This paper describes the synthesis of Al7075 metal matrix composites reinforced with SiC, and the characterization of their microstructure and mechanical behavior. The mechanically milled Al7075 micron-sized powder an...This paper describes the synthesis of Al7075 metal matrix composites reinforced with SiC, and the characterization of their microstructure and mechanical behavior. The mechanically milled Al7075 micron-sized powder and SiC nanoparticles are dynamically compacted using a drop hammer device. This compaction is performed at different temperatures and for various volume fractions of SiC nanoparticles. The relative density is directly related to the compaction temperature rise and indirectly related to the content of SiC nanoparticle reinforcement, respectively. Furthermore, increasing the amount of SiC nanoparticles improves the strength, stiffness, and hardness of the compacted specimens. The increase in hardness and strength may be attributed to the inherent hardness of the nanoparticles, and other phenomena such as thermal mismatch and crack shielding. Nevertheless, clustering of the nanoparticles at aluminum particle boundaries make these regions become a source of concentrated stress, which reduces the load carrying capacity of the compacted nanocomposite.展开更多
In this research, development of AI356-AI203 nanocomposite coatings has been investigated. AI356-AI203 composite powders were prepared by mechanical milling of AI356 powder and 5 vol.% micro and nanoscaled alumina par...In this research, development of AI356-AI203 nanocomposite coatings has been investigated. AI356-AI203 composite powders were prepared by mechanical milling of AI356 powder and 5 vol.% micro and nanoscaled alumina particles. The milled powders were used as feedstock to deposit composite coatings on A356-T6 aluminum alloy substrate using high velocity oxy-fuel （HVOF） process. X-ray diffractometry, optical and scanning electron microscopy, microhardness and wear tests were used to characterize the composite powders and coatings. The hardness of composite coatings containing micro and nanosized AI203 were 114.1 ± 5.9 HV and 138.4 ± 6.9 HV, respectively which were higher than those for substrate （79.2 ± 1.1 HV）. Nano and microcomposite coatings revealed low friction coefficients and wear rates, which were significantly lower than those obtained for AI356-T6 substrate. Addition of 5 vol.% micro and nanoscaled alumina particles improved the wear resistance by an average of 85% and 91%, respectively. This is mainly caused by the presence of AI203 in matrix and nanocrystalline structure of matrix. Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.展开更多
Grain growth of nanostructured Al6061produced by cryorolling and aging process was investigated during isothermalheat treatment in100?500°C temperature range.Transmission electron microscopy(TEM)observations demo...Grain growth of nanostructured Al6061produced by cryorolling and aging process was investigated during isothermalheat treatment in100?500°C temperature range.Transmission electron microscopy(TEM)observations demonstrate that aftercryorolling and aging at130°C for30h,the microstructure contains61nm grains with dispersed50?150nm precipitates and0.248%lattice strain.In addition,an increase in tensile strength up to362MPa because of formation of fine strengtheningprecipitation and nano-sized grains was observed.Thermal stability investigation within100?500°C temperature range showedrelease of lattice strain,dissolution of precipitates and grain growth.According to the X-ray diffraction(XRD)analysis,Mg2Siprecipitates disappeared after annealing at temperatures higher than300°C.According to the results,due to the limited grain growthup to200°C,there would be little decrease in mechanical properties,but within300?500°C range,the grain growth,dissolution ofstrengthening precipitates and decrease in mechanical properties are remarkable.The activation energies for grain growth werecalculated to be203.3kJ/mol for annealing at100?200°C and166.34kJ/mol for annealing at300?500°C.The effect ofprecipitation dissolution on Al lattice parameter,displacement of Al6061(111)XRD peak and Portevin?LeChatelier(PLC)effect onstress?strain curves is also discussed.展开更多
