Introducing a strain-hardening capability to improve the ductility of bulk metallic glasses via severe plastic deformation

The great technological potential for bulk metallic glasses (BMGs) arises primarily because of their superior mechanical properties. To realize this potential, it is essential to overcome the severe ductility limitations of BMGs which are generally attributed to shear localization and strain softening. Despite much international effort, progress in improving the ductility of BMGs has been limited to certain alloys with specific compositions. Here, we report that severe plastic deformation of a quasi-constrained volume, which prevents brittle materials from fracture during the plastic deformation, can be used to induce strain hardening and to reduce shear localization in BMGs, thereby giving a significant enhancement in their ductility. Structural characterizations reveal the increased free volume and nanoscale heterogeneity induced by severe plastic deformation are responsible for the improved ductility. This finding opens a new and important pathway towards enhanced ductility of BMGs.     

A universal relation for edge chipping from sharp contacts in brittle materials: A simple means of toughness evaluation

An analysis of chipping fracture in brittle solids is presented. Cracks are introduced into the edges of selected materials, including soda-lime glass and fine-grain ceramics, using a Vickers indenter in monotonic loading. The ensuing chip morphology is examined and shown to exhibit a certain geometrical similarity, independent of material properties. A simple universal relation is derived for the critical chipping load in terms of indent location and material toughness. It is suggested that the method could be used as a simple and quick means for evaluating toughness values for glasses and fine-grain brittle materials.     

Effects of alloying elements on glass formation,mechanical and soft-magnetic properties of Fe-based metallic glasses

Effects of alloying additions on glass formation, mechanical and soft-magnetic properties of Fe-(Si,P,C,B)-based bulk metallic glasses (BMGs) were systemically studied in detail. It was found that the glass-forming ability (GFA) and the optimum doping content strongly depend on the electronegativity of the alloying elements, which are discussed in terms of liquid phase stability and crystallization resistance of the competing crystalline phases. These BMGs exhibit high fracture strength ranging from 2800 to 3800 MPa, which closely relates to the atomic size distribution in the alloys. Furthermore, appropriate additions of Co, Ga and Cu could improve not only the GFA but also the saturation magnetization due to different coupling mechanisms.     

Toughness,extrinsic effects and Poisson’s ratio of bulk metallic glasses

A range of bulk metallic glasses has been cast and their mode II fracture toughness has been estimated from the length scale of shear band vein patterns on fracture surfaces. As-cast rare-earth and Mg-based bulk metallic glasses invariably consist of oxide particles dispersed in a glassy matrix and the apparent brittleness of these alloys is partly extrinsic in nature, caused by these inclusions. The intrinsic toughness of these glasses is higher than previous reports suggest. An attempt has been made to correlate the toughness of a variety of glassy alloys, including La- and Mg-based systems, with their Poisson’s ratio (ν). The findings show that mode II toughness increases with ν, though gradually, instead of an abrupt transition occurring at a critical value of ν. Certain glassy alloys are profoundly brittle, irrespective of ν, and this seems to be related to alloy chemistry.     

预应变下高强结构钢低温断裂性能

为了测试高强结构钢Q420力学性能和断裂韧度,对其原材料和塑性变形材料分别进行了不同温度下的拉伸试验和断裂韧性试验.结果表明,结构钢的断裂韧性随着温度的降低显著减小,使其倾向于脆性断裂;而预应变虽然提高了钢材的屈服强度与抗拉强度,但显著降低了钢材的塑性及断裂韧性,进一步增加了脆性断裂发生的可能性.同时利用有限元分析得出...     

