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In this study, the role of twin-twin interactions on the distributions of local defects (e.g., dislocations) and stress fields in a magnesium alloy is investigated. A co-zone (101¯2)-(1¯012) tensile twin junction in a deformed Mg-3wt.%Y alloy is analyzed using transmission electron microscopy (TEM). The results show that the morphology of the impinging (1¯012) twin is asymmetric, and the non-interacting boundary of the recipient (101¯2) twin is irregular. Detailed analysis of TEM images reveals that type-II pyramidal [1¯21¯3](12¯12) dislocations concentrate in the vicinity of the twin-twin junction site. The same 〈c + a〉 dislocations are also observed inside the interacting twin domains along with a few 〈a〉 dislocations. The 〈c + a〉 dislocations emanating from the impinging (1¯012) twin boundary have edge character and are extended with faults parallel to the basal plane. In contrast, the 〈c + a〉 dislocations connected to the recipient (101¯2) twin are predominantly screw orientation and compact. Elasto-viscoplastic fast Fourier transform based crystal plasticity calculations are performed to rationalize the observed twin morphology and local dislocation distribution. The model calculations suggest that the local stress fields generated at the junction site where the two twins meet are responsible for the experimentally observed concentration of 〈c + a〉 dislocations. The calculated stress fields are asymmetric with respect to the junction site, explaining the observed asymmetric morphology of the impinging twin. Overall, these findings show strong effects of twin-twin interactions on the distribution of dislocations as well as the evolution of the twinned microstructure and as such, can help advance understanding of twinning in Mg alloys and their effect on mechanical behavior.

In this study, the role of twin-twin interactions on the distributions of local defects (e.g., dislocations) and stress fields in a magnesium alloy is investigated. A co-zone (10 1 ¯ 2) - (1 ¯ 012) tensile twin junction in a deformed Mg-3wt.%Y alloy is analyzed using transmission electron microscopy (TEM). The results show that the morphology of the impinging (1 ¯ 012) twin is asymmetric, and the non-interacting boundary of the recipient (10 1 ¯ 2) twin is irregular. Detailed analysis of TEM images reveals that type-II pyramidal [ 1 ¯ 2 1 ¯ 3 ] (1 2 ¯ 12) dislocations concentrate in the vicinity of the twin-twin junction site. The same 〈c + a〉 dislocations are also observed inside the interacting twin domains along with a few 〈a〉 dislocations. The 〈c + a〉 dislocations emanating from the impinging (1 ¯ 012) twin boundary have edge character and are extended with faults parallel to the basal plane. In contrast, the 〈c + a〉 dislocations connected to the recipient (10 1 ¯ 2) twin are predominantly screw orientation and compact. Elasto-viscoplastic fast Fourier transform based crystal plasticity calculations are performed to rationalize the observed twin morphology and local dislocation distribution. The model calculations suggest that the local stress fields generated at the junction site where the two twins meet are responsible for the experimentally observed concentration of 〈c + a〉 dislocations. The calculated stress fields are asymmetric with respect to the junction site, explaining the observed asymmetric morphology of the impinging twin. Overall, these findings show strong effects of twin-twin interactions on the distribution of dislocations as well as the evolution of the twinned microstructure and as such, can help advance understanding of twinning in Mg alloys and their effect on mechanical behavior. [ABSTRACT FROM AUTHOR]

• Depth-varied twin/dislocation distribution investigated in laser shock peened Ti. • Twin variants and densities characterized using a novel EBSD data mining approach. • Grain subdivided by twin formations and twin-twin intersections. • Decreased dislocation density along the distance from the shock-penned surface. • Depth-dependent strengthening effects quantified and their mechanisms discussed. 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. [ABSTRACT FROM AUTHOR]