Spheroidization of molybdenum powder by radio frequency thermal plasma

To control the morphology and particle size of dense spherical molybdenum powder prepared by radio frequency（RF） plasma from irregular molybdenum powder as a precursor, plasma process parameters were optimized in this paper. The effects of the carrier gas flow rate and molybdenum powder feeding rate on the shape and size of the final products were studied. The molybdenum powder morphology was examined using high-resolution scanning electron microscopy. The powder phases were analyzed by X-ray diffraction. The tap density and apparent density of the molybdenum powder were investigated using a Hall flow meter and a Scott volumeter. The optimal process parameters for the spherical molybdenum powder preparation are 50 g/min powder feeding rate and 0.6 m^3/h carrier gas rate. In addition, pure spherical molybdenum powder can be obtained from irregular powder, and the tap density is enhanced after plasma processing. The average size is reduced from 72 to 62 μm, and the tap density is increased from 2.7 to 6.2 g/cm^3. Therefore, RF plasma is a promising method for the preparation of high-density and high-purity spherical powders.

Mechanical and corrosion properties of low-carbon steel prepared by friction stir processing

Low-carbon steel plates were successfully subjected to normal friction stir processing(NFSP) in air and submerged friction stir processing(SFSP) under water, and the microstructure, mechanical properties, and corrosion behavior of the NFSP and SFSP samples were investigated. Phase transformation and dynamic recrystallization resulted in fine-grained ferrite and martensite in the processed zone. The SFSP samples had smaller ferrites(5.1 μm), finer martensite laths(557 nm), and more uniform distribution of martensite compared to the NFSP samples. Compared to the base material(BM), the microhardness of the NFSP and SFSP samples increased by 19.8% and 27.1%, respectively because of the combined strengthening effects of grain refinement, phase transformation, and dislocation. The ultimate tensile strengths(UTSs) of the NFSP and SFSP samples increased by 27.1% and 38.7%, respectively. Grain refinement and martensite transformation also improved the electrochemical corrosion properties of the low-carbon steel. Overall, the SFSP samples had better mechanical properties and electrochemical corrosion resistance than the NFSP samples.

The effect of secondary phase on corrosion behaviors of the titanium–zirconium–molybdenum alloy

As the range of applications for molybdenum and its alloys has expanded,the corrosive environment for molybdenum alloys has become more demanding.In the past,the content of doping elements has been studied to investigate their infl uence on the corrosion performance of titanium–zirconium–molybdenum(TZM)alloys.In this paper,it is considered that the second phase in the alloy is the main factor aff ecting the corrosion performance of TZM alloys.By comparing the corrosion behavior of molybdenum metal and TZM alloy,the eff ect of the secondary phase on corrosion behaviors of TZM alloy has been investigated.The results show that the second phase reduces the corrosion resistance of the TZM alloy.The potential of the second phase is 73.7 mV higher than that of the Mo matrix,which contributes to the formation of microscopic electric couples.Under the action of microscopic electric couples,pitting corrosion is preferentially formed at the interface between the second phase and the matrix,which accelerates the corrosion of the matrix.This paper provides a theoretical basis for the application of TZM alloys in corrosive environments.

Photocatalytic applications and modification methods of two-dimensional nanomaterials:a review

Due to its unique electronic structure and special size effect,two-dimensional(2D)nanomaterials have shown great potential far beyond bulk materials in the field of photocatalysis.How to deeply explore the photocatalytic mechanism of 2D nanomaterials and design more efficient 2D semiconductor photocatalysts are research hotspots.This review provides a comprehensive introduction to typical 2D nanomaterials and discusses their current application status in the field of photocatalysis.The effects of material properties such as band structure,morphology,crystal face structure,crystal structure and surface defects on the photocatalytic process are discussed.The main modification methods are highlighted,including doping,noble metal deposition,heterojunction,thickness adjustment,defect engineering,and dye sensitization in 2D material systems.Finally,the future development of 2D nanomaterials is prospected.It is hoped that this paper can provide systematic and useful information for researchers engaged in the field of photocatalysis.

