Abstract
Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc. This microalloying effect raises fundamental questions regarding creep deformation mechanisms. Here, using atomic-scale characterization and first principles calculations, we attribute the 600% increase in creep life in a prototypical Mg–rare earth (RE)–Zn alloy to multiple mechanisms caused by RE–Zn bonding—stabilization of a large volume fraction of strengthening precipitates on slip planes, increase in vacancy diffusion barrier, reduction in activated cross-slip, and enhancement of covalent character and bond strength around Zn solutes along the c-axis of Mg. We report that increased vacancy diffusion barrier, which correlates with the observed 25% increase in interplanar bond stiffness, primarily enhances the high-temperature creep life. Thus, we demonstrate that an approach of local, randomized tailoring of bond stiffness via microalloying enhances creep performance of alloys.
Highlights
Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc
The starting point of this work is our recent exciting discovery that Zn addition to Mg–Nd–La alloy improved the creep life by 600%, which we correlated to the precipitation of large volume fraction of a new phase on the {0002}α basal and {1100}α/{1120}α prismatic planes of hcp-Mg15
While the conventional creep models that invoke inhibition of dislocation motion by precipitates in Mg–Nd–La–Zn alloys predict an increase in creep life, they may not explain the dramatic 600% increase we observed
Summary
Several recent papers report spectacular, and unexpected, order of magnitude improvement in creep life of alloys upon adding small amounts of elements like zinc. Typical mechanisms invoked by conventional creep models are non-conservative vacancy-assisted dislocation climb over obstacles, thermally activated cross-slip, soluteinduced viscous drag on dislocations, jog-assisted dislocation motion, movement through dislocation intersections, and grain boundary sliding[6,7,8,9,10,11,12,13,14,15,16,17,18,19] Such insights have helped us develop creepresistant alloys using fine-scale precipitates and dispersion of hard particles to obstruct dislocation motion in the parent matrix[15,16,17,18,19,20,21]. While clearly illustrating the effects of Zn, these studies fundamentally suggest that Zn tends to occur near rare earth (RE) atoms, which determines the precipitation and creep response
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