Mo-W Alloys Research Articles

Design of a rapidly sintering Mo-W-Cr alloy

Mo-W based materials have excellent mechanical and thermal properties, but are difficult to produce and require either very high temperatures or complicated sintering procedures, and typically have coarse structures. Here the nanophase separation sintering (NPSS) design criteria are used to develop a rapidly sintering ternary Mo-W based alloy. A composition of Mo-25 W-15Cr is chosen and shown to exhibit the critical features of NPSS. The relevant sintering temperatures are shown to be much lower than for comparable refractory alloys. The kinetic mechanisms that facilitate the enhanced sintering are verified, and these involve the transient precipitation and redissolution of a second phase that accelerates sintering. That second phase is fully eliminated at the end of sintering, permitting potential usage of the alloy at temperatures above the sintering range. Finally, the resulting hardness of the material is compared to that of Mo-W alloys produced through other methods, showing a significant strength boost due to the rapid low-temperature sintering and its attendant fine microstructure.

Preparation of fine-grained Mo W solid solution alloys with excellent performances

Molybdenum‑tungsten (MoW) alloy composed of molybdenum (Mo) and tungsten (W), is a popular high-temperature material. In this paper, a new method for preparing MoW solid solution alloy has been proposed. To resolve the problem of low extent of solid solution and large grain size, ultrafine MoW composite powder and Mo-W-Al2O3 composite powder were synthesized by spray drying combined with carbothermic pre-reduction and deep reduction by hydrogen. The results show that the method can effectively promote the formation of MoW solid solution, and the particle sizes of the four composite powders (Mo-30 W, Mo-50 W, Mo-30 W-0.5Al2O3, Mo-50 W-0.5Al2O3) were 50–200 nm. After sintering at 1600 °C, all alloys can achieve a high relative density (97%-98%) while retaining a small grain size and good comprehensive properties. Meanwhile, the effect of Al2O3 doping on the sintering performance of MoW alloys was also investigated in detail. The results showed that doping of 0.5 wt%Al2O3 reduced the grain size of MoW alloy from 5 μm to 3 μm. In addition, the hardness of Mo-30 W alloy and Mo-50 W alloy were increased to 471 HV and 534 HV, while the bending strength was increased to 612 MPa and 647 MPa, respectively.

Thermodynamic analysis and microstructural evolution of the W-Mo-Ni-C system produced by mechanical alloying

In this research, the microstructural evolution of the W-Mo-Ni-C system was studied by conducting a thermodynamic analysis of formation enthalpies using the Miedema model. Samples were synthetized with different milling times of up to 240 h and analyzed using X-ray diffraction to determine the phases that were produced at each stage of the synthesis. The experimental results indicated carbon diffusion into the transition metal lattice, as WC formation was observed at 120 h and NiC formation was observed at 160 h. Nanostructured MoNi3 and MoW alloys were identified at initial stages from 40 h to 120 h, and as carbon diffusion continued, the MoW alloy transformed into WMoC carbide at 200 h. The Miedema model predictions established that the necessary energy for carbide formation is much larger than that for the formation of alloys of transition metals, demonstrating that these alloys must be formed first, which was validated by experimental data. It was found that the mechanical alloying process produced materials that were out of equilibrium.

Application ofab initioand CALPHAD thermodynamics to Mo-Ta-W alloys

A parameter-free electronic structure approach is applied to the study of stability and chemical order in substitutional bcc-based Mo-Ta, Ta-W, and Mo-W alloys. The method is based on a Green's function description of the electronic structure of random alloys. Configurational order is treated within the generalized perturbation method, and temperature effects are accounted for by a generalized mean-field approach. The practical application of this study to phase stability in Mo-Ta-W alloys is tested by converting the ab initio output thermodynamics of the three binary subsystems to a format amenable to a CALPHAD treatment. The results of this conversion are then used to predict isothermal sections of the ternary phase diagram of the Mo-Ta-W system that are compared with the results obtained directly from the ab initio approach with the cluster variation method. It is concluded that the proposed CALPHAD conversion of the ab initio results is a viable scheme to capture the alloy properties predicted from first-principles electronic structure calculations.

