Diffusion bonding of two phase γ-TiAl alloys with duplex microstructure

Abstract

Solid state diffusion bonding of TiAl was carried out with different bonding parameters within the superplastic temperature range. The effect of post-bond heat treatment (PBHT) on the mechanical properties of the bonds was also studied. Defect free sound bonds were achieved within the temperature range 925–1150°C and the pressure range 20–40 MPa. Transverse microtensile specimens extracted from the bonds were tested to evaluate room temperature tensile properties of the bonds and to correlate them with bonding parameters. The bond strength increased with an increase in bonding temperature and pressure. All the specimens of the bond made at 925°C and 40 MPa, and most of the specimens of bonds made at 1000°C and 30 MPa and at 1100°C and 20 MPa, failed in the bond area; all the specimens of bonds made at 1100°C and 30 MPa and at 1150°C and 20 MPa failed in the base metal. Post-bond heat treatment at 1350°C for 1 h led to the transformation of the recrystallised γ grains at the bond inteface formed during bonding to a lamellar microstructure, resulting in an indiscernible bond line in all cases. This resulted in an improvement in the bond strength in most of cases. Moreover, the recrystallised γ grains were also formed away from the bond area in the bonds made at 1100°C and 30 MPa and at 1150°C and 20 MPa. After PBHT, these bonds exhibited slightly lower tensile strength values owing to the change in the base metal microstructure away from the bond area.     

Diffusion bonding of γ-TiAl alloy to Ti-6Al-4V alloy under hot pressure

1 Introduction Alloys based on the ?-TiAl intermetallic compounds have been of much interest in recent years as light-mass structural materials for elevated temperature aerospace applications[1- 5]. Successful joining and cost effective fabrication method…     

Fatigue threshold and crack propagation in γ-TiAl sheets

The fatigue crack propagation behaviour of two different microstructures — a coarse-grained designed fully lamellar (DFL), and a fine-grained near γ (FG) — of a Ti–46.5 at.% Al–4 at.% (Cr, Nb, Ta, B) alloy was studied. Both the threshold of stress intensity range and standard long crack growth behavior were determined. A special technique was applied to separate the different mechanisms — intrinsic and extrinsic effects — and their changes with crack length. The fatigue crack propagation rate of long cracks is much smaller in the DFL microstructure than in the FG microstructure at the same stress intensity range. The effective threshold of stress intensity range of both microstructures is about 1.7 MPa√m. The threshold of stress intensity range shows a strong R-curve behavior. In other words the propagation–non-propagation conditions of cracks are significantly influenced by the crack extension. The long crack thresholds of stress intensity range at the stress ratio 0.1 are relatively large; they are about 4.5 and 8 MPa√m in the DFL and the FG microstructure, respectively. The differences in the crack growth behavior between the two microstructures are mainly induced by extrinsic resistance mechanisms.     

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.     

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.     

Isothermal forging of γ-TiAl based alloys

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

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.     

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…     

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 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.     

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.     

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.     

Oxidation behavior of γ-TiAl coated with zirconia thermal barriers

The applicability of the thermal barrier coating concept, which is an established concept for Ni-base alloys, was investigated for the first time for γ-TiAl. As potential bond coats, the following surface treatments were applied: Al-diffusion coating, Al-diffusion coating combined with short term pre-oxidation in air, short term pre-oxidation in oxygen of a rough surface finish γ-TiAl sample without aluminide coating. For the purpose of lowering the temperature of the γ-TiAl substrate surface, and thus, to prolong the life time, a zirconia thermal barrier coating (TBC) was applied on top of the above-mentioned modified TiAl surfaces. Oxidation resistance of the modified γ-TiAl alloys/TBC systems was evaluated by isothermal oxidation tests at 900 °C for 100 h in air. Whatever the pre-treatment conditions, the TBC was adherent to the oxide scale after 100 h oxidation, which was not the case for untreated γ-TiAl.     

