鎂（Mg）及其合金因其輕質(zhì)、高強度重量比、良好的可回收性，成為了交通運輸中結(jié)構(gòu)組件的候選材料，且由于其可生物降解和相容性行為，在生物醫(yī)學(xué)也應(yīng)用廣泛。

然而，孿晶變形導(dǎo)致了紋理鎂合金的拉伸和壓縮屈服強度以及工作硬化之間的顯著差異，這種明顯的塑性各向異性對鍛制鎂合金的延展性和成形性產(chǎn)生了負面影響。通過孿晶來實現(xiàn)塑性變形，包括兩個連續(xù)步驟：孿晶核形成和孿晶生長（增厚）。

孿晶核形成是一種異質(zhì)過程，發(fā)生在微觀結(jié)構(gòu)中應(yīng)力集中的區(qū)域。一旦孿晶體形成，通常認為孿晶體增厚是由孿晶位錯沿著孿晶面滑移介導(dǎo)的，而這一過程受到了孿晶面和方向上的解析剪應(yīng)力的控制。從多晶體角度來看，用于解釋單個晶粒中孿晶核形成的最常見標準是表觀施密德因子（SF），其基于單個晶粒內(nèi)的應(yīng)力狀態(tài)與宏觀應(yīng)力相同這一假設(shè)。

然而，最近的許多研究揭示，除了SF之外，其他微觀結(jié)構(gòu)特征也對孿晶核形成具有顯著影響。因此，目前對導(dǎo)致孿晶核形成的潛在因素仍無共識，因為孿晶的生長不一定只發(fā)生在大晶粒、晶界或有有利取向的晶粒中。

Fig. 4 Distributions of some microstructural features.

來自西班牙IMDEA材料研究所的Javier Llorca教授課題組，通過電子背散射衍射對拉伸變形的Mg合金孿晶形核進行了研究，分析了晶粒的28個相關(guān)參數(shù)，將其分為四組不同的類別（加載條件、晶粒形狀、表觀施密特因子和晶界特征）。

他們訓(xùn)練了貝葉斯網(wǎng)絡(luò)分類模型，研究發(fā)現(xiàn)較大的晶粒和具有較高施密特因子的晶粒更有利于孿晶的形成。此外，即使對于具有很低甚至負施密特因子的晶粒，如果其至少有一個較小的鄰近晶粒和另一個（或相同的）較剛性的晶粒，也可能形成孿晶。同時，如果小晶粒具有較高的施密特因子，并且具有較低的基面滑移施密特因子以及至少一個具有易于變形的高基面滑移施密特因子的鄰近晶粒，那么小晶粒的孿晶更有利。

這些結(jié)果揭示了許多體系之間的相互作用，例如給定晶粒與其相鄰晶粒之間的剛度和大小差異，以獲取抑制晶粒發(fā)生孿晶的手段。相關(guān)論文近期發(fā)布于npj?Computational Materials?10: 26?(2024)。手機閱讀原文，請點擊本文底部左下角“閱讀原文”，進入后亦可下載全文PDF文件。

Editorial Summary

Magnesium (Mg) and its alloys have emerged as promising candidates for structural components in transport, owing to their light weight, high strength-to-weight ratio, and good recyclability, as well as in biomedical applications due to their biodegradable and biocompatible behavior. However, twinning deformation leads to a large difference between the tensile and the compressive yield strengths and work hardening of textured Mg alloys, and this marked plastic anisotropy has negative effects on the ductility and formability of wrought Mg alloys. The accommodation of plastic deformation by twinning involves two successive steps: twin nucleation and twin growth. Twin nucleation is a heterogeneous process that takes place in regions with large stress concentrations in the microstructure, such as grain boundaries. Once the twin has been formed, it is generally accepted that twin thickening is mediated by the glide of twinning dislocations along the twin planes, and this process is controlled by the resolved shear stress on the twin plane and direction. From the polycrystal viewpoint, the most common criterion used to explain twin nucleation in one grain is the apparent Schmid factor (SF), based on the hypothesis that the stress state in one grain is identical to the macroscopic applied stress. However, many recent works have revealed that other microstructural features, besides the SF, also have remarkable influence on twin nucleation. Thus, there is still no consensus on the underlying factors leading to twin nucleation because extension twins not necessarily occur in all large grains, at all GBs, or in all grains with favorable orientations.?

A?group led by Prof. Javier Llorca, from IMDEA Materials Institute, Spain, studied twin nucleation in textured Mg alloys by means of electron back-scattered diffraction in samples deformed in tension along different orientations, 28 relevant parameters, categorized in four different groups (loading condition, grain shape, apparent Schmid factors, and grain boundary features). They trained Bayesian network (BN) model and found that twin nucleation is favored in larger grains and in grains with high twinning Schmid factors, but also that twins may form in the grains with very low or even negative Schmid factors for twinning if they have at least one smaller neighboring grain and another one (or the same) that is more rigid. These results reveal the role of many-body relationships, such as differences in stiffness and size between a given grain and its neighbors, to assess extension twin nucleation in grains unfavorably oriented for twinning.

原文Abstract及其翻譯

Application of machine learning to assess the influence of microstructure on twin nucleation in Mg alloys (應(yīng)用機器學(xué)習(xí)評估微觀結(jié)構(gòu)對鎂合金孿晶形核的影響)

Biaobiao Yang, Valentin Vassilev-Galindo & Javier Llorca

Abstract?Twin nucleation in textured Mg alloys was studied by means of electron back-scattered diffraction in samples deformed in tension along different orientations in more than 3000 grains. In addition, 28 relevant parameters, categorized in four different groups (loading condition, grain shape, apparent Schmid factors, and grain boundary features) were also recorded for each grain. This information was used to train supervised machine learning classification models to analyze the influence of the microstructural features on the nucleation of extension twins in Mg alloys. It was found twin nucleation is favored in larger grains and in grains with high twinning Schmid factors, but also that twins may form in the grains with very low or even negative Schmid factors for twinning if they have at least one smaller neighboring grain and another one (or the same) that is more rigid. Moreover, twinning of small grains with high twinning Schmid factors is favored if they have low basal slip Schmid factors and have at least one neighboring grain with a high basal slip Schmid factor that will deform easily. These results reveal the role of many-body relationships, such as differences in stiffness and size between a given grain and its neighbors, to assess extension twin nucleation in grains unfavorably oriented for twinning.

摘要該文章通過電子背散射衍射對拉伸變形的Mg合金孿晶形核進行了研究，分析了晶粒的28個相關(guān)參數(shù)，分為四組不同的類別（加載條件、晶粒形狀、表觀施密特因子和晶界特征），樣本多達3000多個晶粒。這些信息被用來訓(xùn)練監(jiān)督式機器學(xué)習(xí)分類模型，以分析微觀結(jié)構(gòu)特征對Mg合金中延伸孿晶的影響。研究發(fā)現(xiàn)，較大的晶粒和具有較高施密特因子的晶粒更有利于孿晶的形成。此外，即使對于具有很低甚至負施密特因子的晶粒，如果其至少有一個較小的鄰近晶粒和另一個（或相同的）較剛性的晶粒，也可能形成孿晶。此外，如果小晶粒具有較高的施密特因子，并且具有較低的基面滑移施密特因子以及至少一個具有易于變形的高基面滑移施密特因子的鄰近晶粒，那么小晶粒的孿晶更有利。這些結(jié)果揭示了許多體系之間的相互作用，例如給定晶粒與其相鄰晶粒之間的剛度和大小差異，以獲取不利于晶粒發(fā)生孿晶的因