Reaction mechanism for in-situ β-SiAlON formation in Fe3Si-Si3N4-Al2O3 composites

In this work, Fe₃Si-Si₃N₄-Al₂O₃ composites were prepared at 1300℃ in an N₂ atmosphere using fused corundum and tabular alumina particles, Al₂O₃ fine powder, and ferrosilicon nitride (Fe₃Si-Si₃N₄) as raw materials and thermosetting phenolic resin as a binder. The effect of ferrosilicon nitride with different concentrations (0wt%, 5wt%, 10wt%, 15wt%, 20wt%, and 25wt%) on the properties of Fe₃Si-Si₃N₄-Al₂O₃ composites was investigated. The results show that the apparent porosity varies between 10.3% and 17.3%, the bulk density varies from 2.94 g/cm3 and 3.30 g/cm3, and the cold crushing strength ranges from 67 MPa to 93 MPa. Under the experimental conditions, ferrosilicon nitride, whose content decreases substantially, is unstable; part of the ferrosilicon nitride is converted into Fe₂C, whereas the remainder is retained, eventually forming the ferrosilicon alloy. Thermodynamic assessment of the Si₅AlON₇ indicated that the ferrosilicon alloy accelerated the reactions between Si₃N₄ and α-Al₂O₃ fine powder and that Si in the ferrosilicon alloy was nitrided directly, forming β-SiAlON simultaneously. In addition, fused corundum did not react directly with Si₃N₄ because of its low reactivity.     

In situ reaction mechanism of MgAlON in Al–Al_2O_3–MgO composites at 1700°C under flowing N_2

The Al–Al_2O_3–MgO composites with added aluminum contents of approximately 0wt%, 5wt%, and 10wt%, named as M_1, M_2, and M_3, respectively, were prepared at 1700°C for 5 h under a flowing N_2 atmosphere using the reaction sintering method. After sintering, the Al–Al_2O_3–MgO composites were characterized and analyzed by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results show that specimen M_1 was composed of MgO and MgAl_2O_4. Compared with specimen M_1, specimens M_2 and M_3 possessed MgAlON, and its production increased with increasing aluminum addition. Under an N_2 atmosphere, MgO, Al_2O_3, and Al in the matrix of specimens M_2 and M_3 reacted to form MgAlON and AlN-polytypoids, which combined the particles and the matrix together and imparted the Al–Al_2O_3–MgO composites with a dense structure. The mechanism of MgAlON synthesis is described as follows. Under an N_2 atmosphere, the partial pressure of oxygen is quite low; thus, when the Al–Al_2O_3–MgO composites were soaked at 580°C for an extended period, aluminum metal was transformed into AlN. With increasing temperature, Al_2O_3 diffused into AlN crystal lattices and formed AlN-polytypoids; however, MgO reacted with Al_2O_3 to form MgAl_2O_4. When the temperature was greater than(1640 ± 10)°C, AlN diffused into Al_2O_3 and formed spinel-structured AlON. In situ MgAlON was acquired through a solid-solution reaction between AlON and Mg Al_2O_4 at high temperatures because of their similar spinel structures.     

Electrical conductivity optimization of the Na_3AlF_6–Al_2O_3–Sm_2O_3 molten salts system for Al–Sm intermediate binary alloy production

Metal Sm has been widely used in making Al–Sm magnet alloy materials. Conventional distillation technology to produce Sm has the disadvantages of low productivity, high costs, and pollution generation. The objective of this study was to develop a molten salt electrolyte system to produce Al–Sm alloy directly, with focus on the electrical conductivity and optimal operating conditions to minimize the energy consumption. The continuously varying cell constant(CVCC) technique was used to measure the conductivity for the Na_3 AlF_6–AlF_3–LiF–MgF_2–Al_2O_3–Sm_2O_3 electrolysis medium in the temperature range from 905 to 1055°C. The temperature(t) and the addition of Al_2O_3(W(Al_2O_3)), Sm_2O_3(W(Sm_2O_3)), and a combination of Al_2O_3 and Sm_2O_3 into the basic fluoride system were examined with respect to their effects on the conductivity(κ) and activation energy. The experimental results showed that the molten electrolyte conductivity increases with increasing temperature(t) and decreases with the addition of Al_2O_3 or Sm_2O_3 or both. We concluded that the optimal operation conditions for Al–Sm intermediate alloy production in the Na_3 AlF_6–AlF_3–LiF–MgF_2–Al_2O_3–Sm_2O_3 system are W(Al_2O_3) + W(Sm_2O_3) = 3wt%, W(Al_2O_3):W(Sm_2O_3) = 7:3, and a temperature of 965 to 995°C, which results in satisfactory conductivity, low fluoride evaporation losses, and low energy consumption.     

