Optimising High Strength Magnesium Alloys for Wrought Production

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Abstract

One of the main limitations of wrought magnesium alloys is their high yield asymmetry and anisotropy due to the intrinsic plastic anisotropy of the magnesium single crystal hexagonal close packed structure. This can lead to strong textures and twinning-dominated deformation. Rare earth additions to magnesium have been shown to reduce this yield asymmetry and anisotropy, though the high price of the rare earth metals and security of supply issues drives the need for alternative alloys that can replicate this e ect. Reducing the grain size of wrought alloys can achieve this to some extent, as can careful choice of precipitates in age-hardenable alloys which can preferentially strengthen weaker deformation systems such as twinning. All three approaches were explored in this thesis in an alloy design context. Magnesium Elektron UK set out an alloy criteria to full for this work with a focus on reduced yield asymmetry and anisotropy, and low cost. Two promising alloying systems were considered for this criteria, both with low rare-earth contents: extruded Mg-Nd-Gn-Zn-Zr (Elektron 21) and rolled Mg-Zn-Ca(-Ce). In addition, modelling methods were developed to design, from metallurgical principles, alloys that full the criteria by exploiting precipitation. This required a determination of the e ect of multiple precipitate types on mechanical asymmetry and anisotropy by extending Orowan strengthening theory to this case, and implementing the results in a polycrystalline crystal plasticity model. Through this modelling work, several extruded Mg-Sn-Zn(-Al-Ca-Na) alloys were designed, produced, and tested against the target properties. TEM observations revealed that ageing the Mg-Sn-Zn(-Al-Ca-Na) alloys at 150°C caused the di erent precipitate types to nucleate together, not independently as was the case when aged at 200°C. This co-precipitation caused a reduction in the predicted mechanical symmetry and isotropy, and strength. Na additions were shown to re ne the precipitate populations in these alloys but strengthen the texture through preferential growth of recrystallised grains during extrusion and solution treatment. The most effective method of reducing mechanical asymmetry and anisotropy was found in this work to be the texture weakening and solid solution strengthening produced by rare-earth additions, followed by grain-size reductions and producing precipitate populations that strengthened weaker deformation systems. Out of the alloys tested, TZA941+0.3Na fullled the mechanicalproperty criteria with a tensile yield stress in the extrusion direction of 360MPa and a minimum yield stress of 222MPa.

Keyword(s)

alloy design; extrusion; magnesium alloy; mechanical testing; modelling; precipitation; rolling; texture; twinning

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