高强度变形铝合金断裂韧度各向异性的机理和预测

本文运用断裂力学的基本理论和结果,结合材料的物理开裂机制,定量分析较弱晶粒界面、裂纹扩展途径偏斜和沿晶分层三种因素对高强度变形铝合金断裂韧度各向异性的影响,预测结果与大量实验数据都比较接近,说明合金强烈的断裂韧度各向异性主要来自以上三种机制,并可根据实际断裂机理可以预估合金半成品的短横向断裂韧度值。     

MECHANISMS AND PREDICTION OF FRACTURE TOUGHNESS ANISOTROPY OF HIGH STRENGTH Al ALLOYS

Contributions of weak grain boundary,cracking path deflection and grain boundarydelamination to fracture toughness anisotropy of high strength Al alloys were evaluated basedupon approaches of fracture mechanics in conjunction with physical cracking mechanisms.The predicted results are close to those experimentally determined in the literature and in thiswork.The strong anisotropy of fracture toughness of high strength Al alloys is therefore attri-buted mainly to weak grain boundary cracking,cracking path deflection and grain boundarydetamination.With the methods of this work,short-transverse fracture toughness values ofsome semi-products can he estimated from in-plane toughness values and corresponding frac-ture characteristics when it is difficult to be determined experimentally.     

Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.     

Mechanical response of metallic glasses: Insights from in-situ high energy X-ray diffraction

The term “metallic glass” usually refers to a metallic alloy rapidly quenched in order to “freeze” its structure from the liquid state. A metallic glass is a metastable alloy, which lacks the symmetry typical for crystalline materials and at room temperature shows an amorphous liquid-like structure. Bulk metallic glasses (BMGs) represent a class of amorphous alloys. The most notable property of BMGs is their ultrahigh (near theoretical) strength and hardness. Because the known BMGs usually miss tensile plasticity and thus exhibit catastrophic failure upon tension it is important to understand deformation mechanisms involved and thus improve their performance. This aricle analyzes the use of synchrotron radiation for evaluating the elastic-plastic response of such materials.     

Progress in studying the fatigue behavior of Zr-based bulk-metallic glasses and their composites

Zr-based amorphous alloys with a high glass-forming ability and thermal stability were discovered in 1990. In the following years, the alloy design increased the critical casting thickness to several centimeters, and a homogeneous dispersion of nanoscale particles was found to improve the ductility. Therefore, Zr-based bulk-metallic glasses (BMGs) are being studied widely because of their potential as structural materials. The fatigue behavior is an important characteristic of structural materials. The current review documents the fatigue studies of Zr-based BMGs and their composites. The fatigue characteristics of these alloys in different loading conditions and environments are summarized and compared in this paper. The factors affecting the fatigue behavior of the Zr-based BMGs and their composites are discussed. The mechanisms of fatigue-crack initiation and propagation in BMGs are addressed in this review. In order to broaden the scope of applications of BMGs, a fundamental understanding of the fatigue behavior is important for the design of new alloy systems and the development of the processing techniques.     

Bending-fatigue behavior of bulk metallic glasses and their composites

Amorphous alloys with high glass-forming ability and thermal stability were discovered in the 1990s. In the following years, the alloy design increased the critical casting thickness to several centimeters, and a homogeneous dispersion of nanoscale particles was found to improve the ductility. Therefore, bulk metallic glasses (BMGs) are being studied widely because of their potential as structural materials. The fatigue behavior is an important characteristic of structural materials. The current review documents the bending-fatigue studies of BMGs and their composites. The fatigue characteristics of these alloys in different loading conditions and environments are summarized and compared in this paper. The factors affecting the fatigue behavior of BMGs and their composites are discussed. The mechanisms of fatigue-crack initiation and propagation in BMGs are addressed in this review. In order to broaden the scope of applications of BMGs, a fundamental understanding of the fatigue behavior is critical for the design of new alloy systems and the development of the processing techniques.     

