Solid state and liquid phase sintering of mechanically activated W–20 wt.% Cu powder mixture

W–20 wt.% Cu powder mixture was mechanically alloyed by high-energy ball milling for various times and the effect of mechanical alloying (MA) on the sintering response of the composite compacts was investigated. The densification, microstructure, hardness and electrical conductivity after solid phase sintering (SPS) and liquid phase sintering (LPS) were examined. It was shown that the microstructure of mechanically alloyed powder profoundly influence the sintering response, i.e. a meaningful relationship between the sintering kinetics and the milling time was observed. It is suggested that MA disintegrates the W–W particle networks and increases the contribution of solid phase sintering (SPS) of nanostructured Cu and W particles on the densification. Higher hardness and conductivity were achieved by prolonged MA and SPS, indicating a lower W–W contiguity of the milled powders compared with the conventionally prepared W–Cu composite. On the other hand, depression of the melting temperature of copper up to 145 °C was noticed by affording a prolonged MA. The lower melting temperature and finer distribution of the Cu particles in the W matrix enhanced the densification during LPS and improved the homogeneity and properties of the final product.     

The effect of sintering temperature on densification of nanoscale dispersed W–20–40%wt Cu composite powders

Nanoscale dispersed particles of W–20–40%wt Cu were synthesized using a chemical procedure including initial precipitating, calcining the precipitates and reducing the calcined powders. The powders were characterized using X-ray diffraction and map analyses. The effect of sintering temperature was investigated on densification and hardness of the powder compacts. Relative densities more than 98% were achieved for the compacts which sintered at 1200 °C. The results showed that in the case of W–20%wt Cu composite powders, the hardness of the sintered compacts increased by elevating the sintering temperature up to 1200 °C while for the compacts with 30 and 40%wt Cu, the sintered specimens at 1150 °C had the maximum hardness value. The microstructural evaluation of the sintered compacts by scanning electron microscopy showed homogenous dispersion of copper and tungsten and a nearly dense structure. A new proposal for the variation of the mean size and morphologies of W-particles with volume percent of copper melt within the composites has been suggested.     

Preparation of W–Al–Mo ternary alloys by mechanical alloying

Single phase W_XAl₅₀Mo_50−X (X = 40, 30, 20 and 10) powders have been synthesized directly by mechanical alloying (MA). The structural evolutions during MA and subsequent as-milled powders by annealing at 1400 °C have been analyzed using X-ray diffraction (XRD). Different from the Mo₅₀Al₅₀ alloy, W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys were stable at 1400 °C under vacuum. The results of high-pressure sintering indicated that the microhardnesses of two compositions, namely W₄₀Al₅₀Mo₁₀ and W₃₀Al₅₀Mo₂₀ alloys have higher values compared with W₅₀Al₅₀ alloy.     

Softening behaviour of nanostructured Al–14 wt% Zn alloy during mechanical alloying

Al and Zn elemental powder mixtures were subjected to high-energy milling to produce Al–14 wt% Zn alloy. The milled powders were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and microhardness measurement. The Al and Zn grain sizes were estimated from broadening of XRD peaks using Williamson–Hall formula. The results showed that in early stage of milling the solubility of Zn in Al is extended compared to the equilibrium value which is accompanied by a decrease in lattice parameter of the Al matrix. However, after longer milling times, decomposition of Al(Zn) supersaturated solid solution appeared to occur leading to an increase in Al lattice parameter and also a decrease in hardness value of as-milled powder. The final product of milling includes both Al and Zn phases having a grain size of 40 nm and 20 nm, respectively.     

Formation of crystalline and amorphous solid solutions of W–Ni–Fe powder during mechanical alloying

The tungsten heavy alloy with the composition of 76.6W–17.3Ni–6.1Fe in atom percent was mechanically alloyed (MA) from the elemental powders of W, Ni and Fe. Nanocrystalline supersaturated solid solutions and amorphous phase were obtained during MA. Phase evolution, grain size and lattice distortion of these powders were determined and discussed. A thermodynamic model was developed based on semi-experimental theory of Miedema to calculate the driving force for phase evolution. The thermodynamic analysis showed that there is no chemical driving force to form the solid solution and the amorphous phase. The effect of the work of milling on the amorphization during MA was discussed and the model of multilayer amorphization during MA was applied to illustrate the feasibility of amorphization of powder with neither ΔH_mix0 nor D_BD_A. The driving force for amorphization is provided not by the negative heat of mixing or the stored energy in the grain boundaries but by the sharp concentration gradients in this system. Amorphization is mechanically driven and not by the negative heat of mixing. Crystallization is suppressed by sharp concentration gradients.     

