Mo Particles Research Articles

Fabrication of Mo-encapsulated stainless steel by powder metallurgy and assessment of localized corrosion resistance

As a source of corrosion inhibitors, 2.5% Mo powder was mixed with Type 304L stainless steel powder and spark-plasma sintered to a fabricate Mo-encapsulated Type 304L stainless steel. The Mo particles formed a core/shell structure. The localized corrosion resistance of the fabricated Mo-encapsulated Type 304L stainless steel was compared with those of spark-plasma-sintered Type 304L and Type 316L stainless steels. Potentiodynamic polarization in 0.1 M NaCl suggested that the Mo cores could release molybdate, which acts as an inhibitor. However, the shells did not dissolve, preventing the Mo-enriched particles from acting as pit initiation sites. Similarly, the surface of the Mo core dissolved during dip-and-dry cycles using 0.1 M NaCl, but the shell was less prone to dissolution. Although the fabricated Mo-encapsulated stainless steel did not contain Mo as a solid solution, its rust resistance was equivalent to that of spark-plasma-sintered Type 316L stainless steel. In potentiodynamic polarization, the initiation potential for localized corrosion of the Mo-encapsulated Type 304L stainless steel was remarkably higher than those of the sintered Type 304L and Type 316L stainless steels. The corrosion inhibition mechanism of the Mo-encapsulated Type 304L stainless steel was discussed from three viewpoints: (1) passive film modification on the steel matrix, (2) cathodic protection of the steel matrix by the Mo core dissolution, and (3) corrosion inhibition by molybdate dissolved from the Mo-enriched particles, and the last one appeared to be the most likely.

The influence of ball milling conditions on the powder characteristics and sintering densification of Mo[sbnd]Cu alloy

Mechanical alloying was utilized to produce Mo-30Cu(wt%) composite powder. The effects of milling conditions on the powder characteristics and sintering densification were investigated. The morphological evolution during the mechanical alloying process can be categorized into four stages: 1-individual Mo and Cu particles, 2-coexistence of Mo particles and blocky MoCu composite particles, 3-coarse irregular composite particles, and 4-refined near-spherical composite particles or lamellar MoCu composite particles, depending on whether process control agent (PCA) is added. The mechanical alloying degree of the MoCu composite powder was deepened gradually from stage 1 to 4, which is influenced by the ball milling parameters. As the milling speed and milling time increase, the lattice strain and the alloying degree of the MoCu composite powders increase while the grain size of molybdenum particles decreases, which is conducive to accelerating the sintering density. Among these ball milling parameters, an elevated milling speed notably accelerates the mechanical alloying process and the sintering density. Under the milling speed of 600 r/min, ball to powder ratio (BPR) of 10:1, and milling time of 4 h, the grain size, lattice strain, and average particle diameter of the composite powder were measured to be 48.4 nm, 0.247 %, and 21.04 μm, respectively, with the particle morphology being nearly spherical. The sintered density of the MoCu composite reached 98.1 %, larger than the other composites.

Effect of Mo nanoparticles on microstructure and mechanical properties of Cu alloys by self-propagating and spark plasma sintering

Cu alloys strengthened with Mo nanoparticle, with Mo contents of 2 at.%, 4 at.%, 6 at.%, 8 at.%, and 10 at.%, were fabricated by self-propagating high-temperature synthesis, followed by two-step hydrogen reduction and spark plasma sintering. The microstructures, mechanical properties, and thermal conductivity of these Cu-Mo alloys were investigated. The results reveal that two-step hydrogen reduction effectively reduces the average Mo particle size to 13.8 nm compared to one-step reduction. With increasing Mo content, the average grain size of Cu initially decreases but then increases due to the segregation of Mo particles. Notably, the Cu-6 at.% Mo alloy exhibits the smallest average grain size of 0.46 μm, with dispersed nanoscale Mo particles of 25.6 nm. The Cu-6 at.% Mo alloy demonstrates superior mechanical properties, with a tensile strength of 405.0 MPa and an elongation of 24.9 %. Furthermore, the Cu-6 at.% Mo alloy has a high thermal conductivity of 320.0 Wm−1K−1 even at 400 °C and a high electrical conductivity of 82.3 % IACS at room temperature. This work offers valuable insights for the design of advanced Cu composites suitable for heat sink applications.

