Processing of Bulk Metallic Glass

Bulk metallic glass (BMG) formers are multicomponent alloys that vitrify with remarkable ease during solidification. Technological interest in these materials has been generated by their unique properties, which often surpass those of conventional structural materials. The metastable nature of BMGs, however, has imposed a barrier to broad commercial adoption, particularly where the processing requirements of these alloys conflict with conventional metal processing methods. Research on the crystallization of BMG formers has uncovered novel thermoplastic forming (TPF)‐based processing opportunities. Unique among metal processing methods, TPF utilizes the dramatic softening exhibited by a BMG as it approaches its glass‐transition temperature and decouples the rapid cooling required to form a glass from the forming step. This article reviews crystallization processes in BMG former and summarizes and compares TPF‐based processing methods. Finally, an assessment of scientific and technological advancements required for broader commercial utilization of BMGs will be made.     

Constitutive description of Bulk Metallic Glass composites at high homologous temperatures

Experimental stress–strain responses of La-based in situ Bulk Metallic Glass (BMG) composites within the supercooled liquid region reveal initial post-yield hardening, followed by softening and subsequent strain-hardening. This behavior contrasts with that of monolithic La-based BMGs, which reach a steady stress level after an initial overshoot. XRD analysis of BMG composites shows the formation of intermetallic compounds during compressive deformation. These intermetallic compound formation/interactions are associated with storage of energy in the material and affect the stress–strain response. In this study, an elastic–viscoplastic, three-dimensional, finite deformation constitutive model is also established to describe the behavior of a recently developed La-based in situ BMG (La–Al–Cu–Ni) composite, within the supercooled liquid region, at ambient pressure and a range of strain rates. The constitutive model is incorporated into a finite element program (ABAQUS/Explicit) via a user-defined material subroutine. Numerical predictions are compared with compression test results on BMG composites cast in-house. The comparison shows that the model is able to describe the material behavior observed.     

A Finite-Deformation Constitutive Model of Bulk Metallic Glass Plasticity

A constitutive model of bulk metallic glass (BMG) plasticity is developed which accounts for finitedeformation kinematics, the kinetics of free volume, strain hardening, thermal softening, rate-dependency and non-Newtonian viscosity. The model has been validated against uniaxial compression test data; and against plate bending experiments. The model captures accurately salient aspects of the material behavior including: the viscosity of Vitreloy 1 as a function of temperature and strain rate; the temperature and strain-rate dependence of the equilibrium free-volume concentration; the uniaxial compression stress-strain curves as a function of strain rate and temperature; and the dependence of shear-band spacing on plate thickness. Calculations suggest that, under adiabatic conditions, strain softening and localization in BMGs is due both to an increase in free volume and to the rise in temperature within the band. The calculations also suggest that the shear band spacing in plate-bending specimens is controlled by the stress relaxation in the vicinity of the shear bands.     

Microstructure Evolution of Zr50Cu18Ni17Al10Ti5 Bulk Metallic Glass during Cold-rolling

Zr50Cu18Ni17Al10Ti5 bulk metallic glass has been rolled at room temperature up to 95% in thickness reduction, and the dependence of microstructure on the strain was investigated. With increasing thickness reduction, the full width at half maximum (FWHM) and crystallization enthalpy decrease gradually till 80%, and then increase evidently at 95%. It is revealed that the reversible transition between the ordered and disordered atomic configurations was found in the metallic glass as the deformation proceeds, which is further verified by the high-resolution transmission electron microscopy images. The final microstructure in metallic glass during cold-rolling is the net result of two competing processes between shear-induced disordering and diffusion controlled reordering.     

Preparation of Cu-based Bulk Metallic Glass Matrix Composites

Cu47Ti34Zr11Ni8 bulk metallic glass （BMG） matrix composites containing in situ formed TiC particles and 5-TiCu dendrite phase were developed by copper mold cast. The thermal stability and microstructure of the composites are investigated. Room temperature compression tests reveal that the composite samples exhibit higher fracture strength and distinct plastic strain of 0.2%-0.5%, comparing with that of the corresponding Cu47Ti34Zr11Ni8 monolithic BMG.     

Ce70Ga8.5Cu18.5Ni3块体非晶合金的室温压缩塑性的显著尺寸效应

以新开发的Ce70Ga8.5Cu18.5Ni3块体非晶合金为研究对象,系统研究了试样尺寸对其室温下压缩力学性能的影响。不同直径的非晶合金采用铜模吸铸法获得,采用万能试验机测量不同尺寸样品的室温压缩力学性能,并用扫描电子显微镜观察断裂后样品的端口形貌,此外,还通过差示扫描量热法分析不同样品的热力学性质。结果显示,直径1.5mm非晶柱状样品具有1%的压缩塑性,并伴随有典型的锯齿流变现象。随着非晶柱状样品尺寸的增加,其塑性逐渐消失。断裂后的样品的端口形貌呈现出典型韧窝状形貌,并且韧窝的密度随着样品尺寸的增加而降低。热分析结果表明,不同尺寸的铸态样品的玻璃转变之前都有一个明显的焓回复过程,而且其焓回复放热量和样品尺寸有近似的反比关系。尺寸较小的非晶合金在制备过程中拥有更快的冷却速率,能够保留更多的自由体积,为剪切带形核提供更多的形核点,表现出较好的宏观塑性变形能力。     

