Boron was found to be a unique grain refiner in cast TiAl alloys in the beginning of 1990 s and has become an element in most of the TiAl alloys developed to date.Over the past 25 or so years,efforts to understand the...Boron was found to be a unique grain refiner in cast TiAl alloys in the beginning of 1990 s and has become an element in most of the TiAl alloys developed to date.Over the past 25 or so years,efforts to understand the role of boron in solidification,solid-phase transformation,thermal and thermomechanical processing and mechanical properties of TiAl alloys and the relevant mechanisms never ceased.As a result,abundant knowledge on boron in TiAl alloys has been accumulated but scattered in various research papers and conference proceedings.This review summarises the progress in understanding boron and its impacts on the TiAl alloy systems.展开更多
High-entropy intermetallic compounds(HEICs)were fabricated by mechanical alloying and spark plasma sintering to fill a knowledge gap between the traditional high-entropy alloys(HEAs)and emerging highentropy ceramics(H...High-entropy intermetallic compounds(HEICs)were fabricated by mechanical alloying and spark plasma sintering to fill a knowledge gap between the traditional high-entropy alloys(HEAs)and emerging highentropy ceramics(HECs).Notably,several four-or five-component equimolar aluminides,such as the B2-phase(Fe1/5 Co1/5 Ni1/5 Mn1/5 Cu1/5)Al,have been made into single-phase HEICs for the first time.Thermodynamic modeling and a reversible,temperature-dependent,phase-stability experiment suggest that such B2-phase HEICs are entropy-stabilized phases.The structure of these HEICs resembles that of HECs with high-entropy mixing of fo ur or five elements of nearly equal fractions in one and only one sublattice,but with significant(10%)anti-site defects(differing from typical HECs).A new phase stability rule for forming single B2-phase HEICs is proposed.Five additional HEICs of predominantly D022 phases have also been made.This study broadens the families of equimola r,single-phase,high-entropy materials that have been successfully fabricated.展开更多
An aluminide（AlFe and α-（FeAl）） surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment（SMAT） combined with a duplex aluminization proce...An aluminide（AlFe and α-（FeAl）） surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment（SMAT） combined with a duplex aluminization process at lower temperatures,i.e.a packed aluminization followed by a diffusion annealing treatment below its tempering temperature.Indentation tests indicated that the lower-Al surface layer formed on the SMAT sample is more resistant to cracking and has better adhesion to the substrate in comparison with the Al 5Fe 2 layer formed on the as-received sample after the duplex aluminization process.Isothermal steam oxidation measurements showed that the oxidation resistance is increased significantly by the lower-Al surface layer due to the formation of a protective（Fe,Cr）Al 2O 4 layer.The rate constant of oxidation was estimated to decrease from-0.849 mg^2 cm^-4h^-1 of the as-received material to^0.011 mg^2 cm^-4 h^-1 of the AlFe layer at 700 ℃.展开更多
Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffu...Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25(at.%) foil was flat and thin,containing Ti19Al6 and Ti2Cu intermetallic compounds; however,the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges,consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However,fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases,but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.展开更多
Casting technology of thin-wall TiAl alloy turbochargers was studied by investment casting and numerical simulation.Misruns and gas holes were the main defects observed in preliminary work due to the poor fluidity of ...Casting technology of thin-wall TiAl alloy turbochargers was studied by investment casting and numerical simulation.Misruns and gas holes were the main defects observed in preliminary work due to the poor fluidity of alloy,and to gas entrapment.In order to eliminate these defects,cast parameters,such as centrifugal rotation rate and mould preheating temperature,were optimized by numerical simulation,meanwhile,the structure of the shell mould was optimized to improve the filling capacity of TiAl alloy.Pouring experiments were carried out by vacuum induction melting furnace equipped with a water-cooled copper crucible based on the above optimization.The quality of the TiAl alloy casting was analyzed by fluorescent penetrant inspection and X-ray detection.The results show that a centrifugal rotation rate of 200 rpm,mould preheating temperature of 600°C,shell preparation through organic fiber addition can dramatically improve the mould filling capacity,and integrated turbochargers were finally prepared.展开更多
