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Functional group approach for grain boundary engineering, A
Rajivmoorthy, Malavikha
Rajivmoorthy, Malavikha
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2022
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Abstract
The phenomenon of solute-induced intergranular embrittlement has been an important subject of research for more than a century. Often tough structural materials like steel may fail catastrophically when contaminated with even minuscule quantities of ubiquitous elements such as phosphorus, sulfur, or hydrogen. Researchers have sought many theoretical and experimental methods to understand the underlying mechanisms and devise approaches to design alloys resistant to these effects - forming the key agenda toward grain boundary engineering. Although some of the methods developed helped in uncovering certain structural and chemical effects, they have not been proven to provide key predictive value in designing materials resistant to embrittlement. In part, the associated discrepancy may be attributed to the fact that fracture processes are not modeled realistically. Any embrittlement process is metallurgically understood to emanate from an atomically sharp crack whose propagation is influenced primarily from the local structure and chemistry ahead of its tip. This investigation aims to arrive at a rigorous chemical definition of structure and associated properties governing embrittlement. Applying the concept of chemical functionality to crystalline and defect structures, we demonstrate that the local structure of metals is mediated by the surrounding neighborhood extending 2 to 3 atomic diameters distant. This neighborhood describes the volume over which strain energy is distributed about a crack tip. Analyses on bismuth doped copper and phosphorus substituted iron leads to the conjecture that embrittling solutes reduce the volume of strain distribution (i.e., increase stress concentration) and further sharpen the crack tip by rendering it more ‘nearsighted’. To further assess grain boundary structure effects on behavior, the electronic structure of experimentally-obtained and computationally-recreated random grain boundaries are compared with symmetric low-energy structures. It is determined through orbital analyses at the grain boundary and bulk crystalline environments that the bonds between atoms are much stiffer in the crystalline environment than at the bulk. We therefore speculate that both intrinsic and extrinsic embrittlement may be dependent on the extent of localization of the atomic wavefunctions.
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