Computational Study of the Thermodynamics and Kinetics of Interfaces in Metals

Abstract

Interfaces are predominant planar defects in metals, playing a significant role in various engineered components designed for structural and functional applications. This study investigates three types of interfaces: (i) the 𝛾/𝛾′ interface in Ni-based alloys, which represents heterophase interfaces between two chemically distinct phases, (ii) the 𝛼/𝛽 interface in titanium alloys, representing heterophase interfaces between two structurally distinct phases, and (iii) the twin boundaries in the hexagonal close-packed (HCP) titanium, which represents homophase interfaces. The objective is to develop an atomistic comprehension of the structure, thermodynamic properties, and kinetic behavior of these interfaces.

Ni-based single crystal superalloys consist of dispersed, coherent 𝛾′ precipitates (Ll₂ Ni₃Al) in a 𝛾 matrix (face-centered cubic (FCC) Ni(Al)). These alloys are widely utilized in structural materials owing to their remarkable mechanical characteristics, including strong resistance to creep and fractures, along with excellent oxidation and corrosion resistance. These superalloys derive their enhanced strength significantly from the coherent 𝛾′ precipitates. The formation and growth of 𝛾′ particles within the 𝛾 matrix are governed by the 𝛾/𝛾′ interface energy. In our research, we engage in an in-depth analysis of the 𝛾/𝛾′ interface through the application of density functional theory (DFT) methods. This investigation focuses on assessing how the chemical potential of different elements impacts the surface energy characteristics of 𝛾′. This study examines the structure and energy characteristics of the interface, focusing on how interfacial translations and variations in stacking sequences influence these aspects. Distinct definitions of the work of adhesion are presented.

In titanium alloys, an illustrative microstructure comprises a mixture of 𝛼 (HCP) and 𝛽 (body-centered cubic, BCC) phases. Manipulating the microstructure of titanium alloys offers a means to fine-tune their mechanical properties, necessitating the comprehension of both thermodynamic and kinetic aspects governing 𝛼/𝛽 interfaces. This study reveals the metastability of 𝛼/𝛽 interfaces and its implication on interface kinetics. We utilize the DFT approach to examine the coherent 𝛼/𝛽 interface in titanium, with a focus on two distinct structural configurations, called the “Hollow” and “Bridge” states. The energy difference of the two states is sensitive to in-plane strain, leading to altered relative stability. DFT-based investigations of interface pseudo-migration show that the interface with Bridge state migrates readily and conservatively during the 𝛽 → 𝛼 phase transformation. In contrast, migration of the interface with the Hollow state results in the formation of 𝜔 phase (hexagonal crystal structure), reducing the driving force and slowing down interface migration kinetics. Additionally, the energy threshold associated with the kinetic Hollow-to-Bridge transition exhibits minimal dependence on applied strain. The alloying effect on the metastability of the 𝛼/𝛽 interface is examined through the virtual crystal approximation (VCA) approach with vanadium doping. The concentration of vanadium modifies the metastability of the two interface states and the energy barrier between them. Based on these findings, a strategy is proposed to control interface kinetics by adjusting the metastability and energy barrier between interface states through strain engineering and alloying.

In most HCP metals, twinning acts as a key deformation mechanism. Experimentally observed twinning modes in HCP titanium include {10^-11} ⟨101^-2^-⟩, {101^-2}⟨101^-1^-⟩, {101^-3} ⟨303^-2^-⟩, {112^-1} ⟨112^-6⟩, {112^-2} ⟨112^-3^-⟩ and {112^-4} ⟨224^-3^-⟩. These twinning modes are crucial for accommodating plastic deformation occurring along the 𝑐-axis in HCP titanium. This study employs atomistic simulations based on the Deep Potential to investigate the migration phenomena linked with shear coupling. The twin boundary (TB) energy is calculated for the TB corresponding to each of the six twinning modes, and the TB migration barrier is derived using the climbing-image nudged elastic band approach. Twinning dislocations (TDs), characterized by the twinning shear (shear-coupling factor), are determined through crystallographic analysis for all six twinning modes. We conduct molecular dynamics simulations to illustrate that the migration of TBs occurs through the nucleation and propagation of the predicted TDs under an applied shear that is consistent with the twinning shear. Interestingly, contrary to common expectations, the observed twinning mode is not necessarily the one corresponding to the minimal twinning shear. A method for selecting the appropriate twinning mode is proposed, which involves consideration of the migration barriers of different TBs.

Date of Award	2 May 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Zhaoxuan WU (Supervisor) & David Joseph SROLOVITZ (Supervisor)