A Comparison of Oxidation Behavior of Al Coatings Prepared by Magnetron Sputtering and Arc Ion Plating on γ-TiAl

利用XRD、EDS和SEM分析研究了磁控溅射和电弧离子镀2种工艺制备的Al涂层的微观结构、形貌和抗氧化性能。磁控溅射技术制备的均匀、致密的Al层拥有更为细小的晶粒组织。在氧化实验后,磁控溅射制备的Al涂层形成了一个由表层氧化层、次表层富Al层和互扩散层的保护性结构。相比之下,电弧离子镀制备的Al涂层表现出了更差的抗氧化性。这是由于在离子镀制备的Al涂层中发现的针孔可以为氧气的侵入提供通道,从而引起涂层的内氧化并最终导致涂层的剥离。结果表明,磁控溅射制备Al涂层具备更好的抗高温氧化性能。     

Manufacture and Properties of SiC(p)/AI Composites Made by Squeeze Casting

SiC(p)/Al composites(volume fraction of SiC V_f=30～40％)were manufactured successfully bysqueeze casting,and its properties were investigated.The reinforcements distributed homogeneously andbonded tightly with the matrix in the material.Compared with the matrix alloy Grade 6061,the elasticmodulus of SiC(p)/6061 composite is increased by 1.17 times,the tensile strength increased by 27％,whilethe coefficient of thermal expansion is decreased by 51％.     

Internal friction measurements of γ-TiAl

Single-phase γ-TiAl polycrystals were prepared by extrusion. The compound possesses an Al content of 54.1 at.% and interstitial impurities typical for technical-purity materials. Owing to dislocation pinning by theses impurities and precipitates, internal friction measurements show no relaxation between room temperature and 950 K. At 1100 K a relaxation maximum is observed with an activation enthalpy of H_eff=(2.97 ± 0.1) eV and a pre-exponential factor of τ₀=2.5×10^−16±1 s, which can be attributed to the reorientation of structural defects. Above 1200 K the damping shows a frequency-dependent strong increase with temperature, which is characteristic for diffusion-controlled climb of dislocations.     

Diffusion bonding of γ-TiAl sheets

Using advanced γ-TiAl based alloys and various ingot and powder metallurgical routes, several aerospace engine components are now being developed. Further application of this material in the aerospace industry can only be implemented provided that successful joining and cost effective fabrication methods for this material are developed. In this context, diffusion bonding and superplastic forming of this material is considered to be of particular interest. The possibility of producing sound bonds in γ-TiAl sheets by industrially available diffusion bonding equipment designed for conventional Ti-alloys was investigated. Defect-free bonds were achieved at 1000°C with pressure levels of 5, 10 and 20 MPa, the holding time varying between 5 and 8 h. The bond qualities were assessed by shear testing at room temperature. Reasonable shear strength levels were obtained by bonding at 1000°C. The bonds were also post bond heat treated at 1430°C for 30 min, which improved the bond quality in all cases.     

Microstructural Study of Oxidized γ-TiAl

The microstructural development of oxidized-TiAl is presented with a focus on oxidation inair. The investigations were carried out usingconventional, analytical, and, especially,energy-filtered transmission electron microscopy (EFTEM). Threeimportant points were studied in detail: (1) thenitrogen effect, (2) thesurface-finish effect, and (3) thesubsurface zone. Nitrogen leads to the formation of TiN andTi₂AlN at the metal-scale interfaceinterrupting alumina and thereby preventing thedevelopment of a continuous alumina layer. TheAl-depletion layer formed during the oxidation process develops from a single-phaselayer, consisting of a cubic phase, to a two-phaselayer, consisting of the cubic phase and₂-Ti₃Al. The cubic phase isnot known in the system Ti-Al-O-N. Oxidation in oxygen depends on the surfacepreparation of the sample with rapid oxidation kineticsfor fine polishing and slow kinetics for a 600-gritSiC-paper finish. The rougher surface finish leads to the development of a recrystallization zonenear the surface and supports the formation of acontinuous alumina layer in the early stages ofoxidation. As for the oxidation in air, the cubic phaseis formed first underneath the oxide scale,followed by ₂-Ti₃Alformation.     

