Doctoral Dissertations

Date of Award

12-2018

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Yanfei Gao

Committee Members

Zhili Feng, Peter K. Liaw, Claudia J. Rawn

Abstract

A number of computational methodologies have been proposed to investigate deformation and damage mechanism of various structural materials and weldments under extreme loading conditions, and also to provide valuable insights on the development of materials with improved performance. Creep strength enhanced ferritic (CSEF) steels containing 9-12wt% chromium have been extensively used in fossil-fuel-fired power plants. Despite their excellent creep resistance at high temperatures, premature failures are often found in the fine grained heat-affected zone (HAZ) of the welded components after years of service, which is referred as Type IV failure. Experiment measurements capture this so-called Type IV failure is preceded by the strain localization in the fine grained HAZ. Here we aim to develop an Integrated Computational Materials Engineering (ICME) approach to determine the micromechanical and microstructural origins of the above deformation and failure process. First, a microstructure-based finite element model is developed for studying the strain localization phenomenon, with the deformation processes resulting from a synergy of thermally activated dislocation movements, diffusional flow, and grain boundary sliding. The comparison of the digital image correlation (DIC) measurement and numerical simulations facilitates the understanding of the effect of pre-welding tempering on the evolution of the strain localization in the HAZ of P91 steels. To meet the design lifetime (>30 years) of thermal power plants, it is crucial to get the fundamental understanding the long-term creep fracture behaviors of power plant steel weldments. Therefore, in the current work an integrated microstructure-micromechanics-based finite element method is presented to analyze the long-term creep fracture behavior (Type IV crack), in which we account for the physical fracture mechanisms at different length scales, including nucleation of grain boundary cavities, their growth, the emergence of microcracks and their evolution to the ultimate failure. The methodology demonstrates excellent computational capabilities in modeling the Type IV failure and also providing high-quality creep rupture life prediction for the CSEF steel and its weldments. These methodologies will be efficient tools to evaluate the very-long-term creep and damage behavior of many other high temperature alloys and their weldments.

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