Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

"Work-Hardenable" ductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.     

A non-viscous-featured fractograph in metallic glasses

A fractograph of non-viscous feature but pure shear-offsets was found in three-point bending samples of a ductile Pd–Cu–Si metallic glass. A sustainable shear band multiplication with large plasticity during notch propagation was observed. Such non-viscous-featured fractograph was formed by a crack propagation manner of continual multiple shear bands formation in front of the crack-tip, instead of the conventional rapid fracture along shear bands. With a 2D model of crack propagation by multiple shear bands, we showed that such fracture process was achieved by a faster stress relaxation than shear-softening effect in the sample. This study confirmed that the viscous fracture along shear bands could be not a necessary process in ductile metallic glasses fracture, and could provide new ways to understand the plasticity in the shear-softened metallic glasses.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

New class of plastic bulk metallic glass

An intrinsic plastic Cu(45)Zr(46)Al(7)Ti(2) bulk metallic glass (BMG) with high strength and superior compressive plastic strain of up to 32.5% was successfully fabricated by copper mold casting. The superior compressive plastic strain was attributed to a large amount of randomly distributed free volume induced by Ti minor alloying, which results in extensive shear band formation, branching, interaction and self-healing of minor cracks. The mechanism of plasticity presented here suggests that the creation of a large amount of free volume in BMGs by minor alloying or other methods might be a promising new way to enhance the plasticity of BMGs.     

Shear and distensile fracture behaviour of Ti-based composites with ductile dendrites

The room-temperature deformation and fracture behaviour of Ti-based composites with ductile dendrites, prepared by copper mold casting and arc-melting techniques, was investigated. Under compressive loading, the Ti-based composites display high fracture strength (about 2000?MPa) and good ductility (about 4 or 10%). The yield strength of the Ti-based composites is relatively low (about 565–923?MPa). However, they have a large strain-hardening ability before failure, due to the interactions between shear bands and dendrites. For the arc-melted Ti-based composites, fracture often occurs in a shear mode with a high plasticity (about 10%). In contrast, the cast Ti-based composites break or split into several parts with a compressive plasticity of 4%, rather than failing in a shear mode. A new fracture mechanism, i.e. distensile fracture, is proposed for the first time to elucidate the failure of the as-cast Ti-based composites. Based on the difference in the fracture modes of the differently prepared composites, the relationships between shear and distensile fracture mechanisms and the corresponding fracture criteria are discussed.     

亚微米尺度金属玻璃的拉伸塑性

文章介绍了在透射电镜中原位拉伸亚微米尺度单相锆基金属玻璃的实验设计和研究进展.研究表明,金属玻璃呈现明显的样品尺寸效应,微观尺度样品不仅表现出稳定可控的形变行为,而且具有良好的拉伸塑性.小尺寸金属玻璃具有良好拉伸塑性的发现,不仅有助于深入理解金属玻璃室温形变的本质,也揭示了金属玻璃在薄膜和微器件中的潜在应用价值.     

块体非晶合金的韧塑化

块体非晶合金因其独特的原子结构而具有许多优异的力学性能,成为近年来材料领域的研究热点之一,但是由于其在变形过程中的室温脆性和应变软化等关键问题一直制约着其实际工程应用.为解决此问题,块体非晶合金领域的研究者们提出了多种方案,包括通过在非晶合金中调控其内禀特性如弹性常数、结构不均匀性,通过外加手段改变其应力及缺陷状态,通过外加和内生的方法在非晶基体中引入晶态增强相等方式,获得了一系列力学性能优异的块体非晶合金及其复合材料.特别是利用"相变诱导塑性"(transformation-induced plasticity,TRIP)概念研制出的块体非晶合金复合材料,同时具有大的拉伸塑性和加工硬化能力.本文围绕块体非晶合金的韧塑化这个关键科学问题,对单相非晶及非晶复合材料的韧塑化方案及机理进行了综述,着重介绍了TRIP韧塑化块体非晶合金复合材料的制备、性能、组织调控及韧塑化机理等,并对此领域的未来发展进行了展望.     

Softening caused by profuse shear banding in a bulk metallic glass

By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.     

