Jun Shen

Exceptionally high glass-forming ability of an FeCoCrMoCBY alloy

It has been well documented that the maximum thickness of as-cast glassy samples attainable through conventional metallurgical routes is the decisive criteria for measuring the glass-forming ability (GFA) of bulk metallic glasses (BMGs). Here we report the exceptionally high GFA of an FeCoCrMoCBY alloy which can be fabricated in the form of glassy rods with a maximum sample thickness of at least 16mm. It is demonstrated that, by substituting Fe with a proper amount of Co in a previously reported Fe-based BMG alloy, the glass formation of the resultant new alloy can be extensively favored both thermodynamically and kinetically. The new ferrous BMG alloy also exhibits a high fracture strength of 3500MPa and Vickers hardness of 1253kgmm−2.It has been well documented that the maximum thickness of as-cast glassy samples attainable through conventional metallurgical routes is the decisive criteria for measuring the glass-forming ability (GFA) of bulk metallic glasses (BMGs). Here we report the exceptionally high GFA of an FeCoCrMoCBY alloy which can be fabricated in the form of glassy rods with a maximum sample thickness of at least 16mm. It is demonstrated that, by substituting Fe with a proper amount of Co in a previously reported Fe-based BMG alloy, the glass formation of the resultant new alloy can be extensively favored both thermodynamically and kinetically. The new ferrous BMG alloy also exhibits a high fracture strength of 3500MPa and Vickers hardness of 1253kgmm−2.

Characterizations of ball-milled nanocrystalline WC–Co composite powders and subsequently rapid hot pressing sintered cermets

Abstract Nanostructured WC–Co cemented carbides, combining high hardness and high toughness, are expected to be widely applicable. In this study, nanocrystalline WC–10Co–0.8VC–0.2Cr3C2 (wt.%) composite powders, whose average grain size is bout 25 nm, were fabricated by a unique ball milling technique with variable rotation rate and repetitious circulation in 32 min. The high energy ball milling process was optimized and the as-prepared nanocrystalline powders were characterized and analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential thermal analysis (DTA). Furthermore, a low temperature, high heating rate, short soaking time and pressure sintering technique, that is, rapid hot pressing sintering, was used in the sintering process, by which nanostructured WC–Co cemented carbides with mean grain size of 250 nm were produced. The material exhibits high hardness of 93.6 HRA and transverse rapture strength of 2746 MPa. In the sintered composites, the distribution of elements W, Co, V and Cr is homogeneous, and it is small quantity of miropores and extraordinary coarse tungsten carbide phase that negatively influence the transverse rapture strength of nanostructured WC–Co cemented carbides.

Binary eutectic clusters and glass formation in ideal glass-forming liquids

In this letter, a physical concept of binary eutectic clusters in “ideal” glass-forming liquids is proposed based on the characteristics of most well-known bulk metallic glasses (BMGs). The authors approach also includes the treatment of binary eutectic clusters as basic units, which leads to the development of a simple but reliable method for designing BMGs more efficiently and effectively in these unique glass-forming liquids. As an example, bulk glass formers with superior glass-forming ability in the Zr–Ni–Cu–Al and Zr–Fe–Cu–Al systems were identified with the use of the strategy.

Crystallization behavior of ZrAlNiCu bulk metallic glass with wide supercooled liquid region

Abstract The crystallization processes of Zr55Al10Ni5Cu30 (at.%) bulk metallic glass from the amorphous state and supercooled liquid region during the continuous heating and isothermal annealing courses were investigated. The apparent activation energy derived from the Kissinger plots for glass transition Eg is higher than that for crystallization Ep and Ex calculated by the peak temperature Tp and onset crystallization temperature Tx during the continuous heating process. The isothermal activation energy obtained using Arrhenius equation shows that it increases accompanying the proceeding of the crystallization transformation. The incubation time of crystallization becomes longer and the crystallization process becomes slower as the annealing temperature is reducing during the isothermal process. The crystallization mechanism was studied using Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. The results indicate that the Avrami exponent n≈2 at the initial crystallization stage, and it alters gradually following the increasing crystallization volume fraction.

Multiplication of shear bands and ductility of metallic glass

The authors find that metallic glass can be controlled to create regularly arrayed fine multiple shear bands under small punch test, indicating that metallic glass essentially has a good plastic deformation ability and thus high ductility under suitable loading condition. The current findings imply that the initiation and propagation of shear bands in metallic glass strongly depends on the stress state and the small punch test can also be regarded as an effective method to characterize the shear deformation ability and distinguish ductile-brittle transition of different metallic glasses.

Nanocrystallization of Zr-Ti-Cu-Ni-Be bulk metallic glass

Abstract The nanocrystallization kinetics of Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 (at.%) bulk metallic glass was investigated by differential scanning calorimetry (DSC) in the mode of continuous heating and isothermal annealing. In the case of continuous heating, three exothermic crystallization peaks can be observed, and the peak temperatures display a strong dependence on the heating rates, which can be fitted by a first order decay equation. The activation energies for crystallization are estimated to be E p1 =166.83±8.85 kJ/mol, E p2 =256.15±9.34 kJ/mol, E p3 =174.36±12.56 kJ/mol, which correspond to peak temperatures of T p1 , T p2 , and T p3 , respectively, indicating the formation of different crystallization phases at different stages. In the case of isothermal annealing, the crystallization products under isothermal annealing were observed and determined by transmission electron microscopy (TEM). The precipitation phases of the sample heated at higher temperatures were identified by X-ray diffraction (XRD), showing that the body-centered tetragonal (bct) Zr 2 Cu and hexagonal ZrBe 2 are the primary phases, in spite of the presence of other phases. These results are consistent with the complexity of the DSC curves obtained during the continuous heating and isothermal annealing.

