GOALI: Atomic Scale Modeling and Experimental Characterization of Non-Basal Deformation Modes in Mg Alloys

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Technical Abstract: This project will provide a scientific basis for a quantitative and systematic approach to design Magnesium alloys with favorable combinations of strength and ductility through selective alloying. The approach is based on electronic-structure calculations of non-basal deformation modes including c+a slip and twinning including accurately accounting for chemistry effects due to alloying coupled with experimental observation and verification of computational predictions. Ab initio and atomistic simulations will be done to investigate (i) Mechanisms for tension twin nucleation and alloys that promote nucleation and growth of twins (ii) the core structures and mobility of c+a dislocations in Mg (iii) effects of alloying on the structure and collective behavior of c+a dislocations. Conventional transmission electron microscopy including bright field, and weak beam techniques will be conducted on deformed single crystal of Mg and binary Mg alloys to understand the effect of chemistry on c+a dislocations -based deformation response for specific forms of loading. This will provide connections to computations. Furthermore, aberration-corrected high resolution electron microscopy of dislocation core structures will enable a more direct connection with results from computations. The experiments and computations together will provide a comprehensive understanding of non-basal deformation in Mg alloys, the effect of chemistry on its relative ease, and a pathway to microstructurally-guided and scientifically-informed alloy design.Non-technical Abstract: The high strength-to-weight ratio of magnesium and magnesium-based alloys makes them excellent candidates for the transportation sector and in particular, the automotive industry that is focused on producing lighter-weight, more fuel-efficient vehicles. However, limited room temperature formability (a feature required to produce useful shapes by forming in a die for example) has prohibited the widespread use of Mg alloys. Forming at elevated temperatures adds cost and makes the material less competitive. Formability is related to the ease of plastic deformation, a phenomenon that is facilitated by atomic level processes called slip and/or twinning. In essence, these atomic level mechanisms are the material's response to application of external forces and these mechanisms enable permanent macroscopic shape change in a material, referred to as plastic deformation. Difficulty in plastic deformation encourages an alternate undesirable response which is premature failure/fracture. In the case of magnesium alloys, the limited formability is related to anisotropic plastic deformation. This means plastic deformation is easy in some directions of the sheet that is being formed but not in others. The cause of this anisotropy lies in the strong differentials in stress (or force) needed to trigger some forms of plastic deformation as opposed to others, a characteristic of this alloy system. This project is focused on identifying alloying elements using a combination of computations and experiments that can reduce the critical stress differential between these deformation modes to facilitate isotropic plastic deformation and thereby improve room temperature formability.