A First-Principle Study Of Stacking-Fault Energy Barriers In Metals And Alloys

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.