Doctoral Dissertations

Date of Award

8-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Hahn Choo

Committee Members

Yanfei Gao, Peter K. Liaw, Zhili Feng

Abstract

The enhancement of formability of advanced high-strength TRIP-assisted steel alloys is a challenging assignment for industrial application due to the cracking phenomenon. The critical factor governing the cracking behavior is residual-stress concentration resulting from the inhomogeneous plastic deformation and microstructural evolution during the forming processes. Martensitic phase transformation kinetics, constituent phases, and crystallographic texture in TRIP-assisted steel alloys are correlated to the microstructure evolution, resulting in phase-specific stress concentration. In the current study, we are aiming at understanding the fundamental mechanisms responsible for the cracking phenomenon and thus improving the formability of TRIP-assisted steel alloys. Four stainless steel (SS) alloys were used in the current study to provide the variables in stability in austenite phase and constituent phases. There are two main objectives: (1) the constitutive behavior of the SS alloy plates during tensile loading, and to provide a basic understanding of the SS alloy behavior, and (2) the correlation of microstructure and its evolution to the cracking behavior in deep-drawn SS alloy cups and formability of the SS alloys during deep-drawing process. Firstly, the effect of phase transformation kinetics, constituent phases, and crystallographic texture on the phase-specific stress partitioning and plastic anisotropy was investigated in SS alloy plates subjected to uniaxial tension using synchrotron x-ray diffraction (S-XRD) and in-situ neutron diffraction. Secondly, the correlation of microstructure and its evolution to the cracking phenomenon and formability of TRIP-assisted steel alloy during deep-drawing process was studied using S-XRD. The results show that the phase-specific stress partitioning behavior is significantly affected by the martensitic phase transformation and constituent phases, resulting in the residual stress concentrating in α’-martensite responsible for the cracking phenomenon in TRIP steel. However, the residual stresses in α’-martensite could be reduced in the duplex TRIP steel due to the local stress partitioning between ferrite and α’-martensite, leading to a better formability in duplex TRIP steel. The textures are correlated to the transformation kinetics, thus influencing the martensite phase fraction and cracking behavior in the TRIP-assisted steel alloys. This study provides the basic idea to improve the formability of high-strength TRIP-assisted steel alloys by manipulating the microstructure to tailor the stress partitioning behavior and plastic anisotropy.

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