Mechanical behavior and related microstructural aspects of a nano-lamellar TiAl alloy at elevated temperatures (original) (raw)
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The effect of lamellar spacing on the creep behavior of a fully lamellar TiAl alloy
Intermetallics, 2000
The compression creep behavior of a cast alloy with the composition of Ti±46Al±2Cr±1.5Nb±1V was studied at 800 C under the stress level of 205 MPa. Dierent heat treatments were devised to produce fully lamellar microstructures with dierent lamellar spacings but remaining similar grain sizes, and the eects of the lamellar spacing on the primary creep strain and minimum creep rate of the alloys were investigated. The results indicated that the primary creep strain and minimum creep rate increase with the increase of the lamellar spacing. TEM observation reveals that the multiple generation of dislocation loops from lamellar interfaces contributes to the creep strain. Lamellar interfaces can also act as the barriers to the dislocation motion and multiple generation.
Acta Materialia, 2020
Abstract Intermetallic γ-TiAl based alloys have found applications in the low-pressure turbine of aircraft engines as well as in the turbocharger unit of automotive engines. However, these light-weight alloys must still be improved, through micro-alloying and tailoring the microstructure, to increase their creep resistance and consequently their maximum working temperature. In this work, a fully nano-lamellar advanced γ-TiAl based alloy doped with small amounts of C and Si is investigated in order to gain a deeper understanding of the atomic mobility mechanisms taking place at high temperature, thus controlling the creep properties. The study was approached through internal friction measurements up to 1223 K. We demonstrate that C has a notable influence on Ti diffusion in α2 phase, leading to an increase of the activation energy for Ti diffusion, which is assessed at ΔETi(α2)=0.32 eV per at% C. An atomic model for the relaxation process is proposed capable to explain this phenomenon. An additional internal friction peak, which, up to now, remained hidden by the high temperature background, was observed in this nano-lamellar TiAl alloy and analyzed through a careful de-convolution of the internal friction spectra. This new relaxation process, with activation energy of 3.70 eV, is attributed to the short distance diffusion of Al atoms in the γ-TiAl lattice. A novel concept of stress-induced cell-lattice reorientation is proposed to explain this relaxation. Finally, a new experimental method to analyze the high temperature internal friction background, which is closely related to the creep behavior, was developed to study the fully nano-lamellar microstructure, whose high temperature background exhibits the highest activation energy ever measured in a γ-TiAl based alloy.
Microstructural Characteristics and Creep Behavior of 45XD TiAl Alloys
MATERIALS TRANSACTIONS, 2004
A near lamellar microstructure and two fine-grained fully lamellar (FGFL) microstructures in Ti-45Al-2Nb-2Mn+0.8 vol%TiB 2 alloy were prepared by selected heat treatments, and the fully lamellar microstructures were aged for stabilizing the lamellar plates. Microstructural examination and tensile creep tests at 760 C showed that the near lamellar microstructure possessed inferior creep resistance due to its coarse lamellar spacing and its larger amount of equiaxed grains at colony boundaries. The fine lamellar spacing as well as the fine lamellar colony size gave a major contribution to make the minimum creep rates smaller in the fully lamellar TiAl alloys. Since aging treatments stabilized the lamellar microstructures and delayed the degradation process during creep deformation, the aged samples exhibited lower minimum creep rate and longer creep life than the corresponding samples without the aging treatment. These results suggest that a fine as well as stabilized fully lamellar structure is a critical factor to improve the creep resistance of TiAl alloys in terms of short and long-term creep.
