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非晶合金结构——性能关联的原子尺度研究
Alternative TitleAtomic-level structure-property relationships of amorphous alloys
魏丹
Thesis Advisor魏炳忱
2019-06-02
Degree Grantor中国科学院大学
Place of Conferral北京
Subtype博士
Degree Discipline一般力学与力学基础
Keyword非晶合金,结构指标,结构-性能关系,激活能,塑性
Abstract

相比于晶体材料,非晶材料中没有明显的晶格和易于辨识的结构缺陷,这导致从结构角度理解非晶合金的性能十分困难。这个困难甚至对凝聚态物理和材料科学领域一直以来的准则——“结构决定性质”,提出挑战。尽管大量的研究尝试去寻找有效的微观结构特征描述非晶合金的性质,但结果并不尽人意。非晶物质神秘的结构-性能关联至今仍是固体力学、凝聚态物理、材料科学领域悬而未决的基本问题之一。本文旨在揭示无序材料结构-性能关联背后的物理机制,采用经典分子动力学和跨时间尺度模拟方法,对该问题开展了系统研究,并取得若干创新性进展。

首先,我们通过Activation-relaxation technique nouveau (ARTn)跨时间尺度分子动力学算法,获得非晶合金体系的原子尺度动力学性质——单原子激活能,并分别尝试将激活能与原子的多种简单短程序和振动均方位移关联,发现最近邻短程结构指标并不能预测体系的微观动力学性质,而振动均方位移这种动力学响应与单原子激活能之间有非常明显的反关联关系。该关联背后的物理机制为振动均方位移的各向异性代表玻璃结构性能的空间关联,在动力学性质预测方面起着关键作用。

然后,我们通过重访四个文献中提出的有效结构指标,建立它们与原子激活能之间的关联关系,并分别比较这四个结构指标和激活能的空间分布特征,发现它们之间有着十分类似的空间斑图,进一步量化它们的空间自关联长度,揭示了非晶局域激活能和四个结构指标均存在一个共同的空间自关联长度,以此作为非晶合金结构-性能强关联背后隐藏的物理因素。然而,最近邻短程序无法达到这种共同的空间自关联长度,因此不能预测动力学性质。

其次,我们基于广泛使用的非晶物质Voronoi多面体结构分析方法,从数学上定义了两个结构测量指标,即非晶结构的“多样性”与“效用”,作为一套分析非晶态材料结构对预测性能有效性的通用策略。发现非晶结构多样性虽然会随着结构振动受限或者结构能量指标变化,但不足以用来说明特定的结构是这些局部物理性质的起因,从而证明非晶物质的短程结构在预测动力学性质上的局限性。

最后,我们通过改变冷却历史获得拥有不同结构无序度的非晶合金体系,以基于香侬构型熵的序参量来衡量体系的无序程度,同时对这些体系进行非热准静态加载,定义其剪切局域化敏感性,从而将序参量与剪切局域化敏感性建立关联。我们发现,当体系越来越有序时,剪切应变越倾向于集中在一个窄的变形带中(剪切带);相反,当有序度降低,剪切应变空间分布趋于均匀,不易形成剪切带,体系塑性较高。并且,我们建立的这种序与剪切局域化敏感性之间的正相关关系适用于相当广泛的非晶合金体系,从而建立了非晶物质剪切局域化与非晶结构的普适关系。

Other Abstract

It is difficult to understand the unconventional properties of amorphous alloys based on the structure, because they do not have distinct lattices or well-recognized structural defects akin to dislocations in crystals. This challenges the prevailing philosophy of “structure determines property” in materials science and condensed matter physics. Despite extensive research attempting to identify possible structural features as property signatures in glasses, the results are unsatisfactory. The mysterious structure–property relationships constitute one of the most intriguing unsolved problems in the broad community of glassy physics. This dissertation investigates this key scientific issue systematacially by combining classic molecular dynamics and long timescale atomistic modelling.

With the accelerated simulation algorithm—Activation-relaxation technique nouveau (ARTn), we obain the atomic-level dynamic property—single-atom activation energy. The activation energy is related to various simple short-range orders and vibrational mean square displacement of atoms. The results show that there exists a strong correlation between the activation energy and vibrational mean square displacement, but not simple short-range orders. In addition, the local anisotropy of vibrational MSD may play an important role in predicting dynamic property for metallic glasses.

We revisit the correlation between the activation energy and four effective structural descriptors in the literature, and find there exists a strong correlation between the activation energy and the four structure descriptors. Though comparing the spatial characteristics of activation energy with that of structure descriptors, we find that the spatial patterns of the structural descriptors are self-similar, and they also resemble the spatial feature of the activation energy. Further, by quantifying the spatial autocorrelation length of the activation energy and the structural descriptors, a hidden rule for the robustness of structure–property relationships in glasses is established, according to which there exists a critical characteristic correlation length. A simple short-range structure does not work in predicting the property, whereas a complex super structure defined by multiple atoms in the cluster up to the common correlation length may be responsible for the property of glasses.

Two measures of structure are defined, “diversity” and “utility,” based on the frequency of Voronoi polyhedra. We show that the change in diversity associated with selecting Voronoi structures with high localization or low energy, while real, is too weak to support claims that specific structures are the prime cause of these local physical properties. Meanwhile, a set of general strategies for the analysis of structure in amorphous materials and a general approach to assessing the utility of any selected structural description.

We propose a general and straightforward approach to quantitatively correlate order and shear localization susceptibility in amorphous solids by examining the correlation between two newly defined physical parameters, i.e., one is the order parameter extracted from the Shannon information entropy about short-range order structures, and the other is the shear localization susceptibility parameter characterized by the ratio between stress overshot and steady-state flow stress of the athermal quasistatic shear protocol. A universal correlation between order parameter and shear localization susceptibility is therefore established robustly in a CuZr model glass prepared by cooling rates over six orders of magnitudes, which yields glass samples with different level of order and mechanical deformation behaviors. The revealed correlation is further generalized to a wide ranged of glasses.

Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/79122
Collection国家微重力实验室
Recommended Citation
GB/T 7714
魏丹. 非晶合金结构——性能关联的原子尺度研究[D]. 北京. 中国科学院大学,2019.
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