Deformation and fracture behavior of commercially pure titanium with gradient nano-to-micron-grained surface layer

Titanium with gradient nano-to-micron scale grains from surface to matrix was fabricated by surface mechanical grinding treatment (SMGT) at room temperature. The SMGT-treated titanium shows higher strength than that of as-received one, but moderate ductility between those of ultra-fine grained (UFG) and coarse-grained titanium. Tensile stress-strain curves of SMGT-treated titanium show double strain hardening regimes. The strain hardening rate (dσ/dɛ) decreases with increasing strain in tensile deformation. The high strain hardening rate at initial yielding is attributed to nano-to-micron-grained surface layer. The low strain hardening rate at large plastic strain regime primarily results from coarse-grained matrix. The SMGT-treated titanium shows a ductile fracture mode with a large number of dimples. The small size of dimples in the treated surface layer is due to the combination of the high strength and strain hardening exponent. The difference between dimple size in nano-to-micron-grained surface layer and coarse-grained matrix is discussed in terms of plastic zone size at the tip of crack in the SMGT-treated titanium.