Formation, Growth, and Interactions of Tension Twins in Magnesium
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Authors
Culbertson, Duke
Issue Date
2021
Type
Dissertation
Language
Keywords
electron backscatter diffraction , ex situ , in situ , magnesium , twinning
Alternative Title
Abstract
In the world of structural metals, magnesium and its alloys are much lighter than other traditionally used metals having a density of ~1/5 that of iron and ~2/3 that of aluminum �" two commonly used metals. Unlike these isotropic cubic metals, magnesium has a hexagonal closed-packed (hcp) crystal structure and is not commonly used. The hcp structure has less symmetry than the face centered and body centered cubic structures, resulting in an anisotropic mechanical response with fewer available slip systems. C-axis deformation is particularly difficult to achieve through dislocation slip at room temperature. To accommodate deformation, tension twinning is activated, which rotates the crystal by almost 90° to better allow the easily activated basal {0001} slip system. As the microstructure dynamically changes by twinning during plastic deformation, so too do the local deformation processes. The microstructural evolution directly affects the mechanical behavior of magnesium under both monotonic and cyclic loading. To better design, process, and utilize the magnesium alloys in engineering applications, a fundamental understanding of the process of twinning and twin-twin interactions is required. The current research aims to capture and explain the microstructural evolution due to twinning during tension, compression, torsion, and uniaxial strain path changes.The nucleation, growth, and interaction of tension twins were directly observed utilizing a hybrid in situ optical microscopy, ex situ electron backscatter diffraction procedure. Cross-grain twin pair formation was captured in extruded pure polycrystalline magnesium where the twin in one grain assists in the nucleation of a similar twin in the adjacent grain. Two assisted formation processes were observed: the commonly observed twin propagation-assisted and the newly recorded twin thickening-assisted mechanisms. A twin chain spanning seven grains was rapidly developed through the twin propagation-assisted mechanism of two smaller chains and eventual conjoining within a middle grain. The two smaller chains form across grains with small misorientations from grain to grain, allowing for easy-cross grain twin formation by a twin-propagation assisted process. In the middle grain where the chains connect, the same twin variant is formed from both chains on either side of the grain, where they both grow and coalesce forming the larger twin chain. The new twin thickening-assisted formation is observed for the first time where the paired twin is formed across a grain boundary by the other twin thickening. Applying the hybrid testing procedure to nearly c-axis tension of single-crystalline pure magnesium reveals that basal slip is activated prior to the nucleation of tension twins. As twinning increases with increasing strain, the initial basal slip bands are deflected within the twinned region relative to the activated variant and the twinning shear. By the final plastic strain of 3.83%, all six tension twin variants are identified within the observed area along with all three types of twin-twin interactions: Type I co-zone, Type II(a) non-co-zone, and Type II(b) non-co-zone. A needle-like Type I interaction is captured by in situ observation for the first time. The initial interaction results in the normally occurring impinging and acute angle twin-twin boundary. On the obtuse angle side, twinning dislocations are deposited near the impinging twin-twin boundary, leading to incoherent curving twin boundaries. The combination of the acute angle twin-twin boundary and incoherent boundaries on the obtuse angle side result in the penetrating structure. Partial penetrating structures are observed in the Type II interactions, along with secondary twinning. Detwinning is quantitatively measured during unloading. Twinning behavior during strain path change and torsional loading was examined using extruded pure polycrystalline magnesium. Compression parallel to the extrusion direction of the bar results in severe twinning up to 95% of its volume by exhaustion. Using companion specimens, pre-compression was applied to -7.3% and -12.8% strains to induce varying twin severity prior to re-loading in tension. The subsequent tension reveals a combination of detwinning and secondary tension twinning of the initial twins formed during compression. Detwinning is more significant than secondary twinning in the -7.3% pre-strain case while the opposite is observed in the -12.8% pre-strain case. After detwinning is exhausted, non-Schmid factor twins are observed along with the retained secondary twins. Twinning during free-end torsion about the extrusion direction cannot reach the same global twin severity as compression as the texture is not entirely favorable for twinning. Grains favorably oriented for tension twinning are severely twinned. Less favorably oriented grains show high variability in terms of twin severity, while the unfavorably oriented grains show very little to some twinning. All six variants are observed in multiple grains, and more significantly, some twinned regions are highly favorable for further twinning, so secondary twinning is very common. By torsional failure, favorable secondary twins can be found fully encompassing their primary variant.