Ti–Al mixed powder（Ti：Al = 3：1 in atomic ratio） and Ti3 Al intermetallic alloy powder mechanically clad hexagonal BN to fabricate Ti Al/BN and Ti3Al/BN composite powders. The corresponding porous abradable seal c...Ti–Al mixed powder（Ti：Al = 3：1 in atomic ratio） and Ti3 Al intermetallic alloy powder mechanically clad hexagonal BN to fabricate Ti Al/BN and Ti3Al/BN composite powders. The corresponding porous abradable seal coatings（named as TAC-1 and TAC-2, respectively） were deposited using vacuum plasma spray（VPS） technology, and their corrosion behavior was studied via salt spray corrosion and electrochemical tests. Phase compositions and microstructures of these coatings before and after corrosion were characterized by X-ray diffraction（XRD） and scanning electron microscopy（SEM） facilitated with energy dispersive X-ray spectrometer（EDS）. The results showed that spontaneous passivation of TAC-1 and TAC-2 granted the coatings excellent corrosion resistance than that of commercial Al/BN coating. Additionally, TAC-2 exhibited higher corrosion potential（Ecorr） and breakdown potential（Ebp） but a lower corrosion current density（icorr） than TAC-1. A small quantity of the corrosion product（Al（OH）3and Al O） could be detected on the surface of TAC-1, while no corrosion product appeared in TAC-2. The non-uniform elements distribution in the metal matrix of TAC-1 resulted in localized corrosion and relatively poor corrosion resistance compared to TAC-2.展开更多
The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron ...The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy dispersive spectroscope （EDS）, and transmission electron microscopy （TEM）. The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47A17Nb2.5V1.0Cr （at%） alloy solidifying through the peritectic reaction. The β-solidifying Ti46A17Nb2.5V1.0Cr （at%） alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48A17Nb2.5V1.0Cr （at%） alloy is equivalent to that of Ti46A17Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.展开更多
This article investigates the low-temperature formation of aluminide coatings on a Ni-base superalloy by pack cementation process. The pack cemented coatings characteristic of high density and homogeneity possess a tw...This article investigates the low-temperature formation of aluminide coatings on a Ni-base superalloy by pack cementation process. The pack cemented coatings characteristic of high density and homogeneity possess a two-layer structure. The top layer mainly consists of Al3Ni2 and Al3Ni,while the bottom layer of Al3Ni2. Great efforts are made to elucidate the effects of different experimental parameters on the microstructure and the constituent distribution of the coatings. The results show that all the parameters exclusive of the pack activator （NH4Cl） content produce effect on the coating thickness,but do not on the microstructure and the constituent distribution. The pack activator （NH4Cl） content affects neither the coating thickness nor structure and constituent distribution. The parabolic relationship between the coating thickness and the deposition time suggests that the process is diffusion-controlled. Furthermore,the article demonstrates a linear relationship between the coating thickness and the re-ciprocal deposition temperature.展开更多
A Cr-modified aluminide coating is prepared on a Ni-based superalloy using arc ion plating and subsequent pack cementation aluminizing.Hot corrosion behavior of the Cr-modified aluminide coating exposed to molten Na2S...A Cr-modified aluminide coating is prepared on a Ni-based superalloy using arc ion plating and subsequent pack cementation aluminizing.Hot corrosion behavior of the Cr-modified aluminide coating exposed to molten Na2SO4/K2SO4（3：1） or Na2SO4/NaCl（3：1） salts at 900 °C in static air are evaluated as well as the aluminide coating.The results indicate that compared with the aluminide coating,the anti-corrosion properties of the Cr-modified aluminide coating in the both salts are improved,which should be attributed to the beneficial effect of the Cr in the coating.The corrosion mechanism of the Cr-modified aluminide coating,especially the role of Cr in the mixture salt corrosion,is discussed.展开更多
The present work explores the feasibility of fabricating porous 3D parts in TiAl intermetallic alloy directly from Tie6Ale4V and Al powders. This approach uses a binder jetting additive manufacturing process followed ...The present work explores the feasibility of fabricating porous 3D parts in TiAl intermetallic alloy directly from Tie6Ale4V and Al powders. This approach uses a binder jetting additive manufacturing process followed by reactive sintering. The results demonstrate that the present approach is successful for realizing parts in TiAl intermetallic alloy.展开更多
