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.     

Crystallization of Mg–Al and Al-based metallic liquids under ultra-high gravity

We present results from high-temperature centrifugal processing, which is a new method to detect the sequence of crystallization and to physically isolate eutectic compositions in multi-component alloys. Alloys of composition Mg₅₀Al₃₀Y₆Li₇Cu₇, Mg₅₀Al₃₀Y₆Li₇Cu₇, and Al_52.6Cu_13.4Ge₂₈Si₆ were processed for 2 h in a centrifuge above their melting point at an inertial acceleration of 60,000 g (g=gravitational acceleration) and then slowly cooled to room temperature during continuous centrifugation. This processing leads to a pronounced stratification of crystalline phases in all three alloys, with primary phases sedimented to the sample ends, and binary and ternary eutectic microstructures solidified in the central region of the sample, as resolved by scanning electron microscopy. Thus, this new method yields deep eutectic compositions in a straightforward way, which is very useful information for the discovery of new bulk metallic glasses.     

3D printing of crack-free high strength Zr-based bulk metallic glass composite by selective laser melting

3D printing of crack-free bulk metallic glasses remains challenge due to the generation of huge thermal stress during the selective laser melting and their intrinsic brittleness. Herein, Zr₅₅Cu₃₀Ni₅Al₁₀ system was selected and 3D printed by selective laser melting technique. The results indicated that bulk metallic glassy composite comprises a large fraction (about 83%) of amorphous phase and minor fraction of intermetallic compounds with free of cracks were successfully fabricated. The 3D printed metallic glassy composite exhibited high strength over 1500 MPa. Experiment combined with finite-element-method simulation not only revealed the mechanism of crystallization at heat affected zones, but demonstrated that low thermal stress reduce the risk of micro-cracks generation and fracture toughness plays a crucial role in suppression the crack propagation during selective laser melting process.     

Effects of alloy addition on the improvement of glass forming ability and plasticity of Mg–Cu–Tb bulk metallic glass

The effects of alloy addition on the glass forming ability, thermal stability and mechanical properties of Mg–Cu–Tb-based bulk metallic glasses were investigated. It was found that appropriate additions of Ag, Zn or Be in Mg₆₅Cu₂₅Tb₁₀ could not only improve the glass forming ability, but also enhance the strength and plasticity of these amorphous metallic alloys within a certain composition range.     

Forming of bulk metallic glass microcomponents

The present article considers forward extrusion, closed-die forging and backward extrusion processes for fabrication of individual microcomponents from two bulk metallic glass (BMG) compositions: Mg₆₀Cu₃₀Y₁₀ and Zr₄₄Cu₄₀Ag₈Al₈. Two types of tooling were used in the present work: relatively massive die sets characteristic of cold forming operations for crystalline metals and lightweight die sets adapted to the special characteristics of BMGs. In addition to demonstrating that microcomponents of several geometries can be readily fabricated from BMGs, rheological properties are combined with crystallization kinetics to formulate a generally applicable method that can guide selection of optimal forming parameters. Finally, the use of particulate-based lubricants for BMG forming is shown to result in individual lubricant particles becoming mechanically locked into the BMG surface.     

Enthalpy relaxation in Cu46Zr45Al7Y2 and Zr55Cu30Ni5Al10 bulk metallic glasses by differential scanning calorimetry (DSC)