Mechanical properties of γ-TiAl cast alloys

At present, much attention is being devoted to evaluation of the properties of -TiAl alloys in various structural states characteristic for the stage of their production (casting or powder metallurgy) and for subsequent treatment (deformation or heat treatment by various regimes). Specifically, in addition to preparing thin sheets, where the high toughness of the TiAl alloys is realized most efficiently, some works are devoted to casting articles and parts with a shape close to the final one. Therefore, it is of interest to investigate the properties of actual cast alloys in two conditions, namely, after crystallization with different cooling rates (in the initial state, with all the defects in the form of chemical inhomogeneities, micropores, different-sized grains, texture) and after complete degradation of the cast structure in the process of long-term operation at temperatures up to 1050°C. The paper concerns the properties and structures of cast and annealed -TiAl alloys alloyed with Hf, V, Mn, and B.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 11 – 14, April, 1996.     

Isothermal forging of γ-TiAl based alloys

1　INTRODUCTIONBecauseoftheirattractive properties ,γ TiAlbasedalloysareconsideredforhightemperatureap plicationsinaerospaceandautomotiveindustries .Thesepropertiesincludelowdensity ,highspecificyieldstrength ,highspecificstiffness ,goodoxidationresistance ,andgoodcreeppropertiesuptohightem peratures.Researchanddevelopmentonγ TiAlbasedalloyshaveprogressedsignificantlywithinthelastdecade .Thisresearchhasledtoabetterunder standingofthefundamentalcorrelationsamongalloycompositionandmicros…     

Wear Behavior of P/M High Nb Containing γ-TiAl Alloy in Different Environments

Metals and Materials International - The wear behavior of Ti–45Al–7Nb–0.3W (at%) alloy prepared by powder metallurgy (P/M) has been investigated using pin-on-disk wear tests at...     

Microstructures and Tensile Properties of Ultrafine-Grained Ni–(1–3.5) wt% SiC_(NP) Composites Prepared by a Powder Metallurgy Route

Silicon carbide nanoparticle-reinforced nickel-based composites(Ni–Si CNP),with a Si CNPcontent ranged from1 to 3.5 wt%,were prepared using mechanical alloying and spark plasma sintering.In addition,unreinforced pure nickel samples were also prepared for comparative purposes.To characterize the microstructural properties of both the unreinforced pure nickel and the Ni–Si CNPcomposites transmission electron microscopy(TEM) was used,while their mechanical behavior was investigated using the Vickers pyramid method for hardness measurements and a universal tensile testing machine for tensile tests.TEM results showed an array of dislocation lines decorated in the sintered pure nickel sample,whereas,for the Ni–Si CNPcomposites,the presence of nano-dispersed Si CNPand twinning crystals was observed.These homogeneously distributed Si CNPwere found located either within the matrix,between twins or on grain boundaries.For the Ni–Si CNPcomposites,coerced coarsening of the Si CNPassembly occurred with increasing Si CNPcontent.Furthermore,the grain sizes of the Ni–Si CNPcomposites were much finer than that of the unreinforced pure nickel,which was considered to be due to the composite ball milling process.In all cases,the Ni–Si CNPcomposites showed higher strengths and hardness values than the unreinforced pure nickel,likely due to a combination of dispersion strengthening(Orowan effects) and particle strengthening(Hall–Petch effects).For the Ni–Si CNPcomposites,the strength increased initially and then decreased as a function of Si CNPcontent,whereas their elongation percentages decreased linearly.Compared to all materials tested,the Ni–Si CNPcomposite containing 1.5% Si C was found more superior considering both their strength and plastic properties.     

