The effects of addition of minor amount of(0.5 wt.%) antimony(Sb) or gadolinium(Gd) and combined addition of Sb and Gd(0.5 wt.%,respectively) on the hot tearing susceptibility(HTS) of Mg-5Al-3Ca alloy were investigate...The effects of addition of minor amount of(0.5 wt.%) antimony(Sb) or gadolinium(Gd) and combined addition of Sb and Gd(0.5 wt.%,respectively) on the hot tearing susceptibility(HTS) of Mg-5Al-3Ca alloy were investigated experimentally using a “T-shaped” hot tearing measuring system. Various solidification parameters of the alloys were measured and calculated through thermal analysis experiments. The microstructure, grain size, and morphology of the crack zone were characterized by scanning electron microscopy and electron backscatter diffraction, and the crystal phases of the alloys were analyzed by X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of 0.5 wt.% Gd resulted in the increase in the vulnerable temperature range(Tv) and reduced the eutectic structure content that could participate in feeding, thereby improving the HTS of the alloy. However, addition of 0.5 wt.% Sb or combined addition of Gd and Sb(0.5 wt.%, respectively) to the Mg-5Al-3Ca alloy shortened the Tvand improved the skeleton strength of the alloy, thereby reducing HTS. Moreover, significantly refined structure of Mg-5Al-3Ca-0.5Gd-0.5Sb alloy improved the feeding ability of the eutectic structure, thus the alloy exhibited the lowest HTS.展开更多
The microstructures and mechanical properties of as-cast Mg-5 Sn-1 Si magnesium alloy modified with trace elements Y,Bi,Sb and Sr were investigated and compared.Results show that the microstructure of the as-cast Mg-5...The microstructures and mechanical properties of as-cast Mg-5 Sn-1 Si magnesium alloy modified with trace elements Y,Bi,Sb and Sr were investigated and compared.Results show that the microstructure of the as-cast Mg-5 Sn-1 Si alloy consists ofα-Mg,Mg_(2) Si,Mg_(2) Sn and Mg_(2)(Si_xSn_(1-x))phases.After adding 0.8 wt.%Y,0.3 wt.%Bi,0.9 wt.%Sb and 0.9 wt.%Sr,respectively into the Mg-5 Sn-1 Si magnesium alloy,Mg_(24)Y_(5),Mg_(3) Bi_(2),Mg_(3) Sb_(2) and Mg_(2) Sr phases are precipitated accordingly.Trace elements can refineα-Mg grain and Chinese scriptshaped Mg_(2) Si phase.Refinement efficiency of different trace elements onα-Mg grain and Mg_(2) Si phase is varied.Sr element has the best refinement effect,followed by Sb and Bi,while Y has the least refinement effect.Mg-5 Sn-1 Si-0.9 Sr alloy has higher tensile properties than the other three modified alloys.The refinement mechanism of Y,Bi and Sr elements on Mg-5 Sn-1 Si magnesium alloy can be explained by the growth restriction factors and the solute undercooling.For Mg-5 Sn-1 Si-0.9 Sb alloy,the heterogeneous nuclei of Mg_(3) Sb_(2) phase is the main reason for the refinement of grains and second phases.展开更多
The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5 Zn-3.5 Sn-1 Mn-0.5 Ca-0.5 Cu plate is studied.The simulation results show decreasing ...The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5 Zn-3.5 Sn-1 Mn-0.5 Ca-0.5 Cu plate is studied.The simulation results show decreasing temperature gradient in the plate with decreasing quenching rate,which leads to weakened inhomogeneous plastic deformation and decreased residual stress.No dynamic precipitation on the grain boundary happens after either cold water cooling or air cooling,however,air cooling leads to dynamic precipitation of Mg-Zn phase on Mn particles around which a low-density precipitate zone develops after aging treatment.Moreover,the fine and densely distributed Mg-Zn precipitates observed after aging treatment of the cold water cooled alloy are replaced by coarse precipitates with low density for the air cooled alloy.Both the low-density precipitate zone near Mn particles and the coarsening of precipitates are the source of the decrease in hardness and tensile properties of the air cooled alloy.The residual stress drops faster than the hardness with decreasing quenching rate,which makes it possible to lower the residual stress without sacrificing too much age-hardening ability of the alloy.展开更多
