Investigation of primary and secondary creep deformation mechanism of TiAl

Creep deformation behaviors in lamellar TiAl alloys have been investigated. As in the case with metals, the normal primary creep stage was observed. As creep strain increased within the primary regime, dislocation density decreased, and creep activation energy increased from 300 kJ/mol, the activation energy of the self-diffusion of Ti in TiAl, to about 380 kJ/mol, that of steady state creep deformation. During primary creep deformation of lamellar TiAl, as the initial dislocation density decreased, the α₂ -phase was found to transform to a γ-phase, generating new dislocations which contributed to the creep deformation. In other words, this phase transformation is the source of the dislocation generation for continuous creep deformation. Therefore, we suggest that phase transformation is the rate controlling processes having an activation energy of about 400 kJ/mol, which is higher than that of self-diffusion. A small amount of prestrain was found to be responsible for the reduction of initial dislocation density. In addition, this prestrained specimen showed significantly reduced primary creep strain, and the creep activation energy in the primary stage was measured to be about 380 kJ/mol. These results clearly confirm the suggested creep deformation mechanism of lamellar TiAl alloys.     

Thermal stability of lamellar structure of PST crystal and lamellar boundary design for creep resistant TiAl alloy

The stability of lamellar structure is crucial for the creep resistance of TiAl alloys, but degradation of the lamellar structure is unavoidable at high temperatures. The degradation of the lamellar structure in PST crystals of Ti-48mol.%Al was studied during high temperature exposure (annealing and creep testing) to examine how to make a stable lamellar structure with high creep deformation resistance. Since the six orientation variants of γ lamellae are nucleated independently of the adjoining lamellae, pseudo twin and 120° rotational fault boundaries are most frequently observed at the initial stage of lamellar formation. The preferential removal of high energy (pseudo twin and 120° rotational fault) boundaries during the evolution of lamellar structure results in the highly probable appearance of a true twin boundary at a later stage of lamellar evolution. The coarsening of lamellar spacing and the spheroidization of the lamellae are the major degradation events occurring during creep deformation, and the migration of the lamellar boundaries brings both of them about. The lamellar structures of TiAl alloy contain four types of lamellar boundaries. The stability of the four types of boundaries decreases in the following order: γ/α₂ > true twin > pseudo twin > or=120° rotational fault boundaries. The γ/α₂ boundary has the highest stability (lowest mobility), and the high density of γ/α₂ boundaries is proposed to make a stable lamellar structure with good creep resistance. A material having the high density of γ/α₂ boundaries was produced through the heat treatment of a PST crystal in the α+γ two-phase regime. The excellent creep properties of the material were proven through creep tests of hard oriented PST crystals made of the material. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Microstructure and creep behavior of directionally solidified TiAl-base alloys

Tensile creep tests were conducted on directionally solidified TiAl alloys to discern the effect of alloying and lamellar orientation. A seeding technique was used to align the TiAl/Ti₃Al lamellar structure parallel to the growth direction for alloys of Ti–47Al, Ti–46Al–0.5Si–0.5X (X=Re, W, Mo, and Cr), and Ti–46Al–1.5Mo–0.2C (at.%). Tensile creep tests were performed at 750 °C using applied stresses of 210 and 240 MPa. Aligning the lamellar microstructure greatly enhances the creep resistance which can further be improved by additional alloying.     

Characteristics of dislocation structure in creep deformed lamellar tial alloy within primary regime

In this investigation, dislocations of a lamellar TiAl alloy are analyzed after creeping in the primary range at 800°C/200MPa in order to interpret their mobility It was found that the dislocation density in γ-laths decreased as the creep deformation proceeds within primary creep regime Schmid factor analysis suggests that the creep deformation in the early stage of the primary creep regime is controlled by the gliding of some of the initial dislocations which have a high enough Schmid factor As the creep deformation progressed, those dislocations with high Schmid factors slip preferentially to be annihilated at the α-γ interface For further continuous deformation, dislocation generation is required, and for this, α-phase is transformed to γ-phase in order to generate new dislocations A slow dislocation generation process by phase transformation of α-phase compared with the absorbing rate to sinks is responsible for the decreasing dislocation density as the creep strain increases     

Orientation control of a lamellar microstructure in a Ti−Al intermetallic compound by high-temperature compression in an alpha single-phase and/or in a two-phase region and the alpha re-heat treatment

In order to control the spatial Ti−Al distribution of the α₂+γ γ lamellar microstructure in Ti−Al alloys, processes consisting of a high-temperature deformation in an alpha single-phase region, and an annealing and deformation in the α₂+γ ψ region were examined. The compression deformation temperatures and strain rates were from 1573 to 1623 K and from 1.0×10⁻⁴ to 5.0×10⁻³ s⁻¹, respectively. Dynamic recrystallization occurred during the compressive deformation in the alpha single-phase region. The uniaxial compression resulted in the formation of a fiber texture. It was found that more than half of the lamellar microstructure can be arranged within 30 degrees from the specimen surface by these processes. It was also shown that the orientation control of the lamellar microstructure in a polycrystalline Ti−Al intermetallic compound is effective in improving the creep resistance of this material.     

