Date of Award

2017-01-01

Degree Name

Doctor of Philosophy

Department

Material Science and Engineering

Advisor(s)

Devesh Misra

Abstract

The concept of phase reversion involving severe cold deformation of metastable austenite to generate strain-induced martensite, followed by temperature-time annealing sequence, was used to obtain varying grain size from nanograined/ultrafine-grained (NG/UFG) to coarse-grained (CG) regime. This concept was used to obtain â??high strength-high ductilityâ?? combination in nano/ultrafine-grained (NG/UFG) austenitic stainless steel. Using this concept, the objective of the study is to elucidate the dependence of grain size on deformation mechanisms and deformation-induced microstructural changes. The objective was accomplished by combining depth-sensing nanoindentation experiments conducted at various strain rates, and interrupted tensile testing at various strain and post-mortem analyses of deformed Fe-17Cr-7Ni (AISI 301LN) austenite alloy using transmission electron microscopy (TEM). In the high strength NG/UFG steel, deformation twinning contributed to excellent ductility, while in the low strength coarse-grained (CG) steel, ductility was also good, but due to strain-induced martensite, implying clear distinction and fundamental transition in the deformation behavior of NG/UFG and CG austenitic stainless steels. The study underscores that irrespective of the grain structure and operating deformation mechanisms (twinning versus strain-induced martensite), the generic nature of strain hardening is unaltered. In the NG/UFG structure, there was marked increase in stacking faults and twin density at high strain rates, and high strains. TWIP effect was observed in NG/UFG steel, whereas TRIP effect was evidenced in CG alloy. The observed change in the deformation mechanism with change in grain size is attributed to increased stability of austenite with decrease in grain size, and is explained in terms of austenite stability-strain energy relationship. The insights on the relationship between grain structure (and strength) and deformation mechanisms are envisaged to be important in providing a new direction for the futuristic design of high strength-high ductility NG/UFG materials such as austenitic stainless steel.

Language

en

Provenance

Received from ProQuest

File Size

158 pages

File Format

application/pdf

Rights Holder

Venkata Sai Ananth Challa

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