The effect of ultrasonic power and treatment time on degassing of Mg-6Zn-1Ca alloy was studied in this paper. The degassing effect was characterized by measuring densities of ingots. The results show that proper ultra...The effect of ultrasonic power and treatment time on degassing of Mg-6Zn-1Ca alloy was studied in this paper. The degassing effect was characterized by measuring densities of ingots. The results show that proper ultrasonic treatment can remove hydrogen from the melt of the Mg-6Zn-1Ca alloy. The ultrasonic degassing effect is closely related to the ultrasonic power density and treatment time. The degassing efficiency increases with an increase in ultrasonic power density when the melt is treated at 690 °C for 120 s, reaching its highest value at 1.2 W·cm-3. When the power density is 1.2 W·cm-3, with an increase in ultrasonic treatment time, the degassing efficiency increases at first, reaches its peak value at 120 s, then decreases as the ultrasonic treatment is further prolonged. In this experiment, the optimum degassing effect with an efficiency of 67.5 % is obtained by ultrasonic treatment with the power density of 1.2 W·cm-3 for 120 s. The maximum density of ingot can be increased from 1.8069 g·cm-3 to 1.8146 g·cm-3(increased by 0.43%).展开更多
The sliding friction and wear behaviors of Mg-11Y-5Gd-2Zn-0.5Zr (wt%) alloy were investigated under oil lubricant condition by pin-on-disk configuration with a constant sliding distance of 1,000 m in the temperature...The sliding friction and wear behaviors of Mg-11Y-5Gd-2Zn-0.5Zr (wt%) alloy were investigated under oil lubricant condition by pin-on-disk configuration with a constant sliding distance of 1,000 m in the temperature range of 25-200℃. Results indicate that the volumetric wear rates and average friction coefficients decrease with the increase of sliding speeds, and increase with the increase of test temperature below 150℃. The hard and thermally stable Mg12(Y,Gd)Zn phase with long-period stacking order structure in the alloy presents significant wear resistance, The wear mechanism below 100℃ is abrasive wear as a result of plastic extrusion deformation. The corporate effects of severe abrasive, oxidative, and delaminating wear result in the tribological mechanism above 100℃.展开更多
Fine grained Mg-7Gd-5Y-1.2Nd-0.5Zr alloy was investigated by dynamic compression tests using a Split Hopkinson Pressure Bar under the strain rates in the range 1000-2000 s^(-1) and the temperature range 293-573 K alon...Fine grained Mg-7Gd-5Y-1.2Nd-0.5Zr alloy was investigated by dynamic compression tests using a Split Hopkinson Pressure Bar under the strain rates in the range 1000-2000 s^(-1) and the temperature range 293-573 K along the normal direction.The microstructure was measured by optical microscopy,electron back-scattering diffraction,transmission electron microscopy and X-ray diffractometry.The results showed that Mg-7Gd-5Y-1.2Nd-0.5Zr alloy had the positive strain rate strengthening effect and thermal softening effect at high temperature.The solid solution of Gd and Y atoms in Mg-7Gd-5Y-1.2Nd-0.5Zr alloy reduced the asymmetry of α-Mg crystals and changed the critical shear stress of various deformation mechanisms.The main deformation mechanisms were prismatic slip and pyramidal(a)slip,{102}tension twinning,and dynamic recrystallization caused by local deformation such as particle-stimulated nucleation.c 2020 Published by Elsevier B.V.on behalf of Chongqing University.展开更多
To understand the solidification pathway and microstructure evolution of Mg-9Al-2Ca alloy,the cooling curve of the alloy solidified under furnace cooling was measured and the water-quenched samples were observed.The e...To understand the solidification pathway and microstructure evolution of Mg-9Al-2Ca alloy,the cooling curve of the alloy solidified under furnace cooling was measured and the water-quenched samples were observed.The experimental results show that the matrix phase of α-Mg dendrites is first generated at 596℃ during the solidification process,then the eutectic phases of Al_(2)Ca and Mg_(17)Al_(12) are formed at 518 and 447℃,respectively,and the solidification is terminated at 436℃.In the process of solidification,the seaweed dendrites of α-Mg get coarser and are gradually transformed into the global dendrites;besides,the secondary dendrite arms spacing(SDAS)of α-Mg as well as the solid fraction are both increased,while the increasing rate of SDAS of α-Mg and the solid fraction in the temperature region of 600-550℃ is faster than that in the temperature region of 550-436℃.And a power function relationship can be used to illustrate the change of the SDAS and the solid fraction with the temperature of solidification.展开更多