A low-diffusion Ni Re Pt Al coating((Ni,Pt)Al outer layer in addition to a Re-rich diffusion barrier layer)was prepared on a Ni_(3)Al-base single crystal(SC)superalloy via electroplating and gaseous aluminizing treatm...A low-diffusion Ni Re Pt Al coating((Ni,Pt)Al outer layer in addition to a Re-rich diffusion barrier layer)was prepared on a Ni_(3)Al-base single crystal(SC)superalloy via electroplating and gaseous aluminizing treatments,wherein the electroplating procedures consisted of the composite deposition of Ni-Re followed by electroplating of Pt.In order to perform a comparison with conventional Ni Al and(Ni,Pt)Al coatings,the cyclic oxidation performance of the Ni Re Pt Al coating was evaluated at 1100 and 1150℃.We observed that the oxidation resistance of the Ni Re Pt Al coating was significantly improved by the greater presence of the residualβ-Ni Al phase in the outer layer and the lesser outward-diffusion of Mo from the substrate.In addition,the coating with the Re-rich diffusion barrier demonstrated a lower extent of interdiffusion into the substrate,where the thickness of the second reaction zone(SRZ)in the substrate alloy decreased by 25%.The mechanisms responsible for improving the oxidation resistance and decreasing the extent of SRZ formation are discussed,in which a particular attention is paid to the inhibition of the outward diffusion of Mo by the Re-based diffusion barrier.展开更多
Two low alloy steels 0.5Cr-0.5Mo-0.25V and H85 were pack-aluminized at 900°for 4 h by using Fe-Al powder mixture containing 48% Fe, 20.6% Al- 29.4% Al2O3 and 2% NH4Cl by weight. The microhardness and oxidation re...Two low alloy steels 0.5Cr-0.5Mo-0.25V and H85 were pack-aluminized at 900°for 4 h by using Fe-Al powder mixture containing 48% Fe, 20.6% Al- 29.4% Al2O3 and 2% NH4Cl by weight. The microhardness and oxidation resistance at 900℃ of the aluminide coatings were studied. It was found that pack-aluminizing improves the microhardness of the 0.5Cro.5Mo-0.25V steel while it reduces the microhardness of the H85 steel. Pack aluminizing highly improves the oxidation resistance after 20h exposure at 900℃ in air for the investigated steels.展开更多
The feasibility to use electron beam welding to join the nominal compositionTi-48Al-2Cr-2Nb (at. percent) alloy was assessed. The microstructure characterization and crackingsusceptibility of the joints were evaluated...The feasibility to use electron beam welding to join the nominal compositionTi-48Al-2Cr-2Nb (at. percent) alloy was assessed. The microstructure characterization and crackingsusceptibility of the joints were evaluated by means of OM, SEM, XRD, and microhardness. It wasfound that the welded microstructure exhibited columnar and dendritic structures. Microstructuralconstituents in the fusion zone were a massive gamma structure and some amount of lamellar structureconsisting of alternating platelets of alpha_2 and gamma. The major contributing factor to thesusceptibility to solidification cracking was microsturctural change in this study for thesuppression of a phase decomposition leading to produce more retained alpha_2 brittle phase.Compared with transgranular cleavage fracture in the base metal, the weld metal exhibited mainlytranslamellar fracture.展开更多
Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ...Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ultra-fine Y2O3 powders were mixed by ball milling. The ultra-fine Y2O3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature （below 600℃） in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM （scanning electron microscope）, AFM （atomic force microscope）, EDS （energy dispersive X-ray spectroscopy）, XRF （X-ray fluorescence spectrometer） and XRD （X-ray diffraction） methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.展开更多
This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombi...This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.展开更多
The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B （at%） titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature ran...The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B （at%） titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature range from 950℃ to 1300℃ and at the strain rates between 10-3 s-1 and 10 s-1. The processing maps have been established to evaluate the optimum hot processing conditions and reveal the instability regions. It is found that the flow stress of the investigated alloy is a strong function of the temperature and the strain rate, The investigated alloy has the optimum hot-working condition at 950℃ and 10-3 s-1, since the material undergoes dynamic recrystallization to produce a fine-grained microstructure. At 1250℃ and 10-3 s-1, the alloy exhibits superplastic deformation. At 2300℃ and 10-1 s-1, the cyclic dynamic recrystallization with high temperature grain coarsening takes place. The material undergoes flow instabilities at lower temperatures and higher strain rates, as predicted by the instability criterion. The processing maps demonstrate that the strain significantly affected the instability regions. The manifestations of the instabilities have been observed in the form of microvoids, wedge cracks, and surface fractures.展开更多
