Date of Award

2021-12-01

Degree Name

Doctor of Philosophy

Department

Material Sciences And Engineering

Advisor(s)

Devesh K. Misra

Abstract

Strain hardening and associated deformation mechanisms play a determining role in the performance of metallic materials. The objective of the doctoral research was to explore and fundamentally understand the strain hardening mechanisms in the next generation of advanced engineering steels. Two main work hardening mechanisms, notably, twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP)) were identified that governed strength and ductility. In the nanograined austenitic steel with high strength-high ductility combination, processed by the novel phase reversion concept, twinning-induced plasticity (TWIP) was the governing deformation mechanism and contributed to good ductility. In striking contrast, transformation-induced plasticity (TRIP) was the deformation mechanism in the low strength coarse-grained counterpart, which contributed to high ductility. The difference in the deformation mechanisms in the nanograined and coarse-grained structures was explained in terms of austenite stability â?? strain energy relationship.In another instance of Ni-free and Ni-bearing steels, two distinct types of nanostructured bainite were obtained through conventional one-stage and two-stage isothermal treatments, with the aim to study the effect of nickel on strain hardening. The presence of Ni led to reduction in the free energy of bainitic transformation, refinement of bainite structure, and extension of bainite nucleation regime. The uniform nanostructure of Ni-bearing steel promoted the formation of stable film-like retained austenite between the bainitic ferrite laths, leading to higher strain hardening rate and stronger transformation-induced plasticity (TRIP) effect, which was explained in terms of a thermodynamic model.

Language

en

Provenance

Received from ProQuest

File Size

168 p.

File Format

application/pdf

Rights Holder

Hangyu Dong

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