Molybdenum Grains Research Articles

Microstructure and high-temperature deformation performance of liquid-dispersed evaporated Nano-ZrTiO4 doping molybdenum alloy

To enhance the low-temperature plasticity and high-temperature strength of molybdenum alloys, we introduce a method of uniformly dispersing nano-ZrTiO4 particles within and between molybdenum grains. This approach effectively eliminates the formation of hard agglomerations between particles and leads to a further refinement of molybdenum grains. The research employs molecular level solid-liquid doping and subsequent liquid dispersion evaporation techniques to successfully synthesize Mo-ZrTiO4 nano-composite powder following H2 reduction. Subsequently, a novel fine-grained Mo-ZrTiO4 alloy sintered billet is successfully fabricated through cold isostatic pressing and high-temperature sintering. To assess the efficacy of the process, three sets of comparative experiments are designed, each employing different methods. The comparative studies have revealed compelling advantages of the liquid dispersed evaporated fine-grained samples of Mo-ZrTiO4. The key advantages identified are as follows:the samples demonstrate a refined grain size, ranging from 2 μm to 4 μm. Within the samples, the nano ZrTiO4 particles have an average size of approximately 200 nm. The density and Vickers hardness of the samples is measured at 10.21 g/cm3 and 268.5 ± 17HV0.2, respectively. The samples exhibit impressive toughness even at room temperature, notably, the high-temperature compressive strength of these samples surpasses that of the deformed samples reported in previous literature concerning this system. The mechanism of liquid dispersion evaporation method in grain nucleation, growth, and powder drying, reduction, and sintering processes was systematically elaborated. Simultaneously, a detailed analysis was conducted on two high-temperature strengthening mechanisms that impact the mechanical properties. The results indicate that this excellent performance can be attributed to the synergistic effect of both fine grain strengthening and uniform dispersion strengthening of second-phase nanoparticles.

Evaluation of tensile property and strengthening mechanism of molybdenum alloy bars doped with different ultrafine oxides

Three kinds of molybdenum alloy are doped with 2.0 wt.% different ultrafine oxide particles (Al2O3, ZrO2 and La2O3) via hydrothermal synthesis, co-precipitation, co-decomposition, powder metallurgy, and rotary swaging. Three nano-oxides refine molybdenum grains by more than 70% and change the fracture modes from brittle fracture to ductile fracture, thereby dramatically improving the integrated mechanical properties of the alloys. Moreover, the trade-off between strength and ductility is offset. Mo−ZrO2 alloy has the highest strength and plasticity, which is attributed to fine ZrO2 particles and a semi-coherent interface between ZrO2 and molybdenum matrix. Moreover, a quantitative strengthening model correlating dislocation density, oxide volume fraction and size, and molybdenum grain size is established.

Effects of oxygen on microstructure and evolution mechanism of body-centred-cubic molybdenum

Molybdenum metal have high brittleness and ductile-to-brittle transition temperature (DBTT) due to the existence of interstitial oxygen impurities, so it is of great scientific significance to strictly control and accurately analyze the oxygen content. In this work, the effects of oxygen on microstructure and evolution mechanism of body-centred-cubic metallic molybdenum are studied. The X-Ray diffraction (XRD), oxygen nitrogen analyzer, scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and electron probe micro-analyzer (EPMA) are used to analyze the chemical composition, microstructure, oxygen existence and distribution form. The results demonstrate that MoO specimens with up to 3300 ppm of oxygen content have single-phase body-centered cubic microstructure with no precipitates. When the oxygen content in molybdenum increases from 44 ppm to 2200– 3300 ppm, more oxygen exists in the pores between molybdenum grains, and the fracture mode changes from intergranular fracture to intergranular and transgranular mixed fracture. The microhardness increases with increase of oxygen content. These results provide new ideas for controlling the change trend of oxygen in molybdenum and regulating the properties of molybdenum.