Microstructure and corrosion behaviors of friction stir-welded Q235 low-carbon steel joint

Friction stir welding(FSW)was used to prepare Q235 low-carbon steel joint,and the microstructure of different zones of the joint was characterized.The electrochemical corrosion behavior of different macroscopic zones of the joint was evaluated in 3.5 wt.%NaCl solution.The results showed that the retreated-side heat-affected zone(HAZRS)and the advanced-side heat-affected zone(HAZAS)did not undergo phase transformation during FSW,and their microstructures were similar to those of the base material(BM),which was mainly composed of blocky ferrite and pearlite.The retreated-side thermo-mechanical affected zone(TMAZRS),the stirring zone(SZ),and the advanced-side thermo-mechanical affected zone(TMAZAS)underwent phase transformation,and the microstructure was mainly composed of proeutectoid ferrite and pearlite.The order of the corrosion resistance of different micro-zones from high to low was:HAZRS>BM>HAZAS>TMAZRS>SZ>TMAZAS.The corrosion mechanism for BM,HAZRS,and HAZAS was mainly the dissolution of ferrite.By contrast,the corrosion mechanism for TMAZRS,SZ,and TMAZAS was mainly galvanic corrosion between proeutectoid ferrite and pearlite.

Friction Stir Processing of Magnesium Alloys:A Review

Magnesium(Mg)alloys have been extensively used in various fields,such as aerospace,automobile,electronics,and biomedical industries,due to their high specific strength and stiff ness,excellent vibration absorption,electromagnetic shielding eff ect,good machinability,and recyclability.Friction stir processing(FSP)is a severe plastic deformation technique,based on the principle of friction stir welding.In addition to introducing the basic principle and advantages of FSP,this paper reviews the studies of FSP in the modification of the cast structure,superplastic deformation behavior,preparation of finegrained Mg alloys and Mg-based surface composites,and additive manufacturing.FSP not only refines,homogenizes,and densifies the microstructure,but also eliminates the cast microstructure defects,breaks up the brittle and network-like phases,and prepares fine-grained,ultrafine-,and nano-grained Mg alloys.Indeed,FSP significantly improves the comprehensive mechanical properties of the alloys and achieves low-temperature and/or high strain rate superplasticity.Furthermore,FSP can produce particle-and fiber-reinforced Mg-based surface composites.As a promising additive manufacturing technique of light metals,FSP enables the additive manufacturing of Mg alloys.Finally,we prospect the future research direction and application with friction stir processed Mg alloys.

Microstructure and tensile properties of Mo alloy synthetically strengthened by nano-Y_2O_3 and nano-CeO_2

Nano-Y2O3 and nano-CeO2 of different weight ratio mixed with deionizing water were doped into MoO2 powder by liquid-solid doping method. The diameter 1.80 and 0.18 mm alloy wires of Mo-0.3Y, Mo-0.3Ce, and Mo-0.15Y-0.15Ce were prepared through reduction, iso- static pressing, sintering, and drawing. Tensile properties, second phase microstructure and fracture surface appear- ance of wires were analyzed. The better refining effect for Mo alloy powder can be gotten after two kinds of nano- particle oxide doped into MoO2 than only one doped. Nano-Y2O3 and nano-CeO2 have different influences on sintering process. For nano-CeO2, the constraining effect of grain growth focuses on the initial sintering stage, nano- Y2O3 plays refining grains roles in the later densification stage. Nano-Y2O3 is undistorted and keeps intact in the process of drawing; and nano-CeO2 is elongated and bro- ken into parts in the drawing direction. The strengthening effect of nano-Y2O3 and nano-CeO2 keeps the finer grains and superior tensile properties for Mo-0.15Y-0.15Ce wire.

Development and Property Tuning of Refractory High-Entropy Alloys:A Review

In the past decade, multi-principal element high-entropy alloys (referred to as high-entropy alloys, HEAs) are an emerging alloy material, which has been developed rapidly and has become a research hotspot in the fi eld of metal materials. It breaks the alloy design concept of one or two principal elements in traditional alloys. It is composed of five or more principal elements, and the atomic percentage (at.%) of each element is greater than 5%but not more than 35%. The high-entropy eff ect caused by the increase of alloy principal elements makes the crystals easy form body-centered cubic or face-centered cubic structures, and may be accompanied by intergranular compounds and nanocrystals, to achieve solid solution strengthening,precipitation strengthening, and dispersion strengthening. The optimized design of alloy composition can make HEAs exhibit much better than traditional alloys such as high-strength steel, stainless steel, copper-nickel alloy, and nickel-based superalloy in terms of high strength, high hardness, high-temperature oxidation resistance, and corrosion resistance. At present,refractory high-entropy alloys (RHEAs) containing high-melting refractory metal elements have excellent room temperature and high-temperature properties, and their potential high-temperature application value has attracted widespread attention in the high-temperature fi eld. This article reviews the research status and preparation methods of RHEAs and analyzes the microstructure in each system and then summarizes the various properties of RHEAs, including high strength, wear resistance, high-temperature oxidation resistance, corrosion resistance, etc., and the common property tuning methods of RHEAs are explained, and the existing main strengthening and toughening mechanisms of RHEAs are revealed. This knowledge will help the on-demand design of RHEAs, which is a crucial trend in future development. Finally, the development and application prospects of RHEAs are prospected to guide future research.

