Date of Award

8-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

George M. Pharr

Committee Members

Easo P. George, Madhu S. Madhukar, Hongbin Bei

Abstract

Compared to decades-old theories of strengthening in dilute solid solutions, the mechanical behavior of concentrated solid solutions is relatively poorly understood. A special subset of these materials includes alloys in which the constituent elements are present in equal atomic proportions, including the high-entropy alloys of recent interest. A unique characteristic of equiatomic alloys is the absence of “solvent” and “solute” atoms, resulting in a breakdown of the textbook picture of dislocations moving through a solvent lattice and encountering discrete solute obstacles. To clarify the mechanical behavior of this interesting new class of materials, we investigate here a family of equiatomic binary, ternary, and quaternary alloys based on the elements Fe, Ni, Co, Cr, and Mn. These subsets were dropp-cast, homogenized, cold-rolled, and further annealed. The recovery, recrystallization, grain growth and phase stability of these alloys were investigated first to identify the alloys with pure FCC crystal structure and their stability and to determine the suitable thermomechanical processing needed to obtain desired microstructures. After this, the mechanical properties of the alloys with FCC crystal structure and comparable grain size were investigated as a function of temperature. The flow stresses were observed to depend to varying degrees on temperature. Lattice friction stress appears to contribute significantly to the temperature-dependent yield stress, possibly because the Peierls barrier height decreases with increasing temperature due to thermal vibration induced increase of dislocation width. In the early stages of plastic flow (5~13% strain, depending on material), the temperature dependence of strain hardening is due mainly to the shear modulus changing with temperature since the curves at different temperatures collapse when the shear modulus corrected flow stress is plotted against strain. In all equiatomic alloys, both strain hardening capability and ductility increase with decreasing temperature, and the formation of deformation twinning could be an important comtribution to this. A statistical analysis is conducted to investigate the alloying (compositional) effects on the mechanical properties. The analysis suggests that, among the factors that have been investigated, the mechanical behavior is most highly correlated with the annealing twin density, which can have effects on strength and strain hardening behavior.

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