Date of Award

2021-08-01

Degree Name

Doctor of Philosophy

Department

Material Sciences And Engineering

Advisor(s)

Devesh Misra

Abstract

Austenitic stainless steels are widely used in our daily life, but their mechanical strength is low. In order to improve their yield strength via grain refinement, an investigation was carried out involving phase reversion annealing concept comprising of severe cold roll reduction followed by annealing at different temperatures for short durations. During annealing reversion of deformation-induced martensite to austenite occurred by shear mechanism, leading to fine-grained structure and high strength-high ductility combination.Nanoscale deformation studies suggested that the deformation mechanism of nanograined structure was different from the coarse-grained counterpart. Post-mortem electron microscopy of plastic zone surrounding the indent indicated that the active deformation mechanism was nanoscale twinning with typical characteristics of a network of intersecting twins in the nanograined structure, while strain-induced martensite transformation was the effective deformation mechanism for the coarse-grained structure. The presence of ~3 wt % Cu in austenitic stainless steel had a moderate effect on strain-rate sensitivity and activation volume at similar grain size in relation to the Cu-free counterpart. The nanoscale twin density was noticeably higher in Cu-bearing austenitic stainless steel as compared to Cu-free counterpart, a behavior that may be related to the increase of stacking fault energy. Furthermore, the synergistic effect of grain boundary and grain orientation on micro-mechanical properties of austenitic stainless steel was studied. Micro/nano-scale deformation behavior including hardness, elastic modulus, and pop-ins, was studied. Relatively higher hardness and modulus was observed near {101} and more pop-ins occurred in this orientation at high loading rate. From the perspective of engineering applications, the wear performance of fine-grained austenitic stainless steel through three-body abrasive wear tests at room and high temperatures was compared with the coarse-grained counterpart. The study demonstrated that fine austenite grains with high yield strength and elongation exhibited superior wear resistance at high temperature (250 °C), which was attributed to deformation twinning-induced plasticity in fine austenite grains. The wear mechanisms were microploughing and microcutting.

Language

en

Provenance

Received from ProQuest

File Size

204 p.

File Format

application/pdf

Rights Holder

Chengyang Hu

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