To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were ...To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s^(-1) under the temperature gradients of 15-20 K·mm^(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V >20 μm·s^(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s^(-1) under a temperature gradient of 15 K·mm^(-1). The increase of temperature gradient to 20 K·mm^(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and展开更多
The effect of excitation current intensity on the mechanical properties of ZL205 A castings solidified under a traveling magnetic field was studied. The results of the experiment indicate that the excitation current i...The effect of excitation current intensity on the mechanical properties of ZL205 A castings solidified under a traveling magnetic field was studied. The results of the experiment indicate that the excitation current intensity of the traveling magnetic field has a great influence on the mechanical properties of the ZL205 A castings. When the excitation current intensity is 15 A, the tensile strength and elongation of ZL205 A alloy castings increase 27.2% and 67.7%, respectively, compared with those of the same alloy solidified under gravity. The improvement of mechanical properties is attributed to the decrease of micro-porosity in the alloy. Under the traveling magnetic field, the feeding pressure in the alloy melt before solidification can be enhanced due to the electromagnetic force. Moreover, the melt flow induced by the traveling magnetic field can decrease the temperature gradient. The feeding resistance will be increased because the temperature gradient decrease. So traveling magnetic field has an optimum effect on feeding.展开更多
Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel ...Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.展开更多
Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density.In this regard,the Al-Cu-Li alloy castings with hi...Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density.In this regard,the Al-Cu-Li alloy castings with high Li content from 1.5 wt.%to 4.5 wt.%were prepared by near-rapid solidification,followed by two-stage homogenization treatment(490℃/16 h and 530℃/16 h).The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software.The results indicate that with the increase of Li content,the grain sizes decrease,the solution ability of Cu in the matrixα-Al phase increases,while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification.Additionally,by the coupling of near-rapid solidification and two-stage homogenization,the metastable precipitated phases(Al7Cu4Li and AlCu3)can be dissolved effectively in the alloys with Li content of 1.5 wt.%-2.5 wt.%;moreover,the stable precipitated phases(Al6CuLi3 and Al2CuLi)uniformly distribute at the grain boundaries in the alloys with Li content of 3.5 wt.%-4.5 wt.%.As a result,the refined and homogenized microstructure can be obtained.展开更多
In the present research, microstructure refinement of a high-Nb TiAl alloy(Ti-48Al-8Nb-0.15B) was realized by means of the electromagnetic continuous casting(EMCC) technique. The microstructure of an ingot obtained by...In the present research, microstructure refinement of a high-Nb TiAl alloy(Ti-48Al-8Nb-0.15B) was realized by means of the electromagnetic continuous casting(EMCC) technique. The microstructure of an ingot obtained by EMCC was analyzed using scanning electron microscopy(SEM). As compared with the raw as-cast al oy, the obtained EMCC alloy presented a much finer microstructure with lamellar colonies with a mean size of about 50-70 μm because the electromagnetic stirring broke initial dendrites and enhanced the heterogeneous nucleation. As the grains were refined, the properties of the TiAl alloy were improved significantly. This implies that the EMCC technique could offer the possibility of application for high-Nb TiAl alloys with a refined microstructure and excel ent properties to be used as a structural material.展开更多
The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidif...The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.展开更多
The effect of growth rates (V=2-50 μm·s-1) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K·mm-1) was ...The effect of growth rates (V=2-50 μm·s-1) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K·mm-1) was investigated. Results indicated that β phase was the primary phase of the directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy. As the growth rate increases, the solid/liquid interface turns from cellular growth to dendric growth. The interlamellar spacing (λs) decreases with the increase of growth rate according to the relationship of λs=3.39V -0.31. The solidification segregation occurs due to the enrichment of β-stabilizing element Nb, Cr in primary β phase during solidification;moreover, the degree of the segregation increases with the growth rate, resulting in the emergence of B2 phase in lamellar colonies at high growth rates. The microhardness (Hv) grows with the growth rate based on the equation of HV=328.69V 0.072, which mainly attributes to the microstructure refinement.展开更多
