Mesoscale deformation mechanisms in relation with slip and grain boundary sliding in TA15 titanium alloy during tensile deformation

Revealing the mesoscale deformation mechanisms of titanium alloy with tri-modal microstructure is of great significance to improve its mechanical properties. In this work, the collective behavior and mechanisms of slip activities, slip transfer, and grain boundary sliding of tri-modal microstructure were investigated by the combination of quasi-in-situ tensile test, SEM, EBSD and quantitative slip trace analyses. It is found that the slip behavior presents different characteristics in the equiaxed α (α_p) and lamellar α (α_l) grains. Under a low level of deformation, almost all the slip deformation is governed by single basal and prismatic slips for both of α_p and α_l, despite small amount of < a >-pyramidal slip exists in α_l grains. As deformation proceeds, < a >-pyramidal and < c + a >-pyramidal slip systems with high Schmid factors were activated in quantities. Specially, certain coarse prismatic slip bands were produced across both of single and colony α_l grains whose major axes tilting about 40 °–70 ° from the tensile axis. Slip transfer occurs at the boundaries of α_p/α_p and α_l/β under the condition that there exists perfect alignment between two slip systems and high Schmid factors of outgoing slip system. The slip transfer across α_l/β boundary can be divided into two types: straight slip transfer and deflect slip transfer with a deviation angle of 5 °–12 °, depending on the alignment of slip planes of two slip systems. The grain boundary sliding along boundaries of α_l/β and α_p/β was captured by covering micro-grid on tensile sample. It is found that the crystallographic orientation and the geometrical orientation related to loading axis play great roles in the occurrence of grain boundary sliding.