Effect of Precipitate Shape and Habit on Mechanical Asymmetry in Magnesium Alloys
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NTRODUCTION
MECHANICAL asymmetry is often observed in wrought magnesium products and is usually detrimental to performance.[1] The origin of these effects is now well understood, which is due to the large differences in critical resolved shear stress (CRSS) for the deformation systems operating in the hexagonal crystal. As a result, the strength in directions in which easy deformation modes (basal slip and f10 12g c-axis tension twinning) can be activated is low compared with the strengths in the directions where the resolved shear stress for these systems is low. Another consequence of the relative ease of activation of basal slip and f10 12g twinning is that strong basal textures develop during thermomechanical processing of magnesium.[2] This means that a loading direction that is in a weak orientation for one grain will be in a weak orientation for the entire grain assembly. One solution to this issue, which has been the subject of recent intensive research, is to produce wrought products with randomized or non-basal texture, so that the grain population always contains a mixture of grains in weak and strong orientations regardless of the direction of the applied load. Various approaches have been used to modify the texture of wrought magnesium alloys; those that have been successful include adding rare-earth elements[3–5] and using more complex strain JOSEPH D. ROBSON, Reader, is with the Manchester Materials Science Centre, University of Manchester, Grosvenor Street, Manchester M1 7HS, UK. Contact e-mail: [email protected] NICOLE STANFORD, Research Fellow, MATTHEW BARNETT, Professor, are with the Centre for Material and Fibre Innovation, Deakin University, Pigdons Road, Geelong 3217, Australia. Manuscript submitted April 23, 2012. Article published online October 17, 2012 2984—VOLUME 44A, JULY 2013
paths.[6] However, these methods have a number of disadvantages. Rare-earth additions are expensive, and using a more complex strain path during processing may not be practical. This article explores the potential to use a different strategy to reduce yield asymmetry. Rather than changing the texture, the aim of the current study is to investigate whether the fundamental source of the problem—the large difference in CRSS of the different deformation modes— can be addressed directly. This necessarily requires finding a method by which the easily activated deformation modes can be strengthened substantially more than the difficult to activate modes, so that the relative difference in CRSS is reduced. One potential route to provide this differential strengthening is to take advantage of the fact that precipitation in magnesium alloys is also highly anisotropic, and precipitate’s shape and habit are expected to have a large effect on the strengthening of any given slip system.[7–9] However, to reduce anisotropy and asymmetry, it is not sufficient to provide differential strengthening against slip alone because magnesium also deforms by twinning, and the f1012g twinning mode in particular requires strong suppression t
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