The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening.Adding a third component to form precipitation can improve their strength,but it usually leads to hig...The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening.Adding a third component to form precipitation can improve their strength,but it usually leads to high-stress corrosion cracking(SCC)sensitivity due to the formation of high-density precipitates at grain boundaries(GBs).So far,it is still challenging to improve the strength of Al-Mg alloys without re-ducing SCC resistance.Herein,a nanostructured Al-5Mg-3 Zn alloy with a good yield strength of 336 MPa and good elongation was successfully produced.By dynamic plastic deformation and appropriate anneal-ing treatment,near-equiaxed nanograins were introduced in the nanostructured Al-5Mg-3 Zn alloy with a high proportion(71%)of the low-angle grain boundary.TEM statistical investigations show that the pre-cipitation of active T’phase at GBs has been greatly suppressed in the nanostructured Al-5Mg-3 Zn alloy at sensitized conditions,and the area fraction of GB precipitates is reduced from 72%to 21%,which sig-nificantly decreases the SCC susceptibility.This study provides guidance for developing advanced Al-Mg alloy with high SCC resistance.展开更多
Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present resear...Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented "CRC" equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE (WE54) alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction (fs) in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.展开更多
In order to study the corrosion resistance of extruded magnesium alloys,the Mg-4Zn-2Gd-0.5Ca alloy was extruded at the speed of 0.01-0.1 mm/s with the temperature of 280-360℃in present study.Hot extrusion results sho...In order to study the corrosion resistance of extruded magnesium alloys,the Mg-4Zn-2Gd-0.5Ca alloy was extruded at the speed of 0.01-0.1 mm/s with the temperature of 280-360℃in present study.Hot extrusion results show that the volume fraction of precipitates(Vpre),VDRX(the dynamic recrystallization rate) and the average size of DRXed grain(dDRX) decrease with the decrease in extrusion speed,and the corrosion rate of the alloy also shows a downward trend.On the contrary,the values of Vpre,VDRX and dDRX increase with the increase in extrusion temperature,and the corrosion resistance of Mg-4Zn-2Gd-0.5Ca alloy decreases.When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280℃,the alloy has the best corrosion resistance.The corrosion of extruded Mg-4Zn-2Gd-0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone.With the further corrosion,the corrosion continues to spread along the phase,and the corrosion area gradually increases.Galvanic corrosion plays a leading role in the corrosion process.Moreover,there are a large number of basal plane textures in the unDRXed region,which is conducive to improving the corrosion resistance of magnesium alloys.In addition,the decrease in grain size also makes the corrosion of magnesium alloy more uniform.展开更多
As reported in our previous works, a Mg-7Gd-5Y-1Nd-0.5Zr alloy recently developed exhibited remarkable age-hardening responses and excellent mechanical properties at both room and elevated temperatures. In Mg-7Gd-5Y-1...As reported in our previous works, a Mg-7Gd-5Y-1Nd-0.5Zr alloy recently developed exhibited remarkable age-hardening responses and excellent mechanical properties at both room and elevated temperatures. In Mg-7Gd-5Y-1Nd-0.5Zr alloy, the β pre- cipitate phase was assumed to he one of the main strengthening phases in peak-aged samples. This study aimed to determine the crys- tal structure and orientation relationship of the β precipitate phase in Mg-7Gd-5Y-INd-0.5Zr alloy using transmission electron mi- croscopy and high-resolution electron microscopy. The results indicated that the β precipitate had a face-centered cubic structure with a lattice parameter of a=2.22 nm. The orientation relationship between the β precipitate phase and the ct-Mg matrix was (i-12)β(1-100)α, [110]β[0001 ]α. Theβ plates formed on prismatic planes could play an important role in alloy strengthening by proving effective barriers to gliding dislocations. A single β plate often contained several domains of (1 11)β twin-related variants. A composition of Mgs(Y0.4Gdo.4Nd0.2) was suggested for the β phase in Mg-7Gd-5Y-1Nd-0.5Zr alloy.展开更多
