On the formation and stability of precipitate phases in a near lamellar γ-TiAl based alloy during creep (original) (raw)
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High temperature creep behaviour of Al-rich Ti-Al alloys
Journal of Physics: Conference Series, 2010
Compared to Ti-rich-TiAl-based alloys Al-rich Ti-Al alloys offer an additional reduction of in density and a better oxidation resistance which are both due to the increased Al content. Polycrystalline material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical, scanning and transmission electron microscopy and XRD analyses. The high temperature creep of two binary alloys, namely Al 60 Ti 40 and Al 62 Ti 38 was comparatively assessed with compression tests at constant true stress in a temperature range between 1173 and 1323 K in air. The alloys were tested in the cast condition (containing various amounts of the metastable phases Al 5 Ti 3 and h-Al 2 Ti) and after annealing at 1223 K for 200 h which produced (thermodynamically stable) lamellar-TiAl + r-Al 2 Ti microstructures. In general, already the as-cast alloys exhibit a reasonable creep resistance at 1173 K. Compared with Al 60 Ti 40 , both, the as-cast and the annealed Al 62 Ti 38 alloy exhibit better creep resistance up to 1323 K which can be rationalized by the reduced lamella spacing. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded apparent activation energies for creep of Q = 430 kJ/mol for the annealed Al 60 Ti 40 alloy and of Q = 383 kJ/mol for the annealed Al 62 Ti 38 material. The latter coincides well with that of Al diffusion in-TiAl, whereas the former can be rationalized by the instability of the microstructure containing metastable phases.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1997
Finite element simulations of the high-temperature behavior of single-phase γ, dual-phase α2+γ, and fully lamellar (FL) α2+γTiAl intermetallic alloy microstructures have been performed. Nonlinear viscous primary creep deformation is modeled in each phase based on published creep data. Models were also developed that incorporate grain boundary and lath boundary sliding in addition to the dislocation creep flow within each phase. Overall strain rates are compared to gain an understanding of the relative influence each of these localized deformation mechanisms has on the creep strength of the microstructures considered. Facet stress enhancement factors were also determined for the transverse grain facets in each model to examine the relative susceptibility to creep damage. The results indicate that a mechanism for unrestricted sliding of γ lath boundaries theorized by Hazzledine and co-workers leads to unrealistically high strain rates. However, the results also suggest that the greater creep strength observed experimentally for the lamellar microstructure is primarily due to inhibited former grain boundary sliding (GBS) in this microstructure compared to relatively unimpeded GBS in the equiaxed microstructures. The serrated nature of the former grain boundaries generally observed for lamellar TiAl alloys is consistent with this finding.
Intermediate temperature creep properties of gamma TiAl
Acta Materialia, 1997
Compressive creep tests of single phase y Ti47AlSIMn2 have been conducted at stresses from 280 to 400 MPa in a temperature range of 5O(r6OO"C. The creep curves exhibited a primary region in which the creep rate decreased rapidly, a secondary region corresponding to a minimum creep rate, and an extended tertiary region in which the creep rate increased steadily with time. Unlike yielding, the creep behavior of this alloy was found to be normal; creep strength decreased with increasing temperature. Temperature change experiments and TEM observations have clearly elucidated the fact that a microstructural steady-state is not attained in the creep of this alloy. Instead, the overall creep performance is dominated by the tertiary creep that results from internal changes in the deformation microstructure. Although several dislocation and twinning mechanisms have been associated with the creep of TiAl, the multiplication and increased mobility of ordinary dislocations was found to play a dominant role in determining the creep behavior of this alloy. 0 1997 Acta Metallurgica Inc.
Materials Science and Engineering: A, 2020
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Creep behavior of single-phase γ-TiAl
Materials Letters, 1994
Creep data of single-phase y-TiAl alloys from different sources can be explained by a single deformation mechanism that incorporates a threshold stress. It is suggested that the creep behavior of single-phase y-TiAl alloys is controlled by a dislocation climb process (n e4.5) with an activation energy for creep of about 3 13 W/mot.