The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed ...The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed in welds of titanium alloy laser welding based on the microscopic characteristics of the porosities. One is the metallurgical porosity with round and smooth inner wall, which results from the surface contamination. The other is the processing porosity with irregular and rough inner wall that displays the trace of the pool flowing, which results from the ruffle on the keyhole wall gathering together locally and closing down the gas in the keyhole into bubbles because of the keyhole fluctuating. The CO2 laser welding could break down easily the surface oxide film and produce little metallurgical porosity, but produces easily processing porosity when partial penetration or unstable-full penetration laser welding is conducted, which always occurs in the center of weld.展开更多
Gamma titanium aluminide (γ-TiAl)intermetallic compounds are spotlighted as lightweight heat-resistant materials, and have been investigated extensively to aiming commercial use.Fundamental approaches in which phase...Gamma titanium aluminide (γ-TiAl)intermetallic compounds are spotlighted as lightweight heat-resistant materials, and have been investigated extensively to aiming commercial use.Fundamental approaches in which phase diagrams of Ti -Al -Cr and TiAl -Nb have been studied based on the experimental data and calculations , have been performed in the national project For industrial approach, ingot process and direct casting process have been developed to provide sheet form of TiAl. By these approaches, γ-TiAl base ,intermetallic compounds show a great possibility to apply in the variety of fields .However, a more detailed understanding of phase equilibrium in Ti-Al -X and the relationship between the mechanical properties and microstructure will be necessary展开更多
文摘Boron was found to be a unique grain refiner in cast TiAl alloys in the beginning of 1990 s and has become an element in most of the TiAl alloys developed to date.Over the past 25 or so years,efforts to understand the role of boron in solidification,solid-phase transformation,thermal and thermomechanical processing and mechanical properties of TiAl alloys and the relevant mechanisms never ceased.As a result,abundant knowledge on boron in TiAl alloys has been accumulated but scattered in various research papers and conference proceedings.This review summarises the progress in understanding boron and its impacts on the TiAl alloy systems.
基金supported by a Vannevar Bush Faculty Fellowship sponsored by the Basic Research Office of the Assistant Secretary of Defense for Research and Engineeringfunded by the Office of Naval Research(N00014-16-1-2569)funding support from State Key Laboratory of High Performance and Complex Manufacturing at Central South University(ZZYJKT2018-04)
文摘High-entropy intermetallic compounds(HEICs)were fabricated by mechanical alloying and spark plasma sintering to fill a knowledge gap between the traditional high-entropy alloys(HEAs)and emerging highentropy ceramics(HECs).Notably,several four-or five-component equimolar aluminides,such as the B2-phase(Fe1/5 Co1/5 Ni1/5 Mn1/5 Cu1/5)Al,have been made into single-phase HEICs for the first time.Thermodynamic modeling and a reversible,temperature-dependent,phase-stability experiment suggest that such B2-phase HEICs are entropy-stabilized phases.The structure of these HEICs resembles that of HECs with high-entropy mixing of fo ur or five elements of nearly equal fractions in one and only one sublattice,but with significant(10%)anti-site defects(differing from typical HECs).A new phase stability rule for forming single B2-phase HEICs is proposed.Five additional HEICs of predominantly D022 phases have also been made.This study broadens the families of equimola r,single-phase,high-entropy materials that have been successfully fabricated.
基金Financial supports from the Ministry of Science and Technology of the People’s Republic of China (No.2012CB932201)the National Natural Science Foundation of China (No.91226204)the Key Research Program of Chinese Academy of Sciences (No.KGZD-EW-T06)
文摘An aluminide（AlFe and α-（FeAl）） surface layer containing lower-Al was formed on ferritic-martensitic steel P92 by means of surface mechanical attrition treatment（SMAT） combined with a duplex aluminization process at lower temperatures,i.e.a packed aluminization followed by a diffusion annealing treatment below its tempering temperature.Indentation tests indicated that the lower-Al surface layer formed on the SMAT sample is more resistant to cracking and has better adhesion to the substrate in comparison with the Al 5Fe 2 layer formed on the as-received sample after the duplex aluminization process.Isothermal steam oxidation measurements showed that the oxidation resistance is increased significantly by the lower-Al surface layer due to the formation of a protective（Fe,Cr）Al 2O 4 layer.The rate constant of oxidation was estimated to decrease from-0.849 mg^2 cm^-4h^-1 of the as-received material to^0.011 mg^2 cm^-4 h^-1 of the AlFe layer at 700 ℃.