Sintering behaviour of liquid phase sintered 90Mo-10(Ni–Cu) alloys

Molybdenum (Mo), a refractory material, boasts an elevated melting point of 2610 °C and is predominantly synthesised through powder metallurgy at high temperatures reaching 2000 °C, necessitating the use of liquid phase sintering (LPS) technique. This study investigates the LPS of Mo with varying proportions of Ni–Cu binders, aiming to understand the mechanism of intermetallic formation and to manufacture low-cost, dense, intermetallic-free and near-net-shape Mo-base alloys. Employing the manufacturing design paradigm for 90W-10(Ni–Cu) tungsten heavy alloys (WHAs) and utilising conventional powder metallurgy techniques, Mo-based 90Mo10(Ni–Cu) alloys were synthesised. Initial findings indicate that the densification of Mo-compacts increases with increasing the Ni–Cu ratio in the binder phase. However, an excess of Ni results in the loss of structural rigidity and geometric distortion during sintering. In Ni-rich alloys, two distinct liquid phases, Ni-rich and Cu-rich, emerge at sintering temperatures. These phases exhibit different solubilities for Mo and solidify into a dual-phase matrix. Intriguingly, in contrast to WHAs, 90Mo-10(Ni–Cu) alloys with a Ni-rich composition manifested irregular Mo particles, a dual-phase matrix, and δ-MoNi intermetallic compound formation. The formation of the intermetallic compound can be avoided by reducing the solubility of Mo in the Ni-rich liquid during sintering. Microstructural evolution and distortion are analysed using stereological quantification of various microstructural parameters. It reveals that alloys with low Mo solubility, high connectivity and high dihedral angle maintain their structural rigidity and resist distortion. Both micro-hardness and bulk hardness increased with increasing Ni–Cu ratios in the binder phase. Based on the present results, a mechanism for liquid phase sintering in the 90Mo-10(Ni–Cu) system in the presence of two liquid phases is postulated.

Exploring Microstructural, Interfacial, Mechanical, and Wear Properties of AlSi7Mg0.3 Composites with TiMOVWCr High-Entropy Alloy Powder.

This study explored the impact of varying weight percentages of TiMoVWCr high-entropy alloy (HEA) powder addition on A356 composites produced using friction stir processing (FSP). Unlike previous research that often focused on singular aspects, such as mechanical properties, or microstructural analysis, this investigation systematically examined the multifaceted performance of A356 composites by comprehensively assessing the microstructure, interfacial bonding strength, mechanical properties, and wear behavior. The study identified a uniform distribution of TiMoVWCr HEA powder in the composition A356/2%Ti2%Mo2%V2%W2%Cr, highlighting the effectiveness of the FSP technique in achieving homogeneous dispersion. Strong bonding between the reinforcement and matrix material was observed in the same composition, indicating favorable interfacial characteristics. Mechanical properties, including tensile strength and hardness, were evaluated for various compositions, demonstrating significant improvements across the board. The addition of 2%Ti2%Mo2%V2%W2%Cr powder enhanced the tensile strength by 36.39%, while hardness improved by 62.71%. Similarly, wear resistance showed notable enhancements ranging from 35.56 to 48.89% for different compositions. Microstructural analysis revealed approximately 1640.59 grains per square inch for the A356/2%Ti2%Mo2%V2%W2%Cr processed composite at 500 magnifications. In reinforcing Al composites with Ti, Mo, V, W, and Cr high-entropy alloy (HEA) particles, each element imparted distinct benefits. Titanium (Ti) enhanced strength and wear resistance, molybdenum (Mo) contributed to improved hardness, vanadium (V) promoted hardenability, tungsten (W) enhanced wear resistance, and chromium (Cr) provided wear resistance and hardness. Anticipating the potential applications of the developed composite, the study suggests its suitability for the aerospace sector, particularly in casting lightweight yet high-strength parts such as aircraft components, engine components, and structural components, underlining the significance of the investigated TiMoVWCr HEA powder-modified A356 composites.

