Search2018 Vol. 25, No. 7
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2018, vol. 25, no. 7, pp.
729-736.
https://doi.org/10.1007/s12613-018-1620-9
Abstract:
Rutile (TiO2) is heavily used in pigments and colorants, and the most abundant source of rutile is ilmenite. Upon oxidation of ilmenite, rutile can be formed with modest energy consumption; furthermore, after leaching, only a few byproducts are formed. Unfortunately, one drawback is the necessarily long oxidative process of typically used methods. In this study, we show that a fluidized bed reactor can be used to oxidize ilmenite ore to rapidly form rutile and pseudobrookite (Fe2TiO5) phases. Ilmenite was oxidized with 5vol% O2 in Ar at temperatures of 1173 K or 1223 K and subsequently leached using a diluted H2SO4 solution to dissolve the pseudobrookite phase. The effects of acid concentration, temperature, and cooling rate after oxidation were investigated. We show that the ilmenite was rapidly oxidized to form rutile and pseudobrookite phases at 1173 and 1223 K in a 5vol% O2/95vol% Ar environment within 40 min. The final maximum rutile yield was 84.2mol% after leaching in (1 + 1) H2SO4 solution at 393 K for 12 h.
Rutile (TiO2) is heavily used in pigments and colorants, and the most abundant source of rutile is ilmenite. Upon oxidation of ilmenite, rutile can be formed with modest energy consumption; furthermore, after leaching, only a few byproducts are formed. Unfortunately, one drawback is the necessarily long oxidative process of typically used methods. In this study, we show that a fluidized bed reactor can be used to oxidize ilmenite ore to rapidly form rutile and pseudobrookite (Fe2TiO5) phases. Ilmenite was oxidized with 5vol% O2 in Ar at temperatures of 1173 K or 1223 K and subsequently leached using a diluted H2SO4 solution to dissolve the pseudobrookite phase. The effects of acid concentration, temperature, and cooling rate after oxidation were investigated. We show that the ilmenite was rapidly oxidized to form rutile and pseudobrookite phases at 1173 and 1223 K in a 5vol% O2/95vol% Ar environment within 40 min. The final maximum rutile yield was 84.2mol% after leaching in (1 + 1) H2SO4 solution at 393 K for 12 h.
2018, vol. 25, no. 7, pp.
737-743.
https://doi.org/10.1007/s12613-018-1621-8
Abstract:
Electrochemical measurements were conducted to study the electrochemical behavior of gold (Au) and its commonly associated minerals in alkaline thiourea solutions. The results indicated that without addition of any stabilizer, selective dissolution of Au from stibnite and pyrite was only possible at relatively low thiourea concentrations. As Na2SiO3 was added, pyrite started to become active and an oxidation peak appeared; the oxidation peaks of arsenopyrite and chalcocite appeared earlier than that of Au. The chalcocite peak shifted in the positive direction and the peak current increased. Stibnite did not show an oxidation peak and its current was nearly zero. Adding Na2SiO3 favored the selective dissolution of Au when its minerals were associated with chalcocite and stibnite. At pH 12, the Au anode dissolution peak current increased with stabilizer concentration. At 0.38 and 0.42 V and for Na2SiO3 concentration below 0.09 M, the current density continuously increased with Na2SiO3 concentration. The Na2SiO3 concentration had to be adequate to stabilize thiourea. When the potential was higher than 0.42 V, the surface of the Au electrode started to passivate. With an additional increase in potential, the presence of Na2SiO3 could not stop the inevitable decomposition of thiourea.
Electrochemical measurements were conducted to study the electrochemical behavior of gold (Au) and its commonly associated minerals in alkaline thiourea solutions. The results indicated that without addition of any stabilizer, selective dissolution of Au from stibnite and pyrite was only possible at relatively low thiourea concentrations. As Na2SiO3 was added, pyrite started to become active and an oxidation peak appeared; the oxidation peaks of arsenopyrite and chalcocite appeared earlier than that of Au. The chalcocite peak shifted in the positive direction and the peak current increased. Stibnite did not show an oxidation peak and its current was nearly zero. Adding Na2SiO3 favored the selective dissolution of Au when its minerals were associated with chalcocite and stibnite. At pH 12, the Au anode dissolution peak current increased with stabilizer concentration. At 0.38 and 0.42 V and for Na2SiO3 concentration below 0.09 M, the current density continuously increased with Na2SiO3 concentration. The Na2SiO3 concentration had to be adequate to stabilize thiourea. When the potential was higher than 0.42 V, the surface of the Au electrode started to passivate. With an additional increase in potential, the presence of Na2SiO3 could not stop the inevitable decomposition of thiourea.