基金financially supported by the State Key Laboratory of Advanced Welding and Joining,Harbin Institute of Technology,Harbin,Chinathe National Natural Science Foundation of China (Grant No.51374048)+1 种基金the National Basic Research Program of China ("973 Program",Grant No.2011CB013402)the Fundamental Research Funds for the Central Universities
文摘Vacuum brazing of TiAl alloy to 40Cr steel sheets was conducted with newly developed CuTiNiZrV amorphous foils. It was found that a diffusion layer,filler metal and reaction layer existed in the brazed seam. The diffusion layer in the joint brazed with Cu43.75Ti37.5Ni6.25Zr6.25V6.25(at.%) foil was flat and thin,containing Ti19Al6 and Ti2Cu intermetallic compounds; however,the diffusion layer brazed with Cu37.5Ti25Ni12.5Zr12.5V12.5 foil was uneven with bulges,consisting of essentially Ti-based solute solution. The foil with 12.5 at.% V showed inferior spreadability compared to that with 6.25 at.% V at brazing temperature. However,fracture happened along the diffusion layer with 6.25 at.% V foil due to the formation of brittle intermetallic phases,but the joints brazed with 12.5 at.% V foil failed through the TiAl substrate. These results show that designing amorphous alloy with less Ti and more V for brazing TiAl alloy to steel is appropriate.
基金financially supported by the Liaoning Natural Science Foundation ( Grant No.20170540888)the Liaoning Science and Technology Project (Grant No.2017221006)
文摘Casting technology of thin-wall TiAl alloy turbochargers was studied by investment casting and numerical simulation.Misruns and gas holes were the main defects observed in preliminary work due to the poor fluidity of alloy,and to gas entrapment.In order to eliminate these defects,cast parameters,such as centrifugal rotation rate and mould preheating temperature,were optimized by numerical simulation,meanwhile,the structure of the shell mould was optimized to improve the filling capacity of TiAl alloy.Pouring experiments were carried out by vacuum induction melting furnace equipped with a water-cooled copper crucible based on the above optimization.The quality of the TiAl alloy casting was analyzed by fluorescent penetrant inspection and X-ray detection.The results show that a centrifugal rotation rate of 200 rpm,mould preheating temperature of 600°C,shell preparation through organic fiber addition can dramatically improve the mould filling capacity,and integrated turbochargers were finally prepared.
基金financially supported by the Fund of State Key Laboratory of Multiphase Complex Systems, IPE, CAS (No. MPCS-2012-A-06)the Natural Science Foundation of Jiangsu Province, China (No. BK2011452)
文摘Ti–Al mixed powder（Ti：Al = 3：1 in atomic ratio） and Ti3 Al intermetallic alloy powder mechanically clad hexagonal BN to fabricate Ti Al/BN and Ti3Al/BN composite powders. The corresponding porous abradable seal coatings（named as TAC-1 and TAC-2, respectively） were deposited using vacuum plasma spray（VPS） technology, and their corrosion behavior was studied via salt spray corrosion and electrochemical tests. Phase compositions and microstructures of these coatings before and after corrosion were characterized by X-ray diffraction（XRD） and scanning electron microscopy（SEM） facilitated with energy dispersive X-ray spectrometer（EDS）. The results showed that spontaneous passivation of TAC-1 and TAC-2 granted the coatings excellent corrosion resistance than that of commercial Al/BN coating. Additionally, TAC-2 exhibited higher corrosion potential（Ecorr） and breakdown potential（Ebp） but a lower corrosion current density（icorr） than TAC-1. A small quantity of the corrosion product（Al（OH）3and Al O） could be detected on the surface of TAC-1, while no corrosion product appeared in TAC-2. The non-uniform elements distribution in the metal matrix of TAC-1 resulted in localized corrosion and relatively poor corrosion resistance compared to TAC-2.
基金financially supported by the National Basic Research Program of China(No.2011CB605503)the Program of Introducing Talents of Discipline to Universities(No.B08040)
文摘The effect of A1 content on the microstructure and solidification characteristics of Ti-A1-Nb-V-Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction （XRD）, scanning electron microscopy （SEM） equipped with energy dispersive spectroscope （EDS）, and transmission electron microscopy （TEM）. The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47A17Nb2.5V1.0Cr （at%） alloy solidifying through the peritectic reaction. The β-solidifying Ti46A17Nb2.5V1.0Cr （at%） alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous finegrained microstructure. However, the crystallization temperature range of Ti48A17Nb2.5V1.0Cr （at%） alloy is equivalent to that of Ti46A17Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.
