Doctoral Dissertations

Date of Award

8-2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Yanfei Gao

Committee Members

Hongbin Bei, Hahn Choo, Claudia Rawn, Anming Hu

Abstract

In the long history of alloy design, bulk metallic glasses (BMGs) and high entropy alloys (HEAs) are two new classes of chemically complex multicomponent alloys with unique structures and promising properties as the structural and functional materials. Due to the dynamic structural nature at atomistic scale, structural heterogeneities in BMGs play a significant role during deformation as plastic flow carriers. Hence, deformation heterogeneities occur as shear banding causes fatal failure in these amorphous alloys. However, the nature of structure and ductilization mechanism of BMGs is still not fully understood. Different deformation heterogeneities exist in crystalline alloys, such as planar slip, twinning and stress-driven phase transformation. As a newer class of materials, more investigations are necessary for understanding deformation behaviors in HEAs, which although have gained extensive investigation of deformation behaviors by applying conventional strengthening methods as well as twinning or phase-transformation induced plasticity (TWIP/TRIP). In this dissertation, we investigated the structural and deformation heterogeneities in BMGs and HEAs. The structural heterogeneities in BMGs were induced by elastic/plastic deformation and ion irradiation. Nanoindentation pop-in tests were performed to probe the atomistic structure which was quantitatively evaluated by numerical study of a unified model. The computational method was proposed by unifying the homogenous shear band nucleation process and heterogeneous shear band nucleation process to characterize the defects as the plastic flow carriers structurally and mechanically. It turns out that deformation induce defect is effective to tuning the amorphous structure in BMGs. Neutron diffraction was utilized to study behaviors of the deformation twinning and texture evolution during tensile deformation at different temperatures in TWIP HEAs. The deformation induced microstructure and texture change of CoCrFeMnNi alloy was investigated by multiple interrupted cryogenic tensile tests. The micromechanical behavior of carbon strengthened CoCrFeMnNi0.5C HEAs was studied by in situ neutron diffraction. Combined with microstructure observation, the role of dislocation slips and twinning of work hardening was clarified. Consequently, the current research provide insight into deep understanding of structure-property relationship of two classes of chemically complex multicomponent alloys.

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