2018, vol. 25, no. 7, pp.
744-751.
https://doi.org/10.1007/s12613-018-1622-7
Abstract:
The sticking phenomenon between molten slag and refractory is one of the crucial problems when preparing ferronickel from laterite ore using rotary hearth furnace or rotary kiln processes. This study aims to ameliorate sticking problems by using silicon dioxide (SiO2) to adjust the melting degree of the briquette during reduction roasting. Thermodynamic analysis indicates that the melting temperature of the slag gradually increases with an increase in the SiO2 proportion (SiO2/(SiO2 + Al2O3 + MgO) mass ratio). Experimental validations also prove that the briquette retains its original shape when the SiO2 proportion is greater than 75wt%, and sticking problems are avoided during reduction. A ferronickel product with 8.33wt% Ni and 84.71wt% Fe was prepared via reductive roasting at 1500℃ for 90 min with a SiO2 proportion of 75wt% and a C/O molar ratio of 1.0 followed by dry magnetic separation; the corresponding recoveries of Ni and Fe reached 75.70% and 77.97%, respectively. The microstructure and phase transformation of reduced briquette reveals that the aggregation and growth of ferronickel particles were not significantly affected after adding SiO2 to the reduction process.
The sticking phenomenon between molten slag and refractory is one of the crucial problems when preparing ferronickel from laterite ore using rotary hearth furnace or rotary kiln processes. This study aims to ameliorate sticking problems by using silicon dioxide (SiO2) to adjust the melting degree of the briquette during reduction roasting. Thermodynamic analysis indicates that the melting temperature of the slag gradually increases with an increase in the SiO2 proportion (SiO2/(SiO2 + Al2O3 + MgO) mass ratio). Experimental validations also prove that the briquette retains its original shape when the SiO2 proportion is greater than 75wt%, and sticking problems are avoided during reduction. A ferronickel product with 8.33wt% Ni and 84.71wt% Fe was prepared via reductive roasting at 1500℃ for 90 min with a SiO2 proportion of 75wt% and a C/O molar ratio of 1.0 followed by dry magnetic separation; the corresponding recoveries of Ni and Fe reached 75.70% and 77.97%, respectively. The microstructure and phase transformation of reduced briquette reveals that the aggregation and growth of ferronickel particles were not significantly affected after adding SiO2 to the reduction process.
2018, vol. 25, no. 7, pp.
752-761.
https://doi.org/10.1007/s12613-018-1623-6
Abstract:
Cold-bonded pellets, to which a new type of inorganic binder was applied, were reduced by H2–CO mixtures with different H2/CO molar ratios (1:0, 5:2, 1:1, 2:5, and 0:1) under various temperatures (1023, 1123, 1223, 1323, and 1423 K) in a thermogravimetric analysis apparatus. The effects of gas composition, temperature, and binder ratio on the reduction process were studied, and the microstructure of reduced pellets was observed by scanning electron microscopy–energy-dispersive spectrometry (SEM-EDS). The SEM-EDS images show that binder particles exist in pellets in two forms, and the form that binder particles completely surround ore particles has a more significant hinder effect on the reduction. The reduction equilibrium constant, effective diffusion coefficient, and the reaction rate constant were calculated on the basis of the unreacted core model, and the promotion effect of temperature on reduction was further analyzed. The results show that no sintering phenomenon occurred at low temperatures and that the increasing reaction rate constant and high gas diffusion coefficient could maintain the promotion effect of temperature; however, when the sintering phenomenon occurs at high temperatures, gas diffusion is hindered and the promotion effect is diminished. The contribution of the overall equilibrium constant to the promotion effect depends on the gas composition.