The effects of heat treatment on microstructures and hardening response of Mg-6Zn4).5Er4).5Ca (wt%) alloy were investigated by optical microscope (OM), scanning elec- tron microscope (SEM), and transmission el...The effects of heat treatment on microstructures and hardening response of Mg-6Zn4).5Er4).5Ca (wt%) alloy were investigated by optical microscope (OM), scanning elec- tron microscope (SEM), and transmission electron microscope (TEM) in this paper. The results show that the Mg(Zn4).5Er- 0.5Ca alloy contains Mg3Zn6Erx quasicrystalline phase (I- phase) and Ca2Mg6Zn3 phase under as-cast condition. Most of the Ca2Mg6Zn3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which in- creases about 40 % compared with that of as-cast alloy.展开更多
The corrosion behaviors of T5 (225 ℃, 6.5 h) and T6 (460 ℃, 2 h + 225 ℃, 12 h) peak-aged Mg-7Gd- 5Y-1Nd-0.5Zr alloys with oxide films were investigated by optical microscope (OM), scanning electron microsco...The corrosion behaviors of T5 (225 ℃, 6.5 h) and T6 (460 ℃, 2 h + 225 ℃, 12 h) peak-aged Mg-7Gd- 5Y-1Nd-0.5Zr alloys with oxide films were investigated by optical microscope (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The weight loss rates and electrochemical tests were also analyzed. The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0 μm, respectively. The components of oxide films mainly consist of O, Mg, Y, Nd, and Gd, and the T6 oxide film results in surfaces with larger peaks than T5 oxide film. In addition, Y, Nd, and Gd peaks are all higher than those of Mg-7Gd-5Y- 1Nd-0.5Zr alloys, but Mg peak is consistently far below than that of the alloys. The specimens could be arranged in de- creasing order of corrosion rates and corrosion current densi- ties: T6 oxide film 〈 T5 oxide film 〈 T6 without oxide film 〈 T5 without oxide film. The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-1Nd-0.5Zr alloys, which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.展开更多
基金financial support from Joint Research Fund Liaoning-Shenyang National Laboratory for Materials Science (2019JH3/30100014)Liaoning Bai Qian Wan Talents Program. Liaoning Revitalization Talents Program (Nos. XLYC1807021 and 1907007)+2 种基金High Level Innovation Team of Liaoning Province(XLYC1908006)Project of Liaoning Education Department(Nos. LQGD2019002, and LJGD2020008)Liaoning Nature Fund Guidance Plan (No. 2019-ZD-0210)。
文摘The effects of addition of minor amount of(0.5 wt.%) antimony(Sb) or gadolinium(Gd) and combined addition of Sb and Gd(0.5 wt.%,respectively) on the hot tearing susceptibility(HTS) of Mg-5Al-3Ca alloy were investigated experimentally using a “T-shaped” hot tearing measuring system. Various solidification parameters of the alloys were measured and calculated through thermal analysis experiments. The microstructure, grain size, and morphology of the crack zone were characterized by scanning electron microscopy and electron backscatter diffraction, and the crystal phases of the alloys were analyzed by X-ray diffraction and energy-dispersive X-ray spectroscopy. The results showed that the addition of 0.5 wt.% Gd resulted in the increase in the vulnerable temperature range(Tv) and reduced the eutectic structure content that could participate in feeding, thereby improving the HTS of the alloy. However, addition of 0.5 wt.% Sb or combined addition of Gd and Sb(0.5 wt.%, respectively) to the Mg-5Al-3Ca alloy shortened the Tvand improved the skeleton strength of the alloy, thereby reducing HTS. Moreover, significantly refined structure of Mg-5Al-3Ca-0.5Gd-0.5Sb alloy improved the feeding ability of the eutectic structure, thus the alloy exhibited the lowest HTS.
