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Stacking faults in the transition metal carbides and nitrides: stability and mechanical properties
Dissertation   Open access

Stacking faults in the transition metal carbides and nitrides: stability and mechanical properties

Hang Yu
Doctor of Philosophy (Ph.D.), Drexel University
Aug 2017
DOI:
https://doi.org/10.17918/etd-7568
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Abstract

Ultra-high-temperature ceramics Transition metal compounds Refractory transition metal compounds Mechanical Engineering
Ultra-high-temperature ceramics (UHTCs) are a class of refractory materials that have melting temperatures above 3000 K. As a major subclass of UHTCs, the transition metal carbides and nitrides (TMC/Ns) are ideal for high temperature applications especially in aerospace field. Despite the fact that the crystal structures and types of bonding are very similar within these materials, their mechanical properties such as hardness and plasticity are different at room temperature. In addition, TMC/Ns suffer from low fracture toughness at room temperature, which may cause unexpected failures during operations and handling, therefore limiting their applications. Recent experimental work has found that the precipitation of certain stacking fault phases (SFPs) may increase the fracture toughness of these materials. However, the stability of these phases and the mechanisms behind the enhanced fracture toughness are still debated. To better understand the interplay between the chemistry, bonding, microstructure, and mechanical properties in TMC/Ns, this thesis focuses on two major topics: (1) the thermodynamic and structural stability of stacking faults and SFPs; (2) the key factors, including the chemistry and bonding, that control the mechanical properties of these materials, such as the slip and fracture toughness. In this thesis, numerical methods including ab-initio simulations are widely used to fill the gap between experimental observations and intrinsic properties of the material. This study serves as a bridge to connect theory to empirical observations, which would provide insights into engineering tougher and stronger TMC/Ns for high temperature applications.

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