Date of Award
Master of Science (MS)
Garrett J Pataky
Grain boundaries play an important role in mechanical properties of metallic alloys. The element segregation of metallic alloys at grain boundary can impact the strength of the alloy by changing the underlying deformation mechanism. Current studies of grain boundary segregation are mainly focused on binary alloys. With the newly developed high entropy alloys, it is necessary to investigate the role of grain boundary segregation on the mechanical properties of the high entropy alloys. Therefore, we conduct an atomistic study of elemental distribution on the grain boundary of the bicrystal CoCrFeMnNi high entropy alloy and investigate the relation between grain boundary segregation and the mechanical strength of the material. Hybrid Monte Carlo (MC) and Molecular Dynamics (MD) simulations are performed to obtain the equilibrium state of elemental distribution within the grain and on the grain boundary of the bicrystal model of CoCrFeMnNi high entropy alloy. The grain boundary is defined as ∑5 (3 1 0) [0 0 1] θ =36.9o. It is found that Cr has a propensity to segregate to the boundary. With the consideration of various dopant elements at the grain boundary of this bi-crystal model, MD/MC simulations are performed to study the role of selected dopant on the grain boundary to the mechanical strength of the material under the uniaxial loading conditions. The results reveal that the presence of Cr on the grain boundary has an embrittling effect to the alloy while Ni leads to an increase in yield and ultimate strength. The embrittling effect is due to the low stress requirement to nucleate dislocations in a Cr dopped grain boundary. Conversely, a higher dislocation density was observed during the deformation of bicrystal with the Ni dopped grain boundary. Such dislocations could traverse between grains without pinning leading to an increase in strength. Under compression, however, there was no significant difference in strength and elastic modulus with respect to different dopants on the grain boundary. This work leads to further study on the tailoring of high entropy alloys' mechanical properties through grain boundary manipulations.
Dhlamini, Zibusiso, "Mechanism Behind Grain Boundary Segregation and Mechanical Properties Enhancement of CoCrFeMnNi High Entropy Alloy Through Molecular Dynamics Simulation" (2021). All Theses. 3550.