鈦合金因其卓越的性能和廣泛應用，對其α/β微觀結構的優化至關重要。界面能量是影響合金性能的關鍵，但直接測定其值極具挑戰。新興技術，包括熱力學積分和基于DFT數據的神經網絡模型，現已使得界面能量的精確計算成為可能，進而推動了材料設計和性能優化的新篇章。

Fig. 1 Temperature dependences of the lattice constants and free?energies of bulk α and β phases.

由香港城市大學材料科學與工程系的Jian Han教授領導的團隊，采用分子動力學、熱力學積分以及經過密度泛函理論（DFT）訓練的深度學習勢模型，深入探討了鈦材料中α/β界面（即和)）的結構和熱力學特性。

研究首先集中于分析鈦的相干α/β界面的熱力學性質，并考察其如何受到應變和溫度的影響。隨后，團隊對半相干界面的結構和屬性進行了細致審查，并將這些發現用于理解β基體中α析出相的成核及生長過程，也就是從高溫狀態開始的冷卻過程。

Fig. 3 Temperature dependence of coherent α/β interface free?energy.

該論文的關鍵發現包括：（1）成功預測了鈦中最關鍵界面（相干和半相干）的自由能，這是首次以接近DFT精確度進行此類計算（值得注意的是，β相在0K下完全不穩定，因而在沒有人為約束的情況下DFT無法直接計算）；（2）模擬結果展示了半相干界面的平衡結構和本質的缺陷結構，這些結構解釋了習慣面的普遍存在；（3）揭示了界面遷移的作用機理，并指出這一機理在不同方向（比如升溫與降溫）會導致界面遷移速率的不同；（4）這些精確的熱力學和結構數據被用于可靠預測α-β相變冷卻過程中析出相的形成。

此項研究不僅為準確預測界面屬性和運動提供了指導，還為理解和預測包括在低溫下不穩定的相和相干性喪失情況下的析出行為提供了實用的參考。該文近期發表于npj Computational Materials 9: 216 (2023)。

Fig. 5 Schematic of the model for simulating semicoherent α/β?interfaces.

Editorial Summary

Titanium alloys, renowned for their exceptional performance and wide-ranging applications, require optimization of their α/β microstructure for enhanced properties. The key to alloy performance, interface energy, presents significant measurement challenges. Emerging technologies, including thermodynamic integration and neural network models trained on DFT data, now enable precise calculations of interface energy, thereby opening a new chapter in material design and performance optimization.

Fig. 6 Semicoherent interface structure.

A team led by Prof. Jian Han from Department of Materials Science and Engineering, City University of Hong Kong, investigated the structure and thermodynamics of the α/β interface in Ti using molecular dynamics, thermodynamic integration and a DFT-trained Deep Potential.?

The authors first focus on the thermodynamic properties of the coherent α/β interface in titanium (i.e., ?and ) as a function of strain and temperature. Next, the authors examine the structure and properties of the semicoherent interface. This information is then applied to understand the nucleation and growth of α precipitates in a β matrix (i.e., cooling from high temperature).?

The main findings in this paper are as follows. (i) The authors predict the free energy of the most important interfaces (coherent and semicoherent) in titanium. This represents the first such calculations with DFT-level accuracy (note that β phase is completely unstable at 0 K and hence inaccessible to DFT without artificial constraints). (ii) The simulations show the equilibrium structure of the semicoherent interface and its intrinsic defect structure that gives rise to the widely-observed habit plane. (iii) The authors demonstrate the mechanism of interface migration and that this mechanism gives rise to different interface mobilities in different directions (heating vs. cooling). (iv) These accurate thermodynamic and structural results are applied to make reliable predictions on how precipitation occurs upon cooling through the α-β phase transition.?

This paper provides a roadmap for accurate prediction of interface properties and motion as well as precipitation in any system, including in systems with phases that are unstable at low temperature and in systems where loss of coherency occurs.?This article was recently published in npj Computational Materials 9: 216 (2023).

原文Abstract及其翻譯

Coherent and semicoherentα/β?interfaces in titanium: structure, thermodynamics, migration (鈦中的相干與半相干α/β界面：結構、熱力學及遷移特性)

Siqi Wang,?Tongqi Wen,?Jian Han?&?David J. Srolovitz?

Abstract?

Theα/β?interface is central to the microstructure and mechanical properties of titanium alloys. We investigate the structure, thermodynamics and migration of the coherent and semicoherent Ti?α/β?interfaces as a function of temperature and misfit strain via molecular dynamics (MD) simulations, thermodynamic integration and an accurate, DFT-trained Deep Potential. The structure of an equilibrium semicoherent interface consists of an array of steps, an array of misfit dislocations, and coherent terraces. Analysis determines the dislocation and step (disconnection) array structure and habit plane. The MD simulations show the detailed interface morphology dictated by intersecting disconnection arrays. The steps are shown to facilitate?α/β?interface migration, while the misfit dislocations lead to interface drag; the drag mechanism is different depending on the direction of interface migration. These results are used to predict the nature of?α?phase nucleation on cooling through the?α–β?phase transition.

摘要

α/β界面是影響鈦合金微觀結構和力學性質的關鍵因素。在本研究中，我們利用分子動力學模擬、熱力學積分方法，以及基于密度泛函理論訓練的高精度深度學習勢能模型，詳細探討了溫度和失配應變條件下鈦的相干與半相干α/β界面的結構、熱力學性質以及遷移行為。研究發現，一個處于平衡狀態的半相干界面由一系列臺階、失配位錯以及相干的平臺構成。通過分析，我們確定了界面上位錯和臺階（斷連）的陣列結構及其習慣面。分子動力學模擬揭示了由斷連陣列交互作用形成的復雜界面形態。研究表明，臺階結構促進了α/β界面的遷移，而失配位錯則產生了對界面遷移的阻力，具體的阻力機制取決于界面遷移的具體方向。這些發現幫助我們預測了在α-β相轉變冷卻過程中α相成核的特性。