Structural relaxation process in Cu₄₆Zr₄₅Al₇Y₂ and Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses during annealing below the glass transition temperature T_g was investigated by differential scanning calorimetry (DSC). The features of enthalpy relaxation are sensitive to both annealing temperature and annealing time. For a given annealing time t_a, the results indicated that the relaxation time t_a decreases with increasing the annealing temperature T_a, in good agreement with results relative to other bulk metallic glasses. Additionally, the enthalpy relaxation behaviour of the bulk metallic glasses appears independent on the cooling rate used before the physical aging experiments, i.e. on the initial as-cast state. The recovered enthalpy evolution of the bulk metallic glasses is well described by the Kohlrausch–Williams–Watts (KWW) exponential relaxation function as ΔH(T_a) = ΔH_eq{1 ? exp[?(t_a/τ)^β]}. Kohlrausch exponent β and enthalpy relaxation time τ are sensitive to the composition of the bulk metallic glasses. Finally, the influence of different heating treatment processes on the enthalpy relaxation in the bulk metallic glasses is presented and shows that this phenomenon is mainly reversible. The structural relaxation behaviour is interpreted by free volume model and quasi-point defects model. Kinetic fragility parameters m in Cu₄₆Zr₄₅Al₇Y₂ and Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glasses are 72 and 69, respectively, indicating therefore that these alloys are intermediate glasses.Crystallization process was also investigated by DSC experiments. According to the Kissinger model, corresponding activation energy is 3.18 eV in Cu₄₆Zr₄₅Al₇Y₂, and 3.19 eV in Zr₅₅Cu₃₀Ni₅Al₁₀, respectively.     

Bulk metallic glass formation in the Mg–Cu–Zn–Y system

The element Cu in the bulk glass-forming alloy Mg₆₅Cu₂₅Y₁₀ was substituted with the element Zn to form a Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, which caused a significant improvement of the glass-forming ability of Mg₆₅Cu₂₅Y₁₀ alloy. For the Mg₆₅Cu₂₀Zn₅Y₁₀ alloy, fully glassy rod with a 6-mm diameter can be obtained by copper mold casting.     

Cast structure and mechanical properties of Zr–Cu–Ni–Al bulk glassy alloys

Homogeneous melting without any nuclei was performed using the cold copper nozzle arc casting furnace and ladle arc-melt type furnace. Casting of a bulk glassy alloy can be achieved by using a copper nozzle arc casting furnace, which eliminates nucleation site (cold spot) for crystallization. Besides, the pouring molten alloy was melted homogeneously by arc heating before casting into the mold, similar to a pseudo float melting state. To produce a bulk glassy alloy sheet, a combination of the ladle arc-melt type furnace and squeeze cast method was used. Using this method, we succeeded in producing Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk glassy alloys in a rod shape by the former method and in a sheet form by the later method. Tensile strength of the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk glassy alloy sheet is about 1900 MPa and the plasticity of the alloy at room temperature is significantly improved by cold rolling.     

Relaxation of rapidly quenched metallic glasses: Effect of the relaxation state on the slow low temperature dynamics

The relaxation spectrum of rapidly quenched Mg₆₅Cu₂₅Y₁₀ metallic glass ribbons is studied by mechanical spectroscopy at temperatures below and around the glass transition. The comparison between hyper-quenched and relaxed samples is used to examine the origin of the low temperature “excess wing” of internal friction commonly observed in mechanical spectroscopy of metallic glasses. The results show that the excess wing can be attributed to access of the system to the broad α-relaxation process while evidence of secondary relaxations is not found. This suggests that in this glassy system the activation energies of structural relaxation and low temperature deformation are directly related to the activation energy of the main relaxation process of the glassy state.     

Fe-Co-Ni-Cr-Mo-C-B-Y系大块金属玻璃的热稳定性、晶化行为、维氏硬度和磁性能

研究Fe41Co7-xNixCr15Mo14C15B6Y2(x=0,1,3,5)大块金属玻璃的热稳定性、晶化行为、维氏硬度和磁性能.通过铜模铸造法制备Fe41Co7-xNixCr15Mo14C15B6Y2(x=0,1,3,5)大块金属玻璃.利用差示扫描量热法和等温热处理法研究这些金属玻璃的热稳定性和晶化行为.在室温下利用维氏硬度计测量试样经过不同温度和时间退火后的硬度,并对它们的磁学性质进行表征.实验结果表明,少量Ni元素的加入没有增大过冷液相区间和玻璃形成能力,但是改变合金的初始晶化行为,增大晶化激活能.少量Ni元素的加入能够细化最终晶化组织中的晶粒大小.初晶相使合金的硬度降低,但随着热处理温度的升高,所有合金的硬度都明显提高,原因是析出了大量的碳化物和硼化物.退火温度对合金的磁性能有很大影响,少量Ni元素的加入阻止了合金在高温退火后从顺磁态向铁磁态的转变.     

