Biaxial tension-torsion fatigue behavior of gradient nano-grained pure titanium fabricated by surface nanocrystallization

A gradient nanostructured surface layer was fabricated in the commercial-purity titanium (Ti) thin-wall tubular sample using the modified surface nanocrystallization (SNC) technique. Biaxial tension-torsion fatigue behavior of the SNC Ti was investigated. The SNC Ti shows significant longer biaxial fatigue lives than the coarse grained Ti (CG Ti) at the same cyclic equivalent stress amplitude. Both CG and SNC Ti display hardening during cyclic deformation, and the hardening level in the SNC Ti is larger than that of the CG one. Microstructural analysis reveals that the SNC Ti shows hierarchical deformation mechanisms in different areas across the wall-thickness of tubular samples during biaxial fatigue. In the nano/ultrafine grain region, the stress-driven nanograin growth is the primary deformation mechanism. In the deformed grain region, the interaction between lamella structure and dislocations is observed. In the coarse grain region, prismatic slip is main deformation mode. The initiation of fatigue cracks is restricted in the SNC Ti since the stress concentration is relieved through stress-driven nanograin growth during cyclic deformation. The initiation mechanism of fatigue cracks transforms from slip band and grain boundary cracking in the CG Ti to the shear band cracking in the SNC Ti. Furthermore, since the superposition of residual compressive stress with the exterior applied stress in the SNC Ti, both maximum shear stress and maximum normal stress are decreased. Consequently, the equivalent driving forces for crack initiation and propagation in the SNC Ti are smaller than those in the CG Ti. The higher resistance to fatigue crack initiation and the lower equivalent driving force for fatigue crack initiation and propagation contribute to the enhancement of fatigue properties in the SNC Ti.