Shock-induced large-depth gradient microstructure in commercial pure titanium subjected to explosive hardening

Explosive hardening of commercial pure titanium plate was conducted by a thin-layer high-detonationvelocity explosive. The nanoindentation and Vickers hardness tests of the treated titanium plates along the depth of shock loading were performed. The microstructures along the depth were characterized by using different methods, including X-Ray Diffraction, Optical Microscopy, Electron Back-Scattered Diffraction and Transmission Electron Microscopy. Experimental results showed that large-depth gradient hardness distribution with depth of 2.5 mm was produced in CP titanium by explosive hardening. There are two gradient hardened layers, ultra-hardened layer with a depth of 0-40 lm and a hardness increase of -400%, sub-hardened layer with a depth of 40 lm-2500 lm and a hardness increase of 54.3%. Gradient microstructures with different characteristics were observed, including gradient a-x phase transition, gradient grain size, gradient twining and gradient dislocation. The area of gradient microstructure is consistence with that of gradient hardness distribution. Grain refinement/a-x phase transition mainly contributed to hardness sharply increasing in ultra-hardened layer. High-density dislocation/twining mainly contributed to hardness increasing in the sub-hardened layer. Finally, the formation mechanism of gradient microstructures and grain refinement mechanism was detailly discussed. (c) 2021 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Explosive hardening of commercial pure titanium plate resulted in a large-depth gradient hardness distribution, with an ultra-hardened layer and a sub-hardened layer exhibiting different gradient microstructures. The hardness increase in the ultra-hardened layer was mainly attributed to grain refinement/a-x phase transition, while the hardness increase in the sub-hardened layer was mainly caused by high-density dislocation/twining. The formation mechanism of gradient microstructures and grain refinement mechanism were discussed in detail.