Segregation behavior at MoW(001) alloy surfaces

The tight-binding Ising model has been used to determine equilibrium segregation profiles at the (001) surface of MoW alloys. The description of the thermodynamic properties of such alloy surfaces is based on a cluster expansion in terms of correlation functions. The expansion coefficients are known as Effective Cluster Interactions and have been computed from their formal definition by means of the Direct Configurational Averaging technique. As observed experimentally strong Mo-segregation occurs at the surface, followed by a depletion of Mo in the second layer. This alloy displays very remarkable behavior: the concentration profile is found to be oscillatory whereas the bulk phase diagram of MoW exhibits phase separation.

Kinetics of Surface Segregation in Metallic Alloys with First-Principles Interaction Parameters

ABSTRACTWe report the results of Monte Carlo simulations of the kinetics of surface segregation at the (001) face of CuNi and MoW alloys. These two systems were selected because they are based on different lattice structures and show contrasting segregation behavior: CuNi exhibits a monotonie profile, while that of MoW is oscillatory. To describe the energetics we have determined a set of effective cluster interactions (ECI) which govern the ordering or clustering tendencies of these alloys. The ECI were obtained by means of tight-binding electronic structure calculations in which no adjustable or experimentally determined parameters were used. Equilibrium segregation profiles are calculated and a series of quenches are performed. The layer concentrations are studied as a function of time and the existence of metastable phases in the surface region is investigated.

High-quality mixed-transition-metal dichalcogenides from solid-state exchange reactions

High-quality molybdenum-tungsten disulfide solid solutions, (Mo,W)S 2 , are rapidly synthesized in self-propagating, solid-state precursor reactions between (MoCl 5 -WCl 6 ) and Na 2 S. The mixed-metal dichalcogenide products can be readily reduced in a dilute hydrogen atmosphere at approximately 900 o C to the corresponding Mo-W alloys. The metal distribution in the Mo-W alloys is examined using the high-angle 123 powder X-ray reflection and can be correlated to the metal distribution in the metal dichalcogenide product

Dendritic segregation of tungsten in molybdenum alloys

1. The primary crystallization structure of Mo-W alloys can be observed by scanning-electron microscopy. It could not be revealed by other methods. The main constituents of this structure are cells and cellular dendrites. 2. The primary dendritic axis has a variable chemical composition along its length, which is caused by its discontinuous growth. 3. There is an interrelationship between the primary crystallization structure and the degree of the intracrystalline segregation, which develops more intensively for cellular growth than when cellular dendrites are formed. 4. Variation in the boron content in alloys of the Mo-W system almost does not have any effect on the degree of development of intracrystalline segregation of W in Mo.

Oxidation of molybdenum-tungsten-chromium-silicon alloys

The oxidation kinetics of (50–60) wt.% Mo-(35–47) wt.% Cr-(2–5) wt.% Si and (30–40) wt.% Mo-(30–40) wt.% W-(27–37) wt.% Cr-(0–3) wt.% Si alloys were studied between 1000 and 1200°C in a pure oxygen atmosphere. The oxidation of Mo-W-Cr-Si alloys resembled that of Mo-Cr-Si alloys but was much more oxidation resistant. In general, oxidation resistance increased with increasing Cr and Si content. Alloys with good oxidation behavior had a thin outer Cr2O3 layer and an internal oxidation zone (in both Mo and Mo-W alloys). Alloys displaying poor oxidation behavior had a porous Cr2O3 layer (in Mo alloys) or layers of oxides of W and Cr (in Mo-W alloys). Although the alloy systems were not truly oxidation resistant, definite improvement in oxidation resistance was achieved.

