Microstructure and strength of brazed joints of Ti3Al-base alloy with NiCrSiB

Brazing of Ti₃Al alloys with the filler metal NiCrSiB was carried out at 1273–1373 K for 60–1800 s. The relationship of brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. When products are brazed, the optimum brazing parameters are as follows: brazing temperature is 1323–1373 K, brazing time is 250–300 s. The maximum shear strength of the joint is 240–250 MPa. Three kinds of reaction products were observed to have formed during the brazing of Ti₃Al alloys with the filler metal NiCrSiB, namely, TiAl₃ (TiB₂) intermetallic compounds formed close to the Ti₃Al alloy. TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds layer formed between TiAl₃ (TiB₂) intermetallic compounds and the filler metal and a Ni[s,s] solid solution formed in the middle of the joint. The interfacial structure of brazed Ti₃Al alloy joints with the filler metal NiCrSiB is Ti₃Al/TiAl₃ (TiB₂)/TiAl₃+AlNi₂Ti (TiB₂)/Ni[s,s] solid solution/TiAl₃+AlNi₂Ti (TiB₂)/TiAl₃ (TiB₂)/Ti₃Al, and this structure will not change with brazing time once it forms. The formation of over many intermetallic compounds TiAl₃+AlNi₂Ti (TiB₂) results in embrittlement of the joint and poor joint properties. The thickness of TiAl₃+AlNi₂Ti (TiB₂) intermetallic compounds increases with brazing time according to a parabolic law. The activation energy Q and the growth velocity K₀ of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) in the brazed joints of Ti₃Al alloys with the filler metal NiCrSiB are 349 kJ/mol and 24.02 mm²/s, respectively, and the growth formula was y²=24.04exp(−41977.39/T)t. Careful control of the growth of the reaction layer TiAl₃+AlNi₂Ti (TiB₂) can influence the final joint strength.     

Zr-Cu-Ni非晶钎料真空钎焊TiAl合金/316L不锈钢接头的界面组织与剪切性能

采用自主设计制备的Zr-42.9Cu-21.4Ni非晶钎料对TiAl合金和316L不锈钢进行真空钎焊,研究钎焊温度和钎焊时间对TiAl合金/316L不锈钢异种金属接头微观组织和剪切性能的影响。结果表明：钎缝界面可以划分为6个不同的反应层。1040 ℃/10 min下制备的钎焊接头从TiAl合金到316L不锈钢侧界面组织依次为γ(TiAl)+AlCuTi/α₂(Ti₃Al)+AlCuTi/AlCu+ZrCuNi+FeZr/Cu₈Zr₃+ZrCuNi+TiFe+Fe₂Zr/FeZr+Fe₂Zr+TiFe₂+ZrCu/α-(Fe, Cr)。随着钎焊温度的升高,接头的抗剪强度先升高后降低。当钎焊温度为1040 ℃和钎焊时间25 min时,接头抗剪强度达到最大值162 MPa。断口分析表明,接头在FeZr+Fe₂Zr+TiFe₂+ZrCu界面处萌生,沿着Cu₈Zr₃+ZrCuNi+TiFe+Fe₂Zr和α-(Fe, Cr)扩展,呈解理断裂。     

Hot pressing diffusion bonding of a titanium alloy to a stainless steel with an aluminum alloy interlayer

The probability and appropriate processing parameters of hot pressing diffusion bonding (HP–DW) of a titanium alloy (TC4) to a stainless steel (1Cr18Ni9Ti) with an aluminum alloy (LF6) interlayer have been investigated. The microstructure of the bonded joints has been observed by optical microscopy, SEM, XRD and EDX, and the main factors affecting hot pressing and diffusion bonding process were analyzed. The results showed that atom diffused well and no intermetallic compound or other brittle compounds appeared at optimum parameters. The fracture way of joints was ductile fracture. With the increment of bonding temperature, large number of intermetallic compounds such as FeAl₆, Fe₃Al, FeAl₂ which were brittle appeared along the interface between the stainless steel and the aluminum alloy interlayer, as a result, the quality of joints was decreased significantly and the fracture way of joints was brittle fracture.     

