Effects of calcium on planar fault energies in ternary magnesium alloys
Effects of calcium on planar fault energies in ternary magnesium alloys
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DOI：
10.1103/physrevmaterials.3.013607
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发表时间：
2019-01-16
影响因子：
3.4
通讯作者：
Paxton, A. T.
中科院分区：
文献类型：
--
作者：
Andritsos, E., I;Paxton, A. T.
我们的动机是需要设计不含稀土添加剂、但可锻造且无强烈基底纹理的镁合金。最近已经清楚地表明，钙是替代具有经济战略意义的稀土元素的有希望的候选者。为此，我们专注于平面故障，通常绑定部分滑移位错的六方晶系晶格。我们已经做了第一性原理计算研究的基础，第一和第二阶锥面的广义层错能（SFE）。我们研究的变化，故障能量和各向异性的合金浓度和低浓度的钙的影响。对于从第一性原理计算的SFE，我们使用非自洽的Harris-Foulkes近似的局域密度泛函理论。我们证明，虽然这种近似导致高的计算效率，有没有显着损失的精度相比，自洽的Hohenberg-Kohn功能。我们超越了所有以前的工作中，合金元素被假定为驻留在故障，而不是解决更现实的情况下，SFE被修改的远程存在的杂质。这使我们能够确定是否隔离是预期的，我们发现，虽然元素隔离的基础故障，他们不金字塔故障。然而，在任何一种情况下，故障能量强烈修改合金化。这表明，无论是长程电子结构效应在发挥作用，或故障能量修改的影响，原子大小的差异，特别是大的情况下，钙。我们发现Mg-Li-Ca和Mg-Zn-Ca合金显示出显著的各向异性降低，这与它们已知的高强度和可成形性一致。在有利的情况下，这是通过加强基底滑移而不是削弱非基底滑移来实现的。Ca的贡献与合金元素的原子尺寸成反比，这使我们能够推测合金化效应通常是原子尺寸效应。
We are motivated by the need to design magnesium alloys that are free of rare-earth additions, but, nevertheless, forgeable and free of strong basal texture. It has become recently clear that calcium is a promising candidate to replace the economically strategic rare-earth elements. To this end, we focus on the planar faults that typically bound partial glide dislocations of the hcp lattice. We have made first-principles calculations to examine the generalized stacking fault energy (SFE) of the basal, first- and second-order pyramidal planes. We examine the changes in fault energy and anisotropy for increasing alloy concentrations and the effect of low concentrations of calcium. For the calculation of the SFEs from first principles, we use the non-self-consistent Harris-Foulkes approximation to the local density functional theory. We demonstrate that while this approximation leads to high computational efficiency, there is no significant loss of precision compared to the self-consistent Hohenberg-Kohn functional. We go beyond all previous work in which the alloying element is assumed to reside within the fault and instead address the more realistic situation in which the SFE is modified by the remote presence of the impurity. This allows us to determine whether segregation is expected and we find that while elements do segregate to the basal fault they do not to pyramidal faults. Nevertheless, in either case, the fault energy is strongly modified by alloying. This argues that either a long-ranged electronic structure effect is in play, or the fault energy modification is affected by the atomic size difference-particularly large in the case of Ca. We find that Mg-Li-Ca and Mg-Zn-Ca alloys show a remarkable decrease in anisotropy, which is consistent with their known high strength and formability. In favorable cases, this comes about by strengthening basal slip rather than weakening nonbasal slip. The Ca contribution increases inversely with the atomic size of the alloying element, allowing us to speculate that alloying effects are generally atomic size effects.