文摘This article investigates the low-temperature formation of aluminide coatings on a Ni-base superalloy by pack cementation process. The pack cemented coatings characteristic of high density and homogeneity possess a two-layer structure. The top layer mainly consists of Al3Ni2 and Al3Ni,while the bottom layer of Al3Ni2. Great efforts are made to elucidate the effects of different experimental parameters on the microstructure and the constituent distribution of the coatings. The results show that all the parameters exclusive of the pack activator （NH4Cl） content produce effect on the coating thickness,but do not on the microstructure and the constituent distribution. The pack activator （NH4Cl） content affects neither the coating thickness nor structure and constituent distribution. The parabolic relationship between the coating thickness and the deposition time suggests that the process is diffusion-controlled. Furthermore,the article demonstrates a linear relationship between the coating thickness and the re-ciprocal deposition temperature.
基金financially supported by the National Natural Science Foundation of China (No.51001106)National Basic Research Program of China (No.2012CB625100)
文摘A Cr-modified aluminide coating is prepared on a Ni-based superalloy using arc ion plating and subsequent pack cementation aluminizing.Hot corrosion behavior of the Cr-modified aluminide coating exposed to molten Na2SO4/K2SO4（3：1） or Na2SO4/NaCl（3：1） salts at 900 °C in static air are evaluated as well as the aluminide coating.The results indicate that compared with the aluminide coating,the anti-corrosion properties of the Cr-modified aluminide coating in the both salts are improved,which should be attributed to the beneficial effect of the Cr in the coating.The corrosion mechanism of the Cr-modified aluminide coating,especially the role of Cr in the mixture salt corrosion,is discussed.
文摘The present work explores the feasibility of fabricating porous 3D parts in TiAl intermetallic alloy directly from Tie6Ale4V and Al powders. This approach uses a binder jetting additive manufacturing process followed by reactive sintering. The results demonstrate that the present approach is successful for realizing parts in TiAl intermetallic alloy.
基金the Key-Area Research and Development Program of Guangdong Province(2019B010936001)financially supported by the National Natural Science Foundation of China(Grant Nos.51671202 and 51301184)。
文摘A low-diffusion Ni Re Pt Al coating((Ni,Pt)Al outer layer in addition to a Re-rich diffusion barrier layer)was prepared on a Ni_(3)Al-base single crystal(SC)superalloy via electroplating and gaseous aluminizing treatments,wherein the electroplating procedures consisted of the composite deposition of Ni-Re followed by electroplating of Pt.In order to perform a comparison with conventional Ni Al and(Ni,Pt)Al coatings,the cyclic oxidation performance of the Ni Re Pt Al coating was evaluated at 1100 and 1150℃.We observed that the oxidation resistance of the Ni Re Pt Al coating was significantly improved by the greater presence of the residualβ-Ni Al phase in the outer layer and the lesser outward-diffusion of Mo from the substrate.In addition,the coating with the Re-rich diffusion barrier demonstrated a lower extent of interdiffusion into the substrate,where the thickness of the second reaction zone(SRZ)in the substrate alloy decreased by 25%.The mechanisms responsible for improving the oxidation resistance and decreasing the extent of SRZ formation are discussed,in which a particular attention is paid to the inhibition of the outward diffusion of Mo by the Re-based diffusion barrier.
文摘Two low alloy steels 0.5Cr-0.5Mo-0.25V and H85 were pack-aluminized at 900°for 4 h by using Fe-Al powder mixture containing 48% Fe, 20.6% Al- 29.4% Al2O3 and 2% NH4Cl by weight. The microhardness and oxidation resistance at 900℃ of the aluminide coatings were studied. It was found that pack-aluminizing improves the microhardness of the 0.5Cro.5Mo-0.25V steel while it reduces the microhardness of the H85 steel. Pack aluminizing highly improves the oxidation resistance after 20h exposure at 900℃ in air for the investigated steels.