The intermetallic Ti-43Al-2Cr-2Nb(at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates(V) ranging from 0.2 to 1.0 mm·min^(-1), at a constant temperature gra...The intermetallic Ti-43Al-2Cr-2Nb(at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates(V) ranging from 0.2 to 1.0 mm·min^(-1), at a constant temperature gradients(G=18 K·mm^(-1)). Macrostructures of the alloy were observed by optical microscopy. Microstructures of the alloy were characterized by scanning electron microscopy(SEM) in back-scattered electron mode and transmission electron microscopy. Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate. Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm·min^(-1). The microstructure of the alloy was composed of α_2/γ lamellar structures and a small number of mixtures of B2 phases and blocky γ phases. The columnar grain size(d) and interlamellar spacing(λ) decrease with an increasing withdrawal rate. The effect of withdrawal rate on microhardness was also investigated. The microhardness of the directional y solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate. This is mainly attributed to the increase of B2 and α_2 phases as well as the refinement of lamellae.展开更多
A transient three-dimensional powder-scale model was established for understanding the flow field and mass transfer within the molten pool during the selective laser melting(SLM)of Inconel 718 alloy by considering som...A transient three-dimensional powder-scale model was established for understanding the flow field and mass transfer within the molten pool during the selective laser melting(SLM)of Inconel 718 alloy by considering some important physical phenomena,such as,a transition from powder to solid,nonlinearities produced by temperature-dependent materials’properties,and fluid flow in the calculation.The influence of laser power or scanning speed on the flow field and cooling rate was discussed in detail.The simulation results reveal that the motion of molten pool and higher cooling rate promote the mass transfer and benefit the solute distribution by increasing laser power.However,with increasing the scanning speed,the melt flow speed and cooling rate are elevated,resulting in an agglomeration of the solute elements,which is ascribed to the shorter dwelling time of liquid.Therefore,the segregation of Nb can be effectively suppressed by increasing laser power or decreasing scanning speed,which can decrease the dwelling time of liquid.展开更多
In order to improve the mechanical properties of Ti Al alloys, especially the ductility at room temperature, and to study the effect of boron(B) on Ti Al alloys, different contents(0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of...In order to improve the mechanical properties of Ti Al alloys, especially the ductility at room temperature, and to study the effect of boron(B) on Ti Al alloys, different contents(0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of B were added into Ti-44Al-6Nb alloys to prepare ingots. The surface quality, macrostructure, microstructure, compressive properties and fracture surface of the ingots were studied. The results show that B has little influence on the surface quality except that there are some dark spots on the surface when the content of B is 0.9%. B can refine the grains. The average grain size decrease from about 0.8 mm to 0.088 mm with increasing B content. Meanwhile, the grain morphology of these ingots changes from big equiaxed grains with lamellars to fine equiaxed grains. When the content of B is 1.2%, the primary Ti B2 phase forms in the liquid phase and increases the nucleation rate, leading to further refinement of the grains. The compressive testing results show that B can increase the strength and the ductility, the compressive strength and compressibility can reach 2,037.8 MPa and 26.7% from 1,156.2 MPa and 10.2% when the boron content is 0.6%, which is resulted from grain refining and grain boundary strengthening. It is found that the compressive strength and the compressibility are relatively stable when the B content is more than 0.3%.展开更多
Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method.The microstructural evolution and orientation characteristics of the starter block and spiral part were s...Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method.The microstructural evolution and orientation characteristics of the starter block and spiral part were studied,and the influence of the competitive growth of crystals on the microstructural characteristics was analyzed.The results show that the divergent grain groups,with small size and randomly oriented grains,appear at the bottom of the start block due to the chilling effect,which is an important area for competitive growth.As the height of the starter block increases,the primary dendrite spacing increases,and the grain density decreases;furthermore,the proportion of grains with an orientation deflection angle less than 10°gradually increases.The<001>texture gradually becomes stronger as the height of the starter block increases,which indicates that the competitive growth of crystals gradually weakens.At the initial stage of the crystal selection in the spiral part,the obstacle of adjacent grains and spiral passage is the main working mechanism.The grains located at the inner side of the front edge of the spiral passage have the growth advantage.The single crystal screening process is achieved at about two-thirds of the spiral height,and the single crystal with the orientation deviation angle of 6.7°from the casting axis is prepared.展开更多
