General grain boundaries in the limelight of quantum mechanical investigation

Significance 

Alloys are metallic compounds made up of one metal infused with one or more metals or non-metal elements. In metallic alloys, grain boundaries govern a plethora of properties and behaviors such as strength, toughness, and corrosion susceptibility. To this end, comprehending the energetics and the atomic-scale behavior of grain boundaries in engineering alloys is therefore of significant interest. In the past, scientists have engaged in atomistic simulations with the aim being to unravel the grain boundary structure and obtain the corresponding energetic cost of creating them. Presently, first principles studies of grain boundary cohesion which are relatively straightforward, and many examples on symmetrical tilt boundaries are available in literature. These first principles simulations have been restricted to symmetrical boundaries which can be calculated using simulation cells with a relatively small number of atoms. A comprehensive literature review of all systems shows that the boundary cohesion is predominantly determined by the bond breaking factor, above the other two. Although this general statement may be true for the symmetrical tilt boundaries that are usually studied, the behavior of complex irrational boundaries has not before been examined.

In this view, researchers from the University of South Australia: Dr. Reza Mahjoub and Professor Nikki Stanford, in collaboration with Dr. Dudekula Althaf Basha at the Indian Institute of Technology Indore, India, Dr. Alok Singh at the National Institute for Materials Science in Japan and Professor Michael Ferry at The University of New South Wales studied carefully the contributions to grain boundary cohesion on complex non-symmetric boundaries. In so doing, they aspired to present an extensive comparison between twin and general grain boundaries. Their work is currently published in the research journal, Materials Science & Engineering A.

In their approach, the research team used a series of experimental observations of grain boundary fracture behavior as exemplars following which boundaries with the same crystallography as the experiments were developed. Additionally, the researchers further interrogated various simulations with the aim being to understand the differences between the boundary types and solute species. The team further examined the boundary cohesion by the parameter known as embrittlement potency. Lastly, as a case study; the two boundaries studied in most detail, the, i.e. the {10 ī 2} twin boundary, and a general grain boundary, were observed experimentally.

The authors reported that there was good agreement between the calculated boundary cohesion values and the experimental observations of fracture behavior. This demonstrated the simulations to be good approximations of real behavior. Moreover, solutes with both larger and smaller radii than magnesium were seen to have a preference for segregation to the grain boundary. Further, it was found that solutes smaller than magnesium had a toughening effect, while those solutes larger than magnesium had more tendency to embrittle.

In summary, the study employed direct experimental observations to examine the grain boundary fracture behavior of magnesium using first principles calculations. The approach involved the application of density functional theory to examine the energetics and cohesion of the simulated volume, resulting in determination of the work of separation. In a statement to Advances in Engineering, Dr. Reza Mahjoub, first author, highlighted that their work provided a good evidence the simulation methodology adequately described the boundary behavior, providing a foundation from which other solutes could be examined in more detail. In addition, he also clarified that although the symmetrical twin boundary behaved in a similar manner to previous reports, the non-symmetrical boundary showed more complex cohesive behavior, highlighting the importance of studying the non-symmetrical boundaries that predominate real materials.

Reference

Reza Mahjoub, Dudekula Althaf Basha, Alok Singh, Michael Ferry, Nikki Stanford. The contrasting fracture behavior of twin boundaries and general boundaries – A first principles study based on experimental observation. Materials Science & Engineering A 781 (2020) 139225.

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