Speciation and chemical behavior of molybdenum in uranium dioxide samples prepared by hydroxide precipitation

This work aims to investigate the behavior of Mo in UO2±x for being an abundant fission product with a high fission yield and a complex speciation linked to its interaction with the fuel and other fission product elements. UO2-based model compounds containing different Mo contents (between 0 and 15 mol%) were synthesized by a wet-chemistry route using hydroxide precipitation. The recovered powders were converted to oxides, pelletized, and sintered to obtain densified pellets of UO2 incorporating Mo. PXRD analyses and Rietveld refinement calculations indicate that Mo has an almost negligible solubility in the fluorite structures of both UO2 and UO2+x samples. SEM, TEM, and EDX characterizations of the produced UO2 + Mo pellets revealed the homogeneous distribution of nanosized metallic Mo particles of spherical geometry inside and outside the UO2 grains and throughout the whole sample pellets, whatever the amount of Mo added, thus confirming that the solubility of Mo in the fluorite structure is way below 0.6 mol% Mo in accordance with the PXRD results. The microstructural properties of the UO2 + Mo pellets, including density, porosity, Mo particle size distribution, and UO2 grain size variation with Mo content, were also determined. The addition of Mo to UO2 reduced the UO2 grain size as compared to UO2 grains in pure pellets, and Mo thus plays an inhibiting role in the UO2 grain growth during sintering. The produced UO2 + Mo pellets exhibit a microstructure similar to that of the real spent nuclear fuel, except that the UO2 grains are smaller. The Mo metallic nanoparticles in these simplified UO2-based model compounds of controlled microstructure could be harnessed as surrogates of the Mo-rich ε-phase metallic nanoparticles in the real spent nuclear fuel for future studies.

Customizing the Young's modulus of Ti–Mo–Zr alloys by in situ additive manufacturing based on Mo spatial concentration modulation

It is well known that common orthopedic implant titanium materials have a higher Young's modulus (100+GPa) than bone. Stress shielding leaves the neighboring bone unstressed for long periods of time and bone resorption occurs around the implant, which triggers implant failure. Here, we developed a novel Ti–Mo–Zr alloy with a customizable Young's modulus by using in situ alloying and laser additive manufacturing (AM) methods. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were used to elucidate the distribution of submicron Mo particles in the materials, and selected laser melting (SLM) was used to customize the spatial concentration of Mo elements in novel alloys in a predictable way. By regulating the concentration and distribution of unmelted Mo particles, it is possible to tailor the grain growth angle and phases of the alloy. This results in excellent comprehensive mechanical properties for the alloys with a customizable Young's modulus ranging from 59.3 to 83.6 GPa, tensile strength from 564 to 1045 MPa, and a fracture strain of up to 13.1%. Orthopedic implants with a customizable Young's modulus can be adapted to bone tissues in different body parts, providing endless possibilities for personalized surgical solutions for patients. The economic potential for such materials in smart medical is enormous.

Microstructures and improved properties of Cu–Mo alloys induced by high current pulsed electron beam irradiation

In this paper, a Cu–Mo alloying layer with improved properties was fabricated by high current pulsed electron beam (HCPEB) irradiation. The microstructure of the modified layer was investigated by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The microhardness and friction properties were also measured. After HCPEB irradiation, nano Mo particles, solid solution, and long-period superlattice structures were generated on the surface of Cu–Mo alloys, together with the formation of defect structures. These microstructures led to a significant increase in the surface hardness. The results of sliding wear tests indicated that the HCPEB-irradiated samples exhibited better properties compared with the initial one, which was attributed to the ultrafine Mo particles and the hardened surface.

Effect of nano-sized Mo particles on microstructure and mechanical properties of Mo/AZ31 composites

It is a challenge to enhance both strength and ductility for the Mg matrix composites. In the present work, nanostructured Mo-reinforced AZ31 composites are synthesized via powder metallurgy method. The addition of nano-sized Mo particles reduces the grain size and decreases the texture strength of the composites. The Al5Mo and Al8Mo3 phases are formed through Al atoms diffusion and interaction with Mo atoms. The synergistically reinforcing effect of Al5Mo, Al8Mo3 and nano-sized Mo particles improves the mechanical properties of the composites. The nano-Mo/AZ31 composite with 1.0 wt% Mo addition exhibits superior mechanical properties, with yield strength, ultimate tensile strength, and elongation of 221 MPa, 328 MPa and 12.4 %, respectively. These results mean the improvements of 20.1 %, 17.1 % and 21.6 % compared to the AZ31 Mg alloy. The strength enhancement is attributed to the grain refinement and Orowan strengthening, while the improvement in ductility is attributed to grain size reduction, texture weakening, and the activation of a broader range of slip systems.

