Asymmetrical Precipitation on the {10-12} Twin Boundary in the Magnesium Alloy
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The combination of predeformation and the aging process has been shown to be an effective way to strengthen magnesium alloys.[1–5] The deformation defects act as heterogeneous nucleation sites and promote precipitation[6]; in turn, these precipitates pin the movement of defects and improve the mechanical properties.[7,8] Similar to other alloy systems, the precipitates in magnesium alloy also often exhibit preferred crystallographic features,[9] such as specific orientation relationship (OR) between precipitates and matrix, interfacial orientation, morphology, and growth direction. These crystallographic features are one of the determinant factors of mechanical properties.[10–13] Therefore, the understanding of precipitation crystallography at deformation defects in magnesium is indispensable in controlling the mechanical properties, but the related studies are limited. The crystallographic restriction of deformation defects on precipitation was studied in the alloy systems mostly with cubic matrix due to the importance of steel, titanium
XIN-FU GU, MAN WANG, ZHANG-ZHI SHI, LENG CHEN, and PING YANG are with the School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China. Contact email: [email protected] Manuscript submitted April 16, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
alloy, etc. Generally speaking, some specific variants, where ‘‘variants’’ means that the transformation products hold crystallographically equivalent OR with respect to the matrix, would be preferred on the deformation defects; i.e., variant selection happens.[14–18] The geometry criterion for the variant selection on dislocations is that the largest misfit direction between the variants and matrix is close to the Burgers vector of the dislocation so that the transformation strain can be effectively accommodated,[14–17] while the criterion for the variant selection on the twin boundary (TB) is that the precipitates at the TB hold the same OR at both sides of the TB, which is close to that precipitated in the matrix, so that interphase boundary with low energies can form at both sides of the TB, such as Cr precipitates (body-centered-cubic (bcc) structure) in Ni matrix (face-centered-cubic (fcc) structure),[15] c austenite (fcc) precipitates in d ferrite matrix (bcc),[19] and a phase (hexagonal-close-packed (hcp) structure) precipitates in b matrix (bcc) of titanium alloys.[18] However, the OR in these cases should be exactly or nearly symmetrical around the TB; though an arbitrary OR will not satisfy this requirement. In this work, we will show such a general case and might shed some light on the effect of defects in the hcp structure on precipitation crystallography. In magnesium alloys, twinning is an important deformation mode to accommodate the deformation strains due to limited slip systems. Since the {10-12}h10-11i extension twinning mode is common not only in Mg alloys but also in other hcp alloys, precipitation on the {10-12} TB is mainly focused here. In this work, the precipita
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