Cold-bonded pellets, to which a new type of inorganic binder was applied, were reduced by H2–CO mixtures with different H2/CO molar ratios (1:0, 5:2, 1:1, 2:5, and 0:1) under various temperatures (1023, 1123, 1223, 1323, and 1423 K) in a thermogravimetric analysis apparatus. The effects of gas composition, temperature, and binder ratio on the reduction process were studied, and the microstructure of reduced pellets was observed by scanning electron microscopy–energy-dispersive spectrometry (SEM-EDS). The SEM-EDS images show that binder particles exist in pellets in two forms, and the form that binder particles completely surround ore particles has a more significant hinder effect on the reduction. The reduction equilibrium constant, effective diffusion coefficient, and the reaction rate constant were calculated on the basis of the unreacted core model, and the promotion effect of temperature on reduction was further analyzed. The results show that no sintering phenomenon occurred at low temperatures and that the increasing reaction rate constant and high gas diffusion coefficient could maintain the promotion effect of temperature; however, when the sintering phenomenon occurs at high temperatures, gas diffusion is hindered and the promotion effect is diminished. The contribution of the overall equilibrium constant to the promotion effect depends on the gas composition.
2018, vol. 25, no. 7, pp.
762-769.
https://doi.org/10.1007/s12613-018-1624-5
Abstract:
Monodispersed copper oxalate particles with flaky morphology were prepared via a simple one-pot synthesis method. Scanning electron microscope (SEM), X-ray diffraction (XRD), and fourier transform infrared (FTIR) spectra were used to characterize particle morphology, size, phase composition, and functional groups. It was found that the presence of ethylenediaminetetraacetic acid (EDTA) and the solution pH value had strong influence on the morphological and size evolution of the precipitated particles. On the basis of controlled release of copper ions from a Cu2+–EDTA complex and Weimarn’s law, a strategy for the controlled synthesis of monodispersed copper oxalate particles was designed by referring to the basic mode of the Stöber method. The inherent nature of crystallization to form the flaky solid in the early stage of precipitation as well as the driving force of the long-lasting low supersaturation in the growth stage was proposed to explain the size and morphological evolution of the copper oxalate precipitates. Thermodynamic equilibrium concentrations of copper(Ⅱ) species in the Cu(Ⅱ)–EDTA–oxalate–H2O solution system were calculated to help explain the possible formation mechanism of copper oxalate precipitates.
Monodispersed copper oxalate particles with flaky morphology were prepared via a simple one-pot synthesis method. Scanning electron microscope (SEM), X-ray diffraction (XRD), and fourier transform infrared (FTIR) spectra were used to characterize particle morphology, size, phase composition, and functional groups. It was found that the presence of ethylenediaminetetraacetic acid (EDTA) and the solution pH value had strong influence on the morphological and size evolution of the precipitated particles. On the basis of controlled release of copper ions from a Cu2+–EDTA complex and Weimarn’s law, a strategy for the controlled synthesis of monodispersed copper oxalate particles was designed by referring to the basic mode of the Stöber method. The inherent nature of crystallization to form the flaky solid in the early stage of precipitation as well as the driving force of the long-lasting low supersaturation in the growth stage was proposed to explain the size and morphological evolution of the copper oxalate precipitates. Thermodynamic equilibrium concentrations of copper(Ⅱ) species in the Cu(Ⅱ)–EDTA–oxalate–H2O solution system were calculated to help explain the possible formation mechanism of copper oxalate precipitates.
2018, vol. 25, no. 7, pp.
770-778.
https://doi.org/10.1007/s12613-018-1625-4
Abstract:
The wear resistances of austempered ductile iron (ADI) were improved through introduction of a new phase (carbide) into the matrix by addition of chromium. In the present investigation, low-carbon-equivalent ductile iron (LCEDI) (CE=3.06%, and CE represents carbon-equivalent) with 2.42% chromium was selected. LCEDI was austenitized at two different temperatures (900 and 975℃) and soaked for 1 h and then quenched in a salt bath at 325℃ for 0 to 10 h. Samples were analyzed using optical microscopy and X-ray diffraction. Wear tests were carried out on a pin-on-disk-type machine. The effect of austenization temperature on the wear resistance, impact strength, and the microstructure was evaluated. A structure–property correlation based on the observations is established.