Microstructure and mechanical properties of WMoCu alloy doped with different sized Mo particles

WMoCu alloys doped with Mo particles in the nanoscale or microscale range were prepared by infiltration process using the element Ni as an activating element. A comparative study was carried out on the effects of doping with nano- and micro-scale Mo particles on the microstructure and mechanical properties of WMoCu alloys. The results show that the microstructure and distribution of Mo particles is more uniform in the W60Mo25Cu15(M) alloy doped with 3 μm sized Mo particles. The addition of the activating element Ni promotes mutual diffusion between W, Mo, and Cu. A WMo transition layer is formed at the interface between W and Cu, and a core-shell structure with a thickness of about 2 μm is formed around the W particles, which has an amorphous structure at the interfaces, which increases the deformation resistance during interfacial sliding and improves the bonding strength of the WMoCu alloy. The tensile strength of the W60Mo25Cu15(M) alloy is higher than that of the W60Mo25Cu15(N) alloy and the WCu alloy both at room temperature and at elevated temperature. Especially at a high temperature of 600 °C, the tensile strength of the alloys W60Mo25Cu15(M) and W60Mo25Cu15(N) is increased by 57.44% and 32.64%, respectively, compared to the alloy W85Cu15. The fracture mechanisms of the WMoCu alloy are mainly intergranular fracture and transgranular fracture.

Mo70Cu30 composites synthesized by infiltration sintering and hot rolling with simultaneously improved mechanical and electrical properties

In this study, Cu@Mo composite powders, prepared using an electroless plating method, were applied to synthesize Mo70Cu30 composites using infiltration sintering and hot rolling technologies. Microstructure, mechanical properties and physical properties of the synthesized Mo70Cu30 composites were studied. Results showed that the prepared Cu@Mo composite powders have a core-shell structure with Mo particles uniformly coated with Cu layers. Density values of Mo70Cu30 composites before and after the hot rolling process are 98.45% and 99.79%, their tensile strengths are 517 MPa and 753 MPa, and their thermal conductivities are 179.7 W/(m·K) and 214.6 W/(m·K), respectively. Formation of network Cu channels in the Mo70Cu30 composite is beneficial for achieving uniform microstructures, increased density, and enhanced thermal conductivity. Rolling process deforms the Cu phase into a fibrous shape, forming a good thermal conductivity channel and refining the Mo phase grains. The enhancement in the mechanical properties of the composites is attributed to the combined mechanisms of deformation strengthening and fine grain strengthening.

Effect of Mo in-situ alloying on microstructure and magnetic properties of (NiFeMo)100−xMox alloy

