金属玻璃中的非晶多形态转变

由于独特的无方向性金属键"无序"密堆而成的原子结构,金属玻璃能够兼具传统玻璃和晶态金属2者的特性,拥有一系列优异的物理、化学和机械性能,被认为具有广阔的应用前景;同时金属玻璃也是研究非平衡无序体系基础科学问题的一个特殊模型,因此获得了广泛的关注。材料相变的研究对于深入理解其原子结构,并实现结构和性能的调控有重要意义。根据过去对非晶多形态的理解和认识,金属玻璃由于其高度致密的结构,在很长时间内被认为不可能具有非晶多形态转变。近年来,随着高压技术与同步辐射X射线原位探测技术的结合,金属玻璃中的非晶多形态转变现象被陆续发现;金属玻璃非晶多形态现象、机理以及伴随相变的各种性能变化得到了较广泛研究。本文简单总结了关于金属玻璃中压力诱导非晶多形态转变研究的已有进展,及其对金属玻璃结构和性能调控的影响。     

深冷处理对6061铝合金冷金属过渡焊接接头组织和力学性能的影响

对6061铝合金板材进行了冷金属过渡(CMT)焊接试验,并对焊接接头进行了不同时间的深冷处理,分析了深冷处理时间对6061铝合金冷金属过渡焊接接头组织和性能的影响。结果表明:深冷处理对改善焊接接头焊缝区性能有显著效果。深冷处理后焊接接头焊缝区有更多的第二相析出,焊接接头的抗拉强度、伸长率和焊缝显微硬度都得到提高,30 h深冷处理后接头的抗拉强度、伸长率和焊缝显微硬度达到最大值,分别为212.4 MPa、12.65%、93.1 HV。     

复合材料深冷处理研究进展

作为一种有效的材料改性工艺,深冷处理引起了越来越广泛的关注。目前在金属合金材料领域的研究报道很多,但现有文献中针对复合材料深冷处理的研究较少,有关机理也不十分清楚。本文结合国内外文献就深冷处理对各种复合材料性能的影响进行了简要介绍。     

金属玻璃结构及其失稳的原子层次研究

金属玻璃无序结构的非均匀性特征给实验研究其原子尺度的结构特性带来了巨大挑战,目前的实验研究手段仍然受限于时空分辨率的不足,很难捕捉到金属玻璃微观结构的局域响应,而计算模拟能够从原子层次上理解非晶结构及其响应规律。但由于元素间相互作用、计算方法和计算能力的限制,用于计算模拟研究的模型体系和真实的金属玻璃材料之间还存在着难以逾越的鸿沟。充分利用和综合现代计算机技术、软件和算法的成果,探索和发展更有效的计算模拟体系应用于金属玻璃计算模拟研究是解决这一困境的可能途径。本文主要综述了近年来我们关于金属玻璃结构与失稳计算模拟研究的重要进展,及其对认识和调控材料性能、优化材料制备方面的影响,并对未来金属玻璃计算模拟研究进行了简要的展望。     

硬质合金深冷处理

本文简要介绍了一种程控深冷处理装置的结构和性能,并介绍了几种不同牌号的硬质合金经深冷处理后性能的变化情况。     

深冷处理对AZ31B镁合金焊接接头疲劳性能的影响

研究了深冷处理对镁合金焊接接头疲劳性能影响。采用TIG焊接AZ31B镁合金,在-160℃、保温12 h深冷工艺下对镁合金焊接接头进行深冷处理。测试未深冷组和深冷组镁合金焊接接头疲劳性能,用最小二乘法拟合实验数据得到S-N曲线,计算2×106循环次数下,未深冷接头疲劳性能为36.42 MPa,深冷接头疲劳性能为41.26 MPa,深冷处理后接头疲劳强度提高13.29%。用盲孔法分别测试未深冷和深冷后垂直焊缝不同位置的残余应力。结果表明:深冷处理后焊接残余应力下降;深冷处理后焊缝晶粒比未深冷焊缝晶粒明显细化。未深冷接头疲劳断口为解理断裂,深冷后接头疲劳断口为韧性断裂,说明深冷处理可提高焊接接头韧性。深冷处理可以提高镁合金焊接接头疲劳性能。     

