Anisotropy and Asymmetry of Yield in Magnesium Alloys at Room Temperature
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THE mechanical properties of wrought magnesium alloys are often characterized by a high degree of plastic anisotropy and asymmetry. In the present work, plastic asymmetry refers to a difference in yield strength when tested in tension and compression along the same axis. This is often undesirable, having a negative effect on both the forming performance of the alloys and the inservice behavior.[1] The fundamental origin of these effects is well understood and can be attributed to the large difference in critical resolved shear stress (CRSS) for the deformation systems in the hexagonal magnesium crystal,[2] which leads to anisotropy, along with the unidirectional nature of twinning, which is responsible for asymmetric behavior. Asymmetry and anisotropy are most pronounced in single magnesium crystals.[2] In such material, the CRSS value for the softest mode (basal slip) is an order of magnitude less than that for any other mode. In polycrystalline magnesium alloys, the relative difference between CRSS values of the different modes is reduced and thus the anisotropy and asymmetry are less pronounced. One reason for this is the strengthening of all modes in the polycrystalline alloy case, which necessarily reduces the relative difference.[2] Nevertheless, even in polycrystalline materials, strong anisotropy and asymmetry remain, especially in JOSEPH ROBSON, Reader, is with the Physical Metallurgy, Manchester Materials Science Centre, University of Manchester, Grosvenor Street, Manchester M1 7HS, U.K. Contact e-mail: [email protected] Manuscript submitted April 9, 2014. Article published online July 16, 2014 5226—VOLUME 45A, OCTOBER 2014
wrought alloys that also develop strong basal textures during thermomechanical processing.[1] A large body of recent research has focussed on texture control in wrought magnesium alloys to produce randomized or non-basal textures, 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[3,4] and using more complex strain paths.[5] These alloys with modified texture often show a marked reduction in anisotropy and asymmetry as would be expected.[4] Other factors that have an important influence on the anisotropy and asymmetry of magnesium alloys are the strengthening mechanisms exploited. Typical strengthening mechanisms used in magnesium include work hardening, grain refinement (to produce Hall–Petch strengthening), solid solution strengthening, and precipitation hardening. The strengthening mechanisms do not influence all deformation systems equally. For example, both solid solution and precipitation hardening have been demonstrated to strengthen some modes more than others. Certain solutes (e.g., zinc) have also been reported to weaken a harder mode (prismatic slip), whilst strengthening a weaker mode (basal slip), reducing the difference in C
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