Chapter 35 Texture Evolution of a Fine-Grained Mg Alloy at Dynamic Strain Rates Christopher S. Meredith and Jeffrey T. Lloyd Abstract AMX602 (Mg-6%Al-0.5%Mn-2%Ca) is a high strength Mg alloy that was manufactured by the spinning water atomization process (SWAP) and extruded into a plate geometry. The processing produces an alloy with a weak rolling texture (for Mg) and grains between 0.5 and 5 μm. Quasi-static and dynamic compression experiments were carried out to probe the material’s mechanical behavior in the three processing directions. The tested plate showed deformation mechanism induced anisotropy consistent with what has been observed for other Mg alloys. The texture evolution was measured after loading in the three directions using X-ray diffraction at the Cornell High Energy Synchrotron Source (CHESS). A computationally efficient crystal plasticity model that demarcates twinning, basal slip, and non-basal slip mechanisms was utilized to predict the texture evolution and compared to the experimental texture. The model was able to predict the reorientation of grains associated with twin dominated yielding in the extrusion and transverse directions, and strengthening of the texture associated with slip dominated deformation in the normal direction. Keywords AMX602 • Dynamic strain rates • Texture evolution • Twinning • Crystal plasticity 35.1 Introduction Magnesium is the lightest metal for structural applications with a specific strength similar to or exceeding aluminum and titanium [1]. However, commercial magnesium alloys have relatively low strength, limited ductility, anisotropic mechanical behavior, and relatively poor corrosion resistance. The strength and ductility can be optimized using alloy development, texture modification and/or grain refinement [3, 4]. For example, microalloying by adding trace amounts of certain elements which creates finely dispersed precipitates recently has been utilized to enhance precipitate hardening [2]. Also, a more random texture increases ductility versus typical wrought Mg alloys because stable plastic deformation is promoted by an increased strain hardening rate [5–7]. Finally, reducing the grain size is an effective way to increase the strength of a material through the well-known Hall–Petch relationship. When twinning dominates yielding the Hall–Petch has the same empirical form as for slip [8], thus twinning is suppressed as the grain size is reduced and is manifested as a higher Hall–Petch constant [8, 9]. Typical processing methods–like rolling and extrusion–limit the ability to reduce the grain size due to Mg’s low ductility, and these methods produce sharp textures associated with increasing mechanical anisotropy. Severe plastic deformation processing methods are effective at refining grain size, but they also produce sharp textures [10–13]. The sharp textures generally result in highly anisotropic mechanical behavior, tension/compression asymmetry and low ductility. Recent work has shown that equal channel angular pressing (ECAP) is able to reduce the grain size and tailor the texture for property improvement. Several authors [14–16] have performed ECAP on AZ31B and showed increased strength and reduced anisotropy while preserving ductility using specific processing routes and temperatures. The resulting basal texture is generally stronger than before, and is generally rotated with respect to a principal processing direction. Therefore, when determining the subsequent strength and ductility, the loading directions were at an angle with respect to the basal texture. When conventional rolled AZ31B is loaded along the processing directions, the loading direction coincides with the principal texture components, which maximizes the measured the anisotropic response. The reduced anisotropic behavior and negligible change in ductility of ECAPed AZ31B is partially a consequence of the chosen loading directions, and not an intrinsic material property. When samples are cut along principle material directions, the measured yield strength differences in different directions can still vary by a factor of 2 or more, and limited ductility is observed for particular loading conditions [14, 15]. In applications with complex loading conditions, where the full anisotropic material response is C.S. Meredith (*) • J.T. Lloyd Weapons and Materials Research Directorate, Army Research Lab, Aberdeen Proving Ground, Aberdeen, MD 21005, USA e-mail: christopher.s.meredith3.civ@mail.mil #The Society for Experimental Mechanics, Inc. 2017 D. Casem et al. (eds.), Dynamic Behavior of Materials, Volume 1, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-41132-3_35 263
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