Mechanical properties and strengthening mechanism of the hydrothermal synthesis of nano-sized α-Al2O3 ceramic particle reinforced molybdenum alloy

Nano-sized α-Al2O3 ceramic-particle-reinforced molybdenum alloy plates were fabricated by a series of processes including hydrothermal synthesis, powder metallurgy, and rolling. The quantitative effect of α-Al2O3 ceramic (particle size and volume fraction) on the microstructures and mechanical properties of molybdenum alloy plates was investigated. Nano-sized α-Al2O3 ceramic particles were obtained and uniformly dispersed in the molybdenum matrix. The α-Al2O3 ceramic refined the molybdenum grains and remarkably increased the microhardness, elastic modulus, and yield strength. The yield strength of the alloy reached a peak value when the α-Al2O3 ceramic content was 1.28 vol% and increased by 61.07%–90.2% compared with that of pure molybdenum after annealing at 1000 °C-1300 °C. A quantitative yield strength model correlating α-Al2O3 particle size, α-Al2O3 volume fraction, molybdenum grain size, and annealing temperature was built. This model attributes the strengthening of alloy to the synergy effects of grain refinement strengthening (σHP) and Orowan strengthening (σp). The addition of fine α-Al2O3 ceramic resulted in Orowan strengthening and fine molybdenum grains, thereby increasing yield strength of alloy.

Effect of Electron-Beam Treatment on Wear-Resistant Coatings Applied by Electroexplosive Sputtering

TiC–Mo, TiC–Ni, TiB2–Mo, and TiB2–Ni coatings applied to the surface of Hardox 450 steel by electroexplosive sputtering are subjected to electron-beam treatment, After electroexplosive application, the surface relief of the coatings includes features such as deformed solidifying microglobules, buildup, microcraters, microcracks, and peeling. After electron-beam treatment, the microglobules, buildup, microcraters, and microcracks disappear from the coating surface. A polycrystalline structure containing cellular elements is formed. After electron-beam treatment, the surface roughness is 1.1–1.2 μm. The thickness of the layers modified by the electron beam in the electroexplosive coatings depends linearly on the surface energy density. The greatest coating thickness is observed when using the TiB2–Mo system; the coating thickness is least for the TiC–Ni system. That may be attributed to the thermophysical properties of the coatings. The following substructures are observed in the coatings: cellular, striated, fragmented, and subgranular. Grains with chaotically distributed dislocations and reticular dislocations are also observed. Electron-beam treatment leads to the formation of composite filled structure over the whole cross section of the remelted layer. The structure formed in this layer is more disperse and uniform than in coatings formed without electron-beam treatment. The inclusions of titanium carbide or titanium diboride in the molybdenum or nickel matrix are 2–4 times smaller than immediately after electroexplosive sputtering. Within the molybdenum or nickel grains and at their boundaries, rounded particles of secondary phase (titanium carbide or titanium diboride) are observed. They may be divided into two classes by size: particles of the initial powder (80–150 nm) that have not dissolved on irradiation; and particles formed on solidification of the melt (10–15 nm). In the electroexplosive powder coatings, the structure is mainly formed by dynamic rotation of the sprayed particles, which form a vertical structure both in the coating and in the upper layers of the substrate. The coatings have excellent operational properties: nano- and microhardness, elastic modulus of the first kind, and wear resistance in dry slipping friction.

Structural, optical and compositional stability of MoS2 multi-layer flakes under high dose electron beam irradiation

MoS2 multi-layer flakes, exfoliated from geological molybdenite, have been exposed to high dose electron irradiation showing clear evidence of crystal lattice and stoichiometry modifications. A massive surface sulfur depletion is induced together with the consequent formation of molybdenum nanoislands. It is found that a nanometric amorphous carbon layer, unwillingly deposited during the transmission electron microscope experiments, prevents the formation of the nanoislands. In the absence of the carbon layer, the formation of molybdenum grains proceeds both on the top and bottom surfaces of the flake. If carbon is present on both the surfaces then the formation of Mo grains is completely prevented.

