Interface effects during consolidation in titanium alloy components locally reinforced with matrix-coated fibre composite

An investigation has been made of diffusion bonding at the interface between a local reinforcing metal matrix composite and a monolithic engineering material. Diffusion bonding occurs during the consolidation of the composite during component manufacture. In this study, the composite is made up from Ti–6Al–4V titanium alloy coated SiC fibres, and the monolithic engineering material is also Ti–6Al–4V, but with a different microstructure.

An interface model is presented which takes account of diffusion bonding and which is able to describe the deformation behaviour at the interface between composite and monolithic material during composite consolidation. The model is developed from an existing diffusion bonding theory, and is implemented into finite element software.

The finite element simulations, and results of experiments, show that diffusion bonding can lead to localised deformation, the inhibition of consolidation, and a resulting inhomogeneous distribution of consolidated and unconsolidated regions during component manufacture. A further effect of the diffusion bonding is to increase the level of component distortion which results from the constraint imposed on the consolidating composite.

The interface model presented enables the simulation of practical forming processes so that process variables such as temperature and pressure can be chosen to ensure appropriate finished component properties.     

High cycle fatigue behaviour of Ti–6Al–4V alloy at elevated temperatures

High cycle fatigue behaviour of Ti–6Al–4V alloy was studied at 623 K and 723 K. Fatigue strength decreased at elevated temperatures compared with at ambient temperature. In the short life regime, fatigue strength was lower at 723 K than at 623 K, but in the long life regime it was nearly the same at both temperatures. At elevated temperatures, cracks were generated earlier at applied stresses below the fatigue limit at ambient temperature, indicating lowered crack initiation resistance. Small cracks grew faster at elevated temperatures than at ambient temperature, which became more noticeable with increasing temperature. After allowing for the elastic modulus, small cracks still grew faster at elevated temperatures.     

A neutron diffraction study of load partitioning in continuous Ti/SiC composites

Neutron diffraction measurements are described of the internal strain response of Ti–6Al–4V/35 vol.% SiC continuous fibre composites to loading axial and transverse to the fibre alignment direction. In the as-fabricated condition large thermal residual strCTains are observed, being equivalent to average axial fibre and matrix stresses of −840 and 450 MPa, respectively. As one might expect, upon loading there are marked differences in load sharing according to whether the fibres are parallel or perpendicular to the loading direction. In the former case, load is transferred towards the fibres; a process which is accelerated when the matrix deforms plastically, while in the latter case, load is transferred to the fibres only at very low loads. At higher loads, the process reverses with the reinforcement shedding load back into the matrix. The measurements suggest that this is caused by matrix/fibre interface failure at transverse loads of around 300 MPa. Simple calculations suggest that this would require a non-zero matrix/fibre normal interface strength of around 100 MPa. The measured thermal and load-induced strains are interpreted in the light of Eshelby-based models throughout.     

Fibre bridging during high temperature fatigue crack growth in Ti/SiC composites

Synchrotron X-ray diffraction has been used to map the crack tip stress field, load redistribution and the variation in interfacial shear stress along bridging fibres local to a matrix crack during fatigue crack growth in Ti-6Al-4V/unidirectional SCS-6 SiC monofilament composite at elevated temperature. Quasi-static fatigue cycles were applied in a diffractometer at the same stress amplitudes and temperatures (120 and 300 °C) as those used in prior off-line fatigue testing. The elastic fibre strains were measured ply by ply along the fibres and in the matrix in the region of the crack. In this manner the crack affected zone was mapped, and subsequently the interfacial shear stress levels deduced as a function of distance from the crack at these temperatures. The results are compared with previous studies of load redistribution at room temperature and the fibre sliding stresses compared with those needed to slide pristine fibres to evaluate degradation of the interfacial shear strength caused by interfacial wear during fatigue. The implications for the use of such composites at elevated temperatures, for example in aero engines, are discussed.     

