Effects of complex strain conditions on macroscopic deformation behavior in basal plane of Mg alloy

초록

Severe plastic deformation increases the defect density and forms an ultrafine grain structure by applying a complex stress state that includes plane and shear deformations to a high strain. However, it is not yet clear how the complex stress state with the added shear strain changes the deformation mechanism. To understand the effect of macroscopically applied shear strain on simple plane strain, tensile test specimens with a special notch (complex stress state) were compared to a general tensile test specimen (simple plane strain) using a Mg alloy. To unify the slip system activation, the normal directions of the tensile test and the notch tensile test were set parallel to the normal direction of the {0001} planes of the strongly textured Mg alloy. The addition of shear strain unexpectedly weakened the average dislocation density and the screening behavior of dislocations, while strain accommodation by deformation twinning accelerated. Additionally, as the equivalent strain increased to 0.2, the general tensile test exhibited de-twinning behavior, whereas the twin fraction continuously increased when a complex strain was applied. The reason for the different deformation mechanisms is considered to be the strong grain rotation behavior under complex strain conditions, which fortifies the stress concentration at grain boundaries. This implies that complex strain with shear strain can accelerate deformation twinning while suppressing dislocation slip when basal slip is dominant.

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