基金the financial support by the Natioal Natural Science Foundation of China(Nos.:51571086 and 51271073)the financial support from the Natural Science Foundation of Henan Polytechnic University(No.:B2010-20)。
文摘The microstructures and mechanical properties of as-cast Mg-5 Sn-1 Si magnesium alloy modified with trace elements Y,Bi,Sb and Sr were investigated and compared.Results show that the microstructure of the as-cast Mg-5 Sn-1 Si alloy consists ofα-Mg,Mg_(2) Si,Mg_(2) Sn and Mg_(2)(Si_xSn_(1-x))phases.After adding 0.8 wt.%Y,0.3 wt.%Bi,0.9 wt.%Sb and 0.9 wt.%Sr,respectively into the Mg-5 Sn-1 Si magnesium alloy,Mg_(24)Y_(5),Mg_(3) Bi_(2),Mg_(3) Sb_(2) and Mg_(2) Sr phases are precipitated accordingly.Trace elements can refineα-Mg grain and Chinese scriptshaped Mg_(2) Si phase.Refinement efficiency of different trace elements onα-Mg grain and Mg_(2) Si phase is varied.Sr element has the best refinement effect,followed by Sb and Bi,while Y has the least refinement effect.Mg-5 Sn-1 Si-0.9 Sr alloy has higher tensile properties than the other three modified alloys.The refinement mechanism of Y,Bi and Sr elements on Mg-5 Sn-1 Si magnesium alloy can be explained by the growth restriction factors and the solute undercooling.For Mg-5 Sn-1 Si-0.9 Sb alloy,the heterogeneous nuclei of Mg_(3) Sb_(2) phase is the main reason for the refinement of grains and second phases.
基金supported by the National Key Research and Development Program of China[2016YFB0301105]the National Key Research and Development Plan[2017YFB0103904]
文摘The effect of the quenching rate after solution treatment on the residual stress and precipitation behavior of a high strength Mg-5 Zn-3.5 Sn-1 Mn-0.5 Ca-0.5 Cu plate is studied.The simulation results show decreasing temperature gradient in the plate with decreasing quenching rate,which leads to weakened inhomogeneous plastic deformation and decreased residual stress.No dynamic precipitation on the grain boundary happens after either cold water cooling or air cooling,however,air cooling leads to dynamic precipitation of Mg-Zn phase on Mn particles around which a low-density precipitate zone develops after aging treatment.Moreover,the fine and densely distributed Mg-Zn precipitates observed after aging treatment of the cold water cooled alloy are replaced by coarse precipitates with low density for the air cooled alloy.Both the low-density precipitate zone near Mn particles and the coarsening of precipitates are the source of the decrease in hardness and tensile properties of the air cooled alloy.The residual stress drops faster than the hardness with decreasing quenching rate,which makes it possible to lower the residual stress without sacrificing too much age-hardening ability of the alloy.