Interface Structure and Energy Calculations for Carbide Precipitates in γ-TiAl
MRS Proceedings, 2002
ABSTRACTTernary carbide precipitates improve the high-temperature creep strength of 2-phase TiAl alloys. The perovskite (P-type) Ti3 AlC nucleates at relatively low temperatures (750 deg. C), whereas hexagonal (H-type) Ti2AlC precipitates occur at somewhat higher temperatures. Calculations are performed, based on first- principles-local-density-functional theory, of the interface structure and energy of these two carbides with a 7-TiAl matrix. Calculations are first done on coherent interfaces, and approximate corrections are then made for the effect of misfit. The perovskite is known to form needle-shaped precipitates oriented along the c-axis of the host. Our calculations yield a relatively low energy for the (100) perovskite-host interface, which is a favorable orientation owing to its low misfit, and because the terminating carbide layer for the coherent interface is pseudomorphic with the host. Predictions are given for the critical thickness for coherence and the critical nucl...
Creep deformation mechanisms in a γ titanium aluminide
Materials Science and Engineering: A, 2016
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2021
γ-titanium aluminide (TiAl) alloys with fully lamellar microstructure possess excellent properties for high-temperature applications. Such fully lamellar microstructure has interfaces at different length scales. The separation behavior of the lamellae at these interfaces is crucial for the mechanical properties of the whole material. Unfortunately, quantifying it by experiments is difficult. Therefore, we use molecular dynamics (MD) simulations to this end. Specifically, we study the high-temperature separation behavior under tensile loading of the four different kinds of lamellar interfaces appearing in TiAl, namely, the c/α 2 , c/c PT , c/c TT , and c/c RB interfaces. In our simulations, we use two different atomistic interface models, a defect-free (Type-1) model and a model with preexisting voids (Type-2). Clearly, the latter is more physical but studying the former also helps to understand the role of defects. Our simulation results show that among the four interfaces studied, the c/α 2 interface possesses the highest yield strength, followed by the c/c PT , c/c TT , and c/c RB interfaces. For Type-1 models, our simulations reveal failure at the interface for all γ/γ interfaces but not for the c/α 2 interface. By contrast, for Type-2 models, we observe for all the four interfaces failure at the interface. Our atomistic simulations provide important data to define the parameters of traction-separation laws and cohesive zone models, which can be used in the framework of continuum mechanical modeling of TiAl. Temperature-dependent model parameters were identified, and the complete traction-separation behavior was established, in which interface elasticity, interface plasticity, and interface damage could be distinguished. By carefully eliminating the contribution of bulk deformation from the interface behavior, we were able to quantify the contribution of interface plasticity and interface damage, which can also be related to the dislocation evolution and void nucleation in the atomistic simulations.
Physical Review B
Solid solution MAX phases offer the opportunity for further tuning of the thermo-mechanical and functional properties of MAX phases, increasing their envelope of performance. Previous experimental results show that the lattice parameters of Ti 3 (Si x Al 1−x)C 2 decrease, while the Young's modulus increases with increased Si content in the lattice. In this work, we present a computational investigation of the structural, electronic, and mechanical properties of Ti 3 (Si x Al 1−x)C 2 (x= 0, 0.25, 0.5, 0.75, and 1). The solid solutions were modeled using special quasirandom structures (SQS) and calculated using Density Functional Theory (DFT), which is implemented in the Vienna Ab initio Simulation Package (VASP). The SQS structures represent random mixing of Al and Si in the A sublattice of 312 MAX phase and their structural, electronic, and mechanical properties were calculated and compared with experiments. We study the cleavage and slip behavior of Ti 3 (Si x Al 1−x)C 2 to investigate the deformation behavior in terms of cleavage and shear. It has been shown that the cleavage between M and A layers results in increasing cleavage stress in Ti 3 (Si x Al 1−x)C 2 as a function of Si content in the lattice. In addition, the shear deformation of hexagonal close packed Ti 3 (Si x Al 1−x)C 2 under 2110 {0001} and 0110 {0001} results in increasing unstable stacking fault energy (USFE) and ideal shear strength (ISS) in Ti 3 (Si x Al 1−x)C 2 as the system becomes richer in Si.