This study aimed to modulate the magnetic properties of selective laser melting NiFeMo alloy. This was achieved via the controlled addition of Mo elemental in-situ alloying content to change the tissue morphology and crystal structure of SLM-formed (NiFeMo)100−xMox alloy (x = 0 wt%, 0.25 wt%, 0.5 wt%, 0.75 wt%, 1.0 wt%, 1.25 wt%, 1.5 wt%, 2.0 wt%). The results of scanning electron microscopy (SEM), X-ray diffraction, electron backscatter diffraction (EBSD), DC B-H hysteresis tester, and transmission electron microscopy (Titan-G2) indicate that all samples are predominantly face-centered-cubic (FCC) γ- (Ni, Fe) solid solutions and typical soft-magnetic hysteresis line features. However, the lattice parameter a of γ- (Ni, Fe) decreased and then increased with increasing Mo content. When the added Mo content is 0–1.25 wt%, the Mo particles inside the (NiFeMo)100−xMox alloy become nucleation sites, creating a diffusive metallurgical behavior with the Ni and Fe elements, and preventing the growth of columnar crystals, leading to the reduction of LAGBs density and the enhancement of the {111} texture orientation in the easy magnetization direction, increasing Bs and the decrease of Hc of the NiFeMo alloy. At the Mo content of 1.25 wt%, the soft magnetic properties were optimal, with Bs and Hc being 0.704 T and 17.31 A/m, respectively. Conversely, when the added Mo content was 1.5–2.0 wt%, the solubility of Mo elements in the γ- (Ni, Fe) solid solution was limited, which resulted in the occurrence of metallurgical defects within the organization such as unfused Mo particles, unfused areas, and porosity. Moreover, when the added Mo content increased to 2.0 wt%, the {111} lattice spacing in the easy magnetization direction increased by 0.2261 nm compared to the NiFeMo alloy, and the average lattice distortion Δε increased to 0.16%, causing a decrease in the soft magnetic performance of the (NiFeMo)98Mo2 alloy, with Bs reducing to 0.6666 T and Hc increasing to 23.44 A/m.

Spray pyrolysis-based synthesis of pure molybdenum powder with nanoscale primary particles and its sintering characteristics

Molybdenum (Mo) is a refractory metal with exceptional high-temperature stability, resistance to creep, and low coefficient of thermal expansion, it thus has great potential for various high-temperature applications. Nevertheless, conventional methods of synthesizing Mo powder, such as mechanical milling and wet chemical processing, have encountered limitations in producing sintered bodies with high relative density and a fine grain structure while maintaining a low sintering temperature. Therefore, it is crucial to develop an efficient powder synthesis method that enables precise and uniform control over the powder's size and shape while minimizing contamination. Such an approach would contribute to achieving excellent formability and sinterability for the resulting Mo sintered body. In this study, we successfully demonstrated the ultrasonic spray pyrolysis (USP) technique in combination with hydrogen reduction for synthesizing pure Mo powder. The resulting Mo powder exhibited micro-sized secondary particles composed of nanoscale primary particles, leading to improved formability and sinterability, essential for fabricating high-quality sintered bodies. In order to comprehensively understand the properties of the as-prepared molybdenum oxide and hydrogen-reduced Mo powder, we employed various methods to analyze its physicochemical characteristics systematically. Subsequently, the fine Mo particles were subjected to a pressureless sintering process after a conventional uniaxial powder molding. Due to its excellent formability and sinterability, we achieved a Mo sintered body with a grain size of approximately 3 μm while maintaining a relatively high sintering density of 98.3% at a comparatively low sintering temperature of 1400 °C. The resulting microstructure contributed to a relatively high hardness of around 234 Hv. These results indicate the successful synthesis of pure Mo powder with desirable properties and highlight its potential in various applications, particularly those requiring high-quality sintered bodies with enhanced hardness.

The investigation of Sn corrosion on Mo meshes under the irradiation of high-flux hydrogen plasma

Liquid-metal divertor consisting of liquid Sn and Mo meshes would face high-flux plasma environments during service. In this work, the Sn corrosion behaviors on initial and pre-irradiated Mo meshes under hydrogen plasma irradiation were investigated to evaluate the reliability of the porous structure. According to the analysis and computation of XRD spectrum, the sub-grain size of the initial meshes would continuously grow during the corrosion process. The SEM images showed that after corrosion, the appearance and disappearance of prismatic Mo grains occurred alternately with the increase of plasma fluence. The thicknesses of corroded wires in the longitudinal and transverse directions displayed a similar repeatable change. In comparison to initial meshes, surface sub-grain growth and damage induced by the pre-irradiation treatment would result in differences in the corrosion process under hydrogen plasma irradiation. The pre-irradiated surface Mo grains with obvious grain boundaries were first dissolved to pebble-like Mo particles by liquid Sn and then stripped from the surface after corrosion. Sub-grain size was reduced from 72.5 nm to 41.1 nm at the fluence of 1.07 × 1026 ions/m2, which was inverse to the increasing phenomenon of the initial sample under same corrosion conditions. Additionally, the corrosion phenomenon, i.e., the presence and subsequent disappearance of prismatic Mo grains, occurred similarly on the surface of pre-irradiated samples. Based on the analysis of initial and pre-irradiated surface morphologies and wire thickness, Mo grains might go through an onion-peeling-like cycle of growth and fracture during the corrosion process. Besides, the wire thicknesses of all pre-irradiated Mo meshes were slightly less than that of the initial samples under the same corrosion conditions. Somehow, the comparative result reflected that the pre-irradiated samples were corroded more severely. Thus, the pre-irradiation of hydrogen plasma in this work would weaken corrosion resistance of Mo meshes in liquid Sn.