In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys,cold crucible directional solidification(CCDS)technology was employed.Aβ-type TiAl-based alloy with the nomina...In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys,cold crucible directional solidification(CCDS)technology was employed.Aβ-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min^-1 solidification rate.Thermo-compression testing was utilized to evaluate the hightemperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy.Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae.Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions.The much higher hardening exponent and deformation activation energy are obtained,corresponding to the excellent high-temperature deformation resistance and creep resistance,which are because of the hard-oriented grains,weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization.Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction,indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.展开更多
Tailoring the alloy composition,which induces the hard secondary phase to increase hardness and strength to improve the wear features,is a feasible approach for developing wear-resistant metal materials.Here,a group o...Tailoring the alloy composition,which induces the hard secondary phase to increase hardness and strength to improve the wear features,is a feasible approach for developing wear-resistant metal materials.Here,a group of(AlCoCrFeNi)_(100–x)Sc_(x)(x=0–2.0,at%)high-entropy alloys(HEAs)are designed and the phase compositions and wear behaviors are explored.Sc-doped HEA series contain the primary body-centered cubic(BCC)and eutectic phases,in which the eutectic phase is composed of the alternately grown BCC and Laves phases.Sc addition promotes the diffusion of Ni atoms from BCC phase to form the Sc-rich Laves phase at the grain boundaries.Vickers hardness increases due to solid solution strengthening and second phase strengthening.And the second phase strengthening plays a more significant role relative to solid solution strengthening.Laves phase and the oxides caused by wear heating prevent the direct contact between friction pair and HEAs,thus inducing a decreased wear rate from 6.82×10^(−5) to 3.47×10^(−5)m^(3)·N^(−1)·m^(−1).Moreover,the wear mechanism changes from adhesive wear,abrasive wear and oxidative wear to abrasive wear and oxidative wear.展开更多
Initially defined high entropy alloys(HEAs)usually exhibit a single-phase solid-solution structure.However,two and/or more types of phases in HE As possibly induce the desired microstructure features,which contribute ...Initially defined high entropy alloys(HEAs)usually exhibit a single-phase solid-solution structure.However,two and/or more types of phases in HE As possibly induce the desired microstructure features,which contribute to improving the wear properties of HE As.Here,we prepare a series of(AlCoCrFeNi)_(100-x)Hf_(x)(x=0,2,4and 6;at%) HEAs and concern their phase compositions,micro structures and wear properties.Hf leads to the formation of(Ni,Co)_(2)Hf-type Laves phase and tailors the microstructure from a body-centered cubic(BCC) singlephase structure to a hypoeutectic structure.An increased hardness from~HV 512.3 to~HV 734.1 is due to solid-solution strengthening,grain refinement strengthening and precipitated phase strengthening.And a few oxides(Al_(2)O_(3)+Cr_(2)O_(3)) caused by the wear heating contribute to an 85.5% decrease in wear rate of the HEA system from6.71×10^(-5) to 0.97×10^(-5) m^(3)·N^(-1)·m^(-1).In addition,Hf addition changes the wear mechanism from abrasive wear,mild oxidative wear and adhesive wear to oxidative wear and adhesive wear.展开更多
The hydrogenation behavior of Ti44A16Nb1Cr2V(at%)alloy at temperature range of 1373-1693 K and its effect on microstructure and room-temperature mechanical properties were studied systematically in this study.The resu...The hydrogenation behavior of Ti44A16Nb1Cr2V(at%)alloy at temperature range of 1373-1693 K and its effect on microstructure and room-temperature mechanical properties were studied systematically in this study.The results show that hydrogen content increases with the increase in temperature,and the maximum hydrogen content is 0.126 wt%at 1693 K.The heat of solution of hydrogen is calculated as 82.9 kJ·mol^(-1)by curve fitting,indicating that hydrogen absorptionin TiAl alloys is endothermic.Hydrogen promotes the lamellar colony size because hydrogen promotes the diffusion of elements.Hydrogen stabilizes B2phase during hydrogenation resulting in more residual B2phase in the hydrogenated alloy.The nanohardness and elastic modulus decrease after hydrogenation due to that hydrogen weakens the bonds.The Ti44A16Nb1Cr2V alloy exhibits higher plasticity and lower flow stress hydrogenation with 0.039 wt%H,and the ultimate compressive strength decreases from 1220 to 1130 MPa,while the fracture strain is enhanced by 26%.展开更多
