Investigating the microstructural evolution during deformation of laser additive manufactured Ti-6Al-4V at 400 °C using in-situ EBSD

A high-temperature in-situ tensile stage setup developed by the authors was combined with electron backscatter diffraction (EBSD) to study the microstructural evolution behavior of Ti-6Al-4V alloy produced by laser direct melting deposition (LDMD) at 400 degrees C. This technique provided direct observation of the microstructure evolution and deformation behavior of materials under dynamic tensile loading at operating temperature. The evolution of grain morphology, misorientation angles of grain boundaries, and Schmid factor (SF) were systematically investigated. The results indicated that the grains no longer sustained the initial lamellar morphology and underwent a severe microstructural evolution during plastic deformation. Various grain morphologies were observed at different tensile strain levels. The statistical analysis revealed the reduction of high-angle grain boundaries' (HAGBs) fraction among alpha/alpha grains, with the opposite trend for low-angle grain boundaries (LAGBs). The increased Schmid factor values (SF 11-20 11-20 11-20 slip system exhibited a sharp drop to SF > 0.4 at higher strain levels. Moreover, it was observed that high-SF grains changed their orientations, while low-SF ones split into smaller grains under tensile load. The kernel average misorientation (KAM) maps showed that deformation promoted dislocation motion, generating many LAGBs. The increasing number of LAGBs and slip openings reduced the boundary resistance and provided an easy path to detour the main crack propagation in response to the applied stress, while the LDMD Ti-6Al-4V specimen showed ductile fracture behavior.