Glass forming ability and mechanical properties characterization on Mg58Cu31Y11−xGdx bulk metallic glasses

Mg–Cu–Y–(Gd) alloy rods are made by arc-melting and injection casting methods in this research. The improvement of glass forming ability and mechanical properties by using Gd to substitute Y in Mg₅₈Cu₃₁Y₁₁ bulk metallic glasses (BMGs) is of interest. The results of thermal analysis present that the Mg–Cu–Y base alloys with the addition of 6 and 8 at% Gd are the best BMG former. The Vickers indentation tests and the compression tests are carried out in order to explore the mechanical properties of alloys. It reveals that there is no obvious change in Young's modulus (45 GPa) of the Gd-containing Mg-based BMG, in contrast with the base alloys. Vickers (micro-) indentation fracture toughness measurements are performed for comparison. Shear bands and the corner cracks around the inverted pyramind mark are showed. An average fracture toughness of Mg–Cu–Y–Gd alloy is calculated as 4 MPa m^1/2, which is a little higher than that of base alloys studied in the paper. Meanwhile, the fracture surface of Mg-based BMGs is dominant by featureless mirror-like and river-like pattern. Only nano-scaled shear bands and vein patterns are displayed, indicating that the plasticity of the Mg–Cu–Y–Gd BMGs are shown in nano-scale indeed.     

Design of Bulk metallic glasses with high glass forming ability and enhancement of plasticity in metallic glass matrix composites: A review

To overcome some of the limits of existing metallic alloys, a new alloy design concept has been introduced recently in order to control the crystallinity, i.e. to utilize crystalline, quasicrystalline, and amorphous structures. In particular, bulk metallic glasses (BMGs) receive great attention because of their unique properties due to their different atomic configuration. Recently, significant progress in enhancing glass forming ability (GFA) has led to the fabrication of BMGs having potential for application as structural and functional materials. Moreover, successful design of BMG matrix composite microstructure suggests that the plasticity of BMGs can be controlled properly. In this review article, we introduce recent research results on the design of BMGs with high GFA and on the enhancement of plasticity in metallic glass matrix composites.     

Crack-resistance curve of a Zr–Ti–Cu–Al bulk metallic glass with extraordinary fracture toughness

For the emerging bulk metallic glasses (BMGs), damage tolerance is a key mechanical property needed for their practical applications. To reach a fracture toughness on a par with, or even better than, conventional engineering alloys, the only route reported so far is to compositionally base the BMG on high-cost palladium (Pd), which has a very high Poisson’s ratio (～0.42). Here we report the discovery of a Zr₆₁Ti₂Cu₂₅Al₁₂ (ZT1) BMG that has a toughness as high as the Pd-based BMG, but at the same time consists of common engineering metals and has robust glass-forming ability. The new BMG, while having an unimpressive Poisson’s ratio of 0.367, derives its high toughness from its high propensity for crack deflection and local loading-mode change at the crack tip due to extensive shear band interactions. The crack-resistance curve (R-curve) of this BMG has been obtained from fatigue pre-crack samples, employing standard “single-specimen” and “multiple-specimen” techniques.     

Cu46Zr46Al8非晶合金电子束焊接特性分析

对Cu₄₆Zr₄₆Al₈非晶合金进行了电子束焊接,并分析了接头微观组织转变、显微硬度分布及拉伸性能.结果表明,Cu₄₆Zr₄₆Al₈非晶合金电子束焊接接头熔化区组织大部分仍为非晶态,过冷液相区内发生晶化形成Cu-Zr金属间化合物.焊接接头熔化区与母材硬度相当,过冷液相区硬度值显著降低.接头抗拉强度及韧性相比母材都明显降低,拉伸断裂于过冷液相区内的脆性化合物层,呈现典型的沿晶脆性断裂特征.     

Correlations between the relaxed excess free volume and the plasticity in Zr-based bulk metallic glasses

Zr-based bulk metallic glasses (BMGs) with Nb minor alloying have been fabricated with different free volume (FV) trapped in. FV is evaluated by the relaxed excess free volume (REFV) after annealing just below T_g through loop thermal expansion tests. The results show that there is a qualitative correlation between the plasticity and REFV in Zr-based BMGs. The larger amount of excess FV the BMGs relax, the better plasticity they exhibit. With 1.5% Nb addition, the brittle Zr₆₅Cu₁₅Ni₁₀Al₁₀ BMGs possess REFV up to about 0.428% and exhibit the relatively good plasticity up to 25.6%. This provides a promising way to estimate the plasticity of BMGs and design new ductile BMGs through the minor alloying.     