基金supported by the Major State Basic Research Development Program of China(2013CB632203)the National Key Technology R&D Program of China(2012BAF09B01)+2 种基金the Liaoning Provincial Natural Science Foundation(Grant No.201202072)the Program for Liaoning Excellent Talents in University(Grant No.LJQ2012023)the Fundamental Research Foundation of Central Universities(N120509002 and N120309003)
文摘The effect of ultrasonic power and treatment time on degassing of Mg-6Zn-1Ca alloy was studied in this paper. The degassing effect was characterized by measuring densities of ingots. The results show that proper ultrasonic treatment can remove hydrogen from the melt of the Mg-6Zn-1Ca alloy. The ultrasonic degassing effect is closely related to the ultrasonic power density and treatment time. The degassing efficiency increases with an increase in ultrasonic power density when the melt is treated at 690 °C for 120 s, reaching its highest value at 1.2 W·cm-3. When the power density is 1.2 W·cm-3, with an increase in ultrasonic treatment time, the degassing efficiency increases at first, reaches its peak value at 120 s, then decreases as the ultrasonic treatment is further prolonged. In this experiment, the optimum degassing effect with an efficiency of 67.5 % is obtained by ultrasonic treatment with the power density of 1.2 W·cm-3 for 120 s. The maximum density of ingot can be increased from 1.8069 g·cm-3 to 1.8146 g·cm-3(increased by 0.43%).
基金supported by the National Natural Science Foundation of China (No. 51074106)Key HiTech Research and Development Program of China (No. 2009AA033501)National Key Technology R&D Program of China (No. 2011BAE22B01-5)
文摘The sliding friction and wear behaviors of Mg-11Y-5Gd-2Zn-0.5Zr (wt%) alloy were investigated under oil lubricant condition by pin-on-disk configuration with a constant sliding distance of 1,000 m in the temperature range of 25-200℃. Results indicate that the volumetric wear rates and average friction coefficients decrease with the increase of sliding speeds, and increase with the increase of test temperature below 150℃. The hard and thermally stable Mg12(Y,Gd)Zn phase with long-period stacking order structure in the alloy presents significant wear resistance, The wear mechanism below 100℃ is abrasive wear as a result of plastic extrusion deformation. The corporate effects of severe abrasive, oxidative, and delaminating wear result in the tribological mechanism above 100℃.
基金National Natural Science Foundation of China(Nos.51571145,51404137)City of Ningbo"science and technology innovation 2025"major special project(new energy vehicle lightweight magnesium alloy material precision forming technology)(No.2018B10045).
文摘Fine grained Mg-7Gd-5Y-1.2Nd-0.5Zr alloy was investigated by dynamic compression tests using a Split Hopkinson Pressure Bar under the strain rates in the range 1000-2000 s^(-1) and the temperature range 293-573 K along the normal direction.The microstructure was measured by optical microscopy,electron back-scattering diffraction,transmission electron microscopy and X-ray diffractometry.The results showed that Mg-7Gd-5Y-1.2Nd-0.5Zr alloy had the positive strain rate strengthening effect and thermal softening effect at high temperature.The solid solution of Gd and Y atoms in Mg-7Gd-5Y-1.2Nd-0.5Zr alloy reduced the asymmetry of α-Mg crystals and changed the critical shear stress of various deformation mechanisms.The main deformation mechanisms were prismatic slip and pyramidal(a)slip,{102}tension twinning,and dynamic recrystallization caused by local deformation such as particle-stimulated nucleation.c 2020 Published by Elsevier B.V.on behalf of Chongqing University.
基金Funded by the Ph D Research Startup Foundation of Jining University(No.2017BSZX02)。
文摘To understand the solidification pathway and microstructure evolution of Mg-9Al-2Ca alloy,the cooling curve of the alloy solidified under furnace cooling was measured and the water-quenched samples were observed.The experimental results show that the matrix phase of α-Mg dendrites is first generated at 596℃ during the solidification process,then the eutectic phases of Al_(2)Ca and Mg_(17)Al_(12) are formed at 518 and 447℃,respectively,and the solidification is terminated at 436℃.In the process of solidification,the seaweed dendrites of α-Mg get coarser and are gradually transformed into the global dendrites;besides,the secondary dendrite arms spacing(SDAS)of α-Mg as well as the solid fraction are both increased,while the increasing rate of SDAS of α-Mg and the solid fraction in the temperature region of 600-550℃ is faster than that in the temperature region of 550-436℃.And a power function relationship can be used to illustrate the change of the SDAS and the solid fraction with the temperature of solidification.