Micromechanical mechanism of texture formation in γ-TiAl

Texture evolution in TiAl depends on the type of deformation such as uniaxial compression, tension or rolling as well as on the initial microstructure. Cast TiAl shows a lamellar microstructure with an alignment of the lamellae and exhibits a strong texture. Deformation parallel to the aligned lamellae results in a plane-strain deformation which leads to a brass type texture. From room temperature up to 700 °C, compression tests of TiAl specimens with equiaxed microstructures result in <110>-fiber textures where the <101> component is shifted to <302>. At higher temperatures the orientation <302> is caused by dynamic recrystallization. In rolled sheets of TiAl with equiaxed microstructure the main texture component is a modified cube texture {100}<010> where the tetragonal c-axis is aligned in transverse direction of the sheet. This texture component is caused by dynamic recrystallization and appears to be stable against further rolling.     

Oxidation protection of NiCoCrAlY coatings on γ-TiAl

1　INTRODUCTIONTiAl intermetallic alloy, based on theγ TiAl,is expected to be one of the candidates for hightemperature systems such as aircraft engine be cause of its high modulus of elasticity, low densityand good creep resistance at high temperature[1, 2].However, there are two major drawbacks withγ TiAl alloy as an engineering material: one is itspoor room temperature ductility and the other isthe poor oxidation resistance above 800 ℃[3]. Alarge am…     

Oxidation protection of NiCoCrAIY coatings on γ-TiAl

Abstract: The effect of NiCoCrAlY overlay coatings on the oxidation resistance of γ-TiAl was studied at 900℃ in static air. To hinder the interdiffusion of the elements, the Al/Al2 O3 layer was added between the coating and the alloy. The results show that the TiAl alloy exhibits poor oxidation resistance. NiCoCrAlY coating can not effectively protect the γ-TiAl substrate from high temperature oxidation because of the serious interdiffusion between the coating and the substrates. With Al/Al2O3 diffusion barrier, the NiCoCrAlY coating exhibits excellent oxidation protection on γ-TiAl alloy.     

Synthesis of γ-TiAl based alloy by mechanical alloying and reactive hot isostatic pressing

Elemental Ti-48Al-2Cr-2Nb powders were mechanically alloyed (MA) for 8 h and 45 h. The MA powders were then consolidated by reactive hot isostatic pressing (HIP). The microstructure of the HIPed materials consisted of equiaxed γ-TiAl and α₂-Ti₃Al phases. During the high-temperature annealing of the HIPed materials, the α₂-Ti₃-Al phase transformed into a lamellar structure consisting of alternating laths of α₂-Ti₃Al and γ-TiAl. It is suggested that a high content of interstitial elements together with the microalloying elements of niobium and/or chromium in MA powders raises α/(α + γ) transus to a higher temperature.     

Modification of the jogged-screw model for creep of γ-TiAl

During the high-temperature creep of the γ-phase (L1₀ structure) of a “near-gamma” Ti–48Al microstructure, observations using transmission electron microscopy indicate that a/2〈110] or “unit” dislocation activity is a dominant deformation mode. These unit dislocations tend to be elongated along the screw orientation, and exhibit a large number of localized pinning points. Tilting experiments demonstrate that these pinning points are associated with jogs on the screw dislocations, suggesting that the jogged-screw model for creep should be appropriate in this case. However, it is shown that in its conventional formulation, the jogged-screw model is not capable of reproducing the measured creep response (i.e. stress exponents or absolute creep rates). Microscopic observations also demonstrate that a spectrum of jog heights are present, with some as large as 40 nm, based on present observations. A modification of the jogged-screw model is proposed in which the average jog height is assumed to depend on stress. This modified model results in good agreement between predicted and measured creep rates while using reasonable model parameters. Additional implications of the model and required experiments to further validate the model are also discussed.     

