Microstructure and High-Temperature Oxidation Behavior of Dy-Doped Nb–Si-Based Alloys

Microstructures and oxidation behaviors of four Dy-doped Nb–Si-based alloys at 1250℃ were investigated. The nominal compositions of the four alloys are Nb–15Si–24Ti–4Cr–2Al–2Hf–xDy(at.%), where x = 0, 0.05, 0.10 and 0.15,respectively. Results showed that the four alloys all consisted of Nbss, αNb_5Si_3 and γNb_5Si_3, and the addition of Dy produced no obvious effect on the phase constitution and the microstructures of Nb–Si-based alloys. After oxidation at 1250℃ for 58 h, it was found that the addition of Dy accelerated the oxidation rate of Nb–Si-based alloys and caused a larger weight gain, accompanied by the formation of a more porous and less protective oxide scale. The oxides of Nb_2O_5,Ti_2Nb_(10)O_(29), TiNb_2O_7, Ti_(0.4)Cr_(0.3)Nb_(0.3)O_2 and glassy SiO_2 were formed on Dy-doped Nb–Si-based alloys. The hightemperature oxidation mechanism of Dy-doped Nb–Si-based alloys was discussed.     

Microstructures and high-temperature oxidation behavior of directionally solidified Nb-Si-based alloys with Re additions

High-temperature oxidation behavior of directionally solidified(DS) Nb-Si-based alloys with Re additions was investigated at 1200 and 1250℃,respectively.Microstructures and high-temperature oxidation behavior of the alloys were characterized.Results show that the microstructures in vertical section of Nb-24Ti-15Si-4Cr-2Al-2Hf-xRe(x=0,1,3;at%) alloys grow parallel to the withdrawal direction and the cross section exhibits bud-like structures.The bud-like structures become finer with more Re addit...     

Microstructure evolution of Ti–46Al–6Nb–(Si,B) alloys during heat treatment with W addition

The cast ingots of Ti-46Al-6Nb-0.25Si-0.2B and Ti-46Al-6Nb-0.5W-0.25Si-0.2B(at%) were made by induction skull melting(ISM) technique.A series of heat treatments(HTs) were conducted to research the microstructure evolution of both alloys.Microstructure and tensile property were examined by scanning electron microscope(SEM),X-ray diffraction(XRD),transmission electron microscope(TEM),and tensile testing machine.The results show that microsegregation(liquid segregation and solid segregation) is exacerbated by the addition of0.5 at%W;the addition of Nb,W in TiAl alloy makes the phase transition difficultly take place;then,the microstructures and tensile properties of both alloys are improved after certain HT processes;finally,the thicknesses of the γ/α_2 lamellae after a certain HT process are significantly affected by the number of residual γ phases before the furnace-cooling moment.     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.     

Microstructure evolution of eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the near-eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf(at%) alloy was directionally solidified at 1900 ℃ with withdrawal rates of 6, 18, 36, 50 mm·min~(-1) and then heat-treated at 1450 ℃ for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified(DS) alloy is composed of Nb_(ss)+Nb_5Si_3 eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes,the morphology and size of Nb_(ss)+Nb_5Si_3 eutectic cells.With the increase in withdrawal rates, the petaloid Nb_(ss)+Nb_5Si_3 eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nbssis formed gradually which isolates the Nb_5Si_3, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb_5Si_3 exist in the heat-treated(HT) samples identified by crystal form and element composition, which are supposed as α-Nb_5Si_3 and γ-Nb_5Si_3, respectively. This study exhibits significant merits in guiding the optimization of Nb–Si-based alloys' mechanical properties.     

