Effect of Loading Type on Deformation and Fracture Behavior of Ti-55531 Alloy with Bimodal Microstructure

A combination of transmission electron microscopy and scanning electron microscopy was used to study deformation and fracture behavior of Ti-55531 alloy with bimodal microstructure (BM Ti-55531 alloy) during tensile and torsion tests at room temperature. Results indicate that loading types have a significant influence on deformation and fracture behavior of BM Ti-55531 alloy. First of all, the tension strength is about 300 MPa higher than torsion strength of BM Ti-55531 alloy, and its ductilty of tension is also higher compared to torsional ductilty. It indicates that BM Ti-55531 alloy is more sensitive to torsional shear stress than to tensile stress. Secondly, deformation mechanisms of both tensile and torsion test are a mixing mode including dislocation slip and shear, while deformation of torsion test is predominantly controlled by shear. The interfaces between net-like grain boundary alpha, primary alpha and retained beta phases are easily filled with high density dislocations. Thirdly, fractographs of tensile and torsion tested specimens exhibit different morphologies. Fractograph of tensile specimen is cliffier than that of torsion specimen. The tensile sample shows a ductile failure, including microvoid coalescence and transgranular fracture mechanism. The fracture of torsion specimen is still a mixture type but with more shear dimples.