Effect of Strain Rate on the Tensile Behavior of CoCrFeNi and CoCrFeMnNi High Entropy Alloys

High entropy alloys (HEAs) are a new class of alloys with the potential to be used in critical load bearing applications instead of conventional alloys. The HEAs studied in this research were CoCrFeNi and CoCrFeMnNi. Both were single-phase face-centered cubic materials. The focus of this study was on the tensile behavior of the two materials at quasi-static and dynamic strain-rates (〖10〗^(-4) to 〖10〗^3 s^(-1)) and the underlying microstructural phenomena driving the behaviors. Electron back-scatter diffraction was performed on both HEAs to study the microstructure before mechanical testing. To study the effect of strain rate, tensile experiments were performed at quasi-static strain rates on hydraulic MTS load frames and dynamic strain-rates on a Split-Hopkinson Pressure Bar. HEAs stress-strain curves, modulus of elasticity, yield strength, ultimate strength, strain-rate sensitivity and work hardening rates were calculated with the data from the tensile experiments. Transmission electron microscopy was performed post-mortem to study the plastic deformation mechanisms activated at different strain rates. The dominant deformation mechanism changed from dislocation slip at quasi-static strain-rates to the addition of deformation nano-twins at dynamic strain-rates. Ultimate strength and ductility both improved with the increase of strain-rate, which can be attributed to the activation of deformation nano-twins in HEAs. CoCrFeNi and CoCrFeMnNi both have low stacking fault energies which encouraged twinning at high strains to accommodate plastic deformation. The strain-rate sensitivity component increased with increasing strain-rate, beginning with negligible strain-rate sensitivity in the quasi-static range to high strain-rate sensitivity in the dynamic range. CoCrFeMnNi showed greater strain-rate sensitivity. CoCrFeNi, with the less configurational entropy, had higher mechanical properties and strain hardening rates at different strain-rates compared to CoCrFeMnNi

Dislocation Slip and Deformation Twinning in Face Centered Cubic Low Stacking Fault Energy High Entropy Alloys

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

Mathematical modeling of glioblastoma tumor growth under the influence of chemotherapy with fluoropyrimidine polymer F10

Glioblastoma multiforme is the most common and most malignant primary tumor of the brain. Optimal therapy results in survival time of 15 months for newly diagnosed cancer and 5-7 months for recurrent disease. Malignant glioblastoma patients demonstrate limited response to conventional therapies that include surgery, radiation, and chemotherapy. New methods of therapy are urgently needed. Mathematical models are often used to understand and describe the behavior of brain tumors. These models can both increase our understanding of tumor growth, as well as aid in the development and preliminary testing of treatment options. In this research five classical cancer growth models, in form of ordinary differential equations (ODEs) were used. These models were exponential, logistic, generalized logistic, Gompertz and Von Bertalanffy growth models. Using data from an in-vivo experiment on glioblastoma, and a nonlinear least-squares solver in MATLAB (lsqcurvefit), the characteristic parameters of each model were found and then the models were compared to find the best fit. In the second part of the research, using Gompertz model, compartment modeling, in-vivo experiment data and lsqcurvefit function in MATLAB, the effect of chemotherapy with fluoropyrimidine polymer F10, was modeled. This model can later be used for preliminary testing and treatment options for glioblastomas