The wear resistances of austempered ductile iron (ADI) were improved through introduction of a new phase (carbide) into the matrix by addition of chromium. In the present investigation, low-carbon-equivalent ductile iron (LCEDI) (CE=3.06%, and CE represents carbon-equivalent) with 2.42% chromium was selected. LCEDI was austenitized at two different temperatures (900 and 975℃) and soaked for 1 h and then quenched in a salt bath at 325℃ for 0 to 10 h. Samples were analyzed using optical microscopy and X-ray diffraction. Wear tests were carried out on a pin-on-disk-type machine. The effect of austenization temperature on the wear resistance, impact strength, and the microstructure was evaluated. A structure–property correlation based on the observations is established.
2018, vol. 25, no. 7, pp.
779-787.
https://doi.org/10.1007/s12613-018-1626-3
Abstract:
This work investigated the flow-accelerated corrosion (FAC) behavior of 13Cr in a wet CO2-containing environment at different flowing gas velocities and impinging angles, with the natural-gas pipeline environment simulated by a self-assembled impingement jet system. Surface morphology determination, electrochemical measurements, and hydromechanics numerical analysis were carried out to study the FAC behavior. The results demonstrate that pitting corrosion was the primary mode of corrosion in 13Cr stainless steel. High-flow-rate gas destroyed the passive film and decreased the pitting potential, resulting in more serious corrosion. The corrosion degree with various impact angles showed the following order: 90° > 60° > 45°. The shear force and the electrolyte from the flowing gas were concluded to be the determinant factors of FAC, whereas the shear force was the main factor responsible for destroying the passive film.
This work investigated the flow-accelerated corrosion (FAC) behavior of 13Cr in a wet CO2-containing environment at different flowing gas velocities and impinging angles, with the natural-gas pipeline environment simulated by a self-assembled impingement jet system. Surface morphology determination, electrochemical measurements, and hydromechanics numerical analysis were carried out to study the FAC behavior. The results demonstrate that pitting corrosion was the primary mode of corrosion in 13Cr stainless steel. High-flow-rate gas destroyed the passive film and decreased the pitting potential, resulting in more serious corrosion. The corrosion degree with various impact angles showed the following order: 90° > 60° > 45°. The shear force and the electrolyte from the flowing gas were concluded to be the determinant factors of FAC, whereas the shear force was the main factor responsible for destroying the passive film.
2018, vol. 25, no. 7, pp.
788-799.
https://doi.org/10.1007/s12613-018-1627-2
Abstract:
Nickel-based alloys exhibit excellent high-temperature strength and oxidation resistance; however, because of coarse grains and severe segregation in their welding joints, these alloys exhibit increased susceptibility to hot cracking. In this paper, to improve the hot-cracking resistance and mechanical properties of nickel-based alloy welded joints, sodium thiosulfate was used to simulate crystallization, enabling the nucleation mechanism under mechanical vibration to be investigated. On the basis of the results, the grain refinement mechanism during the gas tungsten arc welding (GTAW) of Inconel 601H alloy under various vibration modes and parameters was investigated. Compared with the GTAW process, the low-frequency mechanical vibration processes resulted in substantial grain refinement effects in the welds; thus, a higher hardness distribution was also achieved under the vibration conditions. In addition, the γ' phase exhibited a dispersed distribution and segregation was improved in the welded joints with vibration assistance. The results demonstrated that the generation of free crystals caused by vibration in the nucleation stage was the main mechanism of grain refinement. Also, fine equiaxed grains and a dispersed γ' phase were found to improve the grain-boundary strength and reduce the segregation, contributing to preventing the initiation of welding hot cracking in nickel-based alloys.