Effect of Al_2O_3 on the viscosity of CaO–SiO_2–Al_2O_3–MgO–Cr_2O_3 slags

We investigated the effect of Al_2O_3 content on the viscosity of CaO–SiO_2–Al_2O_3–8wt%MgO–1wt%Cr_2O_3 (mass ratio of CaO/SiO_2is 1.0,and Al_2O_3 content is 17wt%–29wt%) slags.The results show that the viscosity of the slag increases gradually with increases in the Al_2O_3content in the range of 17wt%to 29wt%due to the role of Al_2O_3 as a network former in the polymerization of the aluminosilicate structure of the slag.With increases in the Al_2O_3 content from 17wt%to 29wt%,the apparent activation energy of the slags also increases from 180.85 to 210.23 k J/mol,which is consistent with the variation in the critical temperature.The Fourier-transform infrared spectra indicate that the degree of polymerization of this slag is increased by the addition of Al_2O_3.The application of Iida’s model for predicting the slag viscosity in the presence of Cr_2O_3 indicates that the calculated viscosity values fit well with the measured values when both the temperature and Al_2O_3 content are at relatively low levels,i.e.,the temperature range of 1673 to 1803 K and the Al_2O_3 content range of 17wt%–29wt%in CaO–SiO_2–Al_2O_3–8wt%MgO–1wt%Cr_2O_3 slag.     

Effect of Y_2O_3 content in the pack mixtures on microstructure and oxidation resistance of B–Y modified silicide coating

B–Y modified silicide coatings were prepared on Nb–Si based alloy by pack cementation at 1300 ℃ for 10 h. The effect of Y_2O_3 content in the pack mixtures on microstructure and oxidation resistance of the coatings was investigated. The results show that the four coatings have similar structures, which possess a(Nb,X)Si_2 outer layer and a(Nb,X)_5Si_3 transitional layer. Y_2O_3 content in the pack mixtures has an obvious effect on the Si content in the coating. The mass gains of the coatings prepared with 0.5, 1, 2 and 3 wt% Y_2O_3 in pack mixtures are 2.33, 1.96, 2.05 and 2.86 mg/cm~2 after oxidation at 1250 ℃ for 100 h, respectively. The coating prepared with 1 wt% Y_2O_3 exhibits the best oxidation resistance due to the formation of a dense glass-like borosilicate scale.     

Mesoporous fibrous silicon nitride by catalytic nitridation of silicon

The catalytic effect of metal oxide/alumina whiskers(CeO_2, Mn_3O_4, NiO, Co_3O_4, Fe_2O_3, Cr_2O_3/AW) was evaluated on their ability to drive the nitridation of silicon and to generate mesoporous fibrous silicon networks.Silicon powder with different particles size along with the catalyst was nitridized at 1300 °C for 5 h in nitrogen and nitrogen diluted with 10 vol% ammonia atmospheres. Nitridation degree of silicon up to 99% was recorded using 1.5 wt% CeO_2 and Fe_2O_3 catalysts in nitrogen-ammonia atmosphere. The catalyzed samples contain submicronic silicon nitride fibres with a diameter of 400–500 nm and a length of up to few micrometers. The compressive strength of 46 ± 1 MPa was measured for silicon samples catalyzed with nickel oxide/alumina whiskers and nitridized in N_2/10 vol%NH_3 atmosphere. Porous silicon nitride networks were produced with 45–52% porosity, pore sizes in the range of 370–1200 nm and median pore in the range of 495–1655 nm.     