Preparation by gravity casting and hydrogen-storage properties of Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La

Mg–23.5 wt.%Ni–(5, 10 and 15 wt.%)La alloys were prepared by gravity casting and their hydrogen-storage properties were examined after pulverizing. The gravity cast Mg–23.5Ni–(5, 10 and 15)La alloys consist of α-Mg, Mg₂Ni and Mg₁₇La₂ phases. The activated Mg–23.5Ni–10La alloy has the highest hydrogen-storage capacity of 4.96 wt.%H (from P–C–T curve) and the highest initial hydriding rate (hydrogen content 3.83 wt.%H at 10 min) with an initial hydrogen pressure in the channel of 11 bar H₂ at 573 K. This is attributed to its containing the largest amount of the Mg₁₇La₂ phase, which is easily dissociable during the hydriding reaction.     

Structure and magnetic properties of Fe-based powders prepared by mechanical alloying

FeSiAlCr alloy powders were prepared by mechanical alloying, the milling time were 20 h, 40 h, 60 h and 80~h, respectively. Powders morphology was studied by SEM. Microstructure of powders milled for various times were analyzed by XRD. The complex permittivity and complex permeability of four powders were tested in the frequency range from 0.5 to 18 GHz, and their microwave absorption properties were analyzed. It was found that the particle size of powders milled for 80~h was less than 2μm. Silicon and aluminum atoms were dissolved into the crystal lattice of iron, and chromium atoms can form alloy with iron atoms. The minimum peak value of reflectivity can reach to -11.3 dB at the frequency of 4.3 GHz for 80 h milling powders, and the other one was -6 dB at 16.5 GHz.     

Characterization and frictional behavior of nanostructured Ni–W–MoS2 composite coatings

Ni–W–MoS₂ composite coatings were obtained by pulse plating from a Ni–W electrolyte containing suspended MoS₂ particles. The coating composition, morphology, crystalline structure, microhardness and frictional behavior were studied as a function of MoS₂ concentration. The results obtained in this study indicate that co-deposited lubricant particles strongly influenced the composite Ni–W coating properties. It was found that increasing co-deposited MoS₂ diminished tungsten content in the coating and consequently increased the average grain size. Ni–W nanostructured coatings with high MoS₂ content have a porous sponge-like structure, high surface roughness and irregular frictional behavior. However, the friction coefficient of Ni–W coatings is reduced to half its value with low MoS₂ content.     

Effect of composition and milling parameters on the critical ball milling of Ti–Si–B powders

The absence of brittle phases and elevated temperature during ball milling of a powder mixture containing a large amount of ductile component can contribute to reach an excessive agglomeration denoting a critical ball milling (CBM) behavior. This work reports in the effect of composition and milling parameters on the CBM behavior of Ti–Si–B powders. High-purity elemental Ti–Si–B powder mixtures were processed in a planetary ball mill in order to prepare the Ti₆Si₂B compound and two-phase Ti + Ti₆Si₂B alloys. TiH₂ chips instead of titanium powder were used as a starting material. To avoid elevated temperature in the vials during ball milling of Ti–Si–B powders the process was interrupted after each 10 min followed by air-cooling. Following, the milled powders were hot-pressed at 900 °C for 1 h. Samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). Short milling times followed by air-cooling contributed to obtain a large amount of powders higher than 75% in the vials. Only Ti and TiH₂ peaks were observed in XRD patterns of Ti–Si–B and TiH₂–Si–B, respectively, suggesting that extended solid solutions were achieved. The large amount of Ti₆Si₂B and Ti + Ti₆Si₂B structures were formed during hot pressing from the mechanically alloyed Ti–Si–B and TiH₂–Si–B powders.     

