Zigzag Dissociation Mode of 〈c + a〉 Dislocations on the 101¯1  Plane in Magnesium Alloys

Hao Sun,Zhigang Ding,Shuang Li,Wei Liu,Enrique J Lavernia,Xinping Zhang

doi:10.1155/2020/9646084

Abstract

Fundamental understanding of the dissociation mode of 〈c + a〉 dislocations on the 101¯1 plane is required before the goal of improving the ductility of Mg alloys attained. In this study, our density-functional theory calculations reveal that the atoms in the 101¯1 plane slip along a zigzag trace through a low-energy pathway. We thus propose a novel zigzag dissociation mode based on this slip trace. In particular, the shuffling motion of atoms is observed at the position of stable stacking fault, which is closely related to the c/a ratio of the hexagonal closed-packed lattices.

Highlights

Alloying rare Earth elements is an effective approach to improve the room temperature plasticity of Mg, by activating the pyramidal 〈c + a〉 dislocations [1,2,3]. e activation of pyramidal 〈c + a〉 dislocation can fulfill the von Mises’ criterion for a general plastic deformation. e 〈c + a〉 dislocation usually dissociates at a stable stacking fault due to the large Burgers vectors [4], and it is critical to understand the dissociation modes of 〈c + a〉 dislocations [5]
Our recent work has reported that there are two positions (types A and B in Figure 1(a)) on the 􏼈1011􏼉 plane that can be substituted by alloy elements [15]. e types A and B are different in the 􏼈1011􏼉 plane; the 􏼈1011􏼉 plane is composed of planes of the basal type A and B atoms, and the surrounding environments of the type A and B atoms are different
We found that the stable stacking fault energy (SFE) disappears when type A atoms are substituted by Y atoms (red curve in Figure 1(b)); while when type B atoms are substituted, the stable SFE exists at 0.4 b and the 〈c + a〉 dislocation dissociates at the position of the stable SFE [15]

Summary

Introduction

Alloying rare Earth elements is an effective approach to improve the room temperature plasticity of Mg, by activating the pyramidal 〈c + a〉 dislocations [1,2,3]. e activation of pyramidal 〈c + a〉 dislocation can fulfill the von Mises’ criterion for a general plastic deformation (requirements for five independent slip systems). e 〈c + a〉 dislocation usually dissociates at a stable stacking fault due to the large Burgers vectors [4], and it is critical to understand the dissociation modes of 〈c + a〉 dislocations [5]. Molecular dynamics (MD) simulations revealed that a 〈c + a〉 dislocation on the 􏼈1011􏼉 plane dissociates into two equivalent 1/6[2023] and 1/ 6[0223] partial dislocations [7] In this mode, atoms at a stable stacking fault position are deviated from their stable positions (so-called “off-lattice” position), and atoms move along a high-energy pathway [8]. An alternative dissociation mode was proposed based on density-functional theory (DFT) computations, addressing that 〈c + a〉 dislocation on the 􏼈1011􏼉 plane dissociates into two collinear 1/ 6[1123] partial dislocations [5] In this collinear disassociation mode, atoms are deviated from their stable positions because the stable stacking fault position on the 􏼈1011􏼉 plane is not located at the [1123] direction [9]. Our mode has two prominent features that are different from previous ones: (1) the atoms on the 􏼈1011􏼉 plane are located at (or adjacent to) a stable site and move along a low-energy pathway; and (2) in contrast to the collinear dissociation mode, the atoms slip along a zigzag trace that is more suitable to the corrugated 􏼈1011􏼉 plane

Methods

Results and Discussion

Conclusion