Microstructure and Mechanical Properties of ε-Phase-Reinforced ZnAl4Y Matrix Composite Prepared by Mixed Solid-Liquid Casting

ε-Phase-reinforced ZnAl4Y matrix composite has been fabricated by mixed solid-liquid casting method. The results show that the size of primary η-Zn phase in the composite decreases remarkably with the increase of adding amount of Cu-10wt.%Al powders till it reaches 6.0 wt.% in ZnAl4Y alloy. Besides, a large amount of small ε-phase particles form in ZnAl4Y matrix when the adding amount of Cu-10wt.%Al powders is in the range of 4.0-6.0 wt.% in ZnAl4Y alloy. Coarse ε-phase particles forms when the adding amount of Cu-10wt.%Al powders exceeds 8.0 wt.% in ZnAl4Y alloy. Compared with ZnAl4Y alloy, the composite could obtain optimal mechanical properties when the added amount of Cu-10wt.%Al powders is 6.0 wt.%.     

Electrochemical Behavior of Fe—Based Coatings Containing Amorphous Phase Prepared by Electric Arc Spraying

采用倒相法在Cu箔上制得与Cu箔结合牢固的PAN与石墨多孔复合膜,以其作阴极、纯Sn作阳极进行脉冲电沉积, Sn通过多孔复合膜的微孔沉积在铜箔上,然后在氩气气氛中热处理,得到具有复合结构的Sn基合金电极(碳膜隔离的Cu-Sn合金),用作Li离子电池的负极. SEM和EDS能谱分析以及模拟电池的电化学性能测试结果表明:与通常的在裸Cu箔上直接电沉积Sn并热处理的Sn电极相比,这种具有复合结构的Sn电极热处理后具有更好的循环性能和更高的循环容量,首次放电容量达到538.3 mA·h/g,50次循环后充电(Li离子的脱出)循环容量保持率仍有85.5%.     

On grain refinement of a γ-TiAl alloy using cryo-milling followed by spark plasma sintering

A fully dense fine-grained γ-TiAl based alloy was fabricated by cryo-milling pre-alloyed powders followed by spark plasma sintering. The consolidation was performed at 1050 or 1200 °C. The effect of cryo-milling on the densification kinetics and final microstructure was studied. Results indicate that cryo-milled (8 h) powder is fully densified at a temperature nearly 125 °C lower than that of un-milled powder. The microstructure of the alloy fabricated at different temperatures consisted of fine grains of γ-TiAl and α₂ (Ti₃Al) phases in different volume fractions. The average grain size of cryo-milled powders compacted at 1050 and 1200 °C were determined as 0.6 and 0.9 μm, respectively. It is inferred that the reduction in crystallite and particle size induced by cryo-milling is the likely reason for such enhancement in densification and grain refinement.     

The formation of ordered domains during crystal growth of γ-TiAl

The growth of single crystals of γ-TiAl by a floating zone method is described. It is not possible to grow crystals of the stoichiometric composition from the melt so all single crystals are aluminium rich. The composition range that gives good quality crystals is very limited. Although it is relatively easy to grow γ crystals if the composition is about 56^a/₀ Al, on cooling the crystals appear to undergo additional short-range ordering of the excess Al atoms which makes them less suitable for deformation studies. At the other extreme, alloys containing <50^a/₀ Al, including the public domain alloy 48^a/₀ Al 2 Mn 2^a/₀ Nb, form PST crystals. Binary alloys with 53.2^a/₀ Al appeared to give good crystals according to X-ray Laue photographs but always contained a domain structure, i.e. [110] crystals contained regions with a [011] growth direction. With >54^a/₀ Al it is possible to suppress domain formation by careful control of the growth conditions. A mechanism for domain formation is suggested and the results have implications for the phase diagram.     

Electroplastic Tensile Behavior of 5A90 Al–Li Alloys

The electroplastic(EP) tensile properties of 5A90 Al–Li alloys compared with thermal tension were investigated.The microstructural variation at different conditions was observed by SEM and TEM.The current density significantly influences the elongation and the flow stress.With increasing current density,wider and deeper dimples on the fracture surfaces and less dislocation density and pile-ups in the EP tension samples were observed compared with roomtemperature and thermal tension,which indicates the plasticity improvement and flow stress reduction.The EP effect(EPE) mainly results from a comprehensive function of Joule heating and pure EPE.Among them,Joule heating effect is perhaps a dominant factor.