金属及合金中层错能量势垒的第一性原理研究
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摘要
最新研究表明引入纳米尺度共格界面是实现金属强韧化的有效途径。金属中共格界面的形成与位错滑移、层错、孪生等微观变形机制密切相关。而对广义层错能(GSFE)的研究将有助于揭示微观机制的产生机理,并为控制特定界面结构的形成提供理论基础。因此,本文对金属及合金中层错及孪生机制所对应的GSFE开展了深入研究。
本文采用密度泛函理论结合微动弹性带方法精确计算hcp体系(Mg、Mg-Li和Mg-Al)的基面滑移GSFE以及fcc体系(Al、Al-Li、Al-Mg、Al-Cu、Cu和Cu-Al)的孪生能量势垒。然后基于能量特征建立理论判据,以衡量合金元素对微观变形机制的影响。最后从电子结构角度阐述合金化效应的原因。
结果表明:(1)合金元素Li有利于Mg中位错滑移机制的开动;而合金元素Al有利于在Mg引入层错界面。造成合金化效应差异的原因在于Li和Al倾向于在Mg中诱发不同类型的键合方式。(2)合金元素Mg对Al的变形行为几乎没有影响;合金元素Li能有效改善Al的孪生能力,从而引入孪晶界面;合金元素Cu使Al的滑移方式偏离fcc金属特征。造成合金化效应差异的原因在于Li能辅助电子密度的重新分布从而有利于键角以简单方式改变;Mg对电子结构的影响不足以改变Al的切变模式;Cu掺杂会在滑移面间引入高浓度电子,不利于层错结构稳定。
The recent research revealed that coherent internal boundaries at nanoscale could improve both the strength and ductility of metals. The formation of coherent boundaries in metals closely relates to the microscale deformation mechanisms, such as slipping, faulting and twinning. The study of generalized stacking-fault energy (GSFE) could facilitate the understanding of these microscale mechanisms, and theoretically support the strategy to introduce the specific boundary structures. For this purpose, this thesis work will deeply explore the GSFE associated with the faulting and twinning pathways in metals and alloys.
The basal-plane GSFE in several selected hcp systems, including Mg, Mg-Li and Mg-Al, and the energy barriers of twinning in several selected fcc systems, including Al, Al-Li, Al-Mg, Al-Cu, Cu and Cu-Al, were precisely obtained via density functional theory calculations in combination with nudged elastic band method. Then, based on the energy profiles, theoretical criteria were established to evaluate the influence of the alloy elements to the tendency of microscale deformation. Additionally, the electronic origin of alloying effects was unveiled through the analysis of the electronic structure.
One of the results is that Li-alloying will make dislocation slipping tend to be favorable in Mg, while Al-alloying will help to introduce fault boundaries in Mg, which is fundamentally caused by the fact that Li and Al could induce different ways of bonding in Mg. The second result is that Mg-alloying will contribute little change to the mechanical behaviors of Al; Li-alloying may effectively improve the twinnability of Al, resulting in the spread of twin boundaries; and Cu-alloying will force the slipping of Al to deviate from the fcc-type deformation pathway. The electronic origin of the alloying effects is that Li may help the re-adaptation of charge to accommodate the change of bond angle simply; Mg may disturb the electronic structure too slightly to influence the shear mode of Al; and Cu could lead to high density of charge within the slip planes and expose the fault structures to be unstable.
本文采用密度泛函理论结合微动弹性带方法精确计算hcp体系(Mg、Mg-Li和Mg-Al)的基面滑移GSFE以及fcc体系(Al、Al-Li、Al-Mg、Al-Cu、Cu和Cu-Al)的孪生能量势垒。然后基于能量特征建立理论判据,以衡量合金元素对微观变形机制的影响。最后从电子结构角度阐述合金化效应的原因。
结果表明:(1)合金元素Li有利于Mg中位错滑移机制的开动;而合金元素Al有利于在Mg引入层错界面。造成合金化效应差异的原因在于Li和Al倾向于在Mg中诱发不同类型的键合方式。(2)合金元素Mg对Al的变形行为几乎没有影响;合金元素Li能有效改善Al的孪生能力,从而引入孪晶界面;合金元素Cu使Al的滑移方式偏离fcc金属特征。造成合金化效应差异的原因在于Li能辅助电子密度的重新分布从而有利于键角以简单方式改变;Mg对电子结构的影响不足以改变Al的切变模式;Cu掺杂会在滑移面间引入高浓度电子,不利于层错结构稳定。
The recent research revealed that coherent internal boundaries at nanoscale could improve both the strength and ductility of metals. The formation of coherent boundaries in metals closely relates to the microscale deformation mechanisms, such as slipping, faulting and twinning. The study of generalized stacking-fault energy (GSFE) could facilitate the understanding of these microscale mechanisms, and theoretically support the strategy to introduce the specific boundary structures. For this purpose, this thesis work will deeply explore the GSFE associated with the faulting and twinning pathways in metals and alloys.
The basal-plane GSFE in several selected hcp systems, including Mg, Mg-Li and Mg-Al, and the energy barriers of twinning in several selected fcc systems, including Al, Al-Li, Al-Mg, Al-Cu, Cu and Cu-Al, were precisely obtained via density functional theory calculations in combination with nudged elastic band method. Then, based on the energy profiles, theoretical criteria were established to evaluate the influence of the alloy elements to the tendency of microscale deformation. Additionally, the electronic origin of alloying effects was unveiled through the analysis of the electronic structure.
One of the results is that Li-alloying will make dislocation slipping tend to be favorable in Mg, while Al-alloying will help to introduce fault boundaries in Mg, which is fundamentally caused by the fact that Li and Al could induce different ways of bonding in Mg. The second result is that Mg-alloying will contribute little change to the mechanical behaviors of Al; Li-alloying may effectively improve the twinnability of Al, resulting in the spread of twin boundaries; and Cu-alloying will force the slipping of Al to deviate from the fcc-type deformation pathway. The electronic origin of the alloying effects is that Li may help the re-adaptation of charge to accommodate the change of bond angle simply; Mg may disturb the electronic structure too slightly to influence the shear mode of Al; and Cu could lead to high density of charge within the slip planes and expose the fault structures to be unstable.
引文
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