Diffusion creep of intermetallic TiAl alloys
1999
The creep behaviour of g-TiAl with L1 0 structure without second phases, g-TiAl with precipitated particles of a 2 -Ti 3 Al with D0 19 structure, and g-TiAl with the H-phase Ti 2 AlC has been studied at low stresses in the temperature range 900±1200 C. The obtained data allow the construction of creep deformation mechanism maps for the studied alloys which may be used for an extrapolation of the observed creep behaviour. At higher stresses dislocation creep occurs in all alloys, which is well described by the Dorn equation with stress exponents in the range 3±5. Extended Coble creep with threshold stress was observed only for the studied two-phase alloys. A strong temperature dependence of the threshold stress for Coble creep was found for the TiAl alloy with carbide particles. #
Effects of lamellar boundary structural change on lamellar size hardening in TiAl alloy
Acta Materialia, 2004
Strengthening by refinement of lamellar thickness was studied at room temperature on dual phase Ti-39.4mol%Al alloy over a wide range of average lamellar thickness k from 850 to 20 nm. The relation between yield stress r y and k was examined, paying special attention to the change in lamellar boundary structure. The c/a 2 lamellar boundaries in the alloy are found to be perfectly coherent in thin lamellar structure formed at low aging temperatures. In thick lamellar structure formed at high aging temperatures, misfit dislocations were introduced to relieve the lattice misfit and are found on the lamellar boundaries. Both thin lamellae with coherent boundaries and thick lamellae with dislocated ones are present in a lamellar structure formed at an intermediate aging temperature. The critical thickness of c lamella for the introduction of misfit dislocations is about 50 nm. The dislocated boundaries render a high resistance to dislocation motion across the boundaries. A Hall-Petch relation holds in the range of k > 170 nm, and the Hall-Petch slope takes a large value corresponding to the high boundary resistance. The coherent boundaries provide a relatively low resistance. Another r y-k correlation typical of the coherent boundary appears in the range of k < 100 nm. The yield stress saturates to an upper limit of 1 GPa at k = 70 nm. The transition from the property of dislocated boundary to that of coherent boundary proceeds with an increase in the density of the coherent boundaries within the range of k = 170-100 nm.
Intermetallics, 2005
The structural transformations activated during the formation of the lamellar microstructure have been studied in a Ti 49 Al 47 Cr 2 Nb 2 alloy. By performing statistical analysis of the microstructure, a special attention has been paid to the orientations of the lamellae and to the interface relationships. Experiments were conducted on specimens in an as-HIPed condition as well in an isothermal forged plus annealed condition. In the annealed alloy, three mechanisms of the lamellar formation were evidenced depending upon the nucleation temperature: a heterogeneous transformation occurring at grain boundaries, a homogeneous nucleation in the a 2 matrix and an interfacial nucleation of twinrelated lamellae. The microstructural analysis of the as-HIPed alloy was aimed at determining the activated mechanisms among those evidenced in the annealed alloy. q
Practical Metallography, 2015
In this contribution, the microstructure of two intermetallic γ-TiAl-based alloys with different Al contents were examined after high-temperature deformation. To investigate the dynamic recrystallization of these alloys, isothermal compression tests were performed using a Gleeble® 3500 simulator. For the experiments, a temperature range of 1 150 °C to 1 300 °C and strain rates of 0.005 s−1, 0.05 s−1 and 0.5 s−1 were applied, up to a true strain of 0.9. The deformed microstructural states, particularly the multiphase alloys' dynamically recrystallized grain sizes were characterized via Scanning Electronic Microscopy (SEM) and Electron Back Scatter Diffraction (EBSD). The recrystallized grain sizes obtained from the experiments could be linked with the calculated Zener-Hollomon parameter through a power law.
International Journal of Plasticity
Detailed microstructure characterisation and in-situ micropillar compression were coupled with crystal plasticity-based finite element modelling (CP-FEM) to study the micromechanisms of plastic anisotropy in lamellar TiAl alloys. The consideration of microstructure in both simulation and in-situ experiments enables in-depth understanding of micromechanisms responsible for the highly anisotropic deformation response of TiAl on the intralamella and inter-lamella scales. This study focuses on two specific configurations of / 2 lamellar microstructure with the / 2 interfaces being aligned 25 and 55 to the loading direction. Microstructure-based CP-FEM shows that longituginal slip of super and ordinary dislocations are most responsible for the plastic anisotropy in the 25 micropillar while the anisotropy of the 55 micropillar is due to longitudinal superdislocations and longitudinal twins. In addition, transversal superdislocations were more active, making the deformation in the 25 micropillar less localised than that in the 55 micropillar. Moreover, the CP-FEM 2 / 42 model successfully predicted substantial build-up of internal stresses at / 2 interfaces, which is believed to be detrimental to the ductility in TiAl. However, as evidenced by the model, the detrimental internal stresses can be significantly relieved by the activation of transverse deformation twinning, suggesting that the ductility of TiAl can be improved by promoting transverse twins.
Materials Science and Engineering: A, 2005
The creep properties of an as-cast Ti 48 Al 48 Cr 2 Nb 2 alloy have been studied at 750 • C and 150 MPa. The activation parameters have been measured by stress jumps during the creep test. TEM analyses have been performed to determine the rate-controlling deformation mechanism. Creep is controlled by the mixed climb of ordinary dislocations, which combines climb and glide components. It is shown that that the stress dependence of the creep rate is correlated to this mechanism.