In-situ investigation on the deformation mechanism of duplex microstructure of a near α titanium alloy

The targeted research of equiaxed α particles and lamellar α colonies in duplex microstructure of Ti60 alloy was conducted to more reveal the related mechanisms. For this purpose, in-situ tensile test, the electron probe microanalysis (EPMA), microhardness test, electron backscatter diffraction (EBSD), and Transmission Electron Microscopy (TEM) technologies were used to carry out relevant research in this work. The in-situ tensile result shows that the slip line preferentially occurs in the equiaxed α particle. Prism slip of the (101¯0) plane and [12¯10] direction is the most activated slip system in the equiaxed α particle according to the calculation result of the movement slip system. Single slip was operated in individual equiaxed α particle, as the stress concentration at the grain boundary increases, multiple slips could be activated in the adjacent equiaxed α particle while the slip line was hardly found in colonies. With the deformation amount increases, pyramidal slip mode with high critical resolved shear stress (CRSS) in lamellar α had to be activated to accommodate the deformation. It is concluded from the in-situ tensile observation and EBSD analysis that the lamellar microstructure contributes less to the deformation at room temperature which is attributed to the large size of colonies and limited range of slip systems. Cracks nucleate mainly at the grain boundary of the equiaxed α particle and the colony since the existence of non-deformation area. TEM analysis shows that the dislocation networks were pinning by the silicide at the grain boundary of the equiaxed α particle, thus induces the stress concentration and promotes the crack nucleation. In addition, the blocking effect of the α₂-Ti₃Al particles on the dislocation was also found, indicating that the microcrack could nucleate within the equiaxed α particle. On this basis, two diverse crack initial modes of intergranular fracture at the grain boundary and transgranular with in the equiaxed α particle fracture were proposed.