Strength,plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects

To investigate the flaw sensitivity and reliability of bulk metallic glasses (BMGs), compressive testing was performed on a statistically significant number of specimens. Despite the fact that BMGs exhibit little or no macroscopic plasticity before failure (similar to other brittle materials), we observe surprisingly high uniformity in compressive strength. Weibull analysis was employed to study the statistical dispersion in strength, giving very high Weibull moduli of about 25 for an intrinsically brittle glass, and near 75 for an intrinsically malleable one. This high uniformity is encouraging for the use of BMGs in structural applications. Furthermore, we illustrate that subtle imperfections in the test geometry (i.e. miscut or deviations from orthogonality) dramatically affect the compression response. In brittle glasses these act as failure-critical flaws, whereas in malleable glasses they constrain shear bands, lead to tilting and bending during testing, and give rise to misleading macroscopic measurements of plastic deformation.     

A New Cu-Based Metallic Glass Composite with Excellent Mechanical Properties

A new Cu-based bulk metallic glass composite of nominal composition(at.%) Cu_(41)Ni_(27)Ti_(25)Al_7 with excellent plasticity and a strong work-hardening behavior is fabricated. Strength above 1859 MPa and plasticity more than 11% are achieved under compression and tension modes. The deformation mechanism is proposed to the structural heterogeneities of the composite that promotes multiple shear bands meanwhile inhibits their free propagation,which results in the macroscopically plastic strain and work hardening. The alloy contains relatively cheap metals and has a low cost, which is beneficial to industrial applications.     

Improvement of Plasticity of Ti-Based Bulk Metallic Glasses by Addition of Nb

Ti-based bulk metallic glasses （Ti40Zr25Cu9NisBe18 ）100-x Nbx with x = 0 to 5 at. % are prepared by copper-mold casting. The glass formation ability is almost unchanged by addition of Nb. The compression plasticity is, however, apparently changed, from 3% at x = 0 to 13% at x = 3, about 330% increases at the strain rate of 1 × 10^-4 8^-1. The increment of the plasticity can be attributed to the segregation of Nb in the area of shear bands during the compression processing. An effective way to increase the plasticity of Ti-based bulk metallic glasses is thus proposed.     

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.     

Bridging room-temperature and high-temperature plasticity in decagonal Al–Ni–Co quasicrystals by micro-thermomechanical testing

Ever since quasicrystals were first discovered, they have been found to possess many unusual and useful properties. A long-standing problem, however, significantly impedes their practical usage: steady-state plastic deformation has only been found at high temperatures or under confining hydrostatic pressures. At low and intermediate temperatures, they are very brittle, suffer from low ductility and formability and, consequently, their deformation mechanisms are still not clear. Here, we systematically study the deformation behaviour of decagonal Al–Ni–Co quasicrystals using a micro-thermomechanical technique over a range of temperatures (25–500 °C), strain rates and sample sizes accompanying microstructural analysis. We demonstrate three temperature regimes for the quasicrystal plasticity: at room temperature, cracking controls deformation; at 100–300 °C, dislocation activities control the plastic deformation exhibiting serrated flows and a constant flow stress; at 400–500 °C, diffusion enhances the plasticity showing homogenous deformation. The micrometer-sized quasicrystals exhibit both high strengths of ~2.5–3.5 GPa and enhanced ductility of over 15% strains between 100 and 500 °C. This study improves understanding of quasicrystal plasticity in their low- and intermediate-temperature regimes, which was poorly understood before, and sheds light on their applications as small-sized structural materials.     

High-strength and ductile glassy-crystal Ni–Cu–Zr–Ti composite exhibiting stress-induced martensitic transformation

We present a Ni-based crystal-glassy composite material having superior strength paired with a considerable ductility of 15%. The formation of a metastable crystalline phase in a glassy matrix during solidification has been proven capable of promoting a strain-induced martensitic transformation leading to enhanced plasticity under compression at room temperature. Underlying mechanisms of plastic deformation are discussed in terms of the interplay between dislocation slip in the crystalline phase and shear deformation in the glassy matrix. We suppose that the strain-induced martensitic inclusions serve as strong barriers for shear band propagation, promoting shear band branching and multiple shear band formation, thus extending the ductility and preventing a premature brittle fracture. The acoustic emission technique has been employed to clarify the kinetics of transformation and stages of plastic deformation.     