Ti43Al6Nb-xZr alloys with different additions of zirconium were prepared by vacuum arc melting furnace.The microstructure and compressive properties at room temperature(RT) were investigated.The microstructure shows d...Ti43Al6Nb-xZr alloys with different additions of zirconium were prepared by vacuum arc melting furnace.The microstructure and compressive properties at room temperature(RT) were investigated.The microstructure shows dendrites with addition of 0 at%-2.5at% Zr,and the dendrites are refined with the primary dendrite arms spacing decreasing from 222.64 μm(0 at%Zr) to 92.57 μm(2.0 at% Zr).With Zr addition more than2.5 at%,the microstructure shows equiaxed grains surrounded by y phase.Zr is a y stabilizer and promotes the β/y transition,resulting in the change of microstructure morphology.Zr reaches the maximum solid solubility(about 6.5 at%) in y phase with addition of 2.5 at% Zr;moreover,γ phase increases in quantity,bringing about severe micro-segregation.With addition of Zr,the remained β phase turns into ω phase with B82 structure.Ti43Al6Nb-xZr alloys show brittle fracture.The maximum compressive strength is 2161.69 MPa with addition of 2.5at% Zr and the maximum compressive strain is 30.62%with addition of 0.5 at% Zr,improving by 9.24% and7.33%,respectively.The improvement of compressive strength results from fine-grain strengthening and solution strengthening.Severe micro-segregation is bad for compressive strength,and large solubility of Zr is detrimental to ductility.展开更多
基金Project(2017YFA0403802)supported by the National Key Research and Development Program of ChinaProject(51825401)supported by the National Natural Science Foundation of ChinaProject(2019TQ0076)supported by the China Postdoctoral Science Foundation。
基金Project(51741404)supported by the National Natural Science Foundation of ChinaProject(2017YFA0403802)supported by National Key Research and Development Program of China
基金financial supports from the National Key Research and Development Program of China(2017YFA0403804)the National Natural Science Foundation of China(51425402,51671073)。
基金financially supported by the National Natural Science Foundation of China(Nos.51331005 and 51425402)
文摘To investigate the effect of solidification parameters on the solidification path and microstructure evolution of Ti-45Al-5Nb(at.%) alloy, Bridgman-type directional solidification and thermodynamics calculations were performed on the alloy. The microstructures, micro-segregation and solidification path were investigated.The results show that the β phase is the primary phase of the alloy at growth rates of 5-20 μm·s^(-1) under the temperature gradients of 15-20 K·mm^(-1), and the primary phase is transformed into an α phase at relatively higher growth rates(V >20 μm·s^(-1)). The mainly S-segregation and β-segregation can be observed in Ti-45Al-5Nb alloy at a growth rate of 10 μm·s^(-1) under a temperature gradient of 15 K·mm^(-1). The increase of temperature gradient to 20 K·mm^(-1) can eliminate β-segregation, but has no obvious effect on S-segregation. The results also show that 5 at.% Nb addition can expand the β phase region, increase the melting point of the alloy and induce the solidification path to become complicated. The equilibrium solidification path of Ti-45Al-5Nb alloy can be described as L L→β L+β L+β→αα+β_R β→ααα→γα+γα→α_2+γγ_R+(α_2+γ), in which β_R and γ_R mean the residual β and
基金financially supported by the National Basic Research Program of China(2011CB610406)the Natural Science Foundation of Hei Longjiang Province(JC201209)the National Natural Science Foundation of China(51425402)
文摘The effect of excitation current intensity on the mechanical properties of ZL205 A castings solidified under a traveling magnetic field was studied. The results of the experiment indicate that the excitation current intensity of the traveling magnetic field has a great influence on the mechanical properties of the ZL205 A castings. When the excitation current intensity is 15 A, the tensile strength and elongation of ZL205 A alloy castings increase 27.2% and 67.7%, respectively, compared with those of the same alloy solidified under gravity. The improvement of mechanical properties is attributed to the decrease of micro-porosity in the alloy. Under the traveling magnetic field, the feeding pressure in the alloy melt before solidification can be enhanced due to the electromagnetic force. Moreover, the melt flow induced by the traveling magnetic field can decrease the temperature gradient. The feeding resistance will be increased because the temperature gradient decrease. So traveling magnetic field has an optimum effect on feeding.