A fundamental study on Ti–6Al–4V's thermal and electrical properties and their relation to EDM productivity

There are strong needs for productive/quality machining strategies of notoriously “difficult-to-machine” aerospace materials. The current means of machining these materials is dominated by mechanical cutting methods, which are costly due to high tooling costs, poor surface quality and limitations in the workpiece features and operations that can be machined. The newest EDM technology may be able to circumvent problems encountered in mechanical machining methods. In this paper, the EDM technology has been used to machine titanium alloy Ti–6Al–4V to investigate the effect of Ti–6Al–4V's thermal and electrical properties on the EDM productivity. In the study, temperature measurements have been made for Ti–6Al–4V workpieces with various duty factors to clarify the essential causes of difficulty in machining titanium alloys and observe the optimal duty factor in terms of productivity and quality.     

The performance of Al–Ti–C grain refiners in twin-roll casting of aluminium foilstock

The master alloys based on the Al–Ti–B system have been used extensively for refining the grain structure of aluminum alloys. The quality-related problems linked with the TiB₂ particles, however, have generated an interest in the Al–Ti–C grain refiners as an acceptable replacement for Al–Ti–B master alloys. TiC particles are smaller than the TiB₂ particles and are less prone to agglomeration. Al–3Ti–0.15C grain refiners have been in use for some time in several alloy systems. Much of the work reported on this alloy, however, has been from DC casting while performance data in strip casting is not available. In the present work, a commercial Al–3Ti–0.15C grain refiner was employed in the twin-roll casting of AA8111 foilstock. Its grain refining efficiency was compared with that of the Al–5Ti–0.2B master alloy, the standard grain refiner in aluminium industry for the manufacture of aluminium foil products.     

氧化对WC_p/钢基表层复合材料热裂纹萌生扩展的影响

利用负压实型铸渗工艺,通过涂覆预制块法,成功地制备了以高铬钢为基材,WC为增强颗粒的表层复合材料,通过氧化增重法、热震实验法及扫描电镜等分析测试方法重点研究了氧化对WC/钢基表层复合材料热裂纹萌生及扩展的影响。研究结果表明:WC颗粒在高于600℃时,会氧化成为结构疏松的WO3,并且随着温度的升高,氧化反应的速度加快,而WC的氧化,对热疲劳裂纹的萌生和扩展产生重要的影响。在500℃以上的空气环境中,复合材料基体会在裂纹源的尖端处形成氧化物。结合环境中的氧对裂纹扩展影响的分析可知,生成的氧化物为裂纹的扩展提供了途径,并且使复合材料极易在热应力的作用下导致开裂。     

Laser treatment and PVD TiN coating of Ti–6Al–4V alloy

Laser gas assisted processing can be used to modify the surface properties of Ti–6Al–4V alloy through the use of gaseous interaction with the laser melted surface. Laser surface melting of titanium and its alloys in nitrogen to form a layer of TiN embedded in a metallic matrix which is enriched in alloying elements has attracted considerable interest. The surface roughness of the laser-treated surface is poor, therefore, a secondary processing becomes essential. In the present study, duplex treatment of Ti–6Al–4V alloy was carried out. The alloy surface was melted initially under a controlled nitrogen atmosphere, which in turn resulted in a laser-induced nitrided surface. The resulting workpiece surface, then, was PVD TiN coated. In order to assess the wear properties of the resulting surface, friction tests were carried out. SEM, XRD and microhardness were carried out for microstructural analysis and material characterization. It was found that the adhesion of the TiN coating to the base alloy improved considerably in the case of laser-treated workpieces and smooth transition in plastic shearing resistance between the TiN coating and the base alloy enhanced the wear properties of the laser-treated surface.     

ZL101铝合金低周疲劳行为研究

研究了ZL101-T6铝合金的拉压低周疲劳行为,并重点分析了疲劳裂纹在材料中萌生与扩展的过程。通过扫描电镜和金相显微镜的观察分析表明:共晶硅相的断裂是材料低周疲劳裂纹萌生的主要方式,硅颗粒断裂的数量随疲劳循环周次的增加而增加,应变幅值的增加加快了硅颗粒的断裂速率,使疲劳裂纹的萌生速率加快;疲劳裂纹在循环初期主要通过断裂硅颗粒的互相连接进行扩展,随疲劳裂纹的长大,裂纹可穿过铝基体连接形成主裂纹并导致材料破坏,穿晶断裂为最终断裂的主要特征。     

Dual fatigue failure modes in Ti–6Al–2Sn–4Zr–6Mo and consequences on probabilistic life prediction

The variability in fatigue life of the Ti–6Al–2Sn–4Zr–6Mo (Ti-6-2-4-6) alloy was investigated. Cumulative life distribution plots were found to be composed of two failure mechanisms. The data could be closely represented by a cumulative distribution function (CDF) resulting from the superposition of the CDFs of the individual mechanisms. An approach for life prediction based on the data due to the worst-case mechanism is suggested.     

