Ceramic-ceramic. composites II

The growth mechanisms of the oxide scales of Ti-50Al and Ti-45Al-8Nb (at%) alloys at 900°C in air were investigated. The results revealed that the oxidation of γ-TiAl alloys during experiments was divided into four stages. The Ti-50Al alloy showed severe spallation after 3 h of oxidation. Nb addition to γ-TiAl alloys could improve the oxidation resistance effectively. The oxide scale growth was much slower and every oxide layer was more chemically uniform and homogeneous with Nb addition. The Ti-45Al-8Nb alloy showed no spallation even after 100 h of oxidation.     

Effect of sputtered TiAlCr coatings on oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V

The effect of sputtered Ti-50Al-10Cr and Ti-50Al-20Cr coatings on both isothermal and cyclic oxidation resistance at 800 similar to 900 degreesC and hot corrosion resistance at 850 degreesC of Ti-24Al-14Nb-3V was investigated. Results indicated that Ti-24Al-14Nb-3V alloys exhibited poor oxidation resistance due to the formation of Al2O3+TiO2+AlNbO4 mixed scales in air at 800 similar to 900 degreesC and poor hot corrosion resistance due to the spallation of scales formed in Na2SO4+K2SO4 melts at 850 degreesC. Both Ti-50Al-10Cr and Ti-50Al-20Cr coatings remarkably improved the oxidation and hot corrosion resistance of Ti-24Al-14Nb-3V alloy.     

Protective Effect of the Sputtered TiAlCrAg Coating on the High-Temperature Oxidation and Hot Corrosion Resistance of Ti-Al-Nb Alloy

The effect of a sputtered Ti-48AI-8Cr-2Ag (at. pct) coating on the oxidation resistance of the cast Ti-46.5AI-5Nb (at. pct) alloy was investigated in air at 1000-1100℃. Hot corrosion in molten 75 wt pct Na2SO4+25 wt pct K2SO4 was investigated at 900℃. The scale on the cast TiAINb tends to spall in air, while the scale on coating is very adherent. The sputtered Ti-48AI-8Cr-2Ag coating remarkably improved high temperature oxidation resistance of the cast Ti-46.5AI-5Nb alloy because of the formation of an adherent Al2O3 scale. Due to the inward diffusion of Cr, Kirkendall voids were found at the coating/substrate interface. TiAICrAg coating provided excellent hot corrosion resistance for TiAINb alloy in molten 75 wt pct Na2SO4+25 wt pct K2S04 at 900℃ due to the formation of a continuous Al2O3 scale.     

Creep Behavior of As-Cast Ti-48Al-2Cr Intermetallic Alloy for Aerospace and Automotive Applications

The creep properties of as-cast Ti-48Al-2Cr (at%) alloy which had been strengthen with addition of 2 at% Cr were investigated. Tensile creep experiments were performed in air at temperatures from 600-800°C and initial stresses ranging from 150 to 180 MPa. Stress exponent and activation energy were both measured. Data indicates that the alloy exhibits steady state creep behavior and the steady state creep rate is found to depend on the applied load and temperature. The measured power law stress exponent for steady state creep rate is found to be close to 3 and the apparent activation energy for creep is calculated to be 15.7 kJ/mol. The creep resistance of the present alloy was also compared with binary Ti-48Al (at%) to evaluate the effect of Cr addition on creep resistance of TiAl. It is concluded that addition of 2 at% of Cr does not have significant effect on the creep resistance of TiAl.     

Processing TiAl-Based Alloy by Elemental Powder Metallurgy

1. IntroductionTiAl based alloy has long been considered as apromising candidate for high temperature applications, because it has a high specific strength and goodhigh temperattire properties[1-31. The main methods for the preparation of TiAl based alloys includecasting and powder metallurgy. In general, thermomechanical treatment (such as forging and extrusion)and subsequellt heat treatment should be conductedfor cajst TiAl-based alloys in order to refine the coarsemicrostructure[4,5], whe…     

Microstructures and Tensile Properties of Isothermally-forgedTi-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr Alloys

Microstructures and room-temperature tensile properties of isothermally-forged γ-base (γ + α2)alloys in Ti-Al-Nb-Cr-V system with different heat treatments were investigated. The results show that the microstructures of Ti-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr (at. pct) alloys are mainly determined by heat treating temperature in the (cr + 7) tWo-phase field, and the joint additions of Nb, Cr and V in the Ti-47Al alloy afFect Ta significantIy. The microstructure of Ti-47Al alloy with additions of Nb, Cr and V (1~2 at. pct) can be dupIex or nearly-lamellar by a suitable heat treatment after isothermal forging at 1000℃ for over 50% plastic strains.Therefore its tensile properties can be improved at room temperature.     

