Twin-twin geometric structure effect on the twinning behavior of an Mg-4Y-3Nd-2Sm-0.5Zr alloy traced by quasi-in-situ EBSD

We studied the microstructure evolution of Mg-4Y-3Nd-2Sm-0.5Zr alloy by quasi-in-situ electron backscatter diffraction (EBSD) along with several strains under compression tests, which provided direct evidence for the influence of different twin-twin geometric structure on the twinning behavior. The results showed that the mechanical properties of the alloy were higher than traditional magnesium alloys (the maximum compressive strength reaches 402.5 MPa) due to the strengthening effect of Sm and Nd elements addition on solution strengthening, precipitation strengthening, and grain refinement. Combined with the quasi-in-situ EBSD technique, two different twin-twin geometric structures, ‘parallel structure’ and ‘cross structure’, were observed directly in the alloy. In the later stage of deformation, for ‘parallel structure’, residual stress and a large number of dislocations mainly existed in the twin boundary and tip position. For the ‘cross structure’, there was a lot of dislocation density in the interior of twins after fusion. The twin growth rate of ‘parallel structure’ was much faster than that of ‘cross structure’ because the stress of twins was mainly concentrated on the tip of twin. When the movement for the tip of twin was blocked, the growth rate of twin would be obviously decreased. Moreover, the ‘cross structure’ was easy to produce closed space. Due to the constraints of surrounding twins, the confined space was easy to stress concentration, thus inhibiting the growth of twins. At the same time, the ‘cross structure’ of twins needed a more external force to continue to deform, which also served as a strengthening structure.