An in situ transmission electron microscope study of the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening

We investigate the thermal stability of near-surface microstructures induced by deep rolling and laser-shock peening in AISI 304 stainless steel (AISI 304) and Ti–6Al–4V using in situ transmission electron microscopy. The improvements in fatigue resistance at elevated temperature are related to the high-temperature stability of the work-hardened near-surface microstructure.     

Hydrogen evolution from cathodically charged titanium aluminide alloy Ti–24Al–11Nb

A Ti₃Al-based titanium aluminide alloy, Ti–24Al–11Nb, was cathodically charged with hydrogen in a 5% H₂SO₄ aqueous solution for various charging times, and the formation and dissociation of the hydride, the hydrogen evolution behavior and the total hydrogen uptake were investigated mainly by means of X-ray diffractometry and thermal desorption spectroscopy (TDS). The same kind of hydride phase as observed previously in Ti–25Al alloy (hexagonal hydride) was presumably formed in the Ti–24Al–11Nb alloy after cathodic charging. No damage, such as cracks, was induced by hydrogen charging. Two kinds of TDS peaks, one probably corresponding to hydride dissociation and the other to hydrogen dissolution in the normal lattice site, were found after longer hydrogen charging. It is suggested that niobium addition to Ti₃Al-based titanium aluminide alloy may reduce hydrogen susceptibility during cathodic charging.     

Thermal shock fatigue behavior of TiC/Al2O3 composite ceramics

The thermal shock fatigue behaviors of pure hot-pressed alumina and 30 wt. % TiC/Al2O3 composites were studied. The effect of TiC and Al2O3 starting particle size on the mechanical properties of the composites was discussed. Indentation-quench test was conducted to evaluate the effect of thermal fatigue temperature difference (ΔT) and number of thermal cycles (N) on fatigue crack growth (Δα). The mechanical properties and thermal fatigue resistance of TiC/Al2O3 composites are remarkably improved by the addition of TiC. The thermal shock fatigue of monolithic alumina and TiC/Al2O3 composites is due to a "true" cycling effect (thermal fatigue). Crack deflection and bridging are the predominant reasons for the improvement of thermal shock fatigue resistance of the composites.     

Phase identification and effect of W on the microstructure and micro-hardness of Ti2AlNb-based intermetallic alloys

The O-phase (orthorhombic Ti₂AlNb phase) intermetallic alloy has been considered one of the strongest materials for high temperature application. The primary compositions of the test alloys in this paper are Ti–22Al–27Nb (at.%) and Ti–22Al–20Nb–2W (at.%). By in-situ X-ray diffraction (XRD) tests, each phase is identified and the lattice parameters of each phase are calculated for different temperatures. XRD tests show that a Ti₂AlNb-β phase, partially ordered b.c.c. phase, exists besides O, ₂ and B2 that are usually expected in the Ti–Al–Nb alloy system. In this paper, the effects of tungsten on the microstructure and micro-hardness of a Ti₂AlNb-based (O+b.c.c.) alloy are investigated. The effects of W on Ti₂AlNb-based intermetallic alloys are widely known to cause a refinement of microstructure and improvement of mechanical strength, however, the reason has not yet been reported. In this paper, the role of W in the improvement of mechanical strength is clarified. The main effect of W addition is not the formation of a solid solution or the formation of W-precipitates but the refinement of the Widmanstätten structure.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

Fatigue properties of ferrite/bainite dual-phase X80 pipeline steel welded joints

