3D microscopic analysis of plasticity in Mg alloys

Due to the global tendency to reduce the fuel consumption and CO2 emissions, Magnesium alloys, as very light material, are being very attractive for the transportation industry. Researches of IMDEA Materials Institute in collaboration with prestigious European research centers, have proposed a methodology through experiments and simulation to analyze the plasticity in three dimensions at microscopic level in this kind of materials.

Reducing the weight of transportation vehicles is a world trend aiming to diminish the emissions of greenhouse gases and the amount of required fuel. Magnesium (Mg) alloys are key light-weighting materials, as their specific strength is one of the highest among all structural metals. They are, therefore, currently widely investigated with the aim of increasing their potential for the fabrication of components for the light transport industry. Twinning is a very important deformation mechanism in Mg alloys. The interaction between twins and grain boundaries in these materials still remains unknown.

Ana Fernández, a PhD researcher belonging to the IMDEA Materials Physical Metallurgy group led by Dr. Teresa Pérez-Prado, has investigated the influence of grain boundary misorientation on twin propagation in the Mg alloy AZ31 (Mg-3%Al-1%Zn). The study has been carried out in collaboration with Prof. A. Jérusalem (University of Oxford) and Dr. I. Gutiérrez-Urrutia (Max-Planck Institute for Iron Research, Düsseldorf).

A combined methodology, based on three dimensional electron backscatter diffraction (EBSD) and continuum modeling, has been utilized to characterize how a very favorably oriented tensile twin variant transfers across boundaries with misorientation angles ranging from 15 to 65 degrees. Both experiments and simulations confirm that twin transfer is facilitated by low misorientation angles. When the misorientation increases, high local stresses develop at the grain boundaries with the consequence that the transferred twins are variants which do not have the highest Schmid factor (SF) with respect to the externally applied stress. For angles higher than about 50 degrees, twin transfer does not take place and the large stresses present at the corresponding boundary give rise to non-Schmid plasticity in the original grain. This research also proves that twin morphology is highly influenced by the Schmid factor. Favorably oriented variants have a plate-like shape, where as a decrease in SF leads to very irregular geometries. Models aiming to fully describe the plasticity of light Mg alloys must incorporate these observations.

The study has been accepted for publication in the prestigious scientific journal Acta Materialia. (A. Fernández, A. Jérusalem, I. Gutiérrez-Urrutia, M.T. Pérez-Prado. Three-dimensional investigation of the grain boundary-twin interactions in a Mg AZ31 alloy by electron backscatter diffraction and continuum modeling. Acta Materialia, 61:7679-7692, 2013. DOI:10.1016/j.actamat.2013.09.005)

Figure. 3D-Electron backscatter diffraction (EBSD) map of the investigated volume. The orientation color-coding is included as an inset.