Multiple shear banding in a computational amorphous alloy model

The strain localized phenomenon, so called shear bands (SBs), in an amorphous alloy have received a lot of attention in recent years. In this study, we microscopically investigated the nature and dynamics of multiple SBs using molecular dynamics model. In the SB region, intense shear-induced structural change occurred, typified by the annihilation of pentagonal short-range order, and significant localized heating accompanied with the SB propagation was observed. Moreover, a large number of fine SBs operated simultaneously at a high strain rate, whereas, only a few SBs appeared and propagated abruptly at a low strain rate. These results were discussed with respect to brittle/ductile deformation of bulk metallic glasses. PACS 31.15.xv; 62.20.F-; 81.05.Kf     

Dual self-organised shear banding behaviours and enhanced ductility in phase separating Zr-based bulk metallic glasses

The multiplication and interaction of self-organised shear bands often transform to a stick-slip behaviour of a major shear band along the primary shear plane, and ultimately the major shear band becomes runaway and terminates the plasticity of bulk metallic glasses (BMGs). Here, we examined the deformation behaviours of the nanoscale phase-separating Zr_65–xCu₂₅Al₁₀Fe_x (x = 5 and 7.5 at.%) BMGs. The formation of multi-step phase separation, being mainly governed by nucleation and growth, results in the microstructural inhomogeneity on a wide range of length-scales and leads to obviously macroscopic and repeatable ductility. The good deformability can be attributed to two mechanisms for stabilizing shear banding process, i.e. the mutual interaction of multiple shear bands away from the major shear band and the delaying slip-to-failure of dense fine shear bands around the major shear band, both of which show a self-organised criticality yet with different power-law exponents. The two mechanisms could come into effect in the intermediate (stable) and later plastic deformation regime, respectively. Our findings provide a possibility to enhance the shear banding stability over the whole plastic deformation through a proper design of microstructure heterogeneities.     

双层A3钢靶对平头杆弹的抗侵彻性能研究

 钢板被广泛用于构建防护结构,大量文献报道了单层金属靶的防护性能,而对双层金属靶,特别是大间隙双层金属靶,报道的却很少。在轻气炮上进行了平头杆弹体正撞击由两层5 mm厚A3钢板组成的接触式和具有200 mm间隙的间隙式双层靶的实验研究,得到了两种结构的初始-剩余速度曲线。实验表明：（1）两种形式双层靶均发生了充塞剪切;（2）接触式双层靶的弹道极限是5 mm单层A3靶的1.92倍;（3） 间隙式双层靶的抗侵彻性能具有较大的分散性,通过高速摄像和对回收靶板的分析表明,该分散性产生的原因是,弹体贯穿第一层靶后存在两种典型弹道状态;（4）间隙式双层靶存在两个弹道极限;（5）接触式双层靶的弹道极限接近或者大于间隙式双层靶的弹道极限。使用ABAQUS/EXPLICIT有限元软件进行了相应的数值模拟,得到了与实验一致的现象和结果。     

Local plasticity of Al thin films as revealed by x-ray microdiffraction

Grain-to-grain interactions dominate the plasticity of Al thin films and establish effective length scales smaller than the grain size. We have measured large strain distributions and their changes under plastic strain in 1.5-microm-thick Al 0.5% Cu films using a 0.8-microm-diameter white x-ray probe at the Advanced Light Source. Strain distributions arise not only from the distribution of grain sizes and orientation, but also from the differences in grain shape and from stress environment. Multiple active glide plane domains have been found within single grains. Large grains behave like multiple smaller grains even before a dislocation substructure can evolve.     

Microstructures and Mechanical Properties of AlCrFeNiMo_(0.5)Ti_x High Entropy Alloys

Effects of Ti addition on the microstructures and mechanical properties of AlCrFeNiMo_(0.5) Ti_x(X = 0, 0.25, 0.4,0.5, 0.6, 0.75) high entropy alloys(HEAs) are investigated. All these HEAs of various Ti contents possess dual BCC structures, indicating that Ti addition does not induce the formation of any new phase in these alloys. As Ti addition X varies from 0 to 0.75, the Vickers hardness(HV) of the alloy system increases from 623.7 HV to766.2 HV, whereas the compressive yield stress firstly increases and then decreases with increasing X above 0.5.Meanwhile, the compressive ductility of the alloy system decreases with Ti addition. The AlCrFeNiMo_(0.5)Ti_(0.6) and AlCrFeNiMo_(0.5)Ti_(0.75) HEAs become brittle and fracture with very limited plasticity. In the AlCrFeNiMo_(0.5)Ti_x HEAs, the AlCrFeNiMo_(0.5) HEA possesses the highest compressive fracture strength of 4027 MPa and the largest compressive plastic strain of 27.9%, while the AlCrFeNiMo_(0.5)Ti_(0.5) HEA has the highest compressive yield strength of 2229 MPa and a compressive plastic strain of 10.1%. The combination of high strength and large plasticity of the AlCrFeNiMo_(0.5) Ti_x(X = 0, 0.25, 0.4, 0.5) HEAs demonstrates that this alloy system is very promising for engineering applications.