Processing and Characterization of Fiber Reinforced Intermetallic Matrix Composites

A series of intermetallic matrix composites reinforced with Al₂O₃ based fibers were fabricated by pressure casting. The Al₂O₃ based fibers used were DuPont's 20 μm diameter Fiber FP and PRD-166 fiber, Mitsui's 10 μm diameter Almax fiber, and Saphikon's 125 μm diameter single crystal Al₂O₃ fiber. The intermetallic matrices employed were alloys based on Ni₃Al, NiAl, Fe₃Al, Ti₃Al+TiAl, and Nb₂Al+NbAl₃. Optical, scanning and transmission electron microscopy were used to investigate the microstructure of the composites and the fibers. Tensile testing was conducted to determine the Weibull mean strength of the fibers in the as-received and heat treated conditions. The effect of fiber gage length on the Weibull mean strength of the PRD-166 and Fiber FP was evaluated. Indentation tests were performed to determine the effect of alloying additions on the fiber/matrix bond strength in shear in Saphikon fiber reinforced Ni₃Al composites.     

Microstructure and creep properties of a cast intermetallic Ti–46Al–2W–0.5Si alloy for gas turbine applications

The microstructure and creep properties including minimum creep rate, time to 1% creep deformation and creep fracture time of a cast TiAl-based alloy with nominal chemical composition Ti–46Al–2W–0.5Si (at.%) were investigated. The creep specimens were prepared from investment-cast plate and two large turbine blades. Constant load creep tests were performed in air at applied stresses ranging from 150 to 400 MPa in the temperature range 973–1073 K. The microstructure of the specimens is characterised by optical, scanning and transmission electron microscopy before and after creep deformation. The minimum creep rate is found to depend strongly on the applied stress and temperature. The power law stress exponent of minimum creep rate is n = 7.3 and the apparent activation energy for creep is Q_a = 427 ± 14 kJ/mol. The initial microstructure of the creep specimen is unstable. The ₂(Ti₃Al)-phase transforms to γ(TiAl)-phase and needle-like B2-precipitates during long-term creep testing at all testing temperatures. At lower applied stresses, the creep specimens fail by the growth and coalescence of cavities and small cracks formed along the γ/₂ interfaces. At the highest applied stresses, the specimens fail by nucleation and propagation of cracks.     

Interfacial microstructure and strength of partial transient liquid-phase bonding of silicon nitride with Ti/Ni multi-interlayer

Partial transient liquid-phase bonding (PTLP bonding) of silicon nitride (Si₃N₄) ceramic has been performed using Ti/Ni multi-interlayer in vacuum at 1273–1423 K. Interfacial microstructures were examined by scanning electron microscope, electron probe micro-analysis, and X-ray diffraction. The joint strength has been measured by four-point bending tests from room temperature up to 1000 °C. Interfacial structure of Si₃N₄/TiN/Ti₅Si₃ + Ti₅Si₄ + Ni₃Si/(NiTi)/Ni₃Ti/Ni is formed after bonding process. The NiTi layer is gradually consumed with simultaneous growth of the reaction layer and the Ni₃Ti layer. The room temperature joint strength is significantly affected by the reaction layer thickness, whereas the elevated temperature joint strength significantly depends on whether the low melting point NiTi layer exists in the joint. The joint strength of more than 100 MPa is retained up to 800 °C as the NiTi layer is completely consumed. A model is proposed to optimize the PTLP bonding parameters for optimizing joint strength at both room temperature and elevated temperature.     

碳钢坩埚表面渗铝复合涂层

以碳钢板为基板材料, 通过表面渗铝和高温化学反应在其表面形成复合保护涂层。研究了反应层厚度与反应温度、时间之间的关系, 并用光学显微镜、XRD对涂层形貌、相组成进行了表征。实验结果表明: 反应产物层厚度随反应温度、时间的增加而增加; 复合涂层由过渡层和反应产物层组成, 过渡层组成为Fe₃Al及少量Fe₂Al₅、Fe₁₄Al₈₆、Al₂O₃, 反应产物层组成为TiB₂、MgO和少量的Mg₂TiO₄、Mg₂B₂O₅、Fe₃Al、FeAl、Ti₂B₅。      

Measurement of the compressive creep strain rates of the individual phases within a lamellar microstructure

A fuducial line technique has been developed to determine the creep properties of the constituent phases within a lamellar composite subject to compression creep deformation. The technique can yield information on the total strain, creep rate, and the stress exponent and activation energy for creep of the individual phases within a lamellar microstructure. The contribution of interphase interfacial sliding to the strain of lamellar composites can also be evaluated by using the fiducial line technique. Application of the fiducial line analysis to a two-phase TiAl/Ti₃Al lamellar alloy deformed in compression at 1080K and 380MPa yields good agreement between the creep strain determined using the fiducial line analysis and the value directly measured from the crept specimen. The fiducial line analysis reveals that the TiAl phase within the two-phase TiAl/Ti₃Al lamellar microstructure creeps 2.2 times faster than the Ti₃Al phase and that interfacial sliding does not contribute to creep deformation of this alloy, within the resolution limit of the fiducial line experiment.     