Mineral-phase evolution and sintering behavior of MO–SiO_2–Al_2O_3–B_2O_3 (M = Ca,Ba) glass-ceramics by low-temperature liquid-phase sintering

In this work, network former SiO_2 and network intermediate Al_2O_3 were introduced into typical low-melting binary compositions CaO·B_2O_3, CaO·2B_2O_3, and BaO·B_2O_3 via an aqueous solid-state suspension milling route. Accordingly, multiple-phase aluminosilicate glass-ceramics were directly obtained via liquid-phase sintering at temperatures below 950°C. On the basis of liquid-phase sintering theory, mineral-phase evolutions and glass-phase formations were systematically investigated in a wide MO–SiO_2–Al_2O_3–B_2O_3(M = Ca, Ba) composition range. The results indicate that major mineral phases of the aluminosilicate glass-ceramics are Al_(20)B_4O_(36), CaAl_2Si_2O_8, and BaAl_2Si_2O_8 and that the glass-ceramic materials are characterized by dense microstructures and excellent dielectric properties.     

Effect of Fe_2O_3 on the crystallization behavior of glass-ceramics produced from naturally cooled yellow phosphorus furnace slag

CaO–Al_2O_3–SiO_2(CAS) glass-ceramics were prepared via a melting method using naturally cooled yellow phosphorus furnace slag as the main raw material.The effects of the addition of Fe_2O_3 on the crystallization behavior and properties of the prepared glass-ceramics were studied by differential thermal analysis,X-ray diffraction,and scanning electron microscopy.The crystallization activation energy was calculated using the modified Johnson–Mehl–Avrami equation.The results show that the intrinsic nucleating agent in the yellow phosphorus furnace slag could effectively promote the crystallization of CAS.The crystallization activation energy first increased and then decreased with increasing amount of added Fe_2O_3.At 4wt% of added Fe_2O_3,the crystallization activation energy reached a maximum of 676.374 k J×mol-1.The type of the main crystalline phase did not change with the amount of added Fe_2O_3.The primary and secondary crystalline phases were identified as wollastonite(CaSiO_3) and hedenbergite(Ca Fe(Si_2O_6)),respectively.     

Effect of reinforcement content on the adhesive wear behavior of Cu10Sn5Ni/Si_3N_4 composites produced by stir casting

The main objective of this paper was to fabricate Cu_(10)Sn_5Ni alloy and its composites reinforced with various contents of Si_3N_4 particles(5wt%, 10wt%, and 15wt%) and to investigate their dry sliding wear behavior using a pin-on-disk tribometer. Microstructural examinations of the specimens revealed a uniform dispersion of Si_3N_4 particles in the copper matrix. Wear experiments were performed for all combinations of parameters, such as load(10, 20, and 30 N), sliding distance(500, 1000, and 1500 m), and sliding velocity(1, 2, and 3 m/s), for the alloy and the composites. The results revealed that wear rate increased with increasing load and increasing sliding distance, whereas the wear rate decreased and then increased with increasing sliding velocity. The primary wear mechanism encountered at low loads was mild adhesive wear, whereas that at high loads was severe delamination wear. An oxide layer was formed at low velocities, whereas a combination of shear and plastic deformation occurred at high velocities. The mechanism at short sliding distances was ploughing action of Si_3N_4 particles, which act as protrusions; by contrast, at long sliding distances, direct metal–metal contact occurred. Among the investigated samples, the Cu/10wt% Si_3N_4 composite exhibited the best wear resistance at a load of 10 N, a velocity of 2 m/s, and a sliding distance of 500 m.     

