CAREER: Dislocation-level Understanding of Shear Banding in Magnesium and Magnesium Alloys

PART 1: NON-TECHNICAL SUMMARY

Magnesium (Mg) is a light-weight metal that is nearly four times lighter than steel, making it very attractive for automobile and aerospace applications. However, pure Mg and Mg alloys tend to crack fairly easily at room temperature. When Mg is stretched, it does not evenly distribute the strains it experiences.

Instead, Mg tends to concentrate strains in localized zones called "shear bands". These shear bands become locations where Mg most often breaks and are responsible for why Mg cracks more easily than other metals. This project relates shear banding to the atomic level defects that occur in metals when they are stretched or deformed.

These atomic level defects are called "dislocations". A greater number of dislocations that are more spread out are expected to promote more uniform deformation, or stretching of a material, and thereby delay shear banding. The insights gained from this work will help identify new Mg alloys that exhibit delayed shear banding, increased crack resistance and a greater ability for Magnesium metal to be formed into different shapes.

The findings from this research will also help promote increased use of Mg across a wide variety of automotive and aerospace applications. Moreover, this project also produces interdisciplinary educational opportunities to train students in metallurgy, electron microscopy, and mechanics of solids. This project also provides a platform for professors and graduate students to work closely with underrepresented students through the Louis Stokes Alliances for Minority Participation program to expose them to the topics of this research project.

PART 2: TECHNICAL SUMMARY

Magnesium (Mg) alloys hold great potential for use as lightweight energy-saving materials, but the structural applications of Mg have been hindered by its low strain-to-failure properties at room temperature. The comparatively lower strain-to-failure exhibited by Mg as compared to other metals is chiefly related to the formation of localized shear bands.

The goal of this work is to advance the understanding of deformation and failure mechanisms in Mg and its alloys and, in particular, discover pathways that trigger or delay plastic instabilities in them. The hypothesis examined here is that increased non-basal, <c+a> dislocation activities lead to more uniform deformation and provide more homogeneous strain hardening throughout the microstructure, hence delaying plastic instabilities by delocalizing shear bands in Mg and its alloys.

The objectives of this work are to 1) identify the role of <c+a> dislocations on the formation of shear bands; and 2) identify the effect of temperature (up to 75 degrees Celsius) as well as alloying on <c+a> dislocation activities and shear band characteristics (e.g. size, number density, and stored plastic strain) in quasi-statically deformed Mg and Mg alloys. These objectives will be accomplished by experimentally measuring i) the stresses needed to operate dislocation sources, ii) the dislocation glide distances produced under given stresses, and iii) the related cross-slip frequency.

These latter measures will then be directly correlated to the shear band characteristics mentioned in the second objective. The broader impacts of this project are two-fold. First, identifying pathways to more ductile and malleable Mg and Mg alloys will significantly enhance the opportunity and likelihood for Mg alloy usage across a variety of applications resulting in significant weight-savings and increases in energy efficiencies.

Secondly, this project will facilitate increased educational engagement and outreach to students and persons within the collegiate environment and beyond through i) hands-on experiences for undergraduate students, ii) a digital platform to further undergraduate interest in materials science and iii) expansion of an existing YouTube channel engaging the general public on matters and phenomena related to materials science.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.