To Twin or Not to Twin: Micromechanical Response in Magnesium probed with High Energy X-Rays

Elke Arenholz, CHESS

"Reducing the weight of vehicles translates into energy conservation in transportation which is beneficial for economic and environmental reasons."

Evolution of diffracted intensity from basal lattice planes

Evolution of diffracted intensity from basal lattice planes during low-cycle fatigue of pure magnesium. An increase in intensity from lattice planes perpendicular to loading during compression is indicative of twinning, while a decrease is related to detwinning. Grain orientation maps were collected using electron back-scatter diffraction to help visualize twinning and detwinning during cyclic loading.

What did the Scientists Discover?
The structural evolution of extruded Mg was investigated using in-situ high energy X-ray diffraction (HEXD) at CHESS under fully-reversed low cycle fatigue conditions. At cyclic strains greater than 0.5%, twinning occurs during the compression portion of the cycle and, at early stages of fatigue, most twins are detwinned under reversed loading during the tensile portion of the cycle. As the number of fatigue cycles increases the twin volume fraction increases and the detwinning process is incomplete and a significant fraction of residual twins remains throughout an entire cycle.

Why is this important?
Reducing the weight of vehicles translates into energy conservation in transportation which is beneficial for economic and environmental reasons. Magnesium shows promise as lightweight but strong material to be used in vehicles since it has 2/3 the density of aluminum and an excellent strength-to-weight ratio. However, before it can be widely adopted, its performance during cyclic loading, i.e. fatigue, must be understood.

Schematic of the diffraction experiment detailing the sample geometry. An example of the continuous diffraction rings and the HEDM integration areas (Red boxes) are also shown.
Schematic of the diffraction experiment detailing the sample geometry. An example of the continuous diffraction rings and the HEDM integration areas (Red boxes) are also shown.

What are the broader impacts of this work?
Experiments like the one presented here advance our understanding of the fatigue characteristics of magnesium and will enable its use as strong, lightweight material in vehicles and related applications.

Why did this research need CHESS?
The ability to preform high energy X-ray diffraction (HEXD) experiments during in-situ cyclic mechanical loading at the F2 Station at CHESS were crucial for this research. The sample was illuminated by a 61.332 keV X-ray beam and the diffracted intensity was measured in transmission on an area detector. A sufficient number of grains were illuminated such that nearly complete Debye-Scherer powder rings were captured on the detector. The cyclic loading was performed in displacement control with displacement end points.

Collaborators:

  • Aeriel D. Murphy-Leonard, University of Michigan, Department of Materials Science and Engineering
  • Darren C. Pagan, Cornell High Energy Synchrotron Source (CHESS)
  • Armand Beaudoin, Cornell High Energy Synchrotron Source (CHESS)
  • Matthew P. Miller, Cornell University, Sibley School of Mechanical and Aerospace Engineering
  • John E. Allison, University of Michigan, Department of Materials Science and Engineering

CHESS was supported by NSF award DMR-1332208. A. D. Murphy-Leonard acknowledges the support of the National Science Foundation Fellowship. Part of this work is supported by DOE-BES, Division of Materials Science and Engineering under Award #DE-SC0008637.

Publication Citation:
Murphy-Leonard, A.D., Pagan, D. Beaudoin, A., Miller, M., Allison, J., “Quantification of cyclic twinning-detwinning behavior during low-cycle fatigue of pure magnesium using high energy X-ray diffraction.” International Journal of Fatigue, vol. 125, pp. 314-324, 2019. https://doi.org/10.1016/j.ijfatigue.2019.04.011