深冷处理对镐型截齿硬度、耐磨性和冲击性能的影响及其优化

在镐型截齿传统钎焊—热处理工艺中引入深冷处理技术,形成钎焊—热处理—深冷处理新工艺,并以深冷温度和深冷时间为深冷处理主要工艺参数,使用全因子方法设计深冷处理试验方案,测试镐型截齿齿头部位和齿体部位硬度,测试镐型截齿齿体冲击性能,对镐型截齿整体进行截割岩料试验测试耐磨性,以研究深冷处理对镐型截齿硬度、耐磨性和冲击性能的影响规律,并寻求最优深冷处理工艺。结果表明,深冷处理对镐型截齿硬度影响不显著,对冲击性能有轻微不利影响,但显著提高了镐型截齿的耐磨性。镐型截齿综合性能最优时的深冷处理工艺为深冷温度-196 ℃、深冷时间12 h,此时冲击吸收能量仅降低1.2 J,而耐磨性提高41.6%。     

深冷处理对Cu-1.34Ni-1.02Co-0.61Si合金带材组织性能的影响

本文以时效态Cu-1.34Ni-1.02Co-0.61Si合金带材为研究对象,利用深冷处理、拉伸试验、SEM、TEM、显微硬度和导电率的测量等方法,研究了不同深冷处理保温时间对合金带材组织和性能的影响规律。经研究发现:深冷处理能一定程度地细化合金带材的晶粒度,并减少缺陷数量,使得组织结构更加均匀,同时致密度逐渐提高。另外,深冷处理能促进合金带材进一步析出细小的第二相,提高合金带材的力学性能和导电性能。相比于未经深冷处理的合金带材,深冷处理保温48 h后合金带材的显微硬度、抗拉强度、导电率分别提高了约3.13%、2.93%、4.38%,达到264 HV、879 MPa、48.81%IACS。     

外加磁场和深冷处理在铝合金焊接中的应用现状

文中对铝合金焊接在外加磁场和深冷处理方面的研究进展进行了系统的综述，重点分析了外加磁场和深冷处理对铝合金焊接过程中电弧物理和宏观效果影响及其与焊接工艺之间的关系，并从微观相组织、力学性能、形成机理等不同尺度对微观组织方面的研究成果进行了综述和分析。结果表明，外加磁场的引入改变了焊接电弧的形态、熔滴过渡的方式、熔池中熔融金属的流动，在合适的磁场工艺参数下可以得到良好的焊缝组织和形态；深冷处理后铝合金的晶粒发生转动，并且细化，最终可使深冷处理部位的性能更加优异。该文为铝合金焊接在外加磁场和深冷处理方面的研究和应用提供一定的指导。     

磁场深冷处理对60Si2Mn力学性能影响

通过对60Si2Mn钢进行深冷处理及磁场强化处理，分析探讨 了深冷处理及磁场深冷处理对60Si2Mn钢组织、性能的影响及规律。认为：无论深冷、还是磁场强化对材料性能都有一定程度的改善作用。特别是二者同时作用时对材料性能的改善最为有利。     

Local order influences initiation of plastic flow in metallic glass: Effects of alloy composition and sample cooling history

Bulk metallic glasses (MGs) with tunable plasticity and strength have been reported recently. Using Cu–Zr and Cu–Zr–Al MG models, here we illustrate how and why alloy composition and cooling history influence the initial flow behavior in the early stage of plasticity. Starting from Cu₄₆Zr₅₄, either increasing the Cu concentration, or substituting Al for a few percent of Zr, increases the resistance to the initiation of plastic flow, the softening after the local yielding, and the propensity for strain localization. These effects are shown to be intrinsic to the uniform, fully amorphous MGs and rooted in their internal structure. Our quantitative monitoring of the local environment, especially the role of full-icosahedral clusters in shear transformations, identifies the fertile and resistant structural entities controlling deformation. The structural mechanisms have implications for macroscopic plasticity, and the alloy dependence of the MG structure reveals a microscopic origin underlying the varying mechanical properties.     