Formation of TiC/Ti2AlC and α2+γ in in-situ TiAl composites with different solidification paths

In order to improve the mechanical properties of TiAl alloys, TiAl composites with different solidification paths were synthesized by metallurgical method. Results show the TiC disappears and Ti₂AlC increases when the Al content is more than 42% (at.%, similarly hereinafter). Small TiC particles are located in Ti₂AlC grains with irregular shapes when the Al content is 40%, and they translate into clubbed Ti₂AlC with increasing of Al. This metallurgy method can solve the defects of the Al lacking and the residual TiC. The γ phase increases between lamellar colonies with the increasing of Al. When the Al content is 48%, the fully lamellar structure transforms into a duplex microstructure and there are small Ti₂AlC phases in γ phases, because the forming of Ti₂AlC phase must consume Al. The compressive strength increases up to 1678.68 MPa as Al content is 46 at.%, and then decrease to 1460.22 MPa, the compressive strain increases and then keeps stabilization with the increasing Al. The maximum strength improves 38.82% and the maximum strain improves 121.37%. The Ti₂AlC/TiAl composites fracture behaviors are load transferring behavior, crack deflection, trans-lamellar cracking and extraction of carbide reinforcements. The Ti₂AlC phase and the fully lamellar structure improve the mechanical properties.     

The 1400°C isothermal section of the Ti-Al-Nb ternary system

The 1400‡C isothermal section of Ti-Al-Nb system was determined using the diffusion couple technique, along with optical microscopy and electron probe microanalysis (EPMA). Equilibrated alloys were employed to identify the phase regions using XRD. The isothermal section consists of seven single-phase regions including Β(ΒTi,Nb), α(αTi), γ(TiAl), η((Ti,Nb)Al₃), σ(Nb₂Al), δ(Nb₃Al), and γ₁. The detailed studies on the γ₃₁ phase were made by XRD, DTA, and DSC.     

Mechanical properties of the cast intermetallic alloy Ti-43Al-7(Nb,Mo)-0.2B (at %) after heat treatment

A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate $ \dot \varepsilon A new approach to obtaining fine-grained structure in intermetallic-compound alloys such as γ-TiAl + α₂-Ti₃Al has been suggested. This approach is based on the use of alloys that solidify as the β phase, which contain β-stabilizing additives such as Nb and Mo and are characterized by the small size of crystallites already in the cast state; in these alloys, a simple heat treatment makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase. It is shown on the example of the Ti-43Al-7(Nb,Mo)-0.2B (at %) alloy that this heat treatment ensures superplastic properties in the material in the temperature range of T = 1050–1130°C at a deformation rate = 1.7 × 10⁻⁴ K⁻¹. Under these temperature-strain-rate conditions, relative elongations such as δ = 160–230% and low flow stresses such as σ = 36–100 MPa characteristic of superplastic flow have been obtained. It has been shown for the first time for the intermetallic γ-TiAl + ga₂-Ti₃Al alloy that a sheet semifinished product cut out from an ingot subjected only to heat treatment can have plasticity acceptable for press forming. Original Russian Text ? V.M. Imayev, R.M. Imayev, T.G. Khismatullin, 2008, published in Fizika Metallov i Metallovedenie, 2008, Vol. 105, No. 5, pp. 516–522. The author is also known by the name Imayev. The name used here is a transliteration under the BSI/ANSI scheme adopted by this journal.—Ed.     

Microstructure and technological plasticity of cast intermetallic alloys on the basis of γ-TiAl

This work is devoted to the estimation of the technological plasticity of binary and alloyed γ titanium aluminides by conducting compression tests at T = 1000°C. The technological plasticity was shown to grow with decreasing size of grains and grain colonies and with increasing amount of β-stabilizing elements in the alloys. The best technological properties are characteristic of the alloys that solidify completely through the β phase, containing β-stabilizing additions of niobium and molybdenum and microadditions of boron. These alloys are characterized by a small size of crystallites in the cast state; the use of special heat treatments makes it possible to substantially decrease the fraction of the lamellar component and to increase the content of the β(B2) phase in them. For the most technological alloy, tensile tests in the cast state have been carried out. In the temperature range of T = 900–1100°C, superplastic elongations have been achieved.     