Grain refining of Er added to Ti–22Al–25Nb alloy and morphology of erbium precipitates

In order to investigate the effect of erbium (Er) on the microstructure of orthorhombic Ti2AlNb-based alloys, four testing alloys were prepared by adding different contents of the rare earth metal Er to Ti-22Al-25Nb alloy and optical microscopy, X-ray diffraction, scanning electron microscopy, electron probe microanalysis, energy-dispersive spectrometry, and transmission electron microscopy were used. The results show that the addition of Er is capable of grain refining and the refinement effect is more obvious with increasing content of Er. The Er2O3 dispersoids formed by internal oxidation and Al3Er compound particles are observed in Er-doped alloys and the number of Er precipitates is increased with increasing Er addition. It is likely that the solubility of Er in the Ti2AlNb alloy is very low and Er precipitates tend to segregate at grain boundaries, which together with the surface activity of rare earth elements is supposed to decrease the prior B2 grain size of Ti-22Al-25Nb alloy effectively.     

Oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy in temperature range of 650-850 ℃

The oxidation behavior of a novel multi-element alloyed Ti_2AlNb-based alloy (Ti-22Al-25Nb-1Mo-1V-1Zr-0.2Si) was studied in the temperature range of650-850℃. X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with an energy-dispersive spectroscopy (EDS) were used to identify the phase constituents and microstructure of the scales formed on the specimens after oxidation at different temperatures.Isothermal oxidation tests show that the oxidation rate of the alloyed Ti_2AlNb-based alloy is obviously reduced at all temperatures, and the mass gains are very low for this alloy in comparison with those of Ti-22Al-25Nb alloy. The alloying elements Mo, V, Zr and Si have an obvious affect on the oxidation products of the alloys. The improved oxidation resistance for the alloy is ascribed to the introduction of Mo, V, Zr and Si elements, which are beneficial to the selective oxidation of Al to form protective oxides while are disadvantage of the formation of poor oxidation resistance oxides such as AlNb0_4.     

Hot Corrosion Behavior of SiO_2–Al_2O_3–Glass Composite Coating on Ti–47Al–2Cr–2Nb Alloy: Diffusion Barrier for S and Cl

Type Ⅰ hot corrosion behavior of SiO_2-Al_2O_3-glass composite coating based on Ti-47 Al-2 Cr-2 Nb substrate was investigated in the mixture salt of 25 wt%NaCl + 75 wt%Na_2SO_4 at 850 °C. The results showed that there was a bidirectional ion exchange between composite coating and the film of mixed salts, and the sodium ion in the molten salts penetrated into the glass matrix of composite coating, while the potassium ion in the glass matrix dissolved into the molten salts. A decrease in hot corrosion rate was achieved for the coated alloy in comparison with the bared substrate due to the composite coating acting as a diffusion barrier to sulfur and chlorine and preventing the molten salts from diffusing to the coating/alloy interface during the hot corrosion exposure. Additionally, the composite coating decreased the oxygen partial pressure at the coating/alloy interface and promoted the selective oxidation of Al to form a protective Al_2O_3 layer.     

Microstructure Characterization and Fracture Toughness of Laves Phase-Based Cr–Nb–Ti Alloys

Three Laves phase-based alloys with nominal compositions of Cr2Nb–x Ti(x = 20,30,40,in at%) have been prepared through vacuum non-consumable arc melting.The results show that the microstructures of Cr2Nb-(20,30) Ti alloys are composed of the primary Laves phase C15–Cr2(Nb,Ti) and bcc solid solution phase,while the microstructure of Cr2Nb–40Ti alloy is developed with the eutectic phases C15–Cr2(Nb,Ti)/bcc solid solution.The measured fracture toughness of ternary Laves phase C15–Cr2(Nb,Ti) is about 3.0 MPa m1/2,much larger than 1.4 MPa m1/2for binary Laves phase Cr2 Nb.Meanwhile,the fracture toughness of Cr2Nb–x Ti(x = 20,30,40) alloys increases with increasing Ti content and reaches 10.6 MPa m1/2in Cr2Nb–40Ti alloy.The eutectic microstructure and addition of Ti in Cr2 Nb are found to be effective in toughening Laves phase-based alloys.     

