The formability of Magnesium and Magnesium-Rare Earth alloys under the strain path of cold rolling

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Abstract

Magnesium sheet components hold great potential to reduce the environmental footprint of road transport. However, the industrial introduction of magnesium sheet is currently limited by its low formability under the strain path of cold rolling. In this view, the remarkable formability increases imparted by rare-earth (RE) additions to magnesium have attracted increasing interest over the last few years. Three changes induced by RE additions have been put forth to explain the improvement: (i) weaker texture, (ii) enhanced contraction twinning, and (iii) enhanced non-basal slip. Within this context, this project aims to explore the effect of material preparation on the formability of conventional and Mg-RE alloys in the strain path of cold rolling. Attention is paid to texture and grain size, the main factors affecting magnesium formability according to former research. For this aim, a set of annealing conditions are prepared for two alloys accounting for conventional and RE-modified behaviour, respectively: Mg-0.03Y and Mg-0.6Y. Samples are characterized and subjected to plane-strain compression (PSC) tests reproducing the strain path of cold rolling. The hypothesis that the action of solute drag is related to the RE texture weakening, proposed in recent literature, is checked in parallel using activation energies and in the light of Lücke-Detert’s theory. PSC results show that, whereas the strains-to-failure reached by Mg-0.03Y specimens correlate with greater basal slip and tension twinning enabled by weaker texture, those of Mg-0.6Y are remarkably higher for conditions developing stress saturation stages at peak stress. Absent for Mg-0.03Y, such stages have been associated to the RE promotion of contraction twinning, and found to occur for a minimum initial grain size only. Therefore, a substantially different approach should be employed to optimize the formability of conventional and Mg-RE alloys. Moreover, strain-to-failure has been significantly higher for Mg-0.6Y only in conditions with stress saturation, implying that, among all three mechanisms proposed, it is contraction twinning that essentially explains the formability of Mg-RE alloys. Hence, these results outline the importance of enhancing contraction twinning for magnesium alloy developments. Further, this could also apply to biaxial tension, the other relevant strain path in practice, due to the analogous role therein expected for contraction twinning. In addition, considerably higher activation energy for grain growth is measured for Mg-0.6Y than for Mg-0.03Y. The activation energy of Mg-0.03Y is in line with Lücke-Detert’s breakaway regime, and that of Mg-0.03Y with the drag regime. This confirms that a shift in the boundary migration regime is effectively associated to the RE texture weakening. Further, notice has been taken of the unusual development of a TD-tilted fibre by Mg-0.6Y, mainly observed in ternary Mg-Zn-RE alloys only so far. This finding has been rationalized through a theory unifying texture observations in both alloying systems. Future work aimed at contrasting this theory is encouraged.

Keyword(s)

Cold Processing; Deformation twinning; Formability; Magnesium; Rare earths; Texture

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