High speed ball nose end milling of γ-TiAl alloys

Following a brief review on the development and intended application of γ-TiAl for aeroengine components, the paper details statistically designed machinability experiments involving high speed ball end milling in order to benchmark 4th generation alloy Ti–45Al–8Nb–0.2C against Ti–45Al–2Mn–2Nb +0.8%vol. TiB_2, which has been the prime focus of machinability evaluation and associated publications. Testing utilised 8 mm diameter, solid WC, AlTiN coated ball nose end cutters in a factorial design and involved measurement of cutting temperatures and workpiece surface roughness in addition to tool life and cutting forces. The trials established alloy Ti–45Al–8Nb–0.2C as providing a ∼25% increase in tool life based on a maximum flank wear criteria of 300 μm. On average, alloy Ti–45Al–2Mn–2Nb +0.8%vol. TiB₂ had a surface roughness Ra of ∼0.1 μm higher than Ti–45Al–8Nb–0.2C with values <0.6 μm Ra possible. Maximum cutting temperatures for Ti–45A–8Nb–0.2C measured using infra red pyrometry were ∼250 °C at a cutting speed of 160 m/min for a new tool, rising to ∼430 °C at a cutting speed of 340 m/min with a tool that had 300 μm of flank wear. For both γ-TiAl alloys, fracture/pullout was observed on every milled surface that was assessed and varied from a few microns to a few millimeters depending on test operating levels, within general, higher cutting speeds and tool flank wear levels causing an increased level of damage.     

High-Temperature Oxidation of Double-Glow Plasma Tantalum Alloying on γ-TiAl

Oxidation of Metals - A Ta-modified layer was prepared on γ-TiAl by double-glow plasma surface metallurgy technique. Based on the results of high-temperature oxidation tests at 700, 800, and...     

Oxidation-Resistant Aluminide Coatings on γ-TiAl

The long-term application of TiAl alloys based on the -phase at temperatures above 750–800°C requires suitable surface coatings to provide the needed oxidation resistance. Without a coating, these alloys, containing large amounts of titanium, suffer from rapid oxidation attack at elevated temperatures. The pack-cementation coating process was used to aluminize the surface region of a Ti–50 at.% Al alloy to TiAl₃, the most promising, oxidation-resistant phase in the Ti–Al system. The isothermal oxidation behavior of the coated alloy was studied in the temperature range 800–1000°C in air for up to 300 hr. The aluminide coating greatly improves the oxidation resistance of -TiAl, forming a protective alumina scale. The rapid aluminum interdiffusion between the TiAl₃ coating and the -TiAl substrate determined the effective life of the coating. In addition, the oxidation behavior of the TiAl₂ phase formed by interdiffusion of the coating system was studied by oxidation of cross sections.     

An atomic study of the transitional region between γ/γ laths in γ-TiAl

In the present study, a transitional region between two thick γ laths is investigated with TEM, HAADF-STEM, EDX and HRTEM. The results show that the transitional region consists of three thinner γ laths, and all the thick and thin laths have a true twin relationship. The interface between γ/γ laths is basically a twin-related interface but involving a rigid body translation of the adjacent lamellae, and misfit compensating has been observed along the interface. Ti segregates into the transitional region, which is essential to formation of the three thin γ laths. Based on the characterization of the transitional region, the formation mechanism of the transitional region is revealed.     

Stability and cross-slip of [101] superdislocations in γ-TiAl

The stability of superpartial dislocations and their kink pairs in various slip planes is of great interest since the cross-slip of dislocations in γ-TiAl is profuse at high temperatures. Hence, the fundamental understanding of dislocation stability would promote further understanding of cross-slip behavior and anomalous hardening in this material. First the energy factor and the line tension factor have been calculated using Stroh's sextic formalism from RT to 1173 K. The {111} cross-slip of 〈101] type superdislocations occurs at relatively moderate temperatures while the {010) type cross-slip was observed at high temperatures. The elastic anisotropy effect in terms of interaction forces between superpartials was examined to understand the temperature dependent cross-slip behavior of the superpartial dislocations. The elastic stability of superpartials was checked on (111) and (010) planes. In addition, the interaction energy between a kink-pair formed on superpartials was calculated on various cross-slip planes.