Investigation of Liquid-Metal-Embrittlement Crack-path: Role of Grain Boundaries and Responsible Mechanism
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Date
2020-07-24
Authors
Razmpoosh, Mohammad Hadi
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Liquid metal embrittlement (LME) has been reported in many structural materials, including steel, aluminum, nickel during hot working processes e.g. welding, brazing, heattreatment, leading to abrupt failure. In many of the applications, such as automotive, aerospace, nuclear industries, LME failure is considered as a serious safety concern. Over the last decades, research activities have grown considerably striving to understand the LME phenomenon. However, to date a fundamental understanding of the metallurgical and mechanical micro-events of LME has remained unclear. Moreover, the LME mechanism has been concealed behind the diverse, contradicting propositions without any robust experimental support. Hence, a comprehensive understanding of micro-events of LME calls for in-depth crack-path analysis from macroscopic, microscopic, and atomic viewpoints.
The aim of this research is to explore the role of grain boundary type, and characteristics such as grain boundary misorientation angle, crystallographic plane, and grain boundary microchemistry in LME. Restrained laser beam welding was used to induce LME-cracks in various Zn-coated steels. The crack-path has been characterized to identify types and geometrical characteristics of LME-sensitive grain boundaries. It was found that LME crack-path is a function of misorientation angle and stress component perpendicular to grain boundary plane, where high-angle random (non-ordered) grain boundaries are more LME-sensitive than highly coherent low-Σ coincidence site lattice (CSL) boundaries. At higher misorientation angles, lower tensile stresses trigger grain boundary decohesion. Moreover, liquid metal selectively penetrated the grain boundaries with high-index planes due to their relatively high excess volume. The atomic-scale analysis of LME crack-path provided new insights to the inter-relation between the geometrical configuration and grain boundary chemistry. This validated the grain boundary-based LME mechanism, and revealed the micro-events leading to the embrittler-induced grain boundary decohesion. It was found that grain-boundary engineering techniques can be employed to manipulate frequency of random and CSL boundaries, which resulted in significantly improved resistance against LME.
Description
Keywords
advanced high strength steels, liquid metal embrittlement (LME), Fe-Zn LME couple, grain boundary, fracture, microstructure, welding
LC Keywords
Steel, Brittleness, Liquid metals, Iron alloys, Zinc alloys, Kirkendall effect, Fracture, Microstructure, Welding