Tungsten Fibre Reinforced Zr-Based Bulk Metallic Glass Composites

A Zr-based bulk metallic glass (BMG) alloy with the composition (Zr₅₅Al₁₀Ni₅Cu₃₀)_98.5Si_1.5 was used as the base material to form BMG composites. Tungsten fiber reinforced BMG composites were successfully fabricated by pressure metal infiltration technique, with the volume fraction of the tungsten fiber ranging from 10% to 70%. Microstructure and mechanical properties of the BMG composites were investigated. Tungsten reinforcement significantly increased the material's ductility by changing the compressive failure mode from single shear band propagation to multiple shear bands propagation, and transferring stress from matrix to tungsten fibers.     

钨丝/锆基非晶复合材料研究进展

钨丝/锆基非晶复合材料作为近20年新出现的穿甲工程材料,是一种极有可能替代具有放射性危害的贫铀合金和穿甲威力稍差的钨合金的新兴材料。自从钨丝/锆基非晶复合材料被制备出来以后,各学者先后对材料中钨丝和锆基非晶合金之间的界面接触强度、钨丝/锆基非晶复合材料中钨丝的参数、环境温度、应变率、仿真计算以及穿甲应用等方面进行了研究。研究表明,制备过程中保温温度,钨丝/锆基非晶复合材料中钨丝的排布方式、体积分数、直径,环境温度,应变率等因素都能对钨丝/锆基非晶复合材料的性能产生影响;随着计算机技术的发展,钨丝/锆基非晶复合材料的有限元仿真计算已经从等效模型发展到准细观建模,能够模拟其动静态压缩过程、侵彻过程,仿真结果和试验结果基本吻合;在应用方面,试验研究验证了钨丝/锆基非晶复合材料弹芯在侵彻过程中的"自锐"性能。文章概述了钨丝/锆基非晶复合材料在上述领域的研究进展并展望了该材料的发展方向。     

Composition and size dependent torsion fracture of metallic glasses

The fracture of metallic glasses(MGs)of different compositions and sizes down to micrometers under torsion loading were systematically investigated.Contrary to the flat shear fracture along the circumfer-ential plane as commonly supposed under torsion,we find that the torsion fracture of metallic glasses can deviate from flat shear plane,and the fracture angle is closely dependent on the composition and the size of MG samples.With a conversion method,we show that the torsion fracture of both millimeter-and micrometer-sized MGs can be described by the ellipse fracture criterion as originally proposed for the tension fracture.The deviation from the circumferential shear plane under torsion is further shown to intrinsically relate to the fracture toughness of MGs.The tougher MG tends to have a smaller fracture angle with respect to the maximum shear plane,and vice versa,indicating a correlation between the fracture toughness and pressure/normal stress sensitivity in MGs.Our results provide new insights on the fracture mechanism and are helpful to design and control the deformation and fracture behavior of MGs under torsion loading.     

Crystallization Behavior of Zr_(55)Al_(10)Cu_(30)Ni_5 Bulk Metallic Glass Rolled at Room Temperature

Zr 55 Al 10 Cu 30 Ni 5 bulk metallic glass was rolled up to 95% in thickness reduction at room temperature, and the dependences of microstructure and thermal stability on the strain were investigated. It is revealed that phase transformations do not occur during the rolling, but the split of the crystallization peaks becomes more and more obvious with increasing thickness reduction. Analyses of the radial distribution functions and the pair correlation functions indicate that the rolling has enhanced the sh...     

块体非晶合金韧塑性研究现状

提高室温塑性和断裂韧性是块体非晶合金作为先进结构材料应用亟待解决的关键科学问题,理解应力加载时的室温塑性变形机制是提高其韧塑性的前提。块体非晶合金通过高度局域化的剪切带形成和扩展而产生塑性变形,提高其室温塑性取决于剪切带的均匀化分布程度。研究者们在该领域做了深入细致的研究工作,如喷丸、设计高泊松比的非晶、设计具有微观起伏结构的铸态相分离非晶以及引入晶相增韧等,使块体非晶合金的韧塑性得到有效改善。从第二相韧塑化非晶基复合材料、泊松比判据、尺寸效应、非晶表面涂层增韧、通过预变形预制多重剪切带改善塑性、冷热循环处理抗非晶合金老化等方面,综述了块体非晶合金韧塑化的研究热点,韧塑性判据,控制剪切带形成、扩展和分布的方法,指出获得良好拉伸塑性和断裂韧性仍是不同体系非晶合金的研究目标和重要发展方向,推动着块体非晶合金作为新型功能结构材料的应用和产业化。