Structure and corrosion resistance of Al–Cu–Fe alloys

The microstructure and electrochemical properties of Al–Cu–Fe alloys with the atomic compositions of Al_(65)Cu_(20)Fe_(15),Al_(78)Cu_7Fe_(15)and Al_(80)Cu_5Fe_(14)Si_1have been studied.The alloys were produced by induction melting of pure elements with copper mold casting.The microstructure of the alloys was analyzed by X-ray diffraction and high-resolution transmission electron microscopy.The formation of quasicrystalline phases in the Al–Cu–Fe alloys was confirmed.The presence of intermetallic phases was observed in the alloys after crystallization in a form of ingots and plates.The electrochemical measurements were conducted in 3.5%NaCl solution.The electronic structure of the alloys was determined by X-ray photoelectron spectroscopy.The post corrosion surface of the samples was checked using a scanning electron microscope equipped with the energydispersive X-ray detector.It was observed that the Al_(65)Cu_(20)Fe_(15)alloy had the highest corrosion resistance.The improved corrosion resistance parameters were noted for the plate samples rather than those in the as-cast state.And the hardness of the Al_(65)Cu_(20)Fe_(15)alloy was significantly higher than the other alloy samples.     

Microstructure and electrolysis behavior of self-healing Cu-Ni-Fe composite inert anodes for aluminum electrowinning

The microstructure evolution and electrolysis behavior of(Cu_(52)Ni_(30)Fe_(18))–x Ni Fe_2O_4(x=40wt%,50wt%,60wt%,and 70wt%)composite inert anodes for aluminum electrowinning were studied.Ni Fe_2O_4 was synthesized by solid-state reaction at 950°C.The dense anode blocks were prepared by ball-milling followed by sintering under a N_2 atmosphere.The phase evolution of the anodes after sintering was determined by scanning electron microscopy and energy-dispersive X-ray spectroscopy.The results indicate that a substitution reaction between Fe in the alloy phase and Ni in the oxide phase occurs during the sintering process.The samples were also examined as inert anodes for aluminum electrowinning in the low-temperature KF–NaF–AlF_3 molten electrolyte for 24 h.The cell voltage during electrolysis and the corrosion scale on the anodes were analyzed.The results confirm that the scale has a self-repairing function because of the synergistic reaction between the alloy phase with Fe added and the oxide phase.The estimated wear rate of the(Cu_(52)Ni_(30)Fe_(18))–50Ni Fe_2O_4 composite anode is 2.02 cm·a~(-1).     

Development of non-shrinkable ceramic composites for use in high-power microwave tubes

Al_2O_3–CaO–SiC-based ceramic composites with four different compositions were sintered at 1700℃ for 3 h in an air furnace. The phase analysis, microstructural characterization, and elemental composition determination of the developed composites were performed by X-ray diffraction(XRD), field-emission scanning electron microscopy(FESEM), and energy-dispersive X-ray(EDAX) analysis, respectively. The shrinkage, thermal properties, and electrical resistivity of the composites were also studied. The experimental results showed the effects of adding silicon carbide and calcia to alumina on the thermal, electrical, and shrinkage properties of the resultant composites. Among the four investigated ceramic composites, the one composed of 99 wt% alumina, 0.5 wt% CaO, and 0.5 wt% SiC exhibited the best characteristics for use as a potting material in a dispenser cathode of a microwave tube. The material exhibited slight expansion instead of shrinkage during drying or firing. Other properties of the composite powder, such as its thermal properties and electrical resistivity, were comparable to those of a commercial alumina powder.     

The evolution of microstructure and mechanical properties during high-speed direct-chill casting in different Al–Mg_2Si in situ composites

The effect of high-speed direct-chill(DC) casting on the microstructure and mechanical properties of Al–Mg_2Si in situ composites and AA6061 alloy was investigated. The microstructural evolution of the Al–Mg_2Si composites and AA6061 alloy was examined by optical microscopy, field-emission scanning electron microscopy(FE-SEM) and transmission electron microscopy(TEM). The results revealed that an increase of the casting speed substantially refined the primary Mg_2Si particles(from 28 to 12 μm), the spacing of eutectic Mg_2Si(from 3 to 0.5 μm), and the grains of AA6061 alloy(from 102 to 22 μm). The morphology of the eutectic Mg_2Si transformed from lamellar to rod-like and fibrous with increasing casting speed. The tensile tests showed that the yield strength, tensile strength, and elongation improved at higher casting speeds because of refinement of the Mg_2Si phase and the grains in the Al–Mg_2Si composites and the AA6061 alloy. High-speed DC casting is demonstrated to be an effective method to improve the mechanical properties of Al–Mg_2Si composites and AA6061 alloy billets.     

