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[References: 36 tit.]

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 ~ 100 ?m thick surface layer undergoing uniaxial tension was revealed, using transmission electron microscopy and electron backscatter diffraction. Non-equiaxed 100–200 nm ?-Ti grains composed of 2 nm diameter TiC and Ti2C nanoparticles, ?- and ??-phase crystallites were found in the 10 ?m thick uppermost layer. Fine and coarse ?-Ti grains containing dislocations and twins were observed at depths of 20 and 50 ?m 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 50 ?m was implemented by the glide of dislocations and growth of deformation twins induced by preliminary ultrasonic impact treatment.