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

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• Mesoscale deformation mechanisms (slip activities, slip transfer and grain boundary sliding (GBS)) of Ti-alloy with tri-modal microstructure were investigated. • The slip mode, slip system and activation strain presents different characteristics in the equiaxed α (α p ) and lamellar α (α l ) grains. • Two types (straight and deflect) slip transfer occur at α p /α p and α l /β boundaries depending on the geometric alignment of two slip systems. • GBS along α l /β and α p /β boundaries were captured and found be dependent on the crystallographic and geometrical orientations. 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.