The effect of ultrasonic impact treatment on deformation and fracture of electron beam additive manufactured Ti-6Al-4V under uniaxial tension

The results of experimental investigations and molecular dynamics simulation of deformation and fracture of as-built wire-feed EBAM Ti-6Al-4V samples and the samples subjected to ultrasonic impact treatment are presented. Dislocation deformation of the samples under uniaxial tension is shown to occur through propagation of fine and coarse slip bands and shear bands. The ultrasonic impact treatment results in the formation of a 10 μm thick top layer consisting of the nanocrystalline α-Ti, β-Ti and TiO2 phases and an underlying 30–50 μm thick layer, which microstructure is characterized by curved and fragmented α-phase lamellae. As a consequence, dislocation sliding in the ultrasonically treated surface layer of the wire-feed EBAM Ti-6Al-4V samples is hindered. Molecular dynamics simulation has revealed that the grain boundary sliding and twinning are the main deformation mechanisms of the fragmented α-phase lamellae. The migration of the twin boundaries was happened by means of the development of reversible HCP→BCC→HCP transformations. At the prefracture stage the non-crystallographic shear bands nucleating in the underlying layers propagate in the ultrasonically treated surface layer. Intensive deformation inside the shear bands manifests itself in the formation of regions with ductile microrelief on the brittle fracture surface.