針對金屬玻璃固有的高脆性問題，研究人員在非晶基體中加入可轉(zhuǎn)變沉淀相來抑制剪切帶擴展。這種方法在晶態(tài)–非晶界面具有雙重作用：一方面它由于局部應力集中而成為馬氏體轉(zhuǎn)變的觸發(fā)點，另一方面它也能穩(wěn)定這一轉(zhuǎn)變，后者被認為是因為非晶基體產(chǎn)生的“限域效應”。如何協(xié)調(diào)界面在促進馬氏體轉(zhuǎn)變和剪切帶形核兩個看似矛盾的作用，仍是一個待解的難題。

由廣東工業(yè)大學材料與能源學院付小玲副教授和加州大學伯克利分校Robert O. Ritchie教授領導的研究團隊，采用分子動力學（MD）模擬，基于原子間勢準確揭示了原子自發(fā)行為產(chǎn)生的非均勻彈性場，構建了不同尺寸的可轉(zhuǎn)變球形B2納米顆粒與非晶基體間的模型，從而更為精確地研究了界面彈性應力/應變場分布情況，為揭示該材料復雜的力學行為提供了新的視角。這個模型相較于傳統(tǒng)Eshelby理論，提出了一個新的認識：在晶態(tài)–非晶界面附近存在一個納米尺度的界面區(qū)，其中應變呈梯度變化，從壓縮逐漸過渡到拉伸。在這一特定區(qū)域，研究詳細描述了晶態(tài)–非晶界面、壓縮/膨脹轉(zhuǎn)換過程以及界面處的最大應變，而不是簡單地將其歸為常規(guī)的“界面”概念。

隨著球形B2相沉淀物從小到大逐步增長，壓縮/膨脹轉(zhuǎn)換區(qū)域的位置也相應從非晶區(qū)內(nèi)轉(zhuǎn)移至B2沉淀物內(nèi)部。界面處的應變狀態(tài)也隨之從壓縮（對較小沉淀物）轉(zhuǎn)變?yōu)榕蛎洠▽^大沉淀物）。沉淀物和基體中的壓縮（或膨脹）應力狀態(tài)對馬氏體轉(zhuǎn)變的啟動起到抑制（或促進）作用，并相應地增加（或減少）了轉(zhuǎn)變的成核障礙。此外，B2相沉淀物與非晶基體間較松散（或緊密）的界面相互作用導致轉(zhuǎn)變成核障礙的降低（或升高），進而影響材料的馬氏體開始轉(zhuǎn)變溫度。

最大的界面應變，與“界面應變/應力集中”密切相關，其實位于界面幾埃以內(nèi)的非晶基體中，而非精確的界面處。界面應力集中的數(shù)值明顯高于精確界面的應變，并隨著沉淀物尺寸的增加而增長。通過精準調(diào)控馬氏體轉(zhuǎn)變比例和界面應力集中度，可以大幅提升“轉(zhuǎn)變介導的加工硬化和塑性”效應，有效解決即便在低溫條件下也存在的強度與延展性的平衡問題。該文近期發(fā)表于npj Computational Materials 9: 226 (2023).

Editorial Summary

To address the inherent high brittleness of metallic glasses, researchers have introduced transformable precipitates into the amorphous matrix to inhibit the expansion of shear bands. This approach has a dual role at the crystalline-amorphous interface: on one hand, it becomes a trigger point for martensitic transformation due to local stress concentration, and on the other hand, it stabilizes this transformation, which is considered to be due to the “confinement effect” produced by the amorphous matrix. How to coordinate the interface’s seemingly contradictory roles in promoting martensitic transformation and nucleation of shear bands remains an unresolved issue.

A team led by Prof. Xiaoling Fu from School of Materials and Energy, Guangdong University of Technology and Prof. Robert O. Ritchie from Department of Materials Science & Engineering, University of California, USA, established an innate interfacial elastic strain gradient model of a transformable B2 precipitate embedded in an amorphous matrix based on MD simulations.

Compared to the Eshelby solution, this model proposes a nanometer scale interfacial region adjacent to the crystalline-amorphous interface which experiences gradient strain transitions from compressive to tensile. In this region, the crystalline-amorphous interface, the compressive/dilatative transition, and the interfacial maximum strain were characterized and differentiated instead of addressing them all as “interface” in a conventional fashion.

