Bimodal grain structures and tensile properties of a biomedical Co-20Cr-15W-10Ni alloy with different pre-strains

The influence of pre-strain on the formation of bimodal grain structures and tensile properties of a Co -20Cr-15W-10Ni alloy was investigated. The bimodal grain structures consist of fine grains (FGs; 2-3 mu m in diameter) and coarse grains (CGs; 8-16 mu m in diameter), which can be manipulated by changing the pre-strain (epsilon = 0.3-0.7) and annealing temperatures (1000-1100 degrees C). High pre-strain applied in the samples can intensify the plasticity heterogeneity through increasing the total dislo- cation density and the local volumes of high-density dis- locations. This can essentially result in finer FGs, a higher FG volume fraction, and overall grain refinement in the samples after annealing. High-temperature essentially increases both the size and volume fraction of CGs, leading to an increase in the average grain size. The tensile test suggests that the bimodal grain structured samples exhibited both high strength and ductility, yield strengths of 621-877 MPa and ultimate tensile strengths of 1187-1367 MPa with uniform elongations of 55.0% 71.4%. The superior strength-ductility combination of the samples arises from the specific deformation mechanisms of the bimodal grain structures. The tensile properties strongly depend on the size ratio and volume fraction of FGs/CGs in addition to the average grain size in the bimodal grain structures. The grain structures can be modified via changing the pre-strain and annealing temperature.