Nickel-based alloys exhibit excellent high-temperature strength and oxidation resistance; however, because of coarse grains and severe segregation in their welding joints, these alloys exhibit increased susceptibility to hot cracking. In this paper, to improve the hot-cracking resistance and mechanical properties of nickel-based alloy welded joints, sodium thiosulfate was used to simulate crystallization, enabling the nucleation mechanism under mechanical vibration to be investigated. On the basis of the results, the grain refinement mechanism during the gas tungsten arc welding (GTAW) of Inconel 601H alloy under various vibration modes and parameters was investigated. Compared with the GTAW process, the low-frequency mechanical vibration processes resulted in substantial grain refinement effects in the welds; thus, a higher hardness distribution was also achieved under the vibration conditions. In addition, the γ' phase exhibited a dispersed distribution and segregation was improved in the welded joints with vibration assistance. The results demonstrated that the generation of free crystals caused by vibration in the nucleation stage was the main mechanism of grain refinement. Also, fine equiaxed grains and a dispersed γ' phase were found to improve the grain-boundary strength and reduce the segregation, contributing to preventing the initiation of welding hot cracking in nickel-based alloys.
2018, vol. 25, no. 7, pp.
800-809.
https://doi.org/10.1007/s12613-018-1628-1
Abstract:
The effects of Zn content on the microstructure and the mechanical and corrosion properties of as-cast low-alloyed Mg–xZn–0.2Ca alloys (x=0.6wt%, 2.0wt%, 2.5wt%, hereafter denoted as 0.6Zn, 2.0Zn, and 2.5Zn alloys, respectively) are investigated. The results show that the Zn content not only influences grain refinement but also induces different phase precipitation behaviors. The as-cast microstructure of the 0.6Zn alloy is composed of α-Mg, Mg2Ca, and Ca2Mg6Zn3 phases, whereas 2.0Zn and 2.5Zn alloys only contain α-Mg and Ca2Mg6Zn3 phases, as revealed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Moreover, with increasing Zn content, both the ultimate tensile strength (UTS) and the elongation to fracture first increase and then decrease. Among the three investigated alloys, the largest UTS (178 MPa) and the highest elongation to fracture (6.5%) are obtained for the 2.0Zn alloy. In addition, the corrosion rate increases with increasing Zn content. This paper provides an updated investigation of the alloy composition–microstructure–property relationships of different Zn-containing Mg–Zn–Ca alloys.
The effects of Zn content on the microstructure and the mechanical and corrosion properties of as-cast low-alloyed Mg–xZn–0.2Ca alloys (x=0.6wt%, 2.0wt%, 2.5wt%, hereafter denoted as 0.6Zn, 2.0Zn, and 2.5Zn alloys, respectively) are investigated. The results show that the Zn content not only influences grain refinement but also induces different phase precipitation behaviors. The as-cast microstructure of the 0.6Zn alloy is composed of α-Mg, Mg2Ca, and Ca2Mg6Zn3 phases, whereas 2.0Zn and 2.5Zn alloys only contain α-Mg and Ca2Mg6Zn3 phases, as revealed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. Moreover, with increasing Zn content, both the ultimate tensile strength (UTS) and the elongation to fracture first increase and then decrease. Among the three investigated alloys, the largest UTS (178 MPa) and the highest elongation to fracture (6.5%) are obtained for the 2.0Zn alloy. In addition, the corrosion rate increases with increasing Zn content. This paper provides an updated investigation of the alloy composition–microstructure–property relationships of different Zn-containing Mg–Zn–Ca alloys.
2018, vol. 25, no. 7, pp.
810-816.
https://doi.org/10.1007/s12613-018-1629-0
Abstract:
Ti3SiC2-reinforced Ag-matrix composites are expected to serve as electrical contacts. In this study, the wettability of Ag on a Ti3SiC2 substrate was measured by the sessile drop method. The Ag–Ti3SiC2 composites were prepared from Ag and Ti3SiC2 powder mixtures by pressureless sintering. The effects of compacting pressure (100–800 MPa), sintering temperature (850–950℃), and soaking time (0.5–2 h) on the microstructure and properties of the Ag–Ti3SiC2 composites were investigated. The experimental results indicated that Ti3SiC2 particulates were uniformly distributed in the Ag matrix, without reactions at the interfaces between the two phases. The prepared Ag–10wt%Ti3SiC2 had a relative density of 95% and an electrical resistivity of 2.76×10-3 mΩ·cm when compacted at 800 MPa and sintered at 950℃ for 1 h. The incorporation of Ti3SiC2 into Ag was found to improve its hardness without substantially compromising its electrical conductivity; this behavior was attributed to the combination of ceramic and metallic properties of the Ti3SiC2 reinforcement, suggesting its potential application in electrical contacts.
