Date of Award

12-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Committee Member

Dr. Garrett J. Pataky: Thesis Advisor, Committee Chair

Committee Member

Dr. Huijuan Zhao

Committee Member

Dr. Gang Li

Abstract

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.

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