基金financially supported by the 973 project(2011CB610406)Natural Science Foundation of Hei Longjiang Province(JC201209)
文摘Five equiatomic alloys(Ti Zr Hf VNb, Ti Zr Hf VTa, Ti Zr Nb Mo V, Ti Zr Hf Mo V and Zr Nb Mo Hf V) composed of five elements with high melting temperature, respectively were prepared by arc-melting to develop a novel high temperature alloy. The five alloys exhibit different dendritic and interdendritic morphologies. The Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys formed disordered solid solution phases with body-centered cubic structure, and exhibited high compressive strength and good plasticity. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are composed with Laves phase(Hf Mo2) and disordered solid solution phases with body-centered cubic structure. The Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys are harder and more brittle than the other three alloys due to the existence of hard and brittle Laves phases. At high temperatures, the strength decreases to below 300 MPa for the Ti Zr Hf VNb and Ti Zr Hf Mo V alloys. Solution strengthening is the primary strengthening mechanism of the Ti Zr Hf VNb, Ti Zr Hf VTa and Ti Zr Nb Mo V alloys, and brittle Laves phase is the main cause for the low ductility of the Ti Zr Hf Mo V and Zr Nb Mo Hf V alloys.
基金the National Key Research and Development Program of China(Grant No.2017YFA0403804)the National Natural Science Foundation of China(Grant No.51425402)。
文摘Microstructural improvement of Al-Cu-Li alloys with high Li content plays a critical role for the acquisition of excellent mechanical properties and ultra-low density.In this regard,the Al-Cu-Li alloy castings with high Li content from 1.5 wt.%to 4.5 wt.%were prepared by near-rapid solidification,followed by two-stage homogenization treatment(490℃/16 h and 530℃/16 h).The microstructural evolution and solidification behavior of the as-cast and homogenized alloys with different Li contents were systematically studied by combining experiments with calculations by Pandat software.The results indicate that with the increase of Li content,the grain sizes decrease,the solution ability of Cu in the matrixα-Al phase increases,while the content of secondary dendrites increases and the precipitated phases change from low melting point phases to high melting point phases under the near-rapid solidification.Additionally,by the coupling of near-rapid solidification and two-stage homogenization,the metastable precipitated phases(Al7Cu4Li and AlCu3)can be dissolved effectively in the alloys with Li content of 1.5 wt.%-2.5 wt.%;moreover,the stable precipitated phases(Al6CuLi3 and Al2CuLi)uniformly distribute at the grain boundaries in the alloys with Li content of 3.5 wt.%-4.5 wt.%.As a result,the refined and homogenized microstructure can be obtained.
基金financially supported by the National Science Foundation of China(Grant No.51171053 and No.51471062)
文摘In the present research, microstructure refinement of a high-Nb TiAl alloy(Ti-48Al-8Nb-0.15B) was realized by means of the electromagnetic continuous casting(EMCC) technique. The microstructure of an ingot obtained by EMCC was analyzed using scanning electron microscopy(SEM). As compared with the raw as-cast al oy, the obtained EMCC alloy presented a much finer microstructure with lamellar colonies with a mean size of about 50-70 μm because the electromagnetic stirring broke initial dendrites and enhanced the heterogeneous nucleation. As the grains were refined, the properties of the TiAl alloy were improved significantly. This implies that the EMCC technique could offer the possibility of application for high-Nb TiAl alloys with a refined microstructure and excel ent properties to be used as a structural material.
基金financially supported by the National Natural Science Foundation of China(Grant No.51471062)
文摘The present work focused on the Ni_3Al-based alloy with a high melting point. The aim of the research is to study the effect of withdrawal rate on the microstructures and mechanical properties of directionally solidified Ni-25 Al alloy. Ni_3 Al intermetallics were prepared at different withdrawal rates by directional solidification(DS) in an electromagnetic cold crucible directional solidification furnace. The DS samples contain Ni_3 Al and Ni Al phases. The primary dendritic spacing(λ) decreases with the increasing of withdrawal rate(V), and the volume fraction of Ni Al phase increases as the withdrawal rate increases. Results of tensile tests show that ductility of DS samples is enhanced with a decrease in the withdrawal rate.