Thermal stability of an Ni-Cr-Mo-Nb alloy

A study was conducted to characterize the microstructural changes that occur in an Ni-Cr-Mo-Nb alloy after up to 1000 hours aging at 650°C, 760°C, and 870°C and to determine the corresponding effects on mechanical properties and corrosion resistance. Various techniques of analytical electron microscopy and conventional scanning electron microscopy were used to characterize the microstructure. Mechanical properties were determined from room temperature surface hardness and tensile tests. Resistance to aqueous corrosion was determined from tests in boiling 10% nitric acid. Exposure to all temperatures investigated was found to result in age hardening and a decrease in corrosion resistance of the alloy. However, an overaging effect was observed during the later stages of exposure at 760°C and 870°C. The observed changes in properties were correlated with precipitation of the γ″ and σ phases of Ni₃Nb and an Ni-Mo-Si Laves phase.     

Ti-45Al-10Nb合金的高温氧化行为

研究了Ti—45A1—10Nb(原子分数，％)合金在800-960℃在氧气和空气中的氧化行为．结果表明，该合金具有较好的高温抗氧化性能，其氧化增重速率与铁基耐热不锈钢相似或略优．与Ti—50A1合金相反，Ti-45A1-10Nb合金在空气中的氧化增重速率明显低于在纯氧中的氧化增重速率．X射线衍射与能谱分析表明，Ti-45Al一10Nb合金在氧气中的氧化产物主要有TiO2和Al2O3，但在空气中的氧化产物中有TiN相．这是降低氧化速率的主要原因，同时，合金元素Nb稳定了氧化层中的TiN相，因而提高了合金在空气中的抗氧化性能．     

Biaxial deformation of Ti-6Al-4V and Ti-6Al-4V/TiC composites by transformation-mismatch superplasticity

Gas-pressure bulge forming of unreinforced Ti-6Al-4V and TiC-reinforced Ti-6Al-4V was performed while cycling the temperature around the allotropic transformation range of the alloy (880–1020 °C). The resulting domes exhibited very large strains to fracture without cavitation, demonstrating for the first time the use of transformation-mismatch superplasticity under a biaxial state of stress for both an alloy and a composite. Furthermore, much faster deformation rates were observed upon thermal cycling than for control experiments performed under the same gas pressure at a constant temperature of 1000°C, indicating that efficient superplastic forming of complex shapes can be achieved by transformation-mismatch superplasticity, especially for composites which are difficult to shape with other techniques. However, the deformation rate of the cycled composite was lower than for the alloy, most probably because the composite exhibits lower primary and secondary isothermal creep rates. For both cycled materials, the spatial distribution of principal strains is similar to that observed in domes deformed by isothermal microstructural superplasticity and the forming times can be predicted with existing models for materials with uniaxial strain rate sensitivity of unity. Thus, biaxial transformation-mismatch superplasticity can be modeled within the well-known frame of biaxial microstructural superplasticity, which allows accurate predictions of forming time and strain spatial distribution once the uniaxial constitutive equation of the material is known.     

Microstructural stability of the directionally solidified γ′-base superalloy IC6

The microstructural stability of an Ni---Al---Mo directionally solidified γ′-base superalloy, IC6, was investigated and the effect on microstructure and tensile properties at room temperature and 760°C of high temperature aging at 900–1100°C for up to 200 h was studied. The experimental results show that the tensile strength decreases with aging time and that aging at 1000°C has the strongest effect on tensile properties. This is attributed to the precipitation of large amounts of the δ-NiMo phase. Microstructural changes during high temperature creep tests were also examined and are different from those observed during aging at the same temperature but without any applied stress.     

Thermo-mechanical stability of forged Ti-25Al-11Nb (at.%)

Dynamic material modeling (DMM) has been applied to the analysis of the mechanical behavior of a (Widmanstätten + primary ₂ + β) Ti-25Al-11Nb alloy to establish the flow stress-strain rate-temperature-strain criteria for stable flow.

