Effect of surface mechanical grinding treatment on the microstructural evolution and mechanical properties of commercially pure titanium

Abstract

The evolution of microstructure and mechanical properties of commercially pure titanium subjected to surface mechanical grinding treatment has been investigated to decipher the underlying deformation mechanisms of gradient structure formation during SMGT. SMGT specimens demonstrated obvious grain refinement and an exceptionally increased fraction of low-angle grain boundaries as well as a multi-banded gradient structure with a heterogeneous twin structure. The SMGT process resulted in the formation of a 1000 µm thick gradient structure layer on both sides showing a significant improvement (BM: 164 HV; surface MH: 265 HV improved by 62 %; sub-surface MH: 309 HV improved by 88 %) in microhardness at the surface and sub-surface region, accompanied by a gradual decrease to the center of the specimen. Tensile testing of the SMGT specimens shows a 33 % (BM YS: 327 MPa; SMGT YS: 436 MPa) increase in the yield strength and a 31 % (BM UTS: 392 MPa; SMGT UTS: 514 MPa) increase in UTS with a significant reduction in ductility compared to the virgin sample. The mixed fracture modes (ductile fracture in CG/ML layer; brittle fracture in GS layers) observed in fractographic results suggest the stress-strain partitioning, gradual transition of mechanical properties throughout the gradient thickness, and complex interactions between different banded regions of the sample with heterogeneous microstructure. The strengthening and microstructural evolutions observed in this investigation are collectively attributed to SMGT-induced plastic deformation, compressive residual stress, grain refinement, increased fractions of LAGBs, and cDRX (continuous dynamic recrystallization) facilitated by localized heating (due to sliding friction of tool and specimen).