Deformation and fracture mechanisms of Ti-55531 alloy with a bimodal microstructure under the pre-tension plus torsion composite loading

The deformation and fracture behavior of the Ti-55531 alloy with a bimodal microstructure (BM) under the pre-tension plus torsion composite loading were systematically investigated at room temperature. The results indicate that the pre-tension loading dramatically reduces the subsequent torsional strength of the alloy, while its torsional plasticity is almost not affected. A (101¯0) prismatic slip is initiated inside the primary equiaxed α (α_p) phase during the pre-tension stage. Subsequently, several dislocation jogs form inside the α_p particles due to the crossing of the (0002)[1¯ 2 1¯ 0] basal and (11¯01) [1¯ 2 1¯ 0] pyramidal slip systems during the torsion deformation stage, which may be a novel deformation mechanism of the Ti-55531 alloy. Moreover, the α_p particles are cut by numerous parallel slip bands, resulting in ladder-like structures be formed at the α_p/β_trans (β transformed microstructure) interface, which can promote microcrack initiation at the α_p/β_trans interfaces. Furthermore, {101¯1}_α deformation twins are only detected inside secondary α (α_s) phase just during the subsequent torsion deformation stage. In the main bearing phase, the α_p particles elongate along specific directions in different deformation regions due to a change in the maximum shear stress. Thus, the cross-section profile from the surface to the center of the specimens after the pre-tension plus torsion deformation can be divided into three regions: torsion, tension plus torsion, and tension deformation regions. These findings can provide a theoretical basis for understanding the deformation damage of aerospace components under complex loads and optimizing their structural design.