Microstructure and High-Temperature Frictional Wear Property of Mo-Based Composites Reinforced by Aluminum and Lanthanum Oxides

In this article, we developed Mo-based composites reinforced with aluminum and lanthanum oxides using a sol-gel method combined with a sintering process and researched the microstructure and frictional wear properties under high-temperature conditions. The microstructure of composites was characterized by α-Al2O3 and composite oxide (La0.62Mo0.38)AlO3, which were uniformly distributed in the molybdenum matrix. The composite oxide (La0.62Mo0.38)AlO3 was mainly attached to α-Al2O3. The interfaces of α-Al2O3/Mo and α-Al2O3/(La0.62Mo0.38)AlO3 were well bonded. The α-Al2O3 refined the molybdenum grains, increased the relative density and hardness of Mo-based composites, and had an obvious effect on the frictional wear properties of the composites. With increasing α-Al2O3 content, the friction coefficient first increased and then decreased, and the wear weight loss decreased continuously. The wear failure mode varied from microcutting to fatigue with increasing α-Al2O3 content. The better wear resistance of Mo-based composites, compared to pure molybdenum, was mainly attributed to the characteristics of α-Al2O3, such as high microhardness, good morphology, well-bonded phase interface, and high hardness matrix due to the effects of α-Al2O3 reinforcement.

Investigating the kinetics and features of the reduction mechanism of ammonium paramolybdate in hydrogen-nitrogen media

Kinetic features of the reduction of ammonium paramolybdate in pure hydrogen and in a mixture of hydrogen and nitrogen with a ratio of 1: 0.5–1.0 in the temperature range of 250–950°C with a gas supply rate of 1–3 L/min are investigated. The optimum conditions for performing this process in hydrogen-nitrogen media that ensure a no lower than 90% recovery of the target phase are established. The evolution of forming the nano- and microcrystalline molybdenum powders via the reduction of ammonium paramolybdate in hydrogen-nitrogen media is investigated. The dependence of the granulometric composition and morphology of powder samples on the type of the reduction atmosphere is shown. During high-temperature (880–900°C) reduction in a hydrogen-nitrogen medium (composition, 1: 1), numerous crystallization centers with the formation of molybdenum grains through pseudomorphic transformation are observed in the powders.

High absorption of the microwave energy in a system with strongly elongated molybdenum grains in aluminum nitride matrix at frequencies of 9.5–10.5 GHz

High absorption of microwave energy in a composite with an aluminum nitride matrix and strongly elongated molybdenum grains at frequencies of 9.5–10.5 GHz has been discussed. A possibility to use such composites as bulk absorbers inside the vacuum for high-power microwave devices has been considered.

Influence of Mo back contact porosity on co-evaporated Cu(In,Ga)Se2 thin film properties and related solar cell

The present study aims at investigating the influence of Ar sputtering gas pressure on the properties of molybdenum back contact (deposited on soda-lime glass) and consequences on co-evaporated Cu(In,Ga)Se2 (CIGSe) absorber layer and related solar cell. Films 300 nm thick have been grown with argon pressure between 0·75 and 11·25 mTorr; these films have been characterized by several techniques showing that the increase of the sputtering pressure yields wider amorphous areas, containing oxygen and sodium, between the molybdenum grains, thus higher sheet resistance. The volume ratio of these amorphous areas is referenced to as “porosity”. The structural and morphological properties of co-evaporated CIGSe have not been reliably observed influenced by the molybdenum porosity; the only noticeable change is the sodium content of the absorber, which increases with the porosity of the back contact. The impact of the amount of sodium on the device performance has been observed to be very important. On the one hand, as already reported, sodium is beneficial for the open-circuit voltage. On the other hand, a too high amount of sodium is detrimental for the fill factor (hindered shunt resistance), thus the cell efficiency; this latter observation is interpreted as a change in the grain boundary electrical properties. Copyright © 2011 John Wiley & Sons, Ltd.

Microstructure and Mechanical Properties of Molybdenum Alloys Doped with ZrB<sub>2</sub> Particles

The molybdenum alloys doped with various mass fraction zirconium diboride (ZrB2) particles were successfully prepared by using the powder metallurgy technology. The microstructure, hardness and the tensile properties of the pure molybdenum and molybdenum alloys doped with ZrB2 particles were determined at room temperature. Results showed that the addition of zirconium diboride refined the molybdenum grains obviously, meanwhile, molybdenum grain sizes decrease with the increasing of zirconium diboride mass fraction. Measurements on mechanical properties showed that molybdenum alloys doped with ZrB2 particles had higher hardness and strength than pure molybdenum. The analysis and discussion results show that the strengthening mechanisms of ZrB2-doped molybdenum alloys may be attributed to the fine-grain strengthening, particle dispersion strengthening, interfacial-bond strengthening and oxygen reduction strengthening.