Effect of microstructure on tensile properties and fracture behavior of intermetallic Ti2AlNb alloys

The tensile properties and fracture behaviors of Ti-22Al-27Nb and Ti-22Al-20Nb-7Ta alloys were investigated in the temperature range of 25-800℃ Three typical microstructures were obtained by ifferent thermomechanical processing techniques.The results indicate that the duplex microstructure has an optimum combination of tensile yield strength and ductility both at room and elevated temperatures.Adding Ta to Ti2AlNb alloy can improve the yield strength,especially at high temperature while retain a good ductility.The study on crack initiation and propagation in dedformed microstructure of Ti2AlNb alloys indicates that microstructure has ikmportant effect on the tensile fracture mechanism of the alloys.The cracks initiate within primary O/α2 grains along O/B2 boundaries or O phase laths in B2 matrix,and propagate along primary B2 grain boundaries for the duplex microstructure.The fracture mode is transgranular with ductile dimples for the duplex and the equiaxed microstructures,but intergranular for the lath microstructure.     

Grain refinement of Al-3.5FeNb-1.5C master alloy on pure Al and Al-9.8Si-3.4Cu alloy

The refinement potential of Al-3.5 Fe Nb-1.5 C master alloy on pure aluminium and Al-9.8 Si-3.4 Cu alloy has been investigated. Different amounts of Al-3.5 Fe Nb-1.5 C master alloy were added to estimate the optimal addition level. It was found that the addition of Al-3.5 Fe Nb-1.5 C grain refiner can promote significantly the refinement of grains in the pure aluminium, particularly at 0.1 wt.%, with the mean primary aluminium α-grain size reducing to 187±3 μm from about 1-3 mm. Similarly, the microstructural study of the Al-9.8 Si-3.4 Cu alloy die casting at different weight percentages(viz. 0.0 wt.%, 0.1 wt.% and 1.0 wt.%) of Al-3.5 Fe Nb-1.5 C master alloy shows that the Al-3.5 Fe Nb-1.5 C master alloy as a grain refiner is also acceptable for Al-Si cast alloys when the silicon content is more than 4 wt.%. As a result of inoculation with Al-3.5 Fe Nb-1.5 C master alloy, the average grain size of α-Al is reduced to 22±3 μm from about 71±3 μm and grain refining efficiency is not characterized by any visible poisoning effect, which is the major limitation in the grain refinement of Al-Si cast alloys by applying Al-Ti-B ternary master alloys. Mechanical properties such as ultimate tensile strength and yield strength are significantly improved by 9.6% and 9.7%, respectively.     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Effects of minor yttrium addition on hot deformability of lamellar Ti-45Al-5Nb alloy

The effects of 0.3%(molar fraction,the same below)yttrium addition on hot deformability of lamellar Ti-45Al-5Nb alloywere investigated by simulated isothermal forging tests.The ingots with the nominal compositions of Ti-45Al-5Nb andTi-45Al-5Nb-0.3Y were prepared by induction skull melting.Simulated isothermal forging tests were conducted on Gleeble 1500Dthermo-simulation machine using a 6 mm in diameter and 10 mm in length compressive specimen at the deformation temperatures of1 100,1 150,1 200℃and strain rates of 1.0,0.1,0.01 s-1.The results show that yttrium addition remarkably improves hotdeformability of Ti-45Al-5Nb alloy.An appropriate hot deformation processing parameter of Ti-45Al-5Nb-0.3Y alloy is determinedas 1 200℃,0.01 s-1.The flow stresses are decreased by yttrium addition under the same compressive conditions.The activationenergies of deformation Q are calculated as 448.6 and 399.5 kJ/mol for Y-free and Y-containing alloys,respectively.The deformedmicrostructure observation under 1 200℃,0.01 s-1condition indicates that Ti-45Al-5Nb-0.3Y alloy shows more dynamicrecrystallization.The improvement of hot deformability of Ti-45Al-5Nb-0.3Y alloy induced by yttrium addition should be attributedto that the smaller the original lamellar colonies,the lower the deformation resistance and activation energy of deformation are,andthe more the dynamic recrystallization is.     