基金This research was financially supported by the National Key Research and Development Program of China(Grant No.2016YFB0301201)National Natural Science Foundation of China(Grant Nos.51671073,51425402)+2 种基金Fundamental Research and Development Program of China(Grant No.JCKY2017205B032)Yunnan Rare Metal Materials Genetic Engineering Project(Grant No.2018ZE013)Major Special Science and Technology Project ofYunnan Province(Grant No.2019ZE001).
文摘The effect of growth rates (V=2-50 μm·s-1) on microstructure and microhardness of directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B (at.%) alloy at a constant temperature gradient (G=18 K·mm-1) was investigated. Results indicated that β phase was the primary phase of the directionally solidified Ti-44Al-5Nb-1.5Cr-1.5Zr-1Mo-0.1B alloy. As the growth rate increases, the solid/liquid interface turns from cellular growth to dendric growth. The interlamellar spacing (λs) decreases with the increase of growth rate according to the relationship of λs=3.39V -0.31. The solidification segregation occurs due to the enrichment of β-stabilizing element Nb, Cr in primary β phase during solidification;moreover, the degree of the segregation increases with the growth rate, resulting in the emergence of B2 phase in lamellar colonies at high growth rates. The microhardness (Hv) grows with the growth rate based on the equation of HV=328.69V 0.072, which mainly attributes to the microstructure refinement.
基金supported by the National Natural Science Foundation of China(Grant No.51171053 and No.51471062)
文摘The intermetallic Ti-43Al-2Cr-2Nb(at.%) alloy was directionally solidified in an electromagnetic cold crucible with different withdrawal rates(V) ranging from 0.2 to 1.0 mm·min^(-1), at a constant temperature gradients(G=18 K·mm^(-1)). Macrostructures of the alloy were observed by optical microscopy. Microstructures of the alloy were characterized by scanning electron microscopy(SEM) in back-scattered electron mode and transmission electron microscopy. Results showed that morphologies of macrostructure depend greatly on the applied withdrawal rate. Continuous columnar grains can be obtained under slow withdrawal rates ranging from 0.2 to 0.6 mm·min^(-1). The microstructure of the alloy was composed of α_2/γ lamellar structures and a small number of mixtures of B2 phases and blocky γ phases. The columnar grain size(d) and interlamellar spacing(λ) decrease with an increasing withdrawal rate. The effect of withdrawal rate on microhardness was also investigated. The microhardness of the directional y solidified Ti-43Al-2Cr-2Nb alloy increases with an increase in withdrawal rate. This is mainly attributed to the increase of B2 and α_2 phases as well as the refinement of lamellae.
基金the financial support from the National Natural Science Foundation of China(Nos.51425402 and 51501048)。
文摘A transient three-dimensional powder-scale model was established for understanding the flow field and mass transfer within the molten pool during the selective laser melting(SLM)of Inconel 718 alloy by considering some important physical phenomena,such as,a transition from powder to solid,nonlinearities produced by temperature-dependent materials’properties,and fluid flow in the calculation.The influence of laser power or scanning speed on the flow field and cooling rate was discussed in detail.The simulation results reveal that the motion of molten pool and higher cooling rate promote the mass transfer and benefit the solute distribution by increasing laser power.However,with increasing the scanning speed,the melt flow speed and cooling rate are elevated,resulting in an agglomeration of the solute elements,which is ascribed to the shorter dwelling time of liquid.Therefore,the segregation of Nb can be effectively suppressed by increasing laser power or decreasing scanning speed,which can decrease the dwelling time of liquid.
基金supported by the Program of New Century Excellent Talents in University(NCET-12-0153)National Natural Science of Foundation of China(51274076)National Basic Research Program of China(2011CB605504)
文摘In order to improve the mechanical properties of Ti Al alloys, especially the ductility at room temperature, and to study the effect of boron(B) on Ti Al alloys, different contents(0, 0.1, 0.3, 0.6, 0.9, 1.2, at.%) of B were added into Ti-44Al-6Nb alloys to prepare ingots. The surface quality, macrostructure, microstructure, compressive properties and fracture surface of the ingots were studied. The results show that B has little influence on the surface quality except that there are some dark spots on the surface when the content of B is 0.9%. B can refine the grains. The average grain size decrease from about 0.8 mm to 0.088 mm with increasing B content. Meanwhile, the grain morphology of these ingots changes from big equiaxed grains with lamellars to fine equiaxed grains. When the content of B is 1.2%, the primary Ti B2 phase forms in the liquid phase and increases the nucleation rate, leading to further refinement of the grains. The compressive testing results show that B can increase the strength and the ductility, the compressive strength and compressibility can reach 2,037.8 MPa and 26.7% from 1,156.2 MPa and 10.2% when the boron content is 0.6%, which is resulted from grain refining and grain boundary strengthening. It is found that the compressive strength and the compressibility are relatively stable when the B content is more than 0.3%.