V4Cr4Ti合金和RAFM钢的热等静压扩散连接及其界面特性

低活化铁素体/马氏体(reduced activation ferritic/martensitic,RAFM)钢及钒合金被认为是未来核聚变反应堆第一壁的候选结构材料,性能各有优劣,可满足近中期应用要求. 采用热等静压技术在温度800 ℃、等静压压强150 MPa和保温时间2 h下实现V4Cr4Ti合金和CLF-1钢的固态扩散连接,对其界面微观组织、元素扩散特征以及抗剪强度进行了分析. 结果表明,CLF-1钢在距离连接界面120 μm区域内出现脱碳层,而V4Cr4Ti合金侧存在宽度约1.5 μm的高硬脆碳化物层;V4Cr4Ti合金/CLF-1钢连接界面无缺陷,接头室温抗剪强度最高达238 MPa. 断口分析表明,断裂发生于靠近V4Cr4Ti合金侧的高硬脆碳化物层,断口表现出整体韧性,局部脆性断裂的特征.     

Improving the strength and the toughness of Mg–Cu–(Y,Gd) bulk metallic glass by minor addition of Nb

Mg₆₅Cu₂₅Re₁₀ (Re = Y, Gd) and Mg₆₄Cu₂₅Nb₁Re₁₀ (Re = Y, Gd) bulk metallic glasses (BMGs) have been fabricated by copper mould casting. It is shown that the minor addition of Nb can not only increase the thermal stability but also improve the fracture strength and the toughness of the Mg-based BMG alloys greatly. The fracture strength of Mg₆₄Cu₂₅Nb₁Y₁₀ and Mg₆₄Cu₂₅Nb₁Gd₁₀ reaches as high as 1023 MPa and 973 MPa, respectively. The Young's modulus has also been increased by the addition of Nb. From the fracture morphologies of the Nb bearing Mg-based BMG alloys, it is known that the size of plastic deformation zone can be increased to micrometer scale. This work proves that it is possible to improve the strength and toughness of Mg-based BMG by adding an element having positive heat of mixing with the constituent elements.     

非晶及纳米结构钛合金研究进展

综述了钛基大块非晶的成分-非晶形成能力的对应关系，总结了已知的钛基大块非晶的一次结晶相及力学性能，并阐述了钛基大块非晶的塑性变形机制，以及造成低塑性的因素和改善塑性的方法，着重介绍了铸造纳米结构钛合金的成分、组织和力学性能。通过精巧的成分设计，结合金属模铸造技术，制备出了直径小于5mm，并具有微米尺度β-Ti（M）一次树枝晶相＋纳米尺度棒状β-Ti／γ-TiCu共晶结构的复合组织，其中棒状β-Ti相尺寸在30nm～300nm范围；γ-TiCu共晶基体相由小于10nm的晶粒或亚晶粒组成，这种多层次纳米结构的钛合金具有高强度，同时县有明显的延伸性。     

超高强钴基块体非晶合金的组成设计

块体非晶合金是一类具有高强度、高硬度和大弹性极限的无序金属材料,其优异的力学性能是目前先进金属材料领域研究热点之一,如何提高材料的强度是材料研究领域永恒的主题。系统地总结了已知具有超高强度的一类块体非晶合金材料——钴基块体非晶合金的成分、热学稳定性及力学性能;同时研究了不同非晶合金的断裂强度与其弹性常数、硬度和特征温度的关联。研究结果表明：在非晶合金体系中杨氏模量、维氏硬度、玻璃转变温度与断裂强度之间都存在较好的线性变化关系。基于以上结果,本课题组提出了超高强钴基块体非晶合金的组成设计方法,即选取具有强共价键特性的非金属元素和高模量、高熔点过渡金属元素与钴元素进行组合。