文摘The feasibility to use electron beam welding to join the nominal compositionTi-48Al-2Cr-2Nb (at. percent) alloy was assessed. The microstructure characterization and crackingsusceptibility of the joints were evaluated by means of OM, SEM, XRD, and microhardness. It wasfound that the welded microstructure exhibited columnar and dendritic structures. Microstructuralconstituents in the fusion zone were a massive gamma structure and some amount of lamellar structureconsisting of alternating platelets of alpha_2 and gamma. The major contributing factor to thesusceptibility to solidification cracking was microsturctural change in this study for thesuppression of a phase decomposition leading to produce more retained alpha_2 brittle phase.Compared with transgranular cleavage fracture in the base metal, the weld metal exhibited mainlytranslamellar fracture.
基金the National Natural Science Foundation of China （No. 50271010）.
文摘Nanocrystal ODS （oxide dispersion strengthening） aluminide coatings were produced on a stainless steel and nickel-based superalloy by the pock aluminizing process assisted by ball peening, Pure Al powders and 1% of ultra-fine Y2O3 powders were mixed by ball milling. The ultra-fine Y2O3 powders were dispersed in Al particles. Ball peening welded the Al particles onto the substrate and accelerated the formation of aluminide coating. Nanocrystal ODS aluminide coatings were produced by the outward growth at a much low temperature （below 600℃） in a short treatment time. The effects of the operation temperature and treatment time on the formation of the coatings were analyzed. SEM （scanning electron microscope）, AFM （atomic force microscope）, EDS （energy dispersive X-ray spectroscopy）, XRF （X-ray fluorescence spectrometer） and XRD （X-ray diffraction） methods were applied to investigate the microstructure of the coatings. High-temperature oxidation tests were carried out to evaluate the oxidation resistance of the ODS aluminide coatings.
基金funded by National Project PCCA contract No.65/2012POS-CCE O 2.2.1 project INFRANANOCHEM-No 19/January 3,2009 of the EU(ERDF)and Romanian Government
文摘This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 ＋ γ （Ti3Al ＋ TiAl） and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.
文摘The hot deformation behavior of a Ti-47Al-2Cr-2Nb-0.2W-0.15B （at%） titanium aluminide alloy fabricated by pre-alloyed powder metallurgy has been investigated by using the hot compression tests in the temperature range from 950℃ to 1300℃ and at the strain rates between 10-3 s-1 and 10 s-1. The processing maps have been established to evaluate the optimum hot processing conditions and reveal the instability regions. It is found that the flow stress of the investigated alloy is a strong function of the temperature and the strain rate, The investigated alloy has the optimum hot-working condition at 950℃ and 10-3 s-1, since the material undergoes dynamic recrystallization to produce a fine-grained microstructure. At 1250℃ and 10-3 s-1, the alloy exhibits superplastic deformation. At 2300℃ and 10-1 s-1, the cyclic dynamic recrystallization with high temperature grain coarsening takes place. The material undergoes flow instabilities at lower temperatures and higher strain rates, as predicted by the instability criterion. The processing maps demonstrate that the strain significantly affected the instability regions. The manifestations of the instabilities have been observed in the form of microvoids, wedge cracks, and surface fractures.
文摘The CO2 laser welding of BT20 titanium alloy and Ti-23Al-17Nb titanium aluminide was conducted to investigate into the porosity in titanium alloy weld. The results show that there are two sorts of porosities observed in welds of titanium alloy laser welding based on the microscopic characteristics of the porosities. One is the metallurgical porosity with round and smooth inner wall, which results from the surface contamination. The other is the processing porosity with irregular and rough inner wall that displays the trace of the pool flowing, which results from the ruffle on the keyhole wall gathering together locally and closing down the gas in the keyhole into bubbles because of the keyhole fluctuating. The CO2 laser welding could break down easily the surface oxide film and produce little metallurgical porosity, but produces easily processing porosity when partial penetration or unstable-full penetration laser welding is conducted, which always occurs in the center of weld.
文摘Gamma titanium aluminide (γ-TiAl)intermetallic compounds are spotlighted as lightweight heat-resistant materials, and have been investigated extensively to aiming commercial use.Fundamental approaches in which phase diagrams of Ti -Al -Cr and TiAl -Nb have been studied based on the experimental data and calculations , have been performed in the national project For industrial approach, ingot process and direct casting process have been developed to provide sheet form of TiAl. By these approaches, γ-TiAl base ,intermetallic compounds show a great possibility to apply in the variety of fields .However, a more detailed understanding of phase equilibrium in Ti-Al -X and the relationship between the mechanical properties and microstructure will be necessary