Microstructure and Tensile Properties of Oxide Dispersion Strengthened Molybdenum Alloys Prepared by Different Doping Process

The oxide lanthanum dispersion strengthened molybdenum alloys were prepared by proprietary powder metallurgy technology, in which the Mo-La2O3 powders were prepared by liquid-liquid doping process which the oxide lanthanum was added to ammonium bi-molybdate solutions as aqueous solutions of La(NO3)3 and liquid-solid doping process which the oxide lanthanum was added to molybdenum oxide solid particles as aqueous solutions of La(NO3)3, respectively. The microstructure and tensile properties of the molybdenum alloys were investigated at room temperature. The results show that the molybdenum alloys all have fine molybdenum grains, and the molybdenum alloy prepared by liquid-solid doping process mainly contain fine oxide lanthanum particles of submicron and nano-sized while the alloy prepared by liquid-liquid doping process mainly contain nano-sized fine oxide lanthanum particles. The molybdenum alloys prepared by liquid-liquid doping process have higher yield strength and ductility than yield-solid doping process. The results of strengthen mechanism analysis show that the high strength of the molybdenum alloys can be advisablely explained by the fine grain strengthening and particles dispersion strengthening mechanism through the Hall–Petch relationship and Orowan model.

Anti-bombing insensitivity life of molybdenum cathode doped with La 2O 3 and Y 2O 3

Anti-bombing insensitivity of La 2O 3–Y 2O 3–Mo secondary emitter has been studied in this paper. The variation of maximum secondary emission coefficient δ max with time was measured. The cathode after life experiment was analyzed by means of HRM, SEM, EDS and XRD. The results showed that δ max of La 2O 3–Y 2O 3–Mo cathode operating at 1100 °C under continuous electron bombardment of 300 W/cm 2 was still about 2.5 after 1000 h operation, indicating that this kind of cathode had good anti-bombing insensitivity. In the internal part of the cathode, RE 2O 3 (rare earth oxide) and molybdenum grains distributed alternately and there existed a certain relationship between crystallographic orientation of RE 2O 3 and that of molybdenum. It was found that a RE 2O 3 layer was formed on the surface after operation. The high δ max of La 2O 3–Y 2O 3–Mo cathode was related to the RE 2O 3 layer on the surface and the amount of nanosized La 2O 3 particles on the Y 2O 3 layer.

Transmission electron microscopy of oxide-dispersion strengthened (ODS) molybdenum: effects of irradiation on material microstructure

Oxide-dispersion strengthened (ODS) molybdenum has been characterized using transmission electron microscopy (TEM) to determine the effects of irradiation on material microstructure. This work describes the results-to-date from TEM characterization of unirradiated and irradiated ODS molybdenum. The general microstructure of the unirradiated material consists of fine molybdenum grains (<5 µm average grain size) with numerous low angle boundaries and isolated dislocation networks. ‘Ribbon’-like lanthanum oxides are aligned along the working direction of the product form and are frequently associated with grain boundaries, serving to inhibit grain boundary and dislocation movement. In addition to the ‘ribbons’, discrete lanthanum oxide particles have also been detected. After irradiation, the material is characterized by the presence of non-uniformly distributed large (∼20 to 100 nm in diameter), multi-faceted voids, while the molybdenum grain size and oxide morphology appear to be unaffected by irradiation.

Interlayer transition zones in Mo/Si superlattices

The formation of interlayer transition zones (ITZs) in sputtered Mo/Si multilayer structures was studied by means of cross-section electron microscopy and grazing incidence reflectivity measurements. For the evaluation and calculation of interface effects the multiperiodic design of Mo/Si structure was used. It was found that the thickness asymmetry of ITZs (Mo-on-Si and Si-on-Mo) in Mo/Si multilayer structures depends on the degree of perfection of the crystalline structure of the molybdenum layer. A transition from asymmetrical to symmetrical ITZs with a disordering of the molybdenum crystalline structure was shown. A model for the formation mechanism of asymmetrical ITZs at the different interfaces in Mo/Si multilayer structures is suggested. According to this model, ITZ formation at the Mo-on-Si interface is controlled by the surface diffusion of Si atoms on the growing molybdenum surface. In contrast, ITZ formation at the Si-on-Mo interface is determined by the bulk diffusion of Si atoms in textured molybdenum grains.