Ti3SiC2-reinforced Ag-matrix composites are expected to serve as electrical contacts. In this study, the wettability of Ag on a Ti3SiC2 substrate was measured by the sessile drop method. The Ag–Ti3SiC2 composites were prepared from Ag and Ti3SiC2 powder mixtures by pressureless sintering. The effects of compacting pressure (100–800 MPa), sintering temperature (850–950℃), and soaking time (0.5–2 h) on the microstructure and properties of the Ag–Ti3SiC2 composites were investigated. The experimental results indicated that Ti3SiC2 particulates were uniformly distributed in the Ag matrix, without reactions at the interfaces between the two phases. The prepared Ag–10wt%Ti3SiC2 had a relative density of 95% and an electrical resistivity of 2.76×10-3 mΩ·cm when compacted at 800 MPa and sintered at 950℃ for 1 h. The incorporation of Ti3SiC2 into Ag was found to improve its hardness without substantially compromising its electrical conductivity; this behavior was attributed to the combination of ceramic and metallic properties of the Ti3SiC2 reinforcement, suggesting its potential application in electrical contacts.
2018, vol. 25, no. 7, pp.
817-823.
https://doi.org/10.1007/s12613-018-1630-7
Abstract:
The spontaneous infiltration and wetting behaviors of a Zr-based alloy melt on porous a SiC ceramic plate were studied using the sessile drop method by continuous heating and holding for 1800 s at different temperatures in a high-vacuum furnace. The results showed that the Zr-based alloy melt could partly infiltrate the porous SiC substrate without pressure due to the effect of capillary pressure. Wettability and infiltration rates increased with increasing temperature, and interfacial reaction products (ZrC0.7 and TiC) were detected in the Zr-based alloy/SiC ceramic system, likely because of the reaction of the active elements Zr and Ti with elemental C. Furthermore, the redundant element Si diffused into the alloy melt.
The spontaneous infiltration and wetting behaviors of a Zr-based alloy melt on porous a SiC ceramic plate were studied using the sessile drop method by continuous heating and holding for 1800 s at different temperatures in a high-vacuum furnace. The results showed that the Zr-based alloy melt could partly infiltrate the porous SiC substrate without pressure due to the effect of capillary pressure. Wettability and infiltration rates increased with increasing temperature, and interfacial reaction products (ZrC0.7 and TiC) were detected in the Zr-based alloy/SiC ceramic system, likely because of the reaction of the active elements Zr and Ti with elemental C. Furthermore, the redundant element Si diffused into the alloy melt.
Microstructure and mechanical properties of Nb–Mo–ZrB2 composites prepared by hot-pressing sintering
2018, vol. 25, no. 7, pp.
824-831.
https://doi.org/10.1007/s12613-018-1631-6
Abstract:
Nb–Mo–ZrB2 composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB2 were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB2 composites increases with increasing ZrB2 content; Nb–Mo–30vol%ZrB2 had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB2 composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB2 content. Finally, the fracture modes of the Nb–Mo–ZrB2 composites are also discussed in detail.
Nb–Mo–ZrB2 composites (V(Nb)/V(Mo)=1) with 15vol% or 30vol% of ZrB2 were fabricated by hot-pressing sintering at 2000℃. The phases, microstructure, and mechanical properties were then investigated. The composites contain Nb-Mo solid solution (denoted as (Nb, Mo)ss hereafter), ZrB, MoB, and NbB phases. Compressive strength test results suggest that the strength of Nb–Mo–ZrB2 composites increases with increasing ZrB2 content; Nb–Mo–30vol%ZrB2 had the highest compressive strength (1905.1 MPa). The improvement in the compressive strength of the Nb–Mo–ZrB2 composites is mainly attributed to the secondary phase strengthening of the stiffer ZrB phase, solid-solution strengthening of the (Nb, Mo)ss matrix as well as fine-grain strengthening. The fracture toughness decreases with increasing ZrB2 content. Finally, the fracture modes of the Nb–Mo–ZrB2 composites are also discussed in detail.
2018, vol. 25, no. 7, pp.