Fe/Al异质金属接头界面组织演变、生长动力学及力学性能

采用真空扩散连接方法研究Fe/Al异质金属接头界面组织演变规律、金属间化合物(intermetallic compound,IMC)生长动力学及力学性能。结果表明：焊接温度为550 ℃时,接头界面无IMC生成,当焊接温度超过575 ℃时,界面由Fe₂Al₅及少量FeAl₃ IMC构成,且随焊接温度升高IMC层迅速长大。在120 min保温时间条件下,接头剪切强度随焊接温度的升高先增加后降低,当焊接温度为575 ℃时,接头剪切强度达到最大值37 MPa。在550~625 ℃范围内,基于热力学分析得出Fe₂Al₅的吉布斯自由能ΔG_Fe-Al最低,而FeAl₃的ΔG_Fe-Al次之,在接头界面处IMC生成顺序为Fe₂Al₅→FeAl₃。Fe/Al接头界面IMC的生长随焊接温度呈抛物线规律,其生长激活能为282.6 kJ·mol^-1。在575,600,625 ℃条件下,界面IMC的生长速率分别为1.13×10^-14,3.59×10^-14,1.21×10^-13 m²·s^-1。     

Analysis of the microstructure of Cr–Ni surface layers deposited on Fe3Al by TIG

A series of Cr–Ni alloys were overlaid on a Fe₃Al surface by tungsten inert gas arc welding (TIG) technology. The microstructure of the Cr–Ni surface layers were analysed by means of optical metallography, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that when the appropriate TIG parameters were used and Cr25–Ni13 and Cr25–Ni20 alloys were used for the overlaid materials, the Cr–Ni surface layers were crack-free. The matrix of the surface layer was austenite (A), pro-eutectoid ferrite (PF), acicular ferrite (AF), carbide-free bainite (CFB) and lath martensite (LM), distributed on the austenitic grain boundaries as well as inside the grains. The phase constituents of the Cr25–Ni13 surface layer were γ-Fe, Fe₃Al, FeAl, NiAl, an Fe–C compound and an Fe–C–Cr compound. The microhardness of the fusion zone was lower than that of the Fe₃Al base metal and Cr25–Ni13 surface layer.     

Microstructure and tribological properties of in situ synthesized TiN/Ti3Al intermetallic matrix composite coatings on titanium by laser cladding and laser nitriding

TiN reinforced Ti₃Al intermetallic matrix composite (TiN/Ti₃Al IMC) coatings were in situ synthesized on a pure Ti substrate with Ti + Al mixed powders in nitrogen atmosphere by laser cladding and laser nitriding. It was found that the growth morphologies of the TiN reinforced phase in the TiN/Ti₃Al IMC coatings were granular-like, flake-like, and undeveloped dendrites at lower N₂ flow rate; and granular-like, undeveloped and developed dendrites at higher N₂ flow rate. In addition, the volume fraction of the TiN phase increased with increasing nitrogen flow rate. The hardness of the TiN/Ti₃Al IMC coatings was higher than that of the Ti₃Al coating, which increased with increasing volume fraction of the TiN phase. Friction and wear tests revealed that the wear resistance of TiN/Ti₃Al IMC coatings was superior to those of pure Ti and Ti₃Al coating. It is well worth noting that the TiN/Ti₃Al IMC coatings showed excellent wear resistance under lower normal loads.     

Ti811合金表面激光熔覆Ti2Ni+TiC+Al2O3+CrxSy复合涂层的组织和性能

以TC4+Ni45+Al₂O₃+MoS₂+Y₂O₃混合粉末为熔覆材料,采用同轴送粉技术在Ti811合金表面进行激光熔覆制备复合涂层,使用SEM、EDS和XRD等手段分析了涂层的微观组织,测试了涂层的显微硬度和摩擦磨损性能。结果表明,在激光熔覆过程中Ti811合金中的Ni和C分别与Ti发生反应,原位生成金属间化合物Ti₂Ni和硬质增强相TiC;MoS₂分解后S与Cr发生硫化反应生成了软质润滑相Cr_xS_y。网状形态的Ti₂Ni、近球状和枝晶形态的TiC以及点状的Al₂O₃,均匀分布在熔覆层中。硬质相强化和软质相润滑的共同作用,使激光熔覆层具有较高的显微硬度和较优良的耐磨性能。激光功率为900 W的熔覆层其平均显微硬度值达1303.5HV_0.5,其耐磨性能最佳。     