Microstructure and properties of a wear resistant Al–25Si–4Cu–1Mg coating prepared by supersonic plasma spraying

A high content silicon aluminum alloy(Al–25Si–4 Cu–1Mg) coating was prepared on a 2A12 aluminum alloy by supersonic plasma spraying. The morphology and microstructure of the coating were observed and analyzed. The hardness, elastic modulus, and bonding strength of the coating were measured. The wear resistance of the coating and 2A12 aluminum alloy was studied by friction and wear test. The results indicated that the coating was compact and the porosity was only 1.5%. The phase of the coating was mainly composed of α-Al and β-Si as well as some hard particles(Al_9Si,Al_(3.21)Si_(0.47), and CuAl_2). The average microhardness of the coating was HV 242, which was greater than that of 2 A12 aluminum alloy(HV 110). The wear resistance of the coating was superior to 2A12 aluminum alloy. The wear mechanism of the 2A12 aluminum alloy was primarily adhesive wear, while that of the coating was primarily abrasive wear. Therefore, it is possible to prepare a high content silicon aluminum alloy coating with good wear resistance on an aluminum alloy by supersonic plasma spraying.     

Solid fraction evolution characteristics of semi-solid A356 alloy and in-situ A356–TiB_2 composites investigated by differential thermal analysis

The key factor in semi-solid metal processing is the solid fraction at the forming temperature because it affects the microstructure and mechanical properties of the thixoformed components. Though an enormous amount of data exists on the solid fraction–temperature relationship in A356 alloy, information regarding the solid fraction evolution characteristics of A356-TiB2 composites is scarce. The present article establishes the temperature-solid fraction correlation in A356 alloy and A356-x TiB2(x = 2.5wt% and 5wt%) composites using differential thermal analysis(DTA). The DTA results indicate that the solidification characteristics of the composites exhibited a variation of 2°C and 3°C in liquidus temperatures and a variation of 3°C and 5°C in solidus temperatures with respect to the base alloy. Moreover, the eutectic growth temperature and the solid fraction(fs) vs. temperature characteristics of the composites were found to be higher than those of the base alloy. The investigation revealed that in-situ formed TiB2 particles in the molten metal introduced more nucleation sites and reduced undercooling.     

Development of high-strength,low-cost wrought Mg–2.0 mass% Zn alloy with high Mn content

Mg–Zn–Mn-based alloys have received considerable attention because of their high creep resistance, strength,and good corrosion resistance. The alloying element Mn in Mg–Zn-based alloys is commonly less than 1 wt%. In the present study, the effect of high Mn content(1 wt% and 2 wt%) on the microstructures and mechanical properties of Mg–2Zn–0.3Sr extruded alloy was investigated. The results revealed that the high Mn content significantly increased the ultimate tensile strength, tensile yield strength, compress yield strength, and yield asymmetry of the alloy without affecting its ductility. The dynamically recrystallized(DRXed) grains of Mg–2Zn–0.3Sr were remarkably refined because of the large amount of fine Mn precipitates in the homogenized alloy. The improved strengths were mainly attributed to the fine DRXed grains according to the Hall–Petch effect and to the large amount of spherical and 0001 Mn precipitates through the precipitation and dispersion strengthening. The fine DRXed grains and numerous Mn precipitates effectively suppressed the extension twining, substantially enhanced the compress yield strength, and resulted in improved anisotropy.     