832-839.
https://doi.org/10.1007/s12613-018-1632-5
Abstract:
In this study, the fabrication of multilayer Al(Zn)–Al2O3 with different volume fractions of Al2O3 was investigated. Al and ZnO powders were milled by a planetary ball mill, after which five-layer functionally graded samples were produced through hot pressing at 580℃ and 90 MPa pressure for 30 min. Formation of reinforcing Al2O3 particles occurred in the aluminum matrix via the aluminothermic reaction. Determination of the ignition temperature of the aluminothermic reaction was accomplished using differential thermal and thermogravimetric analyses. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometery analyses were utilized to characterize the specimens. The thermal analysis results showed that the ignition temperatures for the aluminothermic reaction of layers with the highest and lowest ZnO contents were 667 and 670℃, respectively. Microstructural observation and chemical analysis confirmed the fabrication of Al(Zn)–Al2O3 functionally graded materials composites with precipitation of additional Zn in the matrix. Moreover, nearly dense functionally graded samples demonstrated minimum and maximum hardness values of HV 75 and HV 130, respectively.
In this study, the fabrication of multilayer Al(Zn)–Al2O3 with different volume fractions of Al2O3 was investigated. Al and ZnO powders were milled by a planetary ball mill, after which five-layer functionally graded samples were produced through hot pressing at 580℃ and 90 MPa pressure for 30 min. Formation of reinforcing Al2O3 particles occurred in the aluminum matrix via the aluminothermic reaction. Determination of the ignition temperature of the aluminothermic reaction was accomplished using differential thermal and thermogravimetric analyses. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffractometery analyses were utilized to characterize the specimens. The thermal analysis results showed that the ignition temperatures for the aluminothermic reaction of layers with the highest and lowest ZnO contents were 667 and 670℃, respectively. Microstructural observation and chemical analysis confirmed the fabrication of Al(Zn)–Al2O3 functionally graded materials composites with precipitation of additional Zn in the matrix. Moreover, nearly dense functionally graded samples demonstrated minimum and maximum hardness values of HV 75 and HV 130, respectively.
2018, vol. 25, no. 7, pp.
840-848.
https://doi.org/10.1007/s12613-018-1633-4
Abstract:
In this study, we investigated the effect of the addition of Sr (0wt%, 0.1wt%, 0.2wt%, and 0.3wt%) on the microstructure and corrosion behavior of Al3Ti/ADC12 composite by optical microscopy, X-ray diffraction, scanning electron microscopy, and energy diffraction spectroscopy. The results reveal that the α-Al phases were nearly spherical and 40 μm in size and that the eutectic Si phases became round in the composite when the Sr content reached 0.2wt%. The Al3Ti particles were distributed uniformly at the grain boundary. The results of the corrosion examination reveal that the Al3Ti/ADC12 composite exhibited a minimum corrosion rate of 0.081 g·m–2·h–1 for an Sr content of 0.2wt%, which is two thirds of that of unmodified composite (0.134 g·m–2·h–1). This improved corrosion resistance was due to galvanic corrosion, which resulted from the low area ratio of the cathode to anode regions. This caused a low-density corrosion current in the composite, thereby yielding optimum corrosion resistance.
In this study, we investigated the effect of the addition of Sr (0wt%, 0.1wt%, 0.2wt%, and 0.3wt%) on the microstructure and corrosion behavior of Al3Ti/ADC12 composite by optical microscopy, X-ray diffraction, scanning electron microscopy, and energy diffraction spectroscopy. The results reveal that the α-Al phases were nearly spherical and 40 μm in size and that the eutectic Si phases became round in the composite when the Sr content reached 0.2wt%. The Al3Ti particles were distributed uniformly at the grain boundary. The results of the corrosion examination reveal that the Al3Ti/ADC12 composite exhibited a minimum corrosion rate of 0.081 g·m–2·h–1 for an Sr content of 0.2wt%, which is two thirds of that of unmodified composite (0.134 g·m–2·h–1). This improved corrosion resistance was due to galvanic corrosion, which resulted from the low area ratio of the cathode to anode regions. This caused a low-density corrosion current in the composite, thereby yielding optimum corrosion resistance.
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