Charge-transfer-enhanced prism-type local order in amorphous Mg65Cu25Y10: Short-to-medium-range structural evolution underlying liquid fragility and heat capacity

Using classical and ab initio molecular dynamics simulations, we have probed into the atomic and electronic structures of an amorphous Mg alloy, Mg₆₅Cu₂₅Y₁₀, as a representative of Mg alloys that form bulk metallic glasses (MGs). Different from some MGs where the icosahedral motifs are the key coordination polyhedra, here the featured short-range order (SRO) is dominated by Cu-centered bicapped square antiprisms and tricapped trigonal prisms. Bond shortening is observed for Mg–Cu and Y–Cu bonds, due to appreciable charge transfer that imparts an ionic character to the bonding. This enhances their chemical affinity, accentuating Cu-centered motifs analogous to solute-centered prisms in metal–metalloid MGs in this all-metal system. The prism-type SRO is prevalent even at high temperatures in the (supercooled) liquids, as revealed from the inherent structures. A weak temperature dependence is observed for the degree of characteristic SRO with undercooling, as well as for the development of connections of the motifs in the medium range. Such a structural evolution is contrasted with the rapidly ascending icosahedral order in Cu₆₄Zr₃₆ supercooled liquids, and explains the much more shallow specific heat curve as well as the low fragility of the Mg₆₅Cu₂₅Y₁₀ supercooled liquid.     

Room-temperature creep and structural relaxation of Mg–Cu–Y metallic glasses

Structural relaxation of Mg–Cu–Y metallic glasses occurs at room-temperature (∼0.7 T_g) and induces an abrupt embrittlement after periods of time that depend on the relative proportions of Cu and Mg. Internal friction measurements of as-quenched samples show that Mg₆₅Cu₂₅Y₁₀ is in a more relaxed state than Mg₈₅Cu₅Y₁₀, suggesting a more compact structure for the alloy with a larger amount of Cu, which is the smallest atom in the system. Relaxation spectra, calculated from the anelastic component of nanoindentation creep curves, show that the two alloys are in a similar relaxed state, when they become brittle. The results are discussed in terms of the defects characterizing the disordered structure of the metallic glass.     

Phase selection and performance of Mg–Cu–Y amorphous composite with different Mg/Cu ratios

In this article, Mg–Cu–Y alloys with two different Mg/Cu ratios(in at%) were prepared using a watercooled copper mold. Scanning electron microscopy and X-ray diffraction were applied to analyze the microstructure and phase composition. Moreover, corrosion resistance and wear resistance were studied systematically. The results show that both Mg65 Cu25 Y10 and Mg60 Cu30 Y10 alloys could form a composition of crystalline and amorphous phases. Although the microstructure of Mg65 Cu25 Y10 consists of an amorphous phase and a-Mg, Mg2 Cu, and Cu2 Y crystalline phases, the microstructure of Mg60 Cu30 Y10 alloy mainly consists of the amorphous phase and a-Mg, Mg2 Cu. With reducing Mg/Cu ratio, the alloys have better corrosion resistance and wear resistance. The mechanism has also been discussed in detail.     

Mg–Y–Cu bulk metallic glass prepared by mechanical alloying and vacuum hot-pressing

We have studied the amorphization behavior of Mg_85−xY₁₅Cu_x (x=20–40) alloy powders synthesized by mechanical alloying technique. The as-milled powders were mainly amorphous after 10 h of milling. The thermal stability of these Mg_85−xY₁₅Cu_x glassy powders was investigated by differential scanning calorimeter (DSC). The ranges of Tg, T_x and ΔT_x are around 430–459, 467–497, and 30–46 K, respectively. The Mg₄₉Y₁₅Cu₃₆ glassy powders exhibit the largest supercooled region of 46 K. The amorphization behavior of Mg₆₁Y₁₅Cu₂₄ was examined in details. Amorphous phases gradually became dominant after 7.5 h of milling and fully amorphous powders formed at the end of milling. The thermal stability of Mg₆₁Y₁₅Cu₂₄ glassy powders was similar to that of melt-spun Mg₆₀Y₁₅Cu₂₅ amorphous alloys. Mg₆₁Y₁₅Cu₂₄ bulk metallic glass with homogeneously embedded nanocrystalline precipitates was successfully prepared by vacuum hot pressing. It was found that the applied pressure during consolidation could enhance the thermal stability and prolong the existence of amorphous phase inside Mg₆₁Y₁₅Cu₂₄ powders.     