基金This research was financially supported by the National Nat-ural Science Foundation of China(Grant No.52171088)the Young Elite Scientists Sponsorship Program by CAST(grant No.2022QNRC001).We thank X.Si for assistance in sample prepara-tion.
文摘The strength of traditional Al-Mg alloys is relatively low because it mainly relies on solid solution strengthening.Adding a third component to form precipitation can improve their strength,but it usually leads to high-stress corrosion cracking(SCC)sensitivity due to the formation of high-density precipitates at grain boundaries(GBs).So far,it is still challenging to improve the strength of Al-Mg alloys without re-ducing SCC resistance.Herein,a nanostructured Al-5Mg-3 Zn alloy with a good yield strength of 336 MPa and good elongation was successfully produced.By dynamic plastic deformation and appropriate anneal-ing treatment,near-equiaxed nanograins were introduced in the nanostructured Al-5Mg-3 Zn alloy with a high proportion(71%)of the low-angle grain boundary.TEM statistical investigations show that the pre-cipitation of active T’phase at GBs has been greatly suppressed in the nanostructured Al-5Mg-3 Zn alloy at sensitized conditions,and the area fraction of GB precipitates is reduced from 72%to 21%,which sig-nificantly decreases the SCC susceptibility.This study provides guidance for developing advanced Al-Mg alloy with high SCC resistance.
基金financially supported by the National Basic Research Program of China(No.2013CB632202)
文摘Hot tearing is a common and severe defect occurring during solidification of castings. The rational understand- ing of hot tearing formation mechanism is beneficial to the foundry process design. In the present research, a new developed instrumented "CRC" equipment was applied in characterization of hot tearing in sand cast Mg-5 wt.% Y-4 wt.% RE (WE54) alloy with and without Zr addition. Microstructure observation and thermal analysis were carried out to help analyzing the results. The results showed that hot tearing onset occurs at a relatively low solid fraction (fs) in WE54 alloy sand castings, which indicates the participation of remaining liquid during hot tearing formation. Microstructure observation of the hot tearing surface also proves the liquid film existence between solidifying dendrites. The contraction strain caused by casting solidification induces the flowing of remaining liquid between solidifying dendrites and results in formation of interdendritic liquid films. These liquid films are separated by sufficient contraction stress and form hot cracks. The addition of Zr in WE54 alloy significantly refines the alloy microstructure and increases the solid fraction at hot tearing onset, both of which result in increasing of the fracture stress of interdendritic liquid film. Thus the hot tearing susceptibility of WE54 alloy is weakened by Zr addition.
基金financially supported by the National Natural Science Foundation of China(Nos.51771128,51771129 and 51201112)the Projects of International Cooperation in Shanxi(No.201703D421039)the Natural Science Foundation of Shanxi(No.201601D011034).
文摘In order to study the corrosion resistance of extruded magnesium alloys,the Mg-4Zn-2Gd-0.5Ca alloy was extruded at the speed of 0.01-0.1 mm/s with the temperature of 280-360℃in present study.Hot extrusion results show that the volume fraction of precipitates(Vpre),VDRX(the dynamic recrystallization rate) and the average size of DRXed grain(dDRX) decrease with the decrease in extrusion speed,and the corrosion rate of the alloy also shows a downward trend.On the contrary,the values of Vpre,VDRX and dDRX increase with the increase in extrusion temperature,and the corrosion resistance of Mg-4Zn-2Gd-0.5Ca alloy decreases.When the extrusion speed is 0.01 mm/s and the extrusion temperature is 280℃,the alloy has the best corrosion resistance.The corrosion of extruded Mg-4Zn-2Gd-0.5Ca alloy occurs preferentially on the magnesium matrix around W and I phases in the DRXed zone.With the further corrosion,the corrosion continues to spread along the phase,and the corrosion area gradually increases.Galvanic corrosion plays a leading role in the corrosion process.Moreover,there are a large number of basal plane textures in the unDRXed region,which is conducive to improving the corrosion resistance of magnesium alloys.In addition,the decrease in grain size also makes the corrosion of magnesium alloy more uniform.
