In Situ Characterization of the Effect of Twin-Microstructure Interactions on {1 0 1 2} Tension and {1 0 1 1} Contraction Twin Nucleation, Growth and Damage in Magnesium

Abstract

Through in situ electron backscatter diffraction (EBSD) experiments, this paper uncovers dominant damage mechanisms in traditional magnesium alloys exhibiting deformation twinning. The findings emphasize the level of deleterious strain incompatibility induced by twin interaction with other deformation modes and microstructural defects. A double fiber obtained by plane-strain extrusion as a starting texture of AM30 magnesium alloy offered the opportunity to track deformation by EBSD in neighboring grains where some undergo profuse 10-12 twinning and others do not. For a tensile loading applied along extrusion transverse (ET) direction, those experiencing profuse twinning reveal a major effect of grain boundaries on non-Schmid behavior affecting twin variant selection and growth. Similarly, a neighboring grain, with its <c>-axis oriented nearly perpendicular to tensile loading, showed an abnormally early nucleation of 10-11 contraction twins (2% strain) while the same 10-11 twin mode triggering under <c>-axis uniaxial compression have higher value of critical resolved shear stress exceeding the values for pyramidal <c+a> dislocations. The difference in nucleation behavior of contraction vs. compression 10-11 twins is attributed to the hydrostatic stresses that promote the required atomic shuffles at the core of twinning disconnections.

A Review on Capturing Twin Nucleation in Crystal Plasticity for Hexagonal Metals

“…Figure 15 shows that a tilt grain boundaries demonstrate a strong correlation of grain boundary energy with the stress required for exothermic twin nucleation [216]. This is consistent with the experimental observation that shows the correlation of the non-Schmid twin nucleation behavior with the neighboring grains and the grain boundary effects [214,217,218]. Basu et al [218] showed that the twin nucleation was favored for the twin variant that has a higher local Schmid factor.…”

“…Corroborating evidence for the need to including hydrostatic pressure in the flow rule is supported by the results recently reported by Russell et al [214] on in situ EBSD characterization of tension and compression of an AM30 extruded sheet showing a double in-plane basal fiber. It was found that {1 0 1 1} twinning occurs easier under contraction than the compression of the c -axis with nearly an order of magnitude difference in the CRSS levels.…”

“…The thickening of a given twin in a crystal plasticity model, though can be handled with PTR, shows discrepancy if the twin has an excessive source of twinning disconnections compared to other engulfed variant as observed in the experiment [214]. This is the case of an early twin nucleating at a grain boundary segment.…”

“…It was found that {1 0 1 1} twinning occurs easier under contraction than the compression of the c -axis with nearly an order of magnitude difference in the CRSS levels. In a contraction {1 0 1 1} twin, <c> axis compression happens as a result of {1 0 1 0} tension, thus resulting in different stress states than that of actual <c> axis compression [214]. The difference of CRSS in compression and contraction {1 0 1 1} twins was explained by the higher triaxiality naturally induced along the c -axis under contraction than compression.…”

“…This is the case of an early twin nucleating at a grain boundary segment. Twin variants that can thicken along the grain boundary segments showed growth within a grain compared to other variants that are more likely to get engulfed away through twin-twin interactions [214]. A way for crystal plasticity to capture such an effect is to consider slightly higher CRSS for twin nucleation other than grain boundaries.…”