铜锌粉末低温机械合金化

对铜锌粉末在-30℃、-20℃以及常温条件下进行了高能球磨.利用透射电镜、X射线衍射仪对粉末的微观形貌、显微结构、晶体结构进行了研究.结果表明:-30℃低温可显著降低粉末塑性,不添加过程控制剂时,球磨过程可顺利进行,颗粒表面平整,呈脆性断裂特征;常温与-30℃低温球磨10 h产物均为α γ相.-20℃低温不添加过程控制剂时,冷焊作用强,球磨不能顺利进行;添加过程控制剂后,球磨过程顺利进行,颗粒尺寸最小,-20℃球磨10 h产物为α相、γ相与ZnO,继续球磨,最终形成α相、ZnO及未知相.     

A novel (W–Al)–C–Co composite cemented carbide prepared by mechanical alloying and hot-pressing sintering

Novel cemented carbides (W_0.4Al_0.6)C_0.5–Co with different cobalt contents were prepared by mechanical alloying and hot-pressing technique. Hot-pressing technique as a common technique was performed to fabricate the bulk bodies of the hard alloys. The novel cemented carbides have good mechanical properties compared with WC–Co. The density and operation cost of the novel material were much lower than the WC–Co system. It was easy to process submicroscale sintering with the novel materials and obtain the rounded particles in the bulk materials. There is almost no η-phase in the (W_0.4Al_0.6)C_0.5–Co cemented carbides system although the carbon deficient obtains the astonishing value of 50%.     

Preparation of nanocomposite alumina–Sn2Fe by mechanical alloying

Nanocomposites of Al₂O₃ and Sn₂Fe were prepared by ball milling alumina with elemental Sn and Fe. Samples were prepared with molar ratios of Al₂O₃ and Sn₂Fe of 1:1 and 2:1. Materials produced in this way have been characterized by X-ray diffraction techniques as well as ⁵⁷Fe Mössbauer effect spectroscopy as a function of milling time. The nanosized grains show a distribution of particle sizes along with some residual elemental components. Preliminary investigations of electrochemical cells indicate that these materials show promise as possible electrodes for Li-ion cells.     

放电辅助机械合金化快速合成Ti(C,N)粉体(英文)

利用机械合金化方法以Ti粉、石墨粉为原料,采用行星式球磨机在高纯氮气气氛下制备出Ti(C,N)粉体。研究放电处理对机械合金化1～7h试样的相变及显微组织的影响。实验结果表明:球磨1h后的样品在放电辅助下合成了Ti(C,N)粉体;而仅采用机械合金化方法,球磨7h不能合成Ti(C,N)粉体。放电处理产生的等离子体作用于粉体上,提高了原子间的扩散速度和Ti(C,N)在球磨粉体表面的形核速度,这是加速Ti(C,N)合成的主要原因。两种合成方法都遵循机械诱发自蔓延反应机制。     

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al alloy at 973–1073 K in 101 kPa O2

Oxidation of a quaternary two-phase Cu–40Ni–17.5Cr–2.5Al (at.%) alloy was investigated at 973–1073 K in 101 kPa O₂. The alloy is composed of two phases. One light phase with lower Cr content forms the matrix of the alloy, and the other medium gray phase richer in Cr is presented in the form of continuous islands. At 973 and 1073 K, the kinetic curves for the present alloy deviate evidently from the parabolic rate law. They show a large mass gain in initial stage, and then their oxidation rates decrease evidently with time until they become very small up to 24 h. Cross sectional morphologies show the present alloy is able to form continuous external scales of chromia over the alloy surface with a gradual decrease in the oxidation rate. However, the previous studies showed that a ternary two-phase Cu–40Ni–20Cr alloy is unable to form protective external scales of chromia over the alloy surface, but is able to form a thin and very irregularly continuous layer of chromia at the top of the mixed internal oxidation region. Therefore, substituting Cr in Cu–40Ni–20Cr alloy with 2.5 at.% Al is able to decrease the critical content required to form Cr oxide and help to form continuous external scales of chromia under lower Cr content in two-phase alloys.     