Annealing Temperature Effect on the Surface Properties of the MoSe Thin Films

2D transition metal dichalcogenides have been studied extensively in the field of electronics and photonics. Among them, the molybdenum chalcogenides have been receiving considerable attention due to their potential usage in field‐effect transistors and biosensors. Despite such promising aspects of these materials, studies regarding temperature effects on MoSe remain relatively rare. Herein, MoxSey (x = 0 ≈ 10, y = 0 ≈ 2) thin films are fabricated by radio frequency (RF) magnetron cosputtering on silicon and investigated using scanning electron microscopy for various atomic ratios and annealing temperatures from room temperature to 500 °C. Above the melting point of Se, Se evaporates and forms a layer, subsequently leading Mo to be exposed on the surface of the thin films. From the X‐ray diffraction and X‐ray photoelectron spectroscopy results, silicon peaks are observed due to the evaporation of Se. In addition, both Mo and Se are oxidized at above 300 °C. The work functions of the MoxSey thin films show the highest value at 200 °C measured by ultraviolet photoelectron spectroscopy and a Kelvin probe. Above the melting point of Se, there is a tendency for the work function to decrease due to the influence of Mo.

Quench initiation and normal zone propagation characteristics of smart insulated high-temperature superconducting coil

A smart insulation (SI) winding technique utilizing vanadium trioxide (V2O3) as a turn-to-turn insulation material is an effective means of achieving a suitable time constant and thermal stability in high-temperature superconducting (HTS) coils. However, the low electrical conductivity of the V2O3 makes the SI coil vulnerable to damage during quenching. To address this problem, this study explored a mixture of V2O3 and molybdenum (Mo) particles to improve the thermal stability of the SI coil. Two GdBCO coils, insulated with V2O3 and V2O3:Mo, respectively, were fabricated, and quench tests were performed at various operating currents in liquid nitrogen at 77 K. The findings revealed that the V2O3:Mo-insulated coil demonstrated improvements of 82%, 25%, and 160% in the minimum quench energy, and the normal-zone propagation velocities in the longitudinal and transverse directions, respectively, at 60% of the current-carrying capacity of the coil, which are essential performance parameters in the operation of a superconducting magnet.

Sputtering of Molybdenum as a Promising Back Electrode Candidate for Superstrate Structured Sb2 S3 Solar Cells.

Sb2 S3 is rapidly developed as light absorber material for solar cells due to its excellent photoelectric properties. However, the use of the organic hole transport layer of Spiro-OMeTAD and gold (Au) in Sb2 S3 solar cells imposes serious problems in stability and cost. In this work, low-cost molybdenum (Mo) prepared by magnetron sputtering is demonstrated to serve as a back electrode in superstrate structured Sb2 S3 solar cells for the first time. And a multifunctional layer of Se is inserted between Sb2 S3 /Mo interface by evaporation, which plays vital roles as: i) soft loading of high-energy Mo particles with the help of cottonlike-Se layer; ii) formation of surficial Sb2 Se3 on Sb2 S3 layer, and then reducing hole transportation barrier. To further alleviate the roll-over effect, a pre-selenide Mo target and consequentially form a MoSe2 is skillfully sputtered, which is expected to manipulate the band alignment and render an enhanced holes extraction. Impressively, the device with an optimized Mo electrode achieves an efficiency of 5.1%, which is one of the highest values among non-noble metal electrode based Sb2 S3 solar cells. This work sheds light on the potential development of low-cost metal electrodes for superstrate Sb2 S3 devices by carefully designing the back contact interface.