基金supported by the National "Twelfth Five-Year Plan" Key Technologies Program of China (2011DAE22B01)
文摘As reported in our previous works, a Mg-7Gd-5Y-1Nd-0.5Zr alloy recently developed exhibited remarkable age-hardening responses and excellent mechanical properties at both room and elevated temperatures. In Mg-7Gd-5Y-1Nd-0.5Zr alloy, the β pre- cipitate phase was assumed to he one of the main strengthening phases in peak-aged samples. This study aimed to determine the crys- tal structure and orientation relationship of the β precipitate phase in Mg-7Gd-5Y-INd-0.5Zr alloy using transmission electron mi- croscopy and high-resolution electron microscopy. The results indicated that the β precipitate had a face-centered cubic structure with a lattice parameter of a=2.22 nm. The orientation relationship between the β precipitate phase and the ct-Mg matrix was (i-12)β(1-100)α, [110]β[0001 ]α. Theβ plates formed on prismatic planes could play an important role in alloy strengthening by proving effective barriers to gliding dislocations. A single β plate often contained several domains of (1 11)β twin-related variants. A composition of Mgs(Y0.4Gdo.4Nd0.2) was suggested for the β phase in Mg-7Gd-5Y-1Nd-0.5Zr alloy.
基金financial supported by the National Natural Science Foundation of China(No.51301006)the Projects of Beijing Municipal Science and Technology Commission(No.Z131100003213019)+1 种基金the Projects of Beijing Municipal Commission of Education(Nos.KM201310005001 and KM201410005014)Beijing Natural Science Foundation(No.2142005)
文摘The effects of heat treatment on microstructures and hardening response of Mg-6Zn4).5Er4).5Ca (wt%) alloy were investigated by optical microscope (OM), scanning elec- tron microscope (SEM), and transmission electron microscope (TEM) in this paper. The results show that the Mg(Zn4).5Er- 0.5Ca alloy contains Mg3Zn6Erx quasicrystalline phase (I- phase) and Ca2Mg6Zn3 phase under as-cast condition. Most of the Ca2Mg6Zn3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which in- creases about 40 % compared with that of as-cast alloy.
基金financially supported by the National Natural Science Foundation of China(No.51204020)the National Basic Research Program of China(Nos.2013CB632202 and 2013CB632205)
文摘The corrosion behaviors of T5 (225 ℃, 6.5 h) and T6 (460 ℃, 2 h + 225 ℃, 12 h) peak-aged Mg-7Gd- 5Y-1Nd-0.5Zr alloys with oxide films were investigated by optical microscope (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The weight loss rates and electrochemical tests were also analyzed. The thicknesses of T5 and T6 oxide films are roughly 0.6 and 1.0 μm, respectively. The components of oxide films mainly consist of O, Mg, Y, Nd, and Gd, and the T6 oxide film results in surfaces with larger peaks than T5 oxide film. In addition, Y, Nd, and Gd peaks are all higher than those of Mg-7Gd-5Y- 1Nd-0.5Zr alloys, but Mg peak is consistently far below than that of the alloys. The specimens could be arranged in de- creasing order of corrosion rates and corrosion current densi- ties: T6 oxide film 〈 T5 oxide film 〈 T6 without oxide film 〈 T5 without oxide film. The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-1Nd-0.5Zr alloys, which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.