its character. Numerical analysis using a dislocation-model-based strain gradient crystal plasticity calculation makes it possible to characterize the distributions of dislocation density, local stress and local strain in the polycrystalline ε Co–Cr–Mo alloy, and the calculation is largely consistent with the experimental results. This simulation reveals that the activity of the prismatic 〈a〉 slip in addition to the basal 〈a〉 slip contributes to the stress relaxation at the boundary. For this reason, excellent tensile ductility is obtained in the polycrystalline ε Co–Cr–Mo alloy.