Enhancement of mechanical properties of commercially pure titanium by shock induced gradient microstructure with martensitic transformation

In the present work, explosion hardening (EH) technique was used to produce gradient microstructure with martensitic transformation in commercially pure titanium (CP Ti) to improve its mechanical properties. After EH treatment, phase structure, grain size distribution, dislocation structure and twin structure in CP Ti were characterized. Subsequently, microhardness tests, tensile/compressive tests and three-point bending tests were conducted to investigate mechanical properties of EH treated CP Ti along different depth positions from EH treated surface. Experimental results show that gradient microstructure in terms of grain size, alpha-omega martensitic transformation, texture intensity, twin density, dislocation density and hierarchical nanotwins, are observed in EH treated CP Ti. According to microstructure characteristics, four layers including nanostructure surface layer, refined structure layer, deformed coarse grained layer and coarse grained matrix are divided from EH treated surface to matrix. Microhardness tests show a gradient hardness layer with a maximum hardness increase of 83% within 3.5 mm from EH treated surface. Tensile and compressive test results show that the most significant strengthening exists within 2 mm depth from EH treated surface with a maximum tensile yield stress increase of 71% and the total EH affected depth is more than 25 mm. In addition, the gradient microstructure is affected by the number of EH treatments. For example, bending yield stress increases by 41.3% in 1-EH treated specimens and 64.8% in 2-EH treated specimens. Combining microstructure characterization and mechanical properties testing, shock-induced strengthening mechanism of CP Ti is analyzed: for positions close to EH treated surface, formation of alpha-omega martensitic transformation and grain refinement is responsible for the significant strengthening phenomenon; whereas, for positions far away from EH treated surface, the moderate strengthening phenomenon is due to the generation of high-density dislocations/twins.