基金financially supported by the National Natural Science Foundation of China (52071065)the National Key Research and Development Program of China (2016YFB0301201)the Fundamental Research Funds for the Central Universities (N2007007)
文摘Directionally solidified single crystal superalloy test bars were prepared by the spiral grain selection method.The microstructural evolution and orientation characteristics of the starter block and spiral part were studied,and the influence of the competitive growth of crystals on the microstructural characteristics was analyzed.The results show that the divergent grain groups,with small size and randomly oriented grains,appear at the bottom of the start block due to the chilling effect,which is an important area for competitive growth.As the height of the starter block increases,the primary dendrite spacing increases,and the grain density decreases;furthermore,the proportion of grains with an orientation deflection angle less than 10°gradually increases.The<001>texture gradually becomes stronger as the height of the starter block increases,which indicates that the competitive growth of crystals gradually weakens.At the initial stage of the crystal selection in the spiral part,the obstacle of adjacent grains and spiral passage is the main working mechanism.The grains located at the inner side of the front edge of the spiral passage have the growth advantage.The single crystal screening process is achieved at about two-thirds of the spiral height,and the single crystal with the orientation deviation angle of 6.7°from the casting axis is prepared.
基金the National Natural Science Foundation of China(Grant Nos.51825401 and 51601034)the Fundamental Research Funds for the Central Universities(Grant No.N180904005).
文摘In order to improve the high-temperature deformation resistance and creep resistance of TiAl-based alloys,cold crucible directional solidification(CCDS)technology was employed.Aβ-type TiAl-based alloy with the nominal composition of Ti44Al6Nb1Cr2V was prepared using the optimized CCDS parameters of 45 kW input power and 0.5 mm·min^-1 solidification rate.Thermo-compression testing was utilized to evaluate the hightemperature deformation resistance and creep resistance of the CCDS Ti44Al6Nb1Cr2V alloy.Results show that the CCDS Ti44Al6Nb1Cr2V alloy billets contain aligned columnar grains and a high percentage of small-angle lamellae.Thermo-compression testing results in the radial direction of the CCDS alloy show a much higher peak stress than other reported results in similar conditions.The much higher hardening exponent and deformation activation energy are obtained,corresponding to the excellent high-temperature deformation resistance and creep resistance,which are because of the hard-oriented grains,weaker stress-strain coordination capability of lamella structure and relatively more hysteretic dynamic recrystallization.Thermo-compression testing results in the longitudinal direction of the CCDS Ti44Al6Nb1Cr2V alloy show the much higher peak stress than that in the radial direction,indicating the better high-temperature deformation resistance and creep resistance attributed to the hard-oriented lamellae in this condition.
基金This work was financially supported by the National Natural Science Foundation of China(No.51825401)the Postdoctoral Foundation of Heilongjiang Province(No.LBH-Z19154)+1 种基金the National Natural Science Foundation of Heilongjiang Province(No.LH2020E031)the Interdisciplinary Research Foundation of HIT.
文摘Tailoring the alloy composition,which induces the hard secondary phase to increase hardness and strength to improve the wear features,is a feasible approach for developing wear-resistant metal materials.Here,a group of(AlCoCrFeNi)_(100–x)Sc_(x)(x=0–2.0,at%)high-entropy alloys(HEAs)are designed and the phase compositions and wear behaviors are explored.Sc-doped HEA series contain the primary body-centered cubic(BCC)and eutectic phases,in which the eutectic phase is composed of the alternately grown BCC and Laves phases.Sc addition promotes the diffusion of Ni atoms from BCC phase to form the Sc-rich Laves phase at the grain boundaries.Vickers hardness increases due to solid solution strengthening and second phase strengthening.And the second phase strengthening plays a more significant role relative to solid solution strengthening.Laves phase and the oxides caused by wear heating prevent the direct contact between friction pair and HEAs,thus inducing a decreased wear rate from 6.82×10^(−5) to 3.47×10^(−5)m^(3)·N^(−1)·m^(−1).Moreover,the wear mechanism changes from adhesive wear,abrasive wear and oxidative wear to abrasive wear and oxidative wear.