The non-isothermal crystallization kinetics of Ni55Nb35Si10 amorphous alloy,prepared by mechanical alloying,was studied using differential scanning calorimetry.The amorphous alloy showed one-stage crystallization on h...The non-isothermal crystallization kinetics of Ni55Nb35Si10 amorphous alloy,prepared by mechanical alloying,was studied using differential scanning calorimetry.The amorphous alloy showed one-stage crystallization on heating,which led to the formation of nano-intermetallic crystals in amorphous matrix.The apparent activation energy for the crystallization of the alloy,determined by the Kissinger equation,was relatively high(468 kJ/mol),indicating that this amorphous alloy has high thermal stability.Changes in the activation energy during the crystallization process,were also evaluated by iso-conversional methods.The results showed that it decreases slowly from the beginning to crystallized fractionα=0.35 and it remains almost constant to the end of the process.The nano-crystallization mechanism for the non-isothermal crystallization of the amorphous alloy was explained by determining Avrami exponents.Transmission electron microscopy studies revealed the microstructural modification of amorphous alloy via nanocrystallization during annealing.The results suggest that the nucleation rate decreases with increasing time and the crystallization mechanism is governed dominantly by a three-dimensional diffusion-controlled growth.A predictive equation was obtained based on the Sestak-Berggren autocatalytic model to describe quantitatively the non-isothermal crystallization kinetics.展开更多
Bulk Al/Al_3Zr composite was prepared by a combination of mechanical alloying(MA) and hot extrusion processes. Elemental Al and Zr powders were milled for up to 10 h and heat treated at 600℃ for 1 h to form stable Al...Bulk Al/Al_3Zr composite was prepared by a combination of mechanical alloying(MA) and hot extrusion processes. Elemental Al and Zr powders were milled for up to 10 h and heat treated at 600℃ for 1 h to form stable Al_3Zr. The prepared Al_3Zr powder was then mixed with the pure Al powder to produce an Al–Al_3Zr composite. The composite powder was finally consolidated by hot extrusion at 550℃. The mechanical properties of consolidated samples were evaluated by hardness and tension tests at room and elevated temperatures. The results show that annealing of the 10-h-milled powder at 600℃ for 1 h led to the formation of a stable Al_3Zr phase. Differential scanning calorimetry(DSC) results confirmed that the formation of Al_3Zr began with the nucleation of a metastable phase, which subsequently transformed to the stable tetragonal Al_3Zr structure. The tension yield strength of the Al-10wt%Al_3Zr composite was determined to be 103 MPa, which is approximately twice that for pure Al(53 MPa). The yield stress of the Al/Al_3Zr composite at 300℃ is just 10% lower than that at room temperature, which demonstrates the strong potential for the prepared composite to be used in high-temperature structural applications.展开更多
文摘This paper describes the synthesis of Al7075 metal matrix composites reinforced with SiC, and the characterization of their microstructure and mechanical behavior. The mechanically milled Al7075 micron-sized powder and SiC nanoparticles are dynamically compacted using a drop hammer device. This compaction is performed at different temperatures and for various volume fractions of SiC nanoparticles. The relative density is directly related to the compaction temperature rise and indirectly related to the content of SiC nanoparticle reinforcement, respectively. Furthermore, increasing the amount of SiC nanoparticles improves the strength, stiffness, and hardness of the compacted specimens. The increase in hardness and strength may be attributed to the inherent hardness of the nanoparticles, and other phenomena such as thermal mismatch and crack shielding. Nevertheless, clustering of the nanoparticles at aluminum particle boundaries make these regions become a source of concentrated stress, which reduces the load carrying capacity of the compacted nanocomposite.
文摘In this research, development of AI356-AI203 nanocomposite coatings has been investigated. AI356-AI203 composite powders were prepared by mechanical milling of AI356 powder and 5 vol.% micro and nanoscaled alumina particles. The milled powders were used as feedstock to deposit composite coatings on A356-T6 aluminum alloy substrate using high velocity oxy-fuel （HVOF） process. X-ray diffractometry, optical and scanning electron microscopy, microhardness and wear tests were used to characterize the composite powders and coatings. The hardness of composite coatings containing micro and nanosized AI203 were 114.1 ± 5.9 HV and 138.4 ± 6.9 HV, respectively which were higher than those for substrate （79.2 ± 1.1 HV）. Nano and microcomposite coatings revealed low friction coefficients and wear rates, which were significantly lower than those obtained for AI356-T6 substrate. Addition of 5 vol.% micro and nanoscaled alumina particles improved the wear resistance by an average of 85% and 91%, respectively. This is mainly caused by the presence of AI203 in matrix and nanocrystalline structure of matrix. Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.
