块体金属玻璃的加工硬化行为

The Work-hardening Behavior of Bulk Metallic Glasses

块体金属玻璃(BMG)具有高强度、高硬度和大的弹性应变极限等独特的力学性能。然而由于缺乏位错、孪生等晶态缺陷,金属玻璃通过高度局域化的剪切带发生塑性变形,因此其通常不显示加工硬化行为,而发生应变软化和/或热软化。这导致了BMG早期灾难性失效,极大地限制了其广泛的工程应用。然而近年来,人们在一些单相BMG材料中观察到了明显的加工硬化行为。这引起了工程界学者的极大兴趣,也引发了关于金属玻璃加工硬化起源机制的讨论。目前人们对于金属玻璃的结构如何影响其性能和形变行为的理解还非常有限,BMG的加工硬化起源仍是当前颇具争议的研究热点。但总的说来,BMG的加工硬化行为与外加应力(能量)引起的内部结构改变,包括多重剪切带的形成、自由体积的演化和纳米晶化行为等密切相关,并最终涉及其变形过程中的剪切带行为。Cu47.5Zr47.5Al5是被最早报道的可加工硬化的塑性BMG。相关研究认为,合金中存在的不同尺度的化学和/或结构非均匀性促进了材料变形过程中多重剪切带的形成和增殖;而大量剪切带在三维方向上的交互作用导致了材料流变应力的增加,从而引起加工硬化。这就是BMG的加工硬化机理,该理论最早由Das等提出,后来被更多研究所证实。之后,研究者们在某些BMG加载-卸载循环纳米压痕试验中观察到了应变硬化-软化现象,并提出了BMG加工硬化的"自由体积模型"。他们认为,外加剪应力的改变导致了非晶结构内部净自由体积的变化,进而通过其对塑性变形微区剪切带行为的影响引起材料硬度的变化。Chen等在对均质结构的Cu50Zr50非晶条带进行弯曲变形后,检测到剪切带内原位纳米晶化,并基于对剪切带-纳米晶相互作用的实验观察,发展了形变诱导纳米晶化导致的应变硬化机制。这些工作丰富和发展了BMG加工硬化的基本原理及其研究方法。本文简要介绍了通常用来评估金属材料加工硬化能力的方法 /参数,并概述了金属玻璃中的剪切带行为;在此基础上,通过对几种典型的BMG加工硬化行为的分析,归纳性地讨论了BMG加工硬化起源可能的机制,以期为研究BMG的力学行为、开发性能优异的塑性BMG结构材料提供参考。

Bulk metallic glass(BMG)possesses unique mechanical properties,such as high strength,high hardness and large elastic strain limit,etc.However,due to the lack of crystalline defects including dislocation and twinning,plastic deformation of metallic glasses occurs in the form of highly localized shear bands.Consequently,they generally do not exhibit work-hardening,but occur strain softening and/or thermal softening.This leads to the premature catastrophic fracture of BMGs,which greatly hinders their widespread engineering application.However,in recent years,apparent work-hardening behaviors have been observed in some monolithic BMG materials which has attracted significant interest in the field of engineering,and triggered scientific studies on the origin of work-hardening in BMGs.So far,the understanding of how the structure of metallic glasses affects their performance and deformation behavior remains quite limited,thus,the origin of work-hardening of BMGs is still a hot topic of controversy,In general,the work-hardening behavior of BMGs is closely related to their internal structural changes caused by external stress(energy),including the formation of multiple shear bands,the evolution of free volume and the behavior of nanocrystallization,and ultimately associated with the shear-band behavior during plastic deformation.Cu 47.5 Zr 47.5 Al 5 was the first reported plastic BMG which has extensive work-hardening capability.Relevant studies suggested that the chemical and/or structural inhomogeneity with different scales existing in the alloy promoted the formation and multiplication of multiple shear bands during material deformation;while the interaction of massive shear bands in three dimensions increased the flow stress of the materials,resulting in the work-hardening behavior.which is ascribed to the work-hardening mechanism of BMGs.This theory was first proposed by Das et al.,and later confirmed by a variety of other studies.Afterwards,strain hardening and softening phenomena were observed in some BMGs’loading-unloading cycle nanoindentation tests,and a“free volume model”of BMG work-hardening was proposed,they believed that the change of external shear stress lead to the change of net free volume in amorphous structure,and then the variation in hardness of the material caused by the shear band behavior of plastic deformation micro-zone.Chen et al.excluded structural inhomogeneities of amorphous Cu 50 Zr 50 ribbons,detected in situ nanocrystallization behavior at shear bands of bent Cu 50 Zr 50 ribbons,and proposed the strain hardening mechanism associated with deformation induced nanocrystallization based on the experimental observation of strong interactions between shear bands and nanocrystallites.These works enriched the basic principles and developed the related research method of BMGs work-hardening.This paper briefly introduces the methods/parameters commonly used to evaluate the work-hardening capability of metal materials,and outlines the shear bands behaviors in BMGs.On the basis of this,the work-hardening behaviors observed in typical BMGs are analyzed and the possible work-hardening origin mechanisms of BMGs are discussed,which could be the reference for studying the mechanical behavior of BMGs and developing plastic BMGs as structural materials with excellent performance.