Unstable and stable flow regimes predicted by DMM were correlated to microstructural observations. Unstable flow zones were related to transformation of orthorhombic O → ₂, kinking of the ₂ lamellae, shear band formation and coarsening of the dynamically recrystallized grain structure; stable flow regimes were shown to be associated with dissolution of the Widmanstätten ₂, coarsening of primary ₂ and dynamic spheroidization of the lamellar ₂ microconstituents.     

An investigation of the fatigue and fracture behavior of multicomponent Nb-11Al-41Ti-1.5Mo-1.5Crintermetallic

This paper presents the results of a recent study of the fracture and fatigue crack growth behavior of a newly developed multicomponent niobium aluminide intermetallic Nb-11Al-41Ti-1.5Mo-1.5Cr alloy (compositions quoted in at % unless stated otherwise). The alloy is shown to have attractive combinations of room-temperature tensile ductility (approx. 11%) and fracture toughness (approx. scds 83 MPa ) in the as-forged condition. However, the tensile properties and fracture toughness are degraded somewhat by direct aging at 750 °C for 25 h. The direct aged Nb-11Al-41Ti-1.5Mo-1.5Cr intermetallic is also shown to have comparable fatigue crack growth resistance to pure Nb, IN 718 and mill annealed Ti-6Al-4V at room- and elevated-temperature. Fatigue and fracture mechanisms are elucidated prior to a discussion of the implications of the current results for potential high temperature structural applications.     

抗Zr“中毒”Al-Ti-B-C中间合金对7050铝合金力学性能的影响

通过场发射扫描电子显微镜(FESEM),X射线衍射仪(XRD),能量色谱仪(EDS)分析Al-5Ti-1B,Al-4Ti-1C和Al-5Ti-0.8B-0.2C中间合金的微观组织与物相组成,比较研究3种中间合金对7050铝合金晶粒尺寸与力学性能的影响。结果表明:Zr的存在削弱了Al-5Ti-1B和Al-4Ti-1C中间合金的细化效果,而对Al-5Ti-0.8B-0.2C中间合金细化效果影响较小。含掺杂型TiC粒子的Al-5Ti-0.8B-0.2C中间合金具有较好的抗Zr"中毒"能力,加入量为0.2%(质量分数,下同)时,含Zr7050铝合金平均晶粒尺寸由200μm细化至(60±5)μm,室温极限抗拉强度由405MPa提高到515MPa,提高了27.2%,伸长率由2.1%提高到4.1%。而加入0.2%的Al-5Ti-1B或Al-4Ti-1C中间合金时晶粒尺寸较粗大且分布不均匀,表现出明显的细化"中毒"。     

高温熔体反应法制备Al-5Ti-1B细化剂

利用高温熔体反应法成功制备Al-5Ti-1B细化剂。通过热力学计算,确定Al-5Ti-1B细化剂的起始反应温度,研究熔体温度对细化剂组织形貌及吸收率的影响。利用X射线衍射,扫描电子显微镜和X射线能谱仪观察细化剂的相组成和形貌,同时对Al-5Ti-1B细化剂铸锭进行高温挤压,并对挤压出的Ф9.5mm丝材进行微观组织分析和细化实验。结果表明:细化剂主要由TiB_2,TiAl_3,α-Al相组成;850℃制备的细化剂铸锭组织形貌最佳,且Ti和B吸收率达到最佳匹配。挤压后TiAl3相呈细小的块状和TiB_2弥散分布在基体内。添加0.2%(质量分数)细化剂后,纯铝的晶粒尺寸由3.99mm细化到0.45mm。     

铌硅化物基超高温合金Si-Y2O3共渗涂层的组织及其高温抗氧化性能

采用Si-Y₂O₃包埋共渗工艺在铌硅化物基超高温合金表面制备Y改性的硅化物涂层, 研究其在1250℃的恒温氧化性能. 采用扫描电镜(SEM)、能谱(EDS)与X射线衍射(XRD)分析Si-Y₂O₃共渗涂层氧化前后的物相组成和组织变化. 结果表明：涂层具有明显分层的结构, 由外至内依次为(Nb,X)Si₂(X表示Ti, Hf和Cr)外层和(Nb,X)₅Si₃过渡层, 在过渡层与基体之间有不连续分布的细小(Cr,Al)₂(Nb,Ti)块状沉淀. EDS分析表明, 涂层中的Y分布是不均匀的, (Cr,Al)₂(Nb,Ti)相的Y含量为0.94at%左右, 而(Nb,X)Si₂和(Nb,X)₅Si₃相的Y含量为0.46at%～0.57at%. 经1250℃分别氧化5, 10, 20, 50和100h后, Si-Y₂O₃共渗涂层保持其原始的相组成, 并在其表面形成以TiO₂、 SiO₂和Cr₂O₃组成的致密混合氧化膜, 且与基体结合良好.     