Fabrication of aspheric surface using ultraprecision cutting and BMG molding

A new process to replicate an aspheric lens is presented in this study. A mold of oxygen free copper (OFC) is fabricated using ultraprecision machining, which is a popular material for machining due to its good machinability. But OFC has a very low hardness of 1.606 GPa, it is not suitable for the subsequent molding process. Then, this OFC mold (known as the first mold) is used to hot emboss on Mg₅₈Cu₃₁Y₁₁ amorphous alloy to form a secondary mold which is one kind of metallic glass. The hardness of the secondary mold is as high as 3.445 GPa, whereas the glass transition temperature (T_g) of the Mg₅₈Cu₃₁Y₁₁ metallic glass is as low as 413 K (140 °C), at which the Mg₅₈Cu₃₁Y₁₁ metallic glass shows a good glass-forming ability (GFA). In order to perform superplastic microforming, the working temperature must be close to the glass temperature around 413 K. Therefore, in this study, the temperatures of the hot embossing experiments to fabricate the secondary mold are set at 423 K (150 °C). It shows experimentally that the working temperature is dependent on the applied stress level. Since the Mg₅₈Cu₃₁Y₁₁ metallic glass has superplastic property at the supercooled liquid region, it can be easily formed by the master die. This embossing process on the Mg₅₈Cu₃₁Y₁₁ metallic glass makes molding process faster and more diverse applications. Next, the secondary mold is used to emboss on polymethylmethacrylate (PMMA) sheets for replication process. The Mg₅₈Cu₃₁Y₁₁ metallic glass is not only a good material for hot embossing process to fabricate micro-structure directly, but also an excellent fast-molding material for hot embossing process. It is expected that the machining processes described in this paper could be applied to the related fields to fabricate precision components required of micro, sub-micro, or nano order of dimensional accuracy.     

Microstructural analysis of Zr55Cu30Al10Ni5 bulk metallic glasses by laser surface remelting and laser solid forming

The crystallization behavior of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy during laser solid forming (LSF) was analyzed. Since laser surface remelting (LSM) is a key process for the LSF, the crystallization behavior of as-cast Zr₅₅Cu₃₀Al₁₀Ni₅ bulk metallic glasses (BMGs) during LSM was also investigated. It was found that the amorphous state of the as-cast BMGs was maintained when they were repeatedly remelted four times in a single-trace LSM, and as for the LSF of Zr₅₅Cu₃₀Al₁₀Ni₅ bulk amorphous alloy, the crystallization primarily occurred in the HAZ between the adjacent traces and layers after the two layers were deposited. The as-deposited microstructure exhibited a series of phase evolutions from the molten pool to the HAZ as follows: the amorphous → NiZr₂–type nanocrystal + amorphous → NiZr₂–type equiaxed dendrite + amorphous → Cu₁₀Zr₇–type dendrite + NiZr₂–type nanocrystal. Among these microstructural patterns, the NiZr₂–type nanocrystals and equiaxed dendrites primarily formed from the rapid solidification of the remelted liquid in the laser processing process, and the Cu₁₀Zr₇–type dendrites in the HAZ primarily formed by the crystallization of pre-existed nuclei in the already-deposited amorphous substrate.     