Reaction mechanisms between molten CaF_2-based slags and molten 9CrMoCoB steel

Investigating the reaction mechanism between slag and 9CrMoCoB steel is important to develop the proper slag and produce qualified ingots in the electroslag remelting(ESR) process. Equilibrium reaction experiments between molten 9CrMoCoB steel and the slags of 55 wt%CaF_2–20 wt%CaO–3 wt%MgO–22 wt%Al_2O_3–xwt%B_2O_3(x = 0.0, 0.5, 1.0, 1.5, 2.0, 3.0) were conducted. The reaction mechanisms between molten 9 CrMoCoB steel and the slags with different B_2O_3 contents were deduced based on the composition of the steel and slag samples at different reaction times. Results show that B content in the steel can be controlled within the target range when the B_2O_3 content is 0.5 wt% and the FeO content ranges from 0.18 wt% to 0.22 wt% in the slag. When the B_2O_3 content is ≥1 wt%, the reaction between Si and B_2O_3 leads to the increase of the B content of steel. The additions of SiO_2 and B_2O_3 to the slag should accord to the mass ratio of [B]/[Si] in the electrode, and SiO_2 addition inhibits the reaction between Si and Al_2O_3.     

Enhanced microwave absorption properties of rod-shaped Fe2O3/Fe3O4/ MWCNTs composites

A novel type of composite absorber,i.e.Fe_2O_3/Fe_3O_4/MWCNTs composites(0%,1.7%and 5%MWCNTs),with microwave absorption properties was successfully fabricated by a facile hydrothermal method.The preparedα-Fe_2O_3/Fe_3O_4nanoparticles displayed rod-shaped morphology.The complex permittivity and permeability of the Fe_2O_3/Fe_3O_4/MWCNTs composites distinctly increased,furthermore,with the introduction of MWCNTs,the Fe_2O_3/Fe_3O_4/MWCNTs composites exhibited fine microwave absorption performance with strong absorption and wide absorption band.In particular,for Fe_2O_3/Fe_3O_4/1.7%MWCNTs composite with an absorber thickness of 2.5 mm,the reflection loss(RL)reached a minimum of-44.1 d B at 10.4 GHz and the effective absorption bandwidth(RL-10 d B)covered 3.3 GHz.The enhanced microwave absorption performance of the Fe_2O_3/Fe_3O_4/MWCNTs composites was attributed to the high dielectric loss and improved impedance matching which was closely related to the rod-shaped morphology of Fe_2O_3,Fe_3O_4and the introduction of MWCNTs.     

Effects of P addition on the microstructure and mechanical properties of Mg–5%Sn–1.25%Si alloy

The effects of Al-P addition on the microstructure and mechanical properties of as-cast Mg–5%Sn–1.25%Si magnesium alloy were investigated. The results show that the phases of the as-cast alloy are composed of α-Mg, Mg2 Sn, Mg2 Si, little P, and AlP. The Chinese character shape Mg2 Si phase changes into a granular morphology by P addition because AlP can act as a heterogeneous nucleation core for the Mg2 Si phase. When 0.225wt% of Al–3.5%P alloy is added, the mechanical properties of the Mg–5%Sn–1.25%Si alloy are greatly improved, and the tensile strength increases from 156 to 191 MPa, an increase of 22.4% compared to the alloy without P addition. When the amount of Al–3.5%P reaches 0.300wt%, a segregation phenomenon occurs in the granular Mg2 Si phase, and the tensile strength and hardness decrease though the elongation increases.     

Preparation and radar-absorbing properties of Al_2O_3/TiO_2/Fe_2O_3/Yb_2O_3 composite powder

Al_2O_3/Ti O_2/Fe_2O_3/Yb_2O_3 composite powder was synthesized via the sol–gel method. The structure,morphology,and radar-absorption properties of the composite powder were characterized by transmission electron microscopy,X-ray diffraction analysis and RF impedance analysis. The results show that two types of particles exist in the composite powder. One is irregular flakes(100–200 nm) and the other is spherical Al_2O_3 particles(smaller than 80 nm). Electromagnetic wave attenuation is mostly achieved by dielectric loss. The maximum value of the dissipation factor reaches 0.76(at 15.68 GHz) in the frequency range of 2–18 GHz. The electromagnetic absorption of waves covers 2–18 GHz with the matching thicknesses of 1.5–4.5 mm. The absorption peak shifts to the lower-frequency area with increasing matching thickness. The effective absorption band covers the frequency range of 2.16–9.76 GHz,and the maximum absorption peak reaches-20.18 d B with a matching thickness of 3.5 mm at a frequency of 3.52 GHz.