Electropolishing of Al-7Si-0.3Mg cast alloy by using perchloric and nitric acid electrolytes

The aim of the present study was to find a good method for electropolishing Al-7Si-0.3Mg cast alloy. Consideration of the fundamental current density-voltage relation and limiting current density has made it possible to identify an optimum applied voltage for good electropolishing through variation of different parameters (electrolyte, bath temperature, electrode distance, surface roughness before polishing, cathode material, etc.). The significant findings of the present study are that (1) using 30% perchloric acid + 20% glycerol moderator + 50% methanol electrolyte, (2) maintaining 15 mm electrode distance and a temperature of −33 °C, and (3) electropolishing the Al-7Si-0.3Mg cast alloy at 15 V for 2 minutes reveal the microstructures clearly. The limiting current density is about 7 mA/cm² for the preceding conditions for attaining good electropolishing. Electropolishing can also be achieved under the same conditions even at room temperature (26 °C), but the clarity of the microstructure is not as good as that obtained through electropolishing at −33 °C.     

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects of β sta-bilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb (～64J) were two times higher than that ofTi-6Al (～30J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb (15.4J,16.1J and 15.0J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb (46.7J and 48.3J,respectively) were about three times higher than that of Ti-6Al (14.4J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup at α/β interfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion in α grains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.     

Thermal stability of Ti-V-Cr burn-resistant alloys

Ti-35V-15Cr, Ti-25V-15Cr and Ti-25V-15Cr-0.4Si alloys were exposed to different temperatures in air (450–600°C) for aging times between 0 and 200 h and subsequently tensile tested at room temperature with the surface oxide retained or removed. The influence of an applied tensile stress (50–200 MPa) during thermal exposure was also investigated. The results showed that post-exposure tensile properties deteriorated with the increase in exposure temperature and time. The decrease in tensile properties resulted from the combination of surface oxidation and microstructural changes. The main change of the microstructure during thermal exposure is the heterogeneous precipitation of phase on beta grain boundaries. Both increased vanadium content in the alloy and the addition of silicon have shown an adverse effect on alloys' thermal stability.     

Anelastic relaxation peak associated with the presence of incoherent θ phase in Al-4wt.%Cu alloy

Internal friction measurements have been made for Al-4wt.%(1.76at.%)Cu alloy aged at various temperatures. A relaxation peak was observed at around 400 °C (ƒ = 1.6 Hz) after the speciment had been aged within the temperature range 330–450 °C. X-ray diffraction analysis showed that the incoherent θ phase is formed after aging within this temperature range and that it grows and becomes coarsened at increasing aging temperatures according to electron microscopy observations. Consequently the 400 °C internal friction peak observed for the first time is associated with the presence of θ phase in the alloy. Examination of the experimental results showed further that this peak is not due to a process occurring inside the θ phase but is associated with the stress-induced relaxation along phase boundaries between the θ particle and the matrix phase.

The activation energy of this peak is found to be 1.0 eV. The relaxation time and the relaxation strength of this peak calculated on the basis of an equivalent inclusion model are in accord with experimental results.     

TiAl合金热浸镀铝及热扩散处理后的镀层结构

为了提高TiAl基合金的抗高温氧化性能,采用热浸镀铝法和热扩散处理工艺在合金Ti-45Al-2Cr-2Nb-0.15B(原子分数,%)表面制备TiAl_3涂层.用XRD和SEM方法对涂层的结构和成分进行了分析.结果表明:TiAl合金经过热浸镀铝处理,表面生成了双相的富Al涂层,它由纯Al外层和TiAl_3内层组成;在550℃下,无论是在真空或者空气气氛中,进行扩散热处理,TiAl合金表面都形成了单相的TiAl_3涂层.