The electronic origins of the \u201crare earth\u201d texture effect in magnesium alloys

Reza Mahjoub,Nikki Stanford

doi:10.1038/s41598-021-93703-w

Abstract

Although magnesium alloys are lightweight, recyclable and relatively cheap, they suffer from poor ductility. This can be improved by the addition of rare earth (RE) elements, and this is now a well-established criterion for wrought alloy design. It is notable that this behavior is largely restricted to the lanthanides, but no hypothesis is yet available to explain why other elements do not have the same effect. To answer this question, ab initio simulations of crystallographically complex boundaries have been undertaken to examine the electronic origin of the RE effect. While the electronic structure provided strong bonding between the RE elements and their Mg surroundings, local disruption in atomic arrangement at the grain boundaries was found to modify this effect. This work shows quantifiable changes in electronic structure of solutes resulting from grain boundary crystallography, and is suggested to be a contributing factor to the RE texture effect.

Highlights

Magnesium alloys are lightweight, recyclable and relatively cheap, they suffer from poor ductility
Atomic size explains the energetic reasons for these elements to segregate to the grain boundary, it doesn’t explain how their presence changes the preferential growth of certain boundary types
Perplexing is the restriction of this phenomena to the rare earth (RE) group within the periodic table, there is clearly something about the electronic structure of these elements that gives them different properties compared to others when alloyed with magnesium

Summary

The effect of boundary topology on electronic structure

While the notion of electronegativity is commonly used to describe the nature of chemical bonding, it may fail to reveal the subtle differences between species where the electronegativities are close to each other[26]. Where boundaries A and B have similar pDOS to the bulk, grain boundary C develops a marked rise in pDOS near the Fermi level, indicating that for this solute in this particular boundary, different bonding behaviour can be expected. This is the first concrete evidence that electronic structure of Gd is different at grain boundaries of different crystallography, and this may begin to explain why this element can effect grain boundary properties: of Gd is more sensitive to its surroundings than other elements. It is pertinent to note that Zn and Mg showed only small changes in pDOS of the bulk as compared to the different grain boundaries, indicating that these species are less likely to exhibit changes in bonding behavior depending on their surroundings

The effect of solute species on bonding strength

Bonding strength

Closing remarks

Materials and methods

Boundary C

Additional information