Phase Evolution and Transformations During Long-Term Creep of Two-Phase Titanium Aluminides

1999 ◽  
Vol 580 ◽  
Author(s):  
F. Appel ◽  
M. Oehring ◽  
P.J. Ennis
Keyword(s):  
Titanium Aluminide ◽  
Structural Changes ◽  
Titanium Aluminides ◽  
Driving Forces ◽  
Equilibrium Phase ◽  
Phase Evolution ◽  
Strength Properties ◽  
Two Phase ◽  
Energy Conversion Systems ◽  
Term Creep

AbstractTitanium aluminide alloys based on the intermetallic γ (TiAl) and α2 (Ti3Al) phases are being considered as light-weight materials for high-temperature applications in advanced energy conversion systems. However, for such applications the material suffers from insufficient creep resistance at the intended service temperature of 700°C. The paper reports an electron microscope study of diffusion controlled mechanisms which apparently cause the degradation of the strength properties. The processes lead to significant structural changes involving the formation of extended ledges and recrystallization. The driving forces of these mechanisms probably arise from non-equilibrium phase compositions and significant coherency stresses occurring at the interfaces.

An Electron Microscope Study of Diffusion Assisted Dislocation Processes in Intermetallic Gamma TiAl

1999 ◽  
Vol 589 ◽  
Author(s):  
F. Appel ◽  
U. Lorenz ◽  
M. Oehring
Keyword(s):  
Electron Microscope ◽  
Microscope Study ◽  
Electron Microscope Study ◽  
Structural Changes ◽  
Titanium Aluminides ◽  
Elevated Temperatures ◽  
Strength Properties ◽  
Two Phase ◽  
Term Creep ◽  
Dislocation Sources

AbstractThe paper reports an electron microscope study of diffusion controlled deformation mechanisms in two-phase titanium aluminides which apparently cause the degradation of the strength properties at elevated temperatures. Climb velocities were analyzed in terms the critical vacancy supersaturation necessary for the operation of diffusion assisted dislocation sources. Particular emphasis was paid on structural changes occurring during long-term creep, which are apparently associated with dislocation climb.

Twinning in Crack Tip Plasticity of Two-Phase Titanium Aluminides

2000 ◽  
Vol 646 ◽  
Author(s):  
Fritz Appel
Keyword(s):  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Crack Tip ◽  
High Resolution Electron Microscopy ◽  
Strength Properties ◽  
Mechanical Twinning ◽  
Atomic Scale ◽  
Two Phase ◽  
Design Concepts ◽  
Crack Tip Plasticity

ABSTRACTIntermetallic titanium aluminides based on γ(TiAl) are prone to cleavage fracture on low index lattice planes. Unfavourably oriented grains may therefore provide easy crack paths so that the cracks can rapidly grow to a length which is critical for failure. The effect of crack tip plasticity on crack propagation in γ(TiAl) was investigated by conventional and high-resolution electron microscopy. Crack tip shielding due to mechanical twinning was recognized as toughening mechanism, which occur at the atomic scale and apparently is capable to stabilize fastly growing cracks. The potential of the mechanism will be discussed in the context of novel design concepts for improving the strength properties of γ-base titanium aluminide alloys.

Stress Induced Structural Changes of Interphase Boundaries and Mechanical Twins in two-Phase γ-Titanium Aluminides

1996 ◽  
Vol 466 ◽  
Author(s):  
F. Appel ◽  
R. Wagner
Keyword(s):  
Titanium Aluminide ◽  
Structural Changes ◽  
Titanium Aluminides ◽  
Elevated Temperatures ◽  
High Resolution Electron Microscopy ◽  
Two Phase ◽  
Resolution Electron ◽  
Interphase Boundaries ◽  
Deformation Stress ◽  
Mechanical Twins

ABSTRACTConventional and high-resolution electron microscopy has been used to examine the interfacial structures in (α2 + γ) titanium aluminide alloys. Accommodation of misfit which arises because of differences in lattice parameters and crystal structure leads to dense structures of interfacial dislocations and coherency stresses. During deformation stress induced structural changes of misfitting interfaces occur. These are closely related to the generation of perfect and twinning partial dislocations. At elevated temperatures diffusion controlled structural changes take place at an atomic level and seem to limit the structural stability of the material.