Interfacial reaction of Mo-W alloys with silicon

Solid state contact reactions of thin films of Mo x W 100− x ( x =85, 57, 22) with silicon have been studied using Rutherford backscattering spectrometry, X-ray diffraction and scanning electron microscopy. Reactions and silicide formation were obtained by thermal annealing at temperatures between 450 and 900°C for all the alloys. No layer-by-layer phase separation was observed during silicide growth for the alloy-silicon reaction at these temperatures. The reaction temperature increased with increasing tungsten concentration in the alloys. Ion implantation of 4×10 15 cm −2 of silicon at 250 keV into Mo 57W 43/Si and Mo 22W 78/Si reduced their reaction temperatures and also made the reaction products more uniform.

On the low temperature synthesis of alloys by precipitation reactions

The low temperature synthesis of Mo-W alloys by a precipitation reaction has been studied in detail. Electron microscopy, thermogravimetric analysis, and powder X-ray diffraction measurements show that the precipitation of ammonium paramolybdate and ammonium paratungstate from a water-alcohol mixture leads to the formation of a homogeneous heteropolymetallate (NH 4) 6(Mo 7− n W n O24)·4H 2O. Reduction of this intermediate, under hydrogen at 700 °C, yields Mo-W alloys. X-ray photoelectron spectroscopy reveals that the alloy surface is oxidized and enriched in tungsten. The wider implications of this work for other alloy preparations are discussed.

The thermally activated solid-solution hardening of the ternary system: niobium-molybdenum-tungsten

The temperature dependence of the flow stress of ultra-high-vacuumdegassed Nb-Mo-W single crystals has been investigated between 77 and 900 K. Tests of alloys containing up to 9.4 at.% (Mo+W) show that the thermal component of the flow stress exhibits little or no increase with increasing ternary-solute content for most of the alloys tested. Solid-solution softening is observed in the Nb-4.2Mo-W alloys at 77 K. The results are interpreted utilizing a phenomenological analysis of the solid-solution hardening increments. The analysis of the results suggests that a non-random solute distribution is responsible for reducing the thermally-activated, flow-stress component, especially in concentrated alloys with a Mo to W ratio of near unity.

焼結モリブデン合金の低温変形

Low temperature deformation of powder metallurgical pure Mo and dilute Mo-Cr, Mo-W alloys are studied over the temperature range from 77 to 500°K by simple tension, stress relaxation and incremental unloading tests. The experimental results show that substitutional solute atoms do not affect the activation volume V* and the thermal component of the flow stress σ*, but affect flow stress σ through the athermal component σμ. From the relation Hk=4τ*V*, the kink energy Hk is calculated to be 0.58 eV both for pure Mo and Mo alloys. The activation energy H is deduced to be 1.40 eV at 500°K, which is in good agreement with 1.48 eV obtained from Seeger’s equation proposed for the Peierls-Nabarro process at low stress. It is concluded from the present results that low temperature deformation (T<500°K) is controlled by the overcoming of Peierls-Nabaroo hills by the nucleation of double kinks and that substitutional solute atoms increase only the athermal component of the flow stress.

Mechanical properties of Mo?W alloys

The mechanical properties of Mo-W alloys containing 0 to 100% W at 20 to 2000 deg C were studied. It was found that the strength of Mo-W alloys at low (20 to 600 deg C) and high (over 1800 deg C) temperatures increases with the tungsten content, and at moderate temperatures (600 to l800 deg C) the strength is highest for alloys with the maximum alloying. At 1500 to 1800 deg the strength-to-weight ratio is highest for molybdenum alloys with 80, 56, and 32.5% W, and is equal to or larger than that of unalloyed tungsten. The strength of Mo- W alloys can be increased substantially by small additions of titanium and zirconium. (JRD)

Electrotransport and diffusion in molybdenum-tungsten and iron-nickel alloys over wide ranges of temperatures

In this work the results of investigations of electrotransport and diffusion of components of 11 Mo-W alloys and of the alloys of Fe with 2 and 4 at.% of Ni over wide temperature ranges are set forth. Experiments were carried out on the basis of techniques elaborated by the authors, using the tracers Mo 99, W 185, Fe 55 and Ni 63. Using the experimental data obtained, a row of parameters which characterize the condition of ions and current carriers and their interaction in the investigated alloys were estimated by the authors in the approximation of two bands model.