Abstract
The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation. Here, we describe an in-situ transmission electron microscopy experiment on Mg crystals with strategically designed geometries for visualization of a long-proposed but unverified twinning mechanism. Combining with atomistic simulations and topological analysis, we conclude that twin nucleation occurs through a pure-shuffle mechanism that requires prismatic-basal transformations. Also, we verified a crystal geometry dependent twin growth mechanism, that is the early-stage growth associated with instability of plasticity flow, which can be dominated either by slower movement of prismatic-basal boundary steps, or by faster glide-shuffle along the twinning plane. The fundamental understanding of twinning provides a pathway to understand deformation from a scientific standpoint and the microstructure design principles to engineer metals with enhanced behavior from a technological standpoint.
Highlights
The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation
We use a combination of nanomechanical deformation, in-situ transmission electron microscopy, and aeatorlmy-isct-asgcealgersoiwmtuhlaptriooncetsoseissoolfatÈe1a0n1d2Éiddeenftoirfymtahteionnutcwleiantsioinn and Mg
These observations are in agreement with recent in-situ TEM studies on deformation twins, using conventional pillars, in which the twin propagates extremely fast, so fast that the twinning process is completed within 0.04 s, and under these conditions only adult twin with a size over 100 nm can be observed[28]
Summary
The abrupt occurrence of twinning when Mg is deformed leads to a highly anisotropic response, making it too unreliable for structural use and too unpredictable for observation. We verified a crystal geometry dependent twin growth mechanism, that is the early-stage growth associated with instability of plasticity flow, which can be dominated either by slower movement of prismatic-basal boundary steps, or by faster glide-shuffle along the twinning plane. The mechanisms responsible for the nucleation and early-stage growth of deformation twins remain to be clarified. We use a combination of nanomechanical deformation, in-situ transmission electron microscopy, and aeatorlmy-isct-asgcealgersoiwmtuhlaptriooncetsoseissoolfatÈe1a0n1d2Éiddeenftoirfymtahteionnutcwleiantsioinn and Mg. We strategically designed truncated wedge-shaped pillars (TWPs) from single-crystal Mg to generate a steep stress field in the crystal under compression that supports twin nucleation but not rapid pillar-wide twin propagation and growth. For the crystals with a regular rectangular geometry, only an “adult” twin was observed, and its early-stage growth likely occurred via fast shear-shuffling along the twinning plane. In remarkable contradiction to involve creation ocfotnhveenÈt1io0n12aÉl belief, these early stages coherent twin boundary, do not a twinning dislocation, or even basic shear displacements along the conventional twinning shear direction
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