Stress Driven Phase Transformations and Recrystallization Processes in Two-Phase Titanium Aluminide Alloys

2000 ◽  
Vol 652 ◽  
Author(s):  
Fritz Appel ◽  
Michael Oehring
Keyword(s):  
Phase Transformation ◽  
Titanium Aluminide ◽  
Electron Microscope Study ◽  
Phase Boundaries ◽  
Two Phase ◽  
Titanium Aluminide Alloy ◽  
Lower Solubility ◽  
Term Creep ◽  
Titanium Aluminide Alloys

ABSTRACTThe paper presents an electron microscope study of phase transformation and recrystallization in an intermetallic α2(Ti3Al) + γ(TiAl) titanium aluminide alloy, after long-term creep. The mechanisms are closely related to the atomic structure of the α2/γ phase boundaries and are probably driven by a non-equilibrium of the phase composition leading to the dissolution of the α2 phase. The α2 /γ transformation is accompanied by the formation of precipitates, because the γ(TiAl)phase has a significantly lower solubility for interstitial impurities than the α2(Ti3Al) phase.

Atomistic Processes of Phase Transformation and Dynamic Recrystallization during Hot-Working of Intermetallic Titanium Aluminides

2007 ◽  
Vol 558-559 ◽  
pp. 465-470
Author(s):  
Fritz Appel ◽  
Michael Oehring ◽  
Jonathan H.D. Paul
Keyword(s):  
Phase Transformation ◽  
Dynamic Recrystallization ◽  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Equilibrium Phase ◽  
High Resolution Electron Microscopy ◽  
Hot Working ◽  
Atomic Scale ◽  
Resolution Electron ◽  
Deformation State

Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving the phases γ(TiAl), α2(Ti3Al), β, and B2. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working of such an alloy were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by heterogeneities in the deformation state and non-equilibrium phase composition. The β/B2 phase is apparently unstable under tetragonal distortion, which gives rise to the formation of the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation and coalescence of ledges. Large and broad ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.

Static and Dynamic Strain Ageing in Two-Phase Gamma Titanium Aluminides

2000 ◽  
Vol 646 ◽  
Author(s):  
U. Christoph ◽  
F. Appel
Keyword(s):  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Deformation Behaviour ◽  
Dynamic Strain ◽  
Dynamic Strain Ageing ◽  
Two Phase ◽  
Fast Kinetics ◽  
Strain Ageing ◽  
Wide Range ◽  
Dislocation Pinning

ABSTRACTThe deformation behaviour of two-phase titanium aluminides was investigated in the intermediate temperature interval 450–750 K where the Portevin-LeChatelier effect occurs. The effect has been studied by static strain ageing experiments. A wide range of alloy compositions was investigated to identify the relevant defect species. Accordingly, dislocation pinning occurs with fast kinetics and is characterized by a relatively small activation energy of 0.7 eV, which is not consistent with a conventional diffusion process. Furthermore, the strain ageing phenomena are most pronounced in Ti-rich alloys. This gives rise to the speculation that antisite defects are involved in the pinning process. The implications of the ageing processes on the deformation behaviour of two-phase titanium aluminide alloys will be discussed.

Precipitation Phenomena and Strain Hardening of Intermetallic Titanium Aluminides

2002 ◽  
Vol 753 ◽  
Author(s):  
J. Müllauer ◽  
F. Appel
Keyword(s):  
Work Hardening ◽  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Titanium Boride ◽  
Compression Tests ◽  
Two Phase ◽  
Dislocation Glide ◽  
Transmission Electron ◽  
Work Hardening Coefficient ◽  
Precipitation Phenomena

ABSTRACTIn two-phase titanium aluminide alloys, the implementation of precipitation reactions is a widely utilized concept to control the microstructure and strengthen the material. A study has been made on the influence of carbide and boride precipitates on dislocation mobility and strengthening at 300 K. Compression tests were carried out for characterizing the mechanisms determining flow stress and dislocation glide resistance. The interaction mechanisms between the precipitates and dislocations were assessed by thermodynamic glide parameters and transmission electron microscopy. It has been shown that small titanium boride precipitates and carbide precipitates of perovskite type act as long-range dislocation glide obstacles. The interaction between the dislocations and the borides and carbides mainly leads to an athermal stress contribution. However, the dislocation-particle interactions are quite different. Small groups of borides are encircled by dislocations. This gives rise to the formation of loop structures the density of which increases with strain. On the contrary, the carbide precipitates are shearable and can be overcome without Orowan looping. This different behaviour is also reflected in the work hardening characteristics. Whereas the work hardening coefficient of the boron doped material increases with increasing B-concentration, it is independent of concentration in the case of the carbon-doped material.

