Twin and dislocation induced grain subdivision and strengthening in laser shock peened Ti

A significantly refined microstructure with high twin and dislocation densities is a common feature in the Ti surface layers subjected to laser shock peening (LSP). In this study, in order to develop a detailed understanding of the different twin variants and accurate estimation of twin and dislocation densities that result from LSP, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were performed at different depths below the peened surfaces of single and triple LSP impacted Ti. To overcome challenges associated with characterization of the nano-sized LSP-induced twins during the conventional EBSD analysis, a novel data mining strategy is developed to identify the characteristic straight twin boundary traces from the band contrast maps. Results show five types of twins, whose total density initially increases and then monotonically decreases with increasing distance from the LSP treated surface. The intersection of the primary and secondary type twins subdivides the grains and leads to grain refinement. In contrast, the dislocation density is highest at the LSP treated surface but decreases monotonically with the increasing depth. A similar trend in the variation of microhardness as a function of the depth indicates that LSP-induced strengthening is dominated by the dislocations and grain refinement. Further analysis reveals that the former's contribution is more pronounced than that of the latter. The understanding developed in this work can be useful for the design of the alloys and the LSP process for improved mechanical performance.