Atomic-scale bonding of bulk metallic glass to crystalline aluminum

A Ti40Zr25Cu12Ni3Be20 bulk metallic glass (BMG) was welded to a crystalline aluminum by the parallel plate explosive welding method. Experimental evidence and numerical simulation show that atomic-scale bonding between the BMG and the crystalline aluminum can be achieved, and the weldment on the BMG side can retain its amorphous state without any indication of crystallization in the welding process. Nanoindentation tests reveal that the interface of the explosive joints exhibits a significant increase in hardness compared to the matrix on its two sides. The joining of BMG and crystalline materials opens a window to the applications of BMGs in engineering.

Electrochemical hydrogen storage of Ti–V-based body-centered-cubic phase alloy surface-modified with AB5 nanoparticles

A composite of Ti–V-based bcc phase alloy surface-modified with AB5 nanoparticles was prepared by ball milling. The composite showed significantly improved electrochemical hydrogen release capacities. For example, the 30 min ball milled Ti–30V–15Mn–15Cr+10wt%AB5 showed a discharge capacity in the first cycle, at 353 K, of 886mAhg−1, corresponding to 3.38 wtu2009% of hydrogen, with a 45mAg−1 discharge current. It is thought that this high capacity is due to the enhanced electrochemical-catalytic activity from the alloy surface covered with AB5 nanoparticles, which not only have better charge-discharge capacity themselves, acting as both an electrocatalyst and a microcurrent collector, but also result in the greatly enhanced hydrogen atomic diffusivities in the nanocrystalline relative to their conventional coarse-grained counterparts. These results provide new insight for use of Ti–V-based bcc phase alloy for high-energy batteries.

A high strength and high conductivity copper alloy prepared by spray forming

Abstract This paper reports an experimental investigation into the use of the spray forming process to a high strength, high conductivity, copper alloy of composition Cu–1.33xa0wt.% Cr–0.78xa0wt.% Zr–0.09xa0wt.% Mg, with a refined microstructure increased solid solution of alloy elements, reduced oxidation and a high proportion of small precipitates.

Microstructure characteristics of a hypereutectic Al-Si alloy manufactured by rapid solidification/powder metallurgy process

Hypereutectic Al-Si-Fe-Cu-Mg alloys manufactured by rapid solidification and powder metallurgy (RS/PM) process contain high volume fraction and finely dispersed Si particles and Al-Si-Fe intermetallic compounds (1–10). As a result, the unfavorable effects of coarse primary Si and Al-Si-Fe intermetallic compound caused by conventional ingot metallurgy (IM) process on mechanical properties of the alloy are remarkably eliminated (2, 7). In addition, the sizes of the matrix grain and the other second phases are refined and the solid solubility limit of alloying elements is increased by this process, thus the properties of the alloy are much more improved. In this paper, the microstructure and phase morphology of a RS/PM Al-Si-Fe-Cu-Mg alloy with high silicon content were reported. The nominal composition of the alloy was designed Al-17Si-6Fe-4.5Cu-0.5Mg(wt%). The powders were prepared by means of ultrasonic N2-gas atomization process at pressure of 2 MPa and temperature of 900 ◦C. Consolidation of the alloy powders was carried out by canning, degassing and hot extrusion with a reduction ratio 14:1 after holding at 450 ◦C for 1 h. The phases of the extruded alloy were identified by a D/max-rB X-ray diffractometer (XRD) using Cu-Kα1 radiation. The microstructure was analyzed by S-570 scanning electron microscope (SEM). The SEM specimens were cut directly from the extrudate, followed by grinding, polishing and etching with Killer’s reagent. The size and distribution of the second phase particles in the extruded alloy were measured by image analysis technique. The analysis of the phase morphology was performed using Philips-EM420 transmission electron microscope (TEM) attached with energy dispersive spectroscopy (EDS), operating at an accelerating voltage of 100 KV. Thinning of TEM samples after mechanical grinding of the extrudate was performed by ion beam milling. As shown in Fig. 1, the microstructure of the alloy was primarily consisted of Al, Si, Al5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7 phases. In contrast to the alloy powder [11], new phases Al5FeSi and Al7Cu2Fe were formed in the microstructure of the extruded alloy. By comparing the microstructures between the powder and extruded alloy, it can be concluded that in the course of vacuum degassing or heating before extrusion, Al5FeSi was formed by phase transformation, regarding Al9FeSi3 as its master phase and Al7Cu2Fe was precipitated from the matrix. Fig. 2 illustrated the microstructure of the extruded alloy under SEM. A large amount of fine particle phases were uniformly distributed in aluminum matrix. The results of image analysis of the particle phases were shown in Fig. 3. It was shown that the average size of these particles was only 0.5 μm and maximum size was less than 2 μm. Because of their small size, the chemical compositions of the particle phases could not be ascertained by EDS. It was presumed that these phases were mixture of Si, Ai5FeSi, Al7Cu2Fe and Al4Cu2Mg8Si7. According to the previous research results [11], there was a great amount of fine needle-shaped intermetallic compound Al9FeSi3 in the powder alloy, yet herein we found it disappeared after hot extrusion. It was