The effect of ultrasonic impact treatment on the deformation behavior of commercially pure titanium under uniaxial tension
Abstract The deformation behavior of commercially pure titanium specimens subjected to surface hardening by ultrasonic impact treatment followed by uniaxial tension was investigated experimentally and numerically. The microstructure of the ultrasonically treated ~100μm thick surface layer undergoing uniaxial tension was revealed, using transmission electron microscopy and electron backscatter diffraction. Non-equiaxed 100–200nm α-Ti grains composed of 2nm diameter TiC and Ti 2 C nanoparticles, ω- and α″-phase crystallites were found in the 10μm thick uppermost layer. Fine and coarse α-Ti grains containing dislocations and twins were observed at depths of 20 and 50μm below the specimen surface, respectively. A non-crystallographic deformation (shear banding) mechanism at work in the nanostructured surface layer of the specimens under study was revealed. The evolution of shear bands was studied by the finite difference method, with the fine-grained structure being explicitly accounted for in the calculations. Shear band self-organization was described, using the energy balance approach similar to that based on Griffith's energy balance criterion for brittle fracture. The tensile deformation of the hardened layer lying at a depth of 50μm was implemented by the glide of dislocations and growth of deformation twins induced by preliminary ultrasonic impact treatment. Highlights Ultrasonic impact treatment produces a gradient nano-to-micron-grained structure in the surface layer of CP titanium. Dislocation glide, twinning and shear banding mechanisms are at work at different depths below the surface under tension. Shear bands in hardened surface layer develop alternatively in conjugate directions of maximum tangential stresses. Shear band evolution and self-organization are analyzed using the energy balance approach. Graphical abstract [DISPLAY OMISSION]
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