High-Entropy Metallic Glasses

The high-entropy alloys are defined as solid-solution alloys containing five or more than five principal elements in equal or near-equal atomic percent. The concept of high mixing entropy introduces a new way for developing advanced metallic materials with unique physical and mechanical properties that cannot be achieved by the conventional microalloying approach based on only a single base element. The metallic glass (MG) is the metallic alloy rapidly quenched from the liquid state, and at room temperature it still shows an amorphous liquid-like structure. Bulk MGs represent a particular class of amorphous alloys usually with three or more than three components but based on a single principal element such as Zr, Cu, Ce, and Fe. These materials are very attractive for applications because of their excellent mechanical properties such as ultrahigh (near theoretical) strength, wear resistance, and hardness, and physical properties such as soft magnetic properties. In this article, we review the formation and properties of a series of high-mixing-entropy bulk MGs based on multiple major elements. It is found that the strategy and route for development of the high-entropy alloys can be applied to the development of the MGs with excellent glass-forming ability. The high-mixing-entropy bulk MGs are then loosely defined as metallic glassy alloys containing five or more than five elements in equal or near-equal atomic percent, which have relatively high mixing entropy compared with the conventional MGs based on a single principal element. The formation mechanism, especially the role of the mixing entropy in the formation of the high-entropy MGs, is discussed. The unique physical, mechanical, chemical, and biomedical properties of the high-entropy MGs in comparison with the conventional metallic alloys are introduced. We show that the high-mixing-entropy MGs, along the formation idea and strategy of the high-entropy alloys and based on multiple major elements, might provide a novel approach in search for new MG-forming systems with significances in scientific studies and potential applications.     

Atomistic mechanism of elastic softening in metallic glass under cyclic loading revealed by molecular dynamics simulations

Metallic glasses (MGs) have a great potential for structural applications due to their high strength; however, they soften under cyclic loadings and exhibit low fatigue endurance limits. To understand the softening mechanism, molecular dynamics simulations were carried out to study the Cu₅₀Zr₅₀ MG within the nominal elastic regime, which clearly show that the quasi-static elastic modulus of the MG softens with either the decreasing cyclic frequency or increasing stress amplitude. Through the extensive analysis of the atomic trajectories, we found the complex elastic softening behavior is related to the activation of string-like liquid-like sites and atomic bond breaking in the cyclically deformed amorphous structure. Our current finding provides a quantitative insight into the atomistic mechanism of damage in MGs under cyclic loadings, also shedding light on the important mechanisms for fatigue damage initiation in amorphous solids.     

Fractal growth of the dense-packing phase in annealed metallic glass imaged by high-resolution atomic force microscopy

Unlike crystalline metals, which have a well-understood periodical structure, the amorphous structure of metallic glasses (MGs) is still poorly understood, particularly when such a structure rearranges itself at the nanoscale under external agitations. In this article, we provide compelling evidence obtained from a recently developed high-resolution atomic force microscopy (HRAFM) technique that reveals the nanoscale structural heterogeneity after thermal annealing in a Zr–Ni metallic glass. Through the HRAFM technique, we are able to uncover the annealing-induced fractal growth of the dense-packing phases in the binary MG thin film, which exhibits a fractal dimension of ～1.7, in line with a two-dimensional diffusion limited aggregation process. The current findings not only reveal the evolution process of atomic packing in the annealed MG thin film, but also shed light on the possible cooling rate effect on the atomic structure of MGs.     

Flow units perspective on sensitivity and reliability of metallic glass properties

The metastability of metallic glasses (MGs) can be well described by the Weibull distribution function. We study the relationship between flow units and mechanical properties and find that the concentration of flow units also follows the Weibull distribution function as do other properties of MG. The results suggest that the properties' sensitivity and metastability of MGs described by the Weibull distribution function can be explained by the structural heterogeneity (or flow unit) perspective and attributed to the variation of the flow units.     

