Date of Award
Doctor of Philosophy (PhD)
There is an ongoing need for the design and development of metal alloys with improved properties for extreme environment applications. High entropy alloys (HEAs) are a group of metal alloys that in contrary to conventional metal alloys can have multiple principal elements in high concentrations. HEAs show promising properties better than or comparable to conventional metal alloys for a range of temperature down to cryogenic temperature. HEAs are good candidates to be used as structural materials for extreme environments applications such as in aerospace, automotive, transportation, and energy industries, among others. Mechanical behavior and the underlying plastic deformation mechanisms and the factors affecting HEAs need to be fully understood to be able to use these alloys for the mentioned applications and to design and develop further improved metal alloys.Low stacking fault energy face centered cubic (fcc) HEAs show simultaneous high strength and ductility and specially by the decrease in temperature down to cryogenic temperatures, whereas there is usually a tradeoff between strength and ductility in conventional metal alloys. Plastic deformation in low stacking fault energy fcc HEAs starts with dislocation slip and with the increase in stress, deformation twins nucleate and grow as an additional mode of deformation. There have been studies that experimentally and computationally looked at slip and deformation twins and the effect of different parameters on their nucleation and growth in HEAs. However, the critical resolved shear stress for slip which indicates the beginning of the plastic deformation region in some of these HEAs has not been found. Also, different factors in deformation twin nucleation and growth have been studied but the effect of grain boundary (GB) types and elemental segregation at GBs have not been fully investigated. In this research experimental and computational approaches are used to further identify the underlying plastic deformation mechanisms in HEAs giving rise to their improved properties. High resolution digital image correlation and electron backscatter diffraction have been used to find the dislocation slip critical resolved shear stress (CRSS) in Al0.3CoCrFeNi polycrystalline under tension. Molecular dynamics (MD) simulations and Monte Carlo molecular dynamics (MCMD) simulations have been used to identify the effect of different symmetric twist GB types and elemental segregation on deformation twins in CoCrFeNi bicrystals at three different temperatures 77 K, 100 K, and 300 K. Experimentally Al0.3CoCrFeNi polycrystalline was tested under tension at room temperature slip CRSS was found to be 63±2 MPa based on the activated slip system of (-1 1 1)[-1 -1 0] which also had the highest Schmid factor of 0.42. The MD simulations and the MCMD simulations studies on the CoCrFeNi HEA bicrystals confirmed GBs as deformation twin nucleation sites. The mechanical properties and deformation twin nucleation changed with different symmetric twist GBs having different sigma values and misorientation angles. MCMD simulations revealed GBs becoming Cr-rich and Ni-deficient which matches the results from experimental observations and MCMD simulations of HEAs of similar compositions. Temperature also was shown to influence the material properties in this alloy. With the decrease in temperature from 600 K, to 300 K, to 77 K, the yield strength and stress, and the overall plastic flow stress increased, and the modulus of elasticity decreased. The mentioned scientific contributions guide HEA design and development with improved properties through GB engineering by populating the polycrystals with symmetric twist grain boundaries of high angle misorientation angles and segregation engineering and designing chromium-rich GBs. As a next step to this research, experimentally, tensile tests at cryogenic temperatures with further post-mortem microscopy can be performed to find the CRSS at cryogenic temperatures and characterize the slip and deformation twins. Computationally, MCMD chemical equilibrium can be continued and reinforcement learning algorithms can be implemented to optimize the process. Furthermore, other types of GBs can be considered and the effect of GB geometry on the elemental segregation itself can be another route branching from this research.
Shabanisamghabady, Mitra, "Dislocation Slip and Deformation Twinning in Face Centered Cubic Low Stacking Fault Energy High Entropy Alloys" (2020). All Dissertations. 2756.