Materials under extreme loading conditions

The strength of materials at high rates of loading (timescales less than 100 microseconds) are very different from those at low "quasi-static" loading rates. In metals with cubic crystal symmetries, while the elastic modulus may not have significant strain-rate sensitivity, stresses during plastic deformation (named "flow stresses") are rate-dependent. These have classically been attributed to line defects within crystallites at the nanoscale called "dislocations". The kinetics of these dislocations manifest as higher flow stress at macroscopic length scales.
Much of my work focuses on low symmetry materials, specifically magnesium. With a density that is two-thirds that of aluminium, magnesium has been sought after as the next generation structural metal for engineering applications. However, the low symmetry crystal results in significant asymmetry in deformation mechanisms and hence macroscopic plastic flow, severely limiting its applications in industry. Fundamentally, this poses very interesting research questions more so at extreme loading conditions.

The effect of strain-rate on plastic flow of magnesium alloys

Due to lower symmetry of the hexagonal-close-packed crystal than cubic crystals, magnesium exhibits a much more complex range of deformation mechanisms. Using high speed impact experiments, we measured the macroscopic plastic flow behavior of magnesium alloys with temporal resolutions ranging from minutes down to nanoseconds.
Measurements on a polycrysalline magnesium alloy (AZ31B) across eight decades of strain-rate have revealed that the flow stress as well as the work-hardening rate are functions of both loading orientation and strain-rate. The mechanistic origins of this rate-sensitivity are probed using quantitative post-mortem microscopy.

Mechanistic origins of rate-dependent plastic flow in magnesium alloys

The low symmetry of the hexagonal crystal structure of magnesium results in an additional plastic deformation mechanism at the crystal length-scale called "deformation twins". Quantitative microscopy techniques offer great insight into the activity of these deformation mechanisms, but are severely limited in time resolution. However, using careful impact-recovery experiments, it is still possible to extract important information about active deformation mechanisms at faster rates of loading.
To probe the mechanistic origins of rate-dependent plasticity, we perform impact-recovery experiments and prepare these samples for post-mortem microscopy. An electron microscopy technique called Electron Backscatter Diffraction Microscopy (EBSD) offers quantitative information about spatial distribution of crystal orientations in samples. Our measurements point to a transition from grain growth at quasi-static strain rates to stress-driven deformation twinning at high strain rates. These observations have significant potential in building predictive models for the strain rate-dependent strength of magnesium alloys.

Ductile failure in magnesium alloys at high strain rates

Ductile failure at dynamic loading rates is treated as the unstable growth of a defect following spatially-localized plastic deformation resulting in catastrophic failure (or loss of load-bearing capacity of the material). Two most commonly studied mechanisms of ductile failure are "nearly" adiabatic shear localization and spall failure. While the former is due to a localization of shear strains, the latter is classically considered a volumetric effect. However, the asymmetric plastic response of magnesium adds significant complexity to understanding these phenomena.
Our interest is in the former problem, namely "nearly" adiabatic shear localization. In a physical sense, shear bands are a result of local softening of the material that tends to grow in an unstable manner. At dynamic loading rates, this softening is typically due high local temperatures that do not have time to dissipate i.e. thermal softening. However, the presence of deformation twinning may indicate an additional form of softening due to reorientation of the crystal lattice i.e. geometric softening. In-situ high speed imaging reveals significant differences in the nature of localized deformation along two different loading orientations. A thorough understanding of the competition between thermal and geometric softening and its effect on the kinetics of localized failure in low symmetry metals remains to be developed.

Localized deformation in an AZ31B magnesium alloy during dynamic loading (Duration: 36 microseconds; FOV: 3mm x 3.7mm)
In-situ high speed microscopy (Time resolution: 200 ns; Spatial resolution: 4 micrometers/pixel)

Collaborators

Prof. K. T. Ramesh , Department of Mechanical Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University;
Dr. N. M. Krywopusk , Prof. T. P. Weihs , Department of Materials Science and Engineering and Hopkins Extreme Materials Institute, Johns Hopkins University;
Dr. X. M. Ma , Pacific Northwestern Laboratory;
Dr. L. J. Kecskes , Hopkins Extreme Materials Institute, Johns Hopkins University.

Relevant Publications

This research was funded by the US Army Research Laboratory under the Center for Materials in Extreme Dynamic Environments (CMEDE) within the Hopkins Extreme Materials Institute at Johns Hopkins University.