Impedance spectroscopy studies of electroless Ni–P matrix,Ni–W–P,Ni–P–ZrO2, and Ni–W–P–ZrO2 coatings exposed to 3.5% NaCl solution

Ni–P matrix, ternary Ni–W–P and Ni–P–ZrO₂ coatings, and quaternary Ni–W–P–ZrO₂ coatings were deposited using electroless method from a glycine bath. Their corrosion resistance was evaluated by electrochemical impedance spectroscopy (EIS) for various immersion times in a 3.5% NaCl solution. From among the investigated coatings, the ternary Ni–W–P coatings show the highest resistance to corrosion in the first hour of exposure to the 3.5% NaCl medium. An addition of ZrO₂ adversely affects the performance of both the Ni–P coatings and the Ni–W–P coatings. For all the coatings, including the ones containing tungsten, a marked decrease in pore resistance (R_por) over time is observed. This means that their corrosion resistance and capacity to protect the substrate decline. On the other hand, after 24 h immersion in the 3.5% NaCl solution the Ni–W–P coating shows the highest low‐frequency impedance modulus (|Z|_{f = 0.01 Hz}). As regards corrosion resistance, the Ni–P coatings and the Ni–W–P coatings perform best.     

An investigation on the solid state sintering of mechanically alloyed nano-structured 90W–Ni–Fe tungsten heavy alloy

In this study, 90W–7Ni–3Fe heavy alloy was investigated for its microstructure development, mechanical properties and fracture behavior after solid state sintering. The nano-sized powders were synthesized by mechanical alloying (MA). The microstructure of solid state sintered heavy alloys consisted of tungsten matrix. The average tungsten grain size in the range of 1.7–3.0 μm was obtained. It was found that the grain size largely affected the mechanical properties. Tensile strength more than 1200 MPa was achieved at a sintering temperature of 1350 °C. Fracture mechanisms based on microscopical observations on the fracture surfaces were studied. Matrix failure, tungsten-intergranular cleavage and tungsten–matrix interfacial separation were found to be the possible failure mechanisms.     

Phase formation during mechanical alloying and subsequent low-temperature heat treatment of Al–27.4at%Fe–28.7at%C powders

Phase formation during high energy ball milling of a ternary elemental powder mixture with a composition of Al–27.4at%Fe–28.7at%C and during low temperature heat treatment of the milled powder was studied. It was found that an amorphous phase formed during prolonged milling. During heating the shorter time milled powder, Al and Fe reacted first, forming the AlFe phase and then at a higher temperature, AlFe reacts with Fe and C, forming the AlFe₃C_0.5 phase. During heating the longer time milled powder which contains a substantial amount of amorphous phase, the amorphous phase partially crystallizes first, forming the AlFe and AlFe₃C_0.5 phases, and then AlFe reacts with the remaining amorphous phase, forming the AlFe₃C_0.5 phase. Overall, mechanical alloying of Al, Fe and C elemental phases enables formation of an amorphous phase, while low temperature heat treatment of mechanically milled powder facilitates formation of AlFe and AlFe₃C_0.5 phases.     

The third-element effect in the oxidation of Ni–xCr–7Al (x = 0, 5, 10, 15 at.%) alloys in 1 atm O2 at 900–1000 °C

The oxidation in 1 atm of pure oxygen of Ni–Cr–Al alloys with a constant aluminum content of 7 at.% and containing 5, 10 and 15 at.% Cr was studied at 900 and 1000 °C and compared to the behavior of the corresponding binary Ni–Al alloy (Ni–7Al). A dense external scale of NiO overlying a zone of internal oxide precipitates formed on Ni–7Al and Ni–5Cr–7Al at both temperatures. Conversely, an external Al₂O₃ layer formed on Ni–10Cr–7Al at both temperatures and on Ni–15Cr–7Al at 900 °C, while the scales grown initially on Ni–15Cr–7Al at 1000 °C were more complex, but eventually developed an innermost protective alumina layer. Thus, the addition of sufficient chromium levels to Ni–7Al produced a classical third-element effect, inducing the transition between internal and external oxidation of aluminum. This effect is interpreted on the basis of an extension to ternary alloys of a criterion first proposed by Wagner for the transition between internal and external oxidation of the most reactive component in binary alloys.     

Thermodynamic modeling of the Ba–Ni–Ti system

The binary Ba–Ni and Ba–Ti systems are modeled by computational thermodynamics using the CALculation of PHAse Diagram (CALPHAD) method, wherein the thermodynamic parameters of disordered bcc, fcc and hcp phases are evaluated in terms of the first-principles calculations using the special quasirandom structures (SQSs). In combination with the Ni–Ti system modeling in the literature, the phase equilibria of the Ba–Ni–Ti system are predicted. Isothermal section of the ternary system is presented.