A new experimental technique to investigate the α2/γ interface of lamellar TiAl alloy three dimensionally

The abundant two-dimensional defects, lamellar interface, have an important role in the deformation behavior of lamellar TiAl alloys. Transmission electron microscopy (TEM) has been used to investigating the lamellar interface. However, a very limited area only could be observed by using TEM. The observation of interface morphology and dislocations inner α₂ phase of lamellar TiAl is very difficult by using TEM. The purpose of this study is to propose a new experimental method to observe the lamellar interface perpendicularly and investigate the crystallographic morphology of the α₂ lamellar interface. The γ phase of the lamellar TiAl alloy was selectively dissolved and the α₂ phase was separated from the TiAl lamellar alloy without significant damage. These separated α₂ plates were observed with atomic force microscopy (AFM), scanning electron microscopy and TEM for the investigation of dislocations and interface of the α₂ phase in the lamellar TiAl alloy.     

High-temperature phase equilibria in Ti-Al-Mo system

The phase equilibria in a Ti-Al-Mo system have been investigated at 1473 and 1573 K experimentally and theoretically. Phase equilibria were experimentally investigated by in situ x-ray diffraction and thermal analysis as well as by quenching and diffusion couples. From the results, isothermal sections of this system at these two temperatures were constructed and assessed using Thermo-Calc. A part of the ternary phase diagram of Ti-Al-Mo system has revealed the following features: (1) low solution limits of Mo (a few at.% Mo) in both α (disordered hcp structure) and γ (TiAl: the L1₀ structure) phases, (2) the narrow three-phase region of the α+β (bcc Ti terminal solution) +γ phases, and (3) the role of Mo as a strong “β stabilizer.”     

High-temperature phase equilibria in Ti-Al-Mo system

The phase equilibria in a Ti-Al-Mo system have been investigated at 1473 and 1573 K experimentally and theoretically. Phase equilibria were experimentally investigated by in situ x-ray diffraction and thermal analysis as well as by quenching and diffusion couples. From the results, isothermal sections of this system at these two temperatures were constructed and assessed using Thermo-Calc. A part of the ternary phase diagram of Ti-Al-Mo system has revealed the following features: (1) low solution limits of Mo (a few at.% Mo) in both α (disordered hcp structure) and γ (TiAl: the L1₀ structure) phases, (2) the narrow three-phase region of the α+β (bcc Ti terminal solution) +γ phases, and (3) the role of Mo as a strong “β stabilizer.”     

Microstructure control and high temperature properties of TiAl base alloys

An equiaxed fine grain structure, a γ grain structure with the precipitated ₂ laths, and a fully lamellar structure were obtained by the microstructure control using thermomechanical processing and heat treatment. The key to obtaining the equiaxed fine grain structure using isothermal forging is to decompose the lamellar structure and then produce the fine grain microstructure through dynamic recrystallization. TiAl base alloys consisting of fine equiaxed grains, in particular, Ti-39Al-9V consisting of the γ and B2 phases exhibited superplastic elongation of more than 600% at 1423 K. Creep rupture properties of TiAl binary alloys with various microstructures were studied in purified He in the temperature range from 1073 to 1373 K. Above 1173 K the precipitated ₂ phase improved the steady state creep rate and creep life. At 1023 K, the ₂ phase did not improve the creep rate, although the steady state creep rate decreased and the creep life increased as the γ grain size increased.     

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

Thermodynamic and kinetic modeling of the Cr-Ti-V system

A synergistic approach of thermodynamic and kinetic modeling is applied to the Cr-Ti-V system. To assist the design of (α+β) and β titanium alloys for structural applications and vanadium alloys for fusion reactor applications, a set of self-consistent and optimized thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Ti, Cr-V, Ti-V, and Cr-Ti-V systems. The Laves phases, α-Cr₂Ti, β-Cr₂Ti, and γ-Cr₂Ti, are described by a two-sublattice model assuming antistructure atoms on both sublattices. The calculated thermodynamic quantities and phase diagrams are in good accord with the corresponding experimental data. To assist the simulation of the kinetics of diffusional transformations in bodycentered cubic (bcc) alloys, the atomic mobilities of Cr, Ti, and V are modeled. A set of optimized mobility parameters is given. Very good agreement between the calculated and experimental diffusivities was found.     