真空感应熔炼Nb-16Si-22Ti-2Hf-2Cr-2Al合金的组织和性能

The melt with a nominal composition of Nb-16Si-22Ti-2Hf-2Cr-2Al was poured in a ceramic shell mould with a temperature gradient of about 4℃/mm, and the ingot with the dimension of 60 mm×170 mm was obtained. The relationship between the microstructure and mechanical properties was measured, and the effects of the silicide on fracture toughness at ambient temperature and compression strength at high temperature were analyzed. It is revealed that the microstructure of the alloy consists of Nb solid solution and silicides, and the cooling rate can obviously change primary phase and constituent phases. The volume fraction of Nb₃Si formed at rapid cooling rate is significantly increased, and Nb₅Si₃ phase is formed with decreasing in cooling rate, whereas the volume fraction of the eutectic colonies and Nb_SS dendrites increase evidently, especially the second dendrite arm. Fine and uniform eutectic colonies are contributed to the ambient tensile strength, while the coarse primary Nb₃Si decreases the tensile strength but improves the compression strength of the alloy. When the microstructure is mainly composed of fine (Nb_SS+Nb₅Si₃) eutectic colonies, the tensile strength and elongation of the alloy reach 449 MPa and 0.3%, respectively. When lath-like Nb₃Si phase has 80 $\mu$m in width and 50% in volume fraction as well as its long axis is parallel to compression direction, the compression strength of the alloy at 1250℃ is about 650 MPa.     

Improvement of isothermal oxidation resistance of a γ'-strengthened Co-Al-W-Mo-Ta-B alloy at 800℃via doping Ce

Isothermal oxidation resistance,oxide scale evolution and failure mechanism of Ce-doped Co-Al-W-MoTa-B alloy(0.01 at%,0.05 at%,0.10 at% and 0.20 at% Ce)exposed at 800℃ were compared.The 0.01 Ce and 0.05 Ce alloys were consisted of γ/γ' coherent micro structure,while the κ-Co_3 W compound precipitated at the grain boundary of the 0.1 OCe and 0.20 Ce alloys in addition to the γ/γ'microstructure.The oxidation kinetics curves of the Cedoped alloys exhibited a parabolic time dependence on the weight gain.With an increasing nominal Ce content,the weight gain of the Co-Al-W-Mo-Ta-B alloys monotonically decreased.An oxide scale composed of a dense and uniform outer Co_3 O_4+CoO layer,a middle CoAl_2 O_4 and CoWO_4 compound layer and an inner Al_2 O_3 layer.The excellent oxidation resistance of 0.2 Ce alloy was mainly attributed to a shorter incubation stage for the formation of the continuous and protective Al_2 O_3 layer and the thickest Al_2 O_3 layer during entire oxidation process.     

High-temperature oxidation behavior of Nb67-xW15Si18Hfx（X=0.5 and 10） alloys

The oxidation behaviors of Nb67-xW15Si18Hfx （x=0, 5, 10） alloys were studied at 1 250 ℃ in air. It is found that the Nb67W15Si18 alloy has the best oxidation resistance among the three alloys; and Hf addition is harmful to the oxidation resistance of the Nb67W15Si18 alloy. The oxides formed on the Nb67W15Si18 alloy are mainly Nb12WO33 and NbO2, and that on the Nb62W15Si18Hf5 and Nb57W15Si18Hflo alloys is Nb2O5. Effect of Hfon the oxidation behavior of the Nb67-xW15Si18Hfx alloys has been discussed based on mierostructures and kinetics of oxidation.     

Lamellar morphology of directional solidified Ti–45Al–6Nb–xW alloys

Directional solidification of Ti-45Al-6Nb-xW(x=0,0.4,0.8;at%) alloys was performed by Bridgeman method with the stable growth rate of 5,10,and 20 μm·s~(-1).The differential scanning calorimeter(DSC) results indicate that both the eutectic temperature and the transition temperature of α(Ti_3Al) to γ(TiAl) increase with W content increasing from 0 at%to 0.8 at%.For the stable growth rate of 10 μm·s~(-1),the orientations of α_2(Ti_3Al)/γ(TiAl)lamellae change from 45°(0 at%W) to 0° and near 0°(0.4at%and 0.8 at%W) to the crystal growth direction,and the spacing of α_2/γ lamellae decreases with W content increasing from 0 at%to 0.4 at%,while it increases when W content is 0.8 at%.With the increase in growth rate from5 to 10 and 20 μm·s~(-1),the lamellar spacing of α_2/γbecomes smaller,and the lamellar thickness becomes more uniform.     

