Year

2021

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

It has been known for about a century that hydrogen contamination causes severe degradation in the mechanical properties of metals. This phenomenon is generally termed as ‘hydrogen embrittlement’ (HE). In this thesis, the underlying mechanisms behind HE phenomenon were elucidated on an atomic scale. The H segregation at various grain boundaries (GBs) and its influence on the structure, mechanical properties, deformation mechanisms and failure response of GBs were examined by atomistic simulations.

First, H segregation at various GBs was studied in this thesis. The results indicated that H segregation properties were very sensitive to GB structures. The effects of H atoms on the mechanical behaviour and plastic deformation of GBs were then examined. It was shown that H atoms modified the behaviour of dislocation nucleation and caused the yield stress of dislocation nucleation to increase or decrease. Different deformation mechanisms were directly responsible for this modification. In addition, H segregation increased the critical shear stress and impeded the coupled GB motion, irrespective of the GB structures. During GB migration, H vacancy clusters cannot grow, which suggests that the coupled GB motion may help to resist H-induced intergranular embrittlement.

The role of H atoms in changing the interaction of dislocations with GBs was also investigated. Several interaction mechanisms such as dislocation transmission, nucleation and reflection were reported for different glide planes and GB structures. Segregated H atoms transformed these interaction mechanisms into ones involving dislocation absorption for most of GBs. This disordered the atomic structure of GBs and established a local stress state, which promoted the ultimate failure of GBs due to the formation of vacancies.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.