Microstructure and strength of brazed joints of Ti3Al-base alloy with different filler metals

The interfacial microstructure and properties of brazed joints of a Ti₃Al-based alloy were investigated in this paper to meet the requirements of the use of Ti₃Al-based alloy in the aeronautic and space industries. The effects of different brazing fillers on the interfacial microstructure and shear strength were studied. The relationship between brazing parameters and shear strength of the joints was discussed, and the optimum brazing parameters were obtained. The brazed joints were qualitatively and quantitatively analyzed by means of EPMA, SEM and XRD. The results showed that using a AgCuZn brazing filler, TiCu, Ti(Cu,Al)₂ and Ag[s,s] were formed, the shear strength of the joint was decreased because of the formation of TiCu and Ti(Cu,Al)₂; using a CuP brazing filler, Cu₃P, TiCu and Cu[s,s] were formed at the interface of the joint, the former two intermetallic compounds decreased the shear strength. The analysis also indicated that using the TiZrNiCu brazing filler, the optimum parameters were temperature T=1323 K, joining time t=5 min, and the maximum shear strength was 259.6 MPa. For the AgCuZn brazing filler, the optimum parameters were joining temperature T=1073 K, joining time t=5 min, and the maximum shear strength was 165.4 MPa. To the CuP brazing filler, the optimum parameters were joining temperature T=1223 K, joining time t=5 min, and the maximum shear strength is 98.6 MPa. Consulting the results of P. He, J.C. Feng and H. Zhou [Microstructure and strength of brazed joints of Ti₃Al-base alloy with NiCrSiB, Mater. Charact., 52(8) (2004) 309–318], relative to the other brazing fillers, TiZrNiCu is the optimum brazing filler for brazing Ti₃Al-based alloy.     

Effects of aging on martensitic transformation of Ti50Ni25Cu25 shape memory alloy

Martensitic transformation has been studied in Ti₅₀Ni₂₅Cu₂₅ shape memory alloy by internal friction (IF) measurement and X-ray diffraction. It shows that the martensitic transformation proceeds from B2 to B19 for the solution-treated Ti₅₀Ni₂₅Cu₂₅ alloy. B2 phase is stabilized, and aging the alloy at 723–923 K decreases internal friction values. Part of the remaining B2 parent phase transform to B19′ monoclinic martensite at much lower temperatures.     

Effect of Ar gas pressure on growth, structure, and mechanical properties of sputtered Ti, Al, TiAl, and Ti3Al films

Single layers of Ti, Al, TiAl and Ti₃Al were sputter deposited on to 2″ oxidized Si 111 wafers and 7059 Corning Glass to study the effect of film thickness, temperature, and sputtering gas pressure on the mechanical and physical properties. In the present investigation, sputtering gas pressure was varied from 2 mT to 10 mT. The film thickness was varied from 1000 Å to 2 μm. The as-deposited Ti, Al and Ti₃Al films are well crystallized over the entire thickness range. Ti and Ti₃Al films show preferred orientation in the 0002 direction. On the other hand, Al films are random polycrystalline. TiAl films are nearly amorphous for all the thicknesses under consideration. TiAl films show formation of Ti (Al) solid solution phase with increasing Ar pressure. All the materials under consideration, show average film stress to be independent of thickness for thicker films. The nature of the stress (compressive or tensile) depends upon working gas pressure, sputtering power and the target material used. A definite trend is observed in the film stress as a function of Ar gas pressure. Both power and gas pressure influence the energetic bombardment of ions/atoms which in turn influence the average film stress. The nature of the intrinsic stress is explained by the atomic peening model. The Young's modulus of thin films is calculated by using the slope of the stress-temperature plots. The E values seem to change with deposition conditions, however, there is no obvious trend between the sputtering gas pressure and the Young's modulus of these thin films.     

Joining particulate and whisker ceramic composites by plastic flow

Joining of bulk ceramics by plastic flow has been demonstrated for several fine-grained ceramics. We have joined, for example, submicrometer 3 mol% Y₂O₃ partially stabilized ZrO₂ (YSZ) and YSZ-toughened Al₂O₃. The interlayers in these cases consisted of dense, submicrometer-grain-sized pieces of ceramics that have been shown to exhibit superplastic flow. We have extended the joining work to include incorporation of 15 vol.% SiC or 20 vol.% TiC whiskers within the bond layer. Unlike YSZ/Al₂O₃, because of the presence of the whiskers, YSZ/Al₂O₃/SiC and YSZ/Al₂O₃/TiC do not deform superplastically. However, virtually perfect YSZ/Al₂O₃ joints with and without whiskers could be made by compressing at 1300–1350 °C and stresses of <20 MPa.     