When the size of the spherical B2 precipitates gradually increases from small (dL^-1?<?~?0.75), medium-sized precipitates (~0.85?<?dL^-1?<?~?0.94) to very large precipitates (~0.94?<?dL^-1?<?1), the compressive/dilatative transition region locates from inside the amorphous region to inside the B2 precipitate. The actual interface strain transits from compressive (for dL^-1?<?~?0.91) to dilatative (for dL^-1?>?~?0.91). The compressive (dilatative) stress state in the B2 precipitate and amorphous matrix serve to prohibit (assist) the initiation of martensitic transformation and increase (decrease) the nucleation barrier of the transformation. The looser (close-packed) interfacial interaction between the B2 precipitate and the amorphous matrix decreases (increases) the nucleation barrier of the transformation, thus increasing the Ms temperature.

The interfacial maximum strain, which is likely related to the “interfacial strain/stress concentration”, is located a few ?ngstroms away from the interface and inside the amorphous matrix, instead of at the “exact” interface. The value of the “interfacial stress concentration” is always higher than the strain at the “exact” interface and increases as the precipitate sizes in the BMGCs are enlarged. By properly manipulating the transformation fraction of the martensitic transformation and interfacial stress concentration, the “transformation-mediated work hardening and plasticity” effect can be maximized to overcome the strength-ductility trade-off even at cryogenic temperatures. This article was recently published in npj Computational Materials 9: 226 (2023).

原文Abstract及其翻譯

The innate interfacial elastic strain field of a transformable B2 precipitate embedded in an amorphous matrix (嵌入非晶基體中可轉(zhuǎn)變B2相沉淀物的固有界面彈性應變場)

Xiaoling Fu,?Yujun Lin,?Mixun Zhu,?Kai Wang,?Jiaqing Wu,?Xing Tong,?Wenli Song,?Ming Jen Tan,?Yuanzheng Yang,?Jun Shen,?Gang Wang,?Chan Hung Shek?&?Robert O. Ritchie?

Abstract

When a transformable B2 precipitate is embedded in an amorphous matrix, it is often experimentally observed that the crystalline-amorphous interface not only serves as an initiation site for the martensitic transformation due to local stress concentrations, but also as an inhibitor to stabilize the transformation, the latter being attributed to the “confinement effect” exerted by the amorphous matrix, according to the Eshelby solution. These two seemingly incongruous factors are examined in this study using molecular dynamics simulations from an atomic interaction perspective. An innate strain gradient in the vicinity of the crystalline-amorphous interface is identified. The actual interface, the compressive/dilatative transition, and the interfacial maximum strain are investigated to differentiate from the conventional “interface” located within a distance of a few nanometers. Our innate interfacial elastic strain field model is applicable for the design of materials with a higher degree of martensitic transformation and controllable stress concentration, even in cryogenic environments.

摘要

當在非晶基體中嵌入可變形B2相沉淀物時，實驗常常顯示出晶態(tài)–非晶界面具有雙重作用：一方面它由于局部應力集中而成為馬氏體轉(zhuǎn)變的觸發(fā)點，另一方面它也能穩(wěn)定這一轉(zhuǎn)變，后者被認為是因為非晶基體產(chǎn)生的“限域效應”。這一效應得到了Eshelby理論解的支持。本研究通過原子級相互作用的分子動力學模擬，探討了這兩個看起來矛盾的因素。我們發(fā)現(xiàn)在晶態(tài)–非晶界面附近存在固有的應變梯度。通過詳細研究實際的界面，壓縮/膨脹轉(zhuǎn)變區(qū)域，以及界面的最大應變，我們進一步區(qū)分了這些特征與通常位于距離幾納米范圍內(nèi)的傳統(tǒng)“界面”的不同。我們提出的固有界面彈性應變場模型，為設計能在包括低溫環(huán)境在內(nèi)的各種條件下，實現(xiàn)高度馬氏體轉(zhuǎn)變和可控應力集中的新材料提供了理論依據(jù)。