Microstructure and mechanical properties of magnesium–lithium alloy prepared by friction stir processing

In this study, the fine-grained Mg–Li alloy was prepared by friction stir processing(FSP). The microstructure and mechanical properties of the frictionstir-processed(FSPed) Mg–Li alloy were investigated. The result showed that FSP resulted in the grain refinement, and the average grain size of the b-Li phase was about 7.5 lm.Besides the a-Mg and b-Li phases, a small amount of Li_(3)Mg_(7), Li_(2)MgAl and AlLi phases were obtained. Compared with the base metal(BM), the weakening of the crystallographic texture occurred in the FSPed material,and the c-axis of the a-phase and the <001> crystallographic orientation of the b-phase were tilted about 45°with respect to the transverse direction(TD). The average microhardness(HV 67.8) of the stir zone was higher than that of the BM(HV 61.5). The yield strength(YS) and the ultimate tensile strength(UTS) of the FSPed material were higher than those of the BM, while the elongation slightly reduced. Grain refinement had more significant effect on strength improvement compared with the texture variation for the FSPed material. The fracture surfaces of the BM and FSPed materials showed dimple characteristics.

Microstructure and Mechanical Properties in Friction Stir Welded Thick Al–Zn–Mg–Cu Alloy Plate

In this work, 20-mm-thick aluminum-alloy plates were joined via friction stir welding. The temperature gradient was reduced by reducing the surface welding heat input to achieve uniformity of the mechanical properties across the thick plate joints. The welding temperature was measured using thermocouples. The microstructures were observed via electron backscatter diff raction and transmission electron microscopy. The tensile properties of the samples sliced along the thickness direction of the joint were evaluated. The results show that the highest welding peak temperature is 430℃ on the advancing side on the top surface of the joint. The grain size gradually decreased along the thickness direction, and grain refi nement was due to the combination of continuous, discontinuous, and geometric dynamic recrystallization. The tensile properties of the sliced samples were found to be uniform, and the ultimate tensile strength reached 62% of that of the base metal. The main strengthening mechanism of the Al–Zn–Mg–Cu alloy joints consists of precipitation strengthening. In addition, the η ` → η phase transition and grain coarsening in the heat-affected zone were found to be responsible for the fracture of the joints.

Microstructure,Mechanical Properties,and Corrosion Behavior of Mg-Al-Ca Alloy Prepared by Friction Stir Processing

Friction stir processing(FSP)was used to modify the microstructure and improve the mechanical properties and corrosion resistance of an Mg-Al-Ca alloy.The results demonstrated that,after FSP,the grain size of the Mg-Al-Ca alloy was decreased from 13.3 to 6.7μm.Meanwhile,the Al_(8)Mn_(5) phase was broken and dispersed,and its amount was increased.The yield strength and ultimate tensile strength of the Mg-Al-Ca alloy were increased by 17.0%and 10.1%,respectively,due to the combination of fine grain,second phase,and orientation strengthening,while the elongation was slightly decreased.The immersion and electrochemical corrosion rates in 3.5 wt%NaCl solution decreased by 18.4%and 37.5%,respectively,which contributed to grain refinement.However,the stress corrosion cracking(SCC)resistance of the modified Mg-Al-Ca alloy decreased significantly,which was mainly due to the filiform corrosion induced by the Al_(8)Mn_(5) phase.SCC was mainly controlled by anodic dissolution,while the cathodic hydrogen evolution accelerated the SCC process.

Enhanced Mechanical Properties of Pure Zirconium via Friction Stir Processing

Friction stir processing(FSP), as a new kind of severe plastic deformation technique, can refine and homogenize the microstructure of metallic material. In this study, the effect of FSP on the microstructure and mechanical properties of pure Zr was investigated using electron backscatter diffraction analysis, microhardness and room-temperature tensile testing. The fine-grained(FG) structure with an average grain size of ~ 5.3 μm was obtained in the processed zone, where the average microhardness was ~ 198 HV, 1.6 times higher than that of base metal. Furthermore, tensile property of FG pure Zr exhibited obvious anisotropy owing to strong texture. Both grain size and texture had a significant effect on strength and ductility of FG pure Zr. High yield strength(248 MPa) and ultimate tensile strength(483 MPa), as well as good uniform elongation(10%) were achieved, which demonstrated that FSP was an effective method to fabricate bulk FG pure Zr with high strength and good ductility.