Morphological and mechanical evolution of α-Al2O3 reinforced Mo[sbnd]Cu alloy obtained by planetary ball milling

MoCu alloys were fabricated by the mechanical alloying method and consolidated by cold and hot pressing at 950 °C with the addition of ceramic α-Al2O3 (1–2.5 wt%). XRD was used to define the crystallinity and lattice defects of the mixed powder, milled powders, and hot-pressed samples after mechanical alloying experiments. The crystallite size of Mo and Cu significantly decreased after the milling process and the homogeneous phase distribution was found in the hot-pressed specimen, however, large agglomerates of Mo and α-Al2O3 particles were found in the mixed specimens. The lattice parameter and lattice strain values indicated the existence of a finer Cu fraction positioned on the grain boundaries of Cu. Based on the SEM-EBSD analysis, the isotropic crystallographic texture was found in all samples. The hardness (265–266 HB) and yield strength (849–878 MPa) were significantly increased in the bulk samples due to the effect of milling and relative density still reached 95 %. The hardness and yield strength were improved in the milled and mixed samples by increasing the α-Al2O3 content, however, formability was decreased equally.

Role of Precipitates on the Grain Coarsening of 20CrMnTi Gear Steel during Pseudo-Carburizing

The carburizing period for tool steel could be significantly shortened by operating at a higher carburizing temperature. However, grain coarsening happens during the carburizing process, and then results in the deteriorated surface properties in 20CrMnTi gear steel, especially at an elevated carburizing temperature. The relationships between grain coarsening and the precipitates in the developed 20CrMnTi gear steel during pseudo-carburizing were established by microstructure characterization, precipitate analysis and in-situ observation to clarify the coarsening mechanism. The results manifested the Baker–Nutting orientation relationship between the (Ti, Mo)(C, N) particles and the matrix, and then testified to the redissolution and ripening of the (Ti, Mo)(C, N) precipitates pre-formed in the α phase during the carburizing. Coarsening in austenite grain during the carburizing process was mainly caused by the rapid redissolution and ripening of the (Ti, Mo)(C, N) precipitates, although this occurred in a very short pseudo-carburizing time. The area density of the dispersed unripe (Ti, Mo)(C, N) particles markedly decreased from 0.389% in as-hot rolled gear steel to 0.341%, and then from 0.279% in carburized steels at 970 and 980 °C, respectively. Additionally, the redissolution and ripening of the (Ti, Mo)(C, N) precipitates were accelerated by the elevated carburizing temperature of 980 °C, at which time the growing rate in austenite grains was 2.34 μm/min during the prior 1 min (0.79 μm/min during the prior 3 min at 970 °C). The temperature then decreased to 0.003 μm/min in the subsequent carburizing process. The results obtained our current work reflected that the particles with excellent thermal stability should play important roles in the limitation of grain coarsening during the carburizing process.

Exploring avoiding brittle compounds in brazed joints of C/C composites and DD3 alloys through surface modification

Joining C/C composites to Ni-based single crystal alloy DD3 can significantly broaden their industrial application. The currently used active brazing method (ABM) may cause harmful reactions with Ni-based alloys, generating brittle compounds and deteriorating joint performance. In this work, a two-step method involving chromium carbide (Cr–C) surface modification and brazing using pure Cu filler is first developed to explore avoiding brittle compounds in joints. Surprisingly, the brazed seam of joints is mainly composed of (Cu, Ni) solid solution (s, s) without generating brittle compounds. At the DD3 interface, only (Ni, Cu) (s, s) and (Cu, Ni) (s, s) layers are formed. Besides, (W, Mo) (s, s) particles are precipitated nearby the DD3 substrate. For the C/C interface, the Cu filler is tightly joined to the Cr–C layer without forming new phases. The joint microstructure has no significant change with the variation of the Cr–C layer thickness. As increased the Cr–C layer thickness, the joint strength first increases and then decreases. The maximum shear strength of ∼32 MPa is obtained when the Cr–C layer thickness is ∼4.6 μm. The joint shear strength decreases with increased testing temperatures, which can still be maintained at ∼28 MPa at 600 °C. The surface modification and brazing strategy in this work can be further applied to the joining of other carbon materials and ceramics with poor wettability.