Fatigue properties are important parameters for the safety design and security evaluation of pipelines. In this work, fatigue life and fatigue crack propagation of full-thickness X80 pipeline steel joints compared with the base metal (BM) was investigated. Full-thickness BM specimens showed superior fatigue life compared with that of welded joints. The fatigue crack initiation of full-thickness X80 welded joint specimens occurred at the outside weld toe and then grew inward until a fracture was formed. During fatigue crack growth in the heat-affected zone (HAZ), crack growth rate linearly increased with increased ΔK in each HAZ subregion. However, the change rate of fatigue crack growth rate (da/dN) differed among HAZ subregions. This difference was related to the variation in crack path and fracture mode because of the possible microstructural sensitivity of fatigue crack propagation behaviour.     

Fatigue Behavior and Damage Evolution of SiC Fiberreinforced Ti-6Al-4V Alloy Matrix Composites

研究SiC纤维增强钛基复合材料(SiCf/Ti-6Al-4V)室温疲劳行为和损伤演化机制。疲劳试验条件:载荷控制、应力比0.1和加载频率10 Hz。采用疲劳断裂试验建立最大加载应力为600~1200 MPa内SiCf/Ti-6Al-4V的S-N曲线。采用疲劳中止试验以及SEM显微分析研究应力水平对SiCf/Ti-6Al-4V疲劳损伤演化的影响。结果表明,SiCf/Ti-6Al-4V疲劳损伤萌生模式与演化过程与应力水平密切相关。在高应力水平(Smax=1000 MPa),纤维开裂是主要损伤萌生模式。一旦2或3根纤维断裂后,纤维裂纹和基体裂纹开始联接并形成宏观扩展裂纹。在中等应力水平(Smax=800 MPa),基体裂纹萌生与扩展是主要损伤模式。多条基体裂纹萌生于试样外表面棱边和离外表面附近试样内部开裂的纤维基体界面处。基体裂纹均沿垂直于加载方向扩展,且大部分纤维未断裂并纤维桥接基体裂纹。在低应力水平(Smax=600 MPa),仅在C涂层和界面反应层之间和C涂层内部观察到局部界面脱粘现象。     

Tensile flow properties of Al-based matrix composites reinforced with a random planar network of continuous metallic fibres

Squeeze casting was used for processing two new types of composites: pure Al matrix composites reinforced with fibres of Inconel 601, and AS13 (Al–12% Si) matrix composites reinforced with fibres of Inconel 601 or stainless steel 316L. The fibres are continuous with a diameter of 12 μm and their volume fraction in the composites varied from 20 to 80%. The processing conditions were such that no trace of interfacial reaction compound or of matrix precipitate resulting from the dissolution of elements of the fibres could be detected. The quality of the process was attested by Young's modulus and electrical conductivity measurements. Tensile tests were carried out from room temperature up to 300°C. The composites with the pure Al matrix present a remarkable tensile ductility. They thus constitute convenient materials for assessing continuum plasticity models for composites. Properties of composites with the AS13 matrix are much affected by interface adhesion strength.     

Production of Al–Ti–B master alloys from Ti sponge and KBF4

It is of great interest to replace the K₂TiF₆ salt so as to reduce the volume of fluoride-bearing particulate material to be added to the aluminium melt in the popular “halide salt” process. Ti sponge was used in the present work as the source of Ti in the production of an Al–5Ti–1B grain refiner. Addition of Ti granules into molten aluminium, either premixed with or before KBF₄ salt, has produced Al–5Ti–1B alloys where the boride particles were relatively few and predominantly of the AlB₂ type. The grain refining efficiency of these alloys were far from satisfactory. TiB₂ was the dominant boride phase with sufficient number of blocky aluminide particles when Ti, in excess of the TiB₂ stoichiometry was supplied before hand and the balance was reserved for co-addition with KBF₄. Al₃Ti particles were generated soon after the Ti solubility limit was exceeded in the first step while the boride particles were subsequently produced by the reaction between molten aluminium, KBF₄ and K₂TiF₆. The Al–5Ti–1B master alloy thus produced provided an adequate grain refining performance while the amount of particulate material to be added to the aluminium melt was reduced by nearly 30%.