文摘Grain growth of nanostructured Al6061produced by cryorolling and aging process was investigated during isothermalheat treatment in100?500°C temperature range.Transmission electron microscopy(TEM)observations demonstrate that aftercryorolling and aging at130°C for30h,the microstructure contains61nm grains with dispersed50?150nm precipitates and0.248%lattice strain.In addition,an increase in tensile strength up to362MPa because of formation of fine strengtheningprecipitation and nano-sized grains was observed.Thermal stability investigation within100?500°C temperature range showedrelease of lattice strain,dissolution of precipitates and grain growth.According to the X-ray diffraction(XRD)analysis,Mg2Siprecipitates disappeared after annealing at temperatures higher than300°C.According to the results,due to the limited grain growthup to200°C,there would be little decrease in mechanical properties,but within300?500°C range,the grain growth,dissolution ofstrengthening precipitates and decrease in mechanical properties are remarkable.The activation energies for grain growth werecalculated to be203.3kJ/mol for annealing at100?200°C and166.34kJ/mol for annealing at300?500°C.The effect ofprecipitation dissolution on Al lattice parameter,displacement of Al6061(111)XRD peak and Portevin?LeChatelier(PLC)effect onstress?strain curves is also discussed.
基金supported by the Future Material Discovery Program of the National Research Foundation of Korea(NRF) funded by the Ministry of Science,ICT and Future Planning(MSIP)of Korea(2016M3D1A1023532)
文摘The non-isothermal crystallization kinetics of Ni55Nb35Si10 amorphous alloy,prepared by mechanical alloying,was studied using differential scanning calorimetry.The amorphous alloy showed one-stage crystallization on heating,which led to the formation of nano-intermetallic crystals in amorphous matrix.The apparent activation energy for the crystallization of the alloy,determined by the Kissinger equation,was relatively high(468 kJ/mol),indicating that this amorphous alloy has high thermal stability.Changes in the activation energy during the crystallization process,were also evaluated by iso-conversional methods.The results showed that it decreases slowly from the beginning to crystallized fractionα=0.35 and it remains almost constant to the end of the process.The nano-crystallization mechanism for the non-isothermal crystallization of the amorphous alloy was explained by determining Avrami exponents.Transmission electron microscopy studies revealed the microstructural modification of amorphous alloy via nanocrystallization during annealing.The results suggest that the nucleation rate decreases with increasing time and the crystallization mechanism is governed dominantly by a three-dimensional diffusion-controlled growth.A predictive equation was obtained based on the Sestak-Berggren autocatalytic model to describe quantitatively the non-isothermal crystallization kinetics.
文摘Bulk Al/Al_3Zr composite was prepared by a combination of mechanical alloying(MA) and hot extrusion processes. Elemental Al and Zr powders were milled for up to 10 h and heat treated at 600℃ for 1 h to form stable Al_3Zr. The prepared Al_3Zr powder was then mixed with the pure Al powder to produce an Al–Al_3Zr composite. The composite powder was finally consolidated by hot extrusion at 550℃. The mechanical properties of consolidated samples were evaluated by hardness and tension tests at room and elevated temperatures. The results show that annealing of the 10-h-milled powder at 600℃ for 1 h led to the formation of a stable Al_3Zr phase. Differential scanning calorimetry(DSC) results confirmed that the formation of Al_3Zr began with the nucleation of a metastable phase, which subsequently transformed to the stable tetragonal Al_3Zr structure. The tension yield strength of the Al-10wt%Al_3Zr composite was determined to be 103 MPa, which is approximately twice that for pure Al(53 MPa). The yield stress of the Al/Al_3Zr composite at 300℃ is just 10% lower than that at room temperature, which demonstrates the strong potential for the prepared composite to be used in high-temperature structural applications.