Thermodynamic and kinetic modeling of the Cr-Ti-V system

A synergistic approach of thermodynamic and kinetic modeling is applied to the Cr-Ti-V system. To assist the design of (α+β) and β titanium alloys for structural applications and vanadium alloys for fusion reactor applications, a set of self-consistent and optimized thermodynamic model parameters is presented to describe the phase equilibria of the Cr-Ti, Cr-V, Ti-V, and Cr-Ti-V systems. The Laves phases, α-Cr₂Ti, β-Cr₂Ti, and γ-Cr₂Ti, are described by a two-sublattice model assuming antistructure atoms on both sublattices. The calculated thermodynamic quantities and phase diagrams are in good accord with the corresponding experimental data. To assist the simulation of the kinetics of diffusional transformations in bodycentered cubic (bcc) alloys, the atomic mobilities of Cr, Ti, and V are modeled. A set of optimized mobility parameters is given. Very good agreement between the calculated and experimental diffusivities was found.     

Effects of Nb and Al on the microstructures and mechanical properties of high Nb containing TiAl base alloys

The influence of Nb and Al contents on the microstructure and yield strength of high Nb containing TiAl base alloys was investigated. The experimental results show that the yield strength at 900 °C of the alloys with the same type of microstructure, such as fully lamellar (FL), nearly lamellar (NL) and degraded fully lamellar (DFL), increases with increasing Nb content and decreasing Al content in the composition range of 0–10 at.% Nb and 44–49 at.% Al. DFL is the degraded form of FL microstructure after exposure at 1050 °C for 30 h. It is shown that the Nb addition in the alloys increases the value of the σ₀ term in the Hall–Petch relation of yield stress vs. lamellar spacing. This result has been related to TEM observations of dislocation structure in deformed specimens. The observations indicated that high level of Nb solute in the γ-TiAl matrix leads to a high critical resolved shear stress (CRSS) of dislocation loops. High Nb addition also reduces the degradation rate of FL microstructure after exposure at 1050 °C for 30 h. Both effects of high Nb addition are related to the change of the directionality of Ti–Ti (Nb) and Nb–Al bonds in the lattice. The decrease in Al content results in an increase in the volume fraction of α₂ phase, which leads to a decrease in the lamellar spacing of the lamellar structure. The high temperature strength of the alloys is determined by the lamellar spacing λ through the Hall–Petch equation k_λλ^−1/2.     

Acid corrosion behaviour of Ti–48Al–6Nb porous alloys with different phase skeletons

Ti–48Al–6Nb porous alloys with different phase skeletons were synthesised by powder metallurgy at different temperatures. TiAl₃ skeletons achieved at 700°C were covered by corrosion products and skeletons fractures led to pores collapse. TiAl skeletons were synthesised at 900°C with obvious corrosion pits and pore diameter increasing. And Ti₃Al/TiAl skeletons achieved at 1350°C exhibit the best acid corrosion resistance with the least mass loss of 1.853?mg?m^–2. Skeletons and pore diameters remained stable just with fully lamellar structures emerged. In addition, Nb₂Al phases presented better resistance than NbAl₃ in the skeletons contributing to acid corrosion resistance of the porous alloys.     

Analysis of chill-cast NiAl intermetallic compound with copper additions

This study carried out a characterization of chill-cast NiAl alloys with copper additions, which were added to NiAl, such that the resulting alloy composition occurred in the β-field of the ternary NiAlCu phase diagram. The alloys were vacuum induction melted and casted in copper chill molds to produce ingots 0.002 m thick, 0.020 m wide, and 0.050 m long. X-ray diffractometry (XRD) and transmission electron microscopy (TEM) performed in chill-cast ingots identified mainly the presence of the β-(Ni,Cu)Al phase. As-cast ingots showed essentially no ductility at room temperature except for the Ni₅₀Al₄₀Cu₁₀ alloy, which showed 1.79% of elongation at room temperature. Ingots with this alloy composition were then heat treated under a high-purity argon atmosphere at 550 °C (24 h) and cooled either in the furnace or in air, until room temperature was reached. β-(Ni,Cu)Al and γ′(Ni,Cu)₃Al were present in specimens cooled in the furnace and β-(Ni,Cu)Al, γ′(Ni,Cu)₃Al plus martensite-(Ni,Cu)Al were present in specimens cooled in air. Thermogravimetric analysis indicated that martensite transformation was the result of a solid-state reaction with M _s ∼ 470 and M _f ∼ 430 °C. Tensile tests performed on bulk heat-treated ingots showed room-temperature ductility between 3 and 6%, depending on the cooling media.