Correlation between the elastic modulus and the intrinsic plastic behavior of metallic glasses: The roles of atomic configuration and alloy composition

Recent reports suggest that Poisson’s ratio (ν), or the related ratio of shear modulus G to bulk modulus B, indicates the potential of metallic glasses (MGs) to sustain plastic strain. Using molecular dynamics simulations of the Cu₆₄Zr₃₆ MG as a representative, we demonstrate why and how these elastic and plastic properties are correlated, in terms of the common structural origin underlying these mechanical behaviors in MGs. The full icosahedral ordering has been identified as the key structural feature in the Cu–Zr MGs that controls not only the G and the G/B (or ν), but also the initiation of shear localization and the intrinsic plasticity. Additional analysis of the Cu–Zr MGs of different compositions and MGs in different alloy systems reveals a general correlation of the plasticity with the G/B ratio, as the latter is able to represent and couple the effects of both the atomic configuration and the alloy composition.     

Atomic Structure of Cu_(49)Hf_(42)Al_9 Metallic Glass with High Glass-Forming Ability and Plasticity

Electron diffraction was used to study the local atomic structure of Cu_(49)Hf_(42)Al_9 metallic glasses(MGs).The amorphous nature of the MG was fully retained after the compression test.The partial radial distribution functions(PRDFs)of the MG structure obtained from the atomic model using reverse Monte Carlo and density functional theory optimization display that the peaks of the first nearest-neighbour distances for Cu-Cu,Hf-Cu and Hf-Hf atomic pairs were located at 2.56 A,2.78 A and 3.23 A,respectively.The wide distribution of PRDF for Hf-Hf atomic pair explained the high plasticity of the material.     

A fluctuation electron microscopy study of metallic glasses

采用fluctuation electron microscopy (FEM)对Al85Ni5Y 10-xCo x (x=0, 2)和 Cu 46Zr 54-x Al x (x=0, 7)金属玻璃的微观结构进行了表征, 研究了合金成分变化对金属玻璃微观结构的影响. 结果表明, 2种金属玻璃体系中均存在较强的中程有序结构; 少量元素的添加提高了合金微观结构的均匀性. 金属玻璃中程有序结构均匀性的增加有利于改善合金的玻璃形成能力.     

深冷处理对3Cr13组织和力学性能的影响

利用SLX-30程序控制深冷箱、金相显微镜、扫描电镜和洛氏硬度计,研究了刀剪材料3Cr13钢经普通热处理、深冷处理后的微观组织及力学性能变化。结果表明,与普通热处理相比.深冷处理后钢中析出的碳化物数量明显增加,分布更加均匀弥散;深冷处理后洛氏硬度值较普通热处理增加了5.3 HRC,冲击韧度增加了0.8 J。     

Effect of cryogenic and aging treatments on low-energy impact behaviour of Ti–6Al–4V alloy

The objective of this study is to examine the effects of cryogenic and aging treatments on the impact strength and mechanical properties of Ti–6Al–4V alloy. To accomplish that objective, cryogenic treatment (CT), aging treatment (AT) and cryogenic treatment followed by aging treatment (CAT) were conducted on Ti–6Al–4V alloy. Impact tests were performed on heat-treated and untreated samples using different impactor nose geometries (hemispherical, 60° and 90° conical) to determine the effect of impactor nose geometry on the damage characteristic. The findings showed that energy absorption increased and areas of damage decreased as a result of heat treatment in all treated samples. The highest energy absorption was observed in the CAT samples, due to the increase in energy absorption, the smallest damaged area occurred in the CAT sample, and the largest deformation was seen in the untreated samples. Additionally, it was seen that the damaged area and deflection were strongly dependent on impactor nose geometry. The maximum deflection and narrowest deformation area were seen with 60° conical nose geometry. The deformation area increased with increasing impactor nose angle.