Solidification microstructure characteristics of Ti–44Al–4Nb–2Cr–0.1B alloy under various cooling rates during mushy zone

Beta-solidifying TiAl alloy has great potential in the field of aero-industry as a cast alloy.In the present work,the influence of cooling rate during mushy zone on solidification behavior of Ti-44Al-4Nb-2Cr-0.1B alloy was investigated.A vacuum induction heating device combining with temperature control system was used.The Ti-44Al-4Nb-2Cr-0.1B alloy solidified from superheated was melted to β phase with the cooling rates of 10,50,100,200,400 and 700 K·min~(-1),respectively.Results show that with the increase in cooling rate from 10 to 700 K·min~(-1),the colony size of α_2/γ lamella decreases from 1513 to48 urn and the solidification segregation significantly decreases.Also the content of residual B2 phase within α_2/γlamellar colony decreases with the increase in cooling rate.In addition,the alloy in local interdendritic regions would solidify in a hypo-peritectic way,which can be attributed to the solute redistribution and enrichment of Al element in solidification.     

Effect of Nb and alloying elements on interface reaction between high Nb-containing TiAl alloys and ZrO_2-based ceramic moulds

In the present study, Ti-45Al-(6, 7, 8)Nb(at%) and Ti-45Al-8Nb-0.5(Mn, Si, Y, B) alloys were prepared by arc melting and casting into Zr O2(Y2O3 stabilized) ceramic moulds to study the effect of alloying elements Nb and Mn, Si, Y, B on the interfacial reaction between casting Ti Al alloys and ceramic moulds by SEM, and the elements' distribution in the interface reaction layer by line scanning. The results showed that with an increase in Nb content, the interfacial reaction weakened and the thickness of the reaction layer decreased gradually. The interface reaction thickness of the alloys with Nb content of 6, 7, 8at% were 60, 34 and 26 μm, respectively. Clearly, the addition of 8at% Nb to Ti-45 Al is the best for the thickness of the reaction layer. The addition of Nb would form a Nb-rich film in the reaction layer, which could reduce the solubility of oxygen in the interface, and suppress further diffusion of oxygen to the matrix. If the same content of Mn, Si, Y, or B alloying elements were added respectively to Ti-45Al-8Nb, the thickness of the interface reaction layer from large to small was as follows: MnSiYB. The interface reaction thickness increased after 0.5at% Mn added, had no obvious change after 0.5at% Si addition, and decreased after adding 0.5at% Y or B. The introduced elements, which formed a protective film or/and promoted the formation of a dense aluminum oxide layer, would be of benefit to the resistance of interfacial reaction.     

High-temperature Oxidation Behavior of a High Manganese Austenitic Steel Fe–25Mn–3Cr–3Al–0.3C–0.01N

In this paper, a Fe–Mn–Al–C austenitic steel with certain addition of Cr and N alloy was used as experimental material. By using the SETSYS Evolution synchronous differential thermal analysis apparatus, the scanning electron microscope(SEM), the electron microprobe(EPMA) and the X-ray diffraction(XRD), the high-temperature oxidation behavior microstructure and the phase compositions of this steel in air at 600–1,000 °C for 8 h have been studied. The results show that in the whole oxidation temperature range, there are three distinct stages in the mass gain curves at temperature higher than 800 °C and the oxidation process can be divided into two stages at temperature lower than 800 °C.At the earlier stage the gain rate of the weight oxidized in temperature range of 850 °C to 1,000 °C are extremely lower.The oxidation products having different surface microstructures and phase compositions were produced in oxidation reaction at different temperatures. The phase compositions of oxide scale formed at 1,000 °C are composed of Fe and Mn oxide without Cr. However, protective film of Cr oxide with complicated structure can be formed when the oxidation temperature is lower than 800 °C.