Thermal and mechanical properties of Cu60−xZr25Ti15Nix bulk metallic glasses with x=0, 1, 3, 5, 7, 9 and 11 at.%

Systematic investigation on thermal and mechanical properties of Cu_60?xZr₂₅Ti₁₅Ni_x bulk metallic glasses with x = 0, 1, 3, 5, 7, 9 and 11 at.% points out that monolithic Cu₅₃Zr₂₅Ti₁₅Ni₇ and Cu₅₁Zr₂₅Ti₁₅Ni₉ bulk metallic glasses containing optimum Ni content of 7 and 9 at.% are effective to enhance both thermal stability of amorphous structure up to 716 K and plastic strain of 2.4% at room temperature. This indicates that a selection of additional elements such as Ni by considering a mixing enthalpy to the constituent elements is very important to control the thermal stability and plasticity. Moreover, it is believed that the addition of minor Ni can be a trigger to form the chemical heterogeneity upon solidification. Such chemical heterogeneity formed by the selection of the minor elements has a strong influence to cause the oscillation of the shear stress by wavy propagation of the shear bands thus leading the improvement of macroscopic plasticity of the bulk metallic glasses.     

On calorimetric study of the fragility in bulk metallic glasses with low glass transition temperature: (Ce0.72Cu0.28)90−x Al10Fex (x = 0, 5 or 10) and Zn38Mg12Ca32Yb18

Compared with conventional bulk metallic glasses, Ce-based and Zn-based bulk metallic glasses have received considerable attention because of their possible application as structural and functional materials. Kinetic fragility parameter m in amorphous material presents degree of deviations from the Arrhenius law above the glass transition temperature (T_g) of the material. Kinetic fragility parameter (m) and Kauzmann temperature (T_K) in (Ce_0.72Cu_0.28)_90?x Al₁₀Fe_x (x = 0, 5 or 10) and Zn₃₈Mg₁₂Ca₃₂Yb₁₈ bulk metallic glasses have been determined by differential scanning calorimetry (DSC). Results show that Zn₃₈Mg₁₂Ca₃₂Yb₁₈ presents a higher m than (Ce_0.72Cu_0.28)_90?x Al₁₀Fe_x (x = 0, 5 or 10). The activation energies E_g for glass transition are 1.51 eV (x = 0), 1.59 eV (x = 5) and 1.83 eV (x = 10) in (Ce_0.72Cu_0.28)_90?x Al₁₀Fe_x (x = 0, 5 or 10), and 3.59 eV in Zn₃₈Mg₁₂Ca₃₂Yb₁₈, respectively. The values of E_g increase with increasing the Fe content in (Ce_0.72Cu_0.28)_90?x Al₁₀Fe_x (x = 0, 5 or 10) bulk metallic glasses. Kinetic fragility parameter m of bulk metallic glasses increases with the glass transition temperature T_g of bulk metallic glasses, in agreement with previous investigations.     

Laser welding of annealed Zr55Cu30Ni5Al10 bulk metallic glass

Laser welding is one of the promising ways for manufacturing metallic glass products with complicated shape and geometry. In this work we focus on the effect of annealing treatment and welding parameters on laser welding of annealed Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glass as intended and unintended heat treatment occurs in the process. We find that laser welding can produce well welded specimen plates with no obvious welding defects in the joints and high welding speed may lead to better joints. Although higher annealing temperature or longer annealing time leads crystallization, bulk metallic glass material still remains largely amorphous in the heat affected zone. Compared with the welded joint without annealing, the micro-hardness and bending strength are enhanced due to the presence of the nanocrystals occurred in annealed welding joint. Therefore, appropriate annealing treatment with the annealing temperature near the glass transition temperature and annealing time as long as that in hot embossing of BMG parts may play a beneficial role in laser welding of metallic glasses.     

Structural effects of shot-peening in bulk metallic glasses

Shot-peening induces surface compressive stresses in bulk metallic glass components, improving their plasticity. Structural changes in the peened surface of fully glassy and partially crystalline Zr₅₅Al₁₀Cu₃₀Ni₅ are studied by X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. An earlier study is extended by examining the effect of sample temperature. While fully glassy samples show no phase change under peening, partially crystalline samples show either amorphization or crystallization depending on temperature. Peening can induce very large stored energy in metallic glasses rendering them susceptible to crystallization below room temperature, a result which may be relevant for improving the plasticity of these materials.