Twinning and the dynamic behavior of magnesium and its alloys
Embargo until
2020-05-01
Date
2018-12-17
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Johns Hopkins University
Abstract
With a density two-thirds that of aluminum, magnesium has great potential to become the most sought-after structural metal. Naturally, many applications for structural metals require them to remain resilient under extreme conditions of pressure, temperature and loading rates (some common scenarios being car crashes, high speed machining, ballistic impact and micrometeorite impact). Most materials behave very differently when subjected to these extreme conditions in comparison to conventional quasi-static isothermal loading. In the context of magnesium, its asymmetric hexagonal close packed crystal structure results in an anisotropy in plastic deformation which can be linked back to two major plastic deformation mechanisms at the crystal scale: dislocation slip and deformation twinning. In this thesis, we focus on a mechanism-based approach to understand plastic deformation in magnesium under high rates of loading. Special focus has been placed on understanding deformation twinning under these loading conditions.
We first investigate the macroscopic strength and ductility of a textured polycrystalline AZ31B magnesium alloy across 8 decades of strain rate (10^-4-10^4 /s) under uniaxial compression along different loading orientations relative to the material texture. The macroscopic flow stress and strain hardening are found to be a function of both strain rate and loading orientation. Post-mortem microscopy reveals both dislocation slip and twin-dominant deformation, depending on the loading orientation relative to the sample texture. We find that deformation twinning is more active at high strain rates than at quasi-static rates. This tends to affect both material strength and ductility.
The next part of this thesis examines deformation twinning in greater detail. Using high strain rate experiments combined with in-situ high speed microscopy, we capture the dynamic evolution of deformation twins in single crystal magnesium. The measurements reveal the competition between twin nucleation and growth and its relation to macroscopic material response. A theoretical framework to predict twin propagation speeds is developed with significant potential to explain twin-twin and dislocation-twin interactions. Finally, we end with a discussion of the crystallographic nature of twins nucleated under both quasi-static and dynamic loading. The interplay between the mechanics of twinning (i.e. twin nucleation and growth kinetics) and the crystallography offers unique insights and may help improve predictive capabilities for the dynamic behavior of hcp crystals.
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Keywords
magnesium, high strain rates, deformation twinning