Numerical simulation of plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl

Plastic deformation and fracture in polysynthetically twinned (PST) crystals of TiAl have been simulated by using periodic unit cells representing the relaxed-constraint model recently proposed by Lebensohn et al. [Acta Mater. 46 (1998) 4701–4709] for the co-deformation of the lamellar compound of PST-TiAl. The unit cells contain both intermetallic phases, ₂-(Ti₃Al) and γ-(TiAl). Furthermore, the six orientation variants of the γ-phase are also considered. The constitutive behaviour of both phases is described by crystal plasticity, and the damage behaviour has been implemented by means of cohesive elements. The unit cells have been used as submodels for multi-scale finite element simulations of compression tests and fracture mechanics tests of notched micro-bend specimens. It is shown that the anisotropy of plastic deformation and damage in PST-TiAl can be well represented.     

SiC晶须对反应合成多孔TiB_2-TiC复合材料的影响

以Ti、B_4C和SiC晶须(SiC_w)为原料,采用自蔓延高温合成法制备了多孔TiB__2-TiC复合材料。讨论了SiC_w含量对TiB__2-TiC复合材料物相、组织形貌、孔隙率和抗压强度的影响。结果表明:不添加SiC_w时,复合材料中主要物相为贫硼相TiB和Ti_3B_4以及TiC和少量TiB__2;在5Ti+B_4C体系中加入SiC_w后,贫硼相TiB和Ti_3B_4逐渐减少直至消失,而出现富硼相TiB__2和TiC的含量增加。随着SiC_w含量的增加,复合材料的孔隙率逐渐增加,由38.46%增加至5_2.78%。当SiC_w含量小于1.0时,随着SiC_w含量的增加,多孔TiB_2-TiC复合材料的抗压强度明显增加,当SiC_w含量为1.0时,复合材料的抗压强度达到最大值56.04MPa。Ti与SiC_w反应会生成TiC、Ti_3SiC_2和TiSi_2等物相,消耗一定量的Ti,使得与B4C反应的Ti量减少,从而促进富硼相TiB_2形成和TiC的增多。并且在SiC_w表面形成颗粒状TiC或者层片状Ti_3SiC_2,增加SiC_w与TiB_2-TiC基体之间的结合,更有利于发挥SiC_w的强化作用。     

SiC陶瓷与316L不锈钢真空活性钎焊

采用Ag-Cu-Ti活性钎料,通过真空钎焊方法进行了SiC陶瓷与316L不锈钢的连接,研究了接头的界面组织、特征点成分和物相,并探讨了钎焊温度(800~930℃)、保温时间(0~30 min)对接头界面组织和连接强度的影响。结果表明,SiC陶瓷与316L不锈钢钎焊抗剪断口均发生在SiC陶瓷与钎料连接界面处,由于活性元素Ti的作用,在陶瓷与钎料的界面处形成了连续的反应层,反应生成了Ti C和Ti₅Si₃;在316L不锈钢与钎料的界面处,生成了Fe-Ti化合物和Cu-Ti化合物。随着钎焊温度升高及保温时间延长,接头强度均呈现出一个峰值,在温度为900℃,保温20 min的工艺条件下可获得最大接头抗剪强度。     

Comparative study on microstructure and mechanical properties of laser welded–brazed Mg/mild steel and Mg/stainless steel joints

A laser welding–brazing (LWB) technology using Mg based filler has been developed for joining Mg alloy to mild steel and Mg alloy to stainless steel in a lap configuration. Microstructure and mechanical properties of laser welded–brazed lap joints in both cases were comparatively studied. The results indicated that no distinct reaction layer was observed at the interface of Mg/mild steel and subsequently the interface was confirmed as mechanical bonding, whereas an ultra thin reaction layer with a continuous and uniform morphology was evidenced at the Mg/stainless steel interface, which was indicative of metallurgical bonding. The newly formed interfacial layer was indexed as FeAl phase by transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDS). The average tensile–shear strength of Mg/mild steel joint was only 142 N/mm with typical interfacial failure, while that of Mg/stainless steel joint could reach 270 N/mm, representing 82.4% joint efficiency relative to the Mg alloy base metal. The fracture location of Mg/stainless steel joint was at Mg fusion welding side, suggesting the interface was not weak point due to the formation of ultra thin interfacial layer. The role of alloying elements in base metal and bonding mechanism of the interfacial layer were discussed, respectively.