基金financially supported by the National Natural Science Foundation of China (No.51825401)the Postdoctoral Foundation of Heilongjiang Province (No.LBHZ19154)+1 种基金the National Natural Science Foundation of Heilongjiang Province (No.LH2020E031)the Interdisciplinary Research Foundation of HIT。
文摘Initially defined high entropy alloys(HEAs)usually exhibit a single-phase solid-solution structure.However,two and/or more types of phases in HE As possibly induce the desired microstructure features,which contribute to improving the wear properties of HE As.Here,we prepare a series of(AlCoCrFeNi)_(100-x)Hf_(x)(x=0,2,4and 6;at%) HEAs and concern their phase compositions,micro structures and wear properties.Hf leads to the formation of(Ni,Co)_(2)Hf-type Laves phase and tailors the microstructure from a body-centered cubic(BCC) singlephase structure to a hypoeutectic structure.An increased hardness from~HV 512.3 to~HV 734.1 is due to solid-solution strengthening,grain refinement strengthening and precipitated phase strengthening.And a few oxides(Al_(2)O_(3)+Cr_(2)O_(3)) caused by the wear heating contribute to an 85.5% decrease in wear rate of the HEA system from6.71×10^(-5) to 0.97×10^(-5) m^(3)·N^(-1)·m^(-1).In addition,Hf addition changes the wear mechanism from abrasive wear,mild oxidative wear and adhesive wear to oxidative wear and adhesive wear.
基金financially supported by the National Natural Science Foundation of China(No.51274076)the National Science Fund for Distinguished Young Scholars(No.NSFC51425402)。
文摘The hydrogenation behavior of Ti44A16Nb1Cr2V(at%)alloy at temperature range of 1373-1693 K and its effect on microstructure and room-temperature mechanical properties were studied systematically in this study.The results show that hydrogen content increases with the increase in temperature,and the maximum hydrogen content is 0.126 wt%at 1693 K.The heat of solution of hydrogen is calculated as 82.9 kJ·mol^(-1)by curve fitting,indicating that hydrogen absorptionin TiAl alloys is endothermic.Hydrogen promotes the lamellar colony size because hydrogen promotes the diffusion of elements.Hydrogen stabilizes B2phase during hydrogenation resulting in more residual B2phase in the hydrogenated alloy.The nanohardness and elastic modulus decrease after hydrogenation due to that hydrogen weakens the bonds.The Ti44A16Nb1Cr2V alloy exhibits higher plasticity and lower flow stress hydrogenation with 0.039 wt%H,and the ultimate compressive strength decreases from 1220 to 1130 MPa,while the fracture strain is enhanced by 26%.
基金financially supported by the National Natural Science Foundation of China (Nos.51274076, 51425402 and 51331005)。
文摘Ti43Al6Nb-xZr alloys with different additions of zirconium were prepared by vacuum arc melting furnace.The microstructure and compressive properties at room temperature(RT) were investigated.The microstructure shows dendrites with addition of 0 at%-2.5at% Zr,and the dendrites are refined with the primary dendrite arms spacing decreasing from 222.64 μm(0 at%Zr) to 92.57 μm(2.0 at% Zr).With Zr addition more than2.5 at%,the microstructure shows equiaxed grains surrounded by y phase.Zr is a y stabilizer and promotes the β/y transition,resulting in the change of microstructure morphology.Zr reaches the maximum solid solubility(about 6.5 at%) in y phase with addition of 2.5 at% Zr;moreover,γ phase increases in quantity,bringing about severe micro-segregation.With addition of Zr,the remained β phase turns into ω phase with B82 structure.Ti43Al6Nb-xZr alloys show brittle fracture.The maximum compressive strength is 2161.69 MPa with addition of 2.5at% Zr and the maximum compressive strain is 30.62%with addition of 0.5 at% Zr,improving by 9.24% and7.33%,respectively.The improvement of compressive strength results from fine-grain strengthening and solution strengthening.Severe micro-segregation is bad for compressive strength,and large solubility of Zr is detrimental to ductility.