Solution and Precipitation Hardening in Carbon-Doped Two-Phase γ-Titanium Aluminides

1996 ◽  
Vol 460 ◽  
Author(s):  
F. Appel ◽  
U. Christoph ◽  
R. Wagner
Keyword(s):  
High Temperature ◽  
Titanium Aluminide ◽  
Precipitation Hardening ◽  
Titanium Aluminides ◽  
Activation Parameters ◽  
High Temperature Strength ◽  
Thermal Treatments ◽  
Two Phase ◽  
Carbon Doped ◽  
Titanium Aluminide Alloy

ABSTRACTA two-phase titanium aluminide alloy was systematically doped with carbon to improve its high temperature strength. Solid solutions and precipitates of carbon were formed by different thermal treatments. A fine dispersion of perovskite precipitates was found to be very effective for improving the high temperature strength and creep resistance of the material. The strengthening mechanisms were characterized by flow stresses and activation parameters. The investigations were accompanied by electron microscope observation of the defect structure which was generated during deformation. Special attention was paid on the interaction mechanisms of perfect and twinning dislocations with the carbide precipitates.

The Structure and Stress State of Lamellar Interfaces in Two-Phase Titanium Aluminides

1993 ◽  
Vol 318 ◽  
Author(s):  
Fritz Appel ◽  
Ulrich Christoph ◽  
Richard Wagner
Keyword(s):  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Lattice Mismatch ◽  
Deformation Behaviour ◽  
Lamellar Microstructure ◽  
Misfit Strain ◽  
Shear Stresses ◽  
Dislocation Loops ◽  
Two Phase ◽  
Interfacial Dislocations

ABSTRACTTitanium aluminide alloys with compositions slightly on the Ti-rich side of stoichiometry consist of the intermetallic phases α2 (Ti3Al) and γ(TiAl). The two phases form a lamellar microstructure with various types of coherent and semicoherent interfaces. The lattice mismatch occurring at the semicoherent interfaces is largely accommodated by networks of interfacial dislocations. Nevertheless, a significant homogeneous straining seems to remain at these interfaces, resulting in long-range residual stresses. The present paper reports an electron microscope study of the correlation between the misfit strain of adjacent lamellae and the atomic structure of the interfaces. The residual coherency stresses were determined by analyzing the curvature of dislocation loops which were emitted from the network of the interfacial dislocations. The estimated stresses are close to the shear stresses applied during macroscopic deformation experiments. The effects of these stresses on the deformation behaviour of the material are discussed.

Interactions of Dislocations and Deformation Twins with Interfacial Boundaries in Titanium Aluminides

1993 ◽  
Vol 319 ◽  
Author(s):  
Fritz Appel ◽  
Richard Wagner
Keyword(s):  
Shear Deformation ◽  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Deformation Behaviour ◽  
Lamellar Microstructure ◽  
High Resolution Electron Microscopy ◽  
Misfit Dislocations ◽  
Two Phase ◽  
Deformation Twins ◽  
Resolution Electron

AbstractThe deformation behaviour of two-phase titanium aluminide alloys with a lamellar microstructure of the intermetallic phases α2(Ti3Al) and y(TiAl) was studied. The interaction processes of dislocations and deformation twins, respectively, with the lamellar interfaces are investigated by conventional and high-resolution electron microscopy. The mechanisms of translation of shear deformation across, the lamellar boundaries depend on their structure. Semicoherent interfaces are very effective barriers limiting the propagation of shear deformation. The misfit dislocations present at these interfaces support, on the other hand, the generation of dislocations and deformation twins. The observed processes are discussed regarding plastic deformation and crack propagation in the material.

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