Coalescence of tungsten grains around molybdenum grains in the presence of a liquid phase

The microstructural evolution of W–22.4Mo–7.8Ni–3.4Fe was investigated in this study. A first liquid phase primarily composed of Ni, Fe, and Mo resulted in the coexistence of large W–Mo grains having Mo-rich cores and small W grains.

Solid-state diffusion-based processing kinetics for uniaxial self-propagating high-temperature synthesis of MoSi2

An analysis is presented for self-propagating, high-temperature synthesis (SHS) of MoSi2 from a mixture of silicon and molybdenum powders compacted into a semi-infinite, uniaxial bar and ignited at one end. The kinetics of reacting molybdenum grains are controlled by solid-state diffusion through the interposing product shell of MoSi2 that surrounds each shrinking molybdenum grain. The previously determined temperature-dependent microkinetics of this solid particle reaction are coupled with one-dimensional (1-D) heat transfer and storage to describe the time-dependent macrokinetics of the synthesis reaction sequencing through a perfectly insulated bar. The resulting equations are solved numerically to provide computed results of temperature and conversion as a function of time and distance. Preignition time, propagation velocity, and thickness of the reaction front are also determined. Results depend primarily on the initial temperature at the end of the bar, which affects preignition time, and the molybdenum grain radius. The perfectly insulated model was relaxed by limiting the maximum temperature to arbitrary values corresponding with lateral heat dissipation, and these results compare favorably with experimentally measured propagation velocities and maximum temperatures during SHS of MoSi2. The model presented, with MoSi2 as the prototype, is expected to be applicable to the SHS of many other refractory materials.

Progressive dissolution of molybdenum foils in liquid uranium at 1160°C

Immersion experiments have been performed to investigate the progressive dissolution of 0.3-mm-thick foils of molybdenum metal in liquid uranium at 1160 ° C, for immersion times of 3, 6, 10, and 60 min, and 20 h, in a zarconia crucible. The original foils, as-received and heat-treated at 1160 ° C, and the uranium-reacted foils have been studied microscopically (SEM-EDAX) and the internal morphology has been determined. The flow lines in as-received molybdenum disappear under heat-treatment, while the undistinguishable grains recrystallize upon heating into grains with average size of 20 μm, persisting in samples immersed in liquid uranium. The recrystallization is not uranium-assisted, as uranium does not penetrate into intergranular regions. After incubation time of 3–4 min, outer molybdenum grains dissolve in liquid uranium, thereby reducing the foil thickness progressively. Full dissolution occurs for about 15-min immersion.

Structure of rapidly cooled alloys of the Mo?B system

1. In the dependence of grain in cast alloys of the Mo−B system on boron content a minimum is observed with a content of about 0.002% B. 2. It is not possible to determine by traditional methods the true solubility of boron in molybdenum with a high degree of accuracy, i.e., a combination of quenching experiments with integral methods for estimating the structural state (metallographic, x-ray, determination of mechanical properties, etc.). By means of these methods it is possible to determine the limiting boron concentration which it is possible to fix in the internal volumes of molybdenum grains. In our experiments this concentration is about 0.008% B.

Effect of Al2O3 particles on the growth kinetics of molybdenum grains in liquid nickel

Grain growth inhibition by inert third phase particles during liquid phase sintering has been investigated in Mo-Ni alloys containing Al2O3 particles sintered at 1460° C. The grain growth is inhibited slightly by the Al2O3 particles, and this result is explained in terms of the shielding of material precipitation at the contact area and of the suppression of material precipitation around the contact area, which is determined by the dihedral angle between the molybdenum grain and the Al2O3 particle. The result is compared to those obtained earlier with specimens containing highly soluble third phase particles.