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.

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.

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.

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.

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.

Microstructure and interfaces in TiB2/Ti-46Al-3Cr Alloy Composites

1995 ◽  
Vol 410 ◽  
Author(s):  
Weimin Si ◽  
Michael Dudley ◽  
Pengxing Li ◽  
Renjie Wu
Keyword(s):  
Electron Microscopy ◽  
Titanium Aluminide ◽  
Analytical Electron Microscopy ◽  
Lattice Misfit ◽  
High Resolution Electron Microscopy ◽  
Amorphous Layer ◽  
Two Phase ◽  
Titanium Aluminide Alloy ◽  
Resolution Electron ◽  
Crystallographic Orientation Relationship

ABSTRACTA ternary titanium aluminide alloy, Ti-46Al-3Cr (at%), was discontinuously reinforced with 5 vol% titanium diboride (TiB2), by an in-situ synthesis technique, resulting in a two phase γ(TiAl) (mainly) and α2(Ti3Al) matrix with randomly dispersed TiB2 particle. Interfaces of TiB2-TiAl were investigated by Analytical Electron Microscopy (AEM) and High Resolution Electron Microscopy (HREM). No consistent crystallographic orientation relationship was observed between TiB2 particle and TiAl matrix, and there was no evidence of alloying elements (such as Cr) segregation or interphase formation at the TiB2-TiAl interface. HREM results indicated that no semi-coherent interface between TiB2 and TiAl has been observed. There existed a thin amorphous layer (0.5 to 1.3 nm) at the TiB2-TiAl interface, which may accommodate the large lattice misfit across the interface and enhance the interfacial bonding.

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.

Fracture Behaviour of Two-Phase γ-Titanium Aluminides

1994 ◽  
Vol 364 ◽  
Author(s):  
Fritz Appel ◽  
Uwe Lorenz ◽  
Tao Zhang ◽  
Richard Wagner
Keyword(s):  
Fracture Toughness ◽  
Crack Propagation ◽  
Titanium Aluminides ◽  
Process Zone ◽  
Crack Tip ◽  
Lamellar Microstructure ◽  
High Fracture Toughness ◽  
High Fracture ◽  
Two Phase ◽  
Lamellar Interfaces

AbstractTitanium aluminides with a lamellar microstructure consisting of the intermetallic phases ֱ2 (Ti3Al) and γ(TiAl) suffer from brittleness at ambient temperatures but exhibit at the same time a relatively high fracture toughness. This discrepancy indicates particular processes stabilizing crack propagation in the lamellar microstructure. In this context, the toughening mechanisms were investigated in (α2 + γ) TiAl alloys which contained different volume fractions of lamellar colonies. The fracture toughness for crack propagation parallel or across the lamellar interfaces was estimated by using chevron-notched bending bars. Electron microscope studies were performed to characterize the related processes of crack tip plasticity. Special emphasis was paid to the crystallography of crack propagation and to the interaction of crack tips with lamellar interfaces. Accordingly, the lamellar morphology derives some of its toughness from interface-related processes which stabilize crack propagation by deflecting the crack tip and providing the necessary dislocation sources for crack tip shielding in the process zone ahead of the crack tip.

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.

scholarly journals Interfacial Properties and Mechanical Behavior of Titanium Aluminides

1997 ◽  
Vol 492 ◽  
Author(s):  
M. H. Yoo ◽  
C. L. Fu
Keyword(s):  
Titanium Aluminides ◽  
Boundary Energy ◽  
Plastic Anisotropy ◽  
Interfacial Fracture ◽  
Two Phase ◽  
Deformation And Fracture ◽  
Crack Tip Plasticity ◽  
Lamellar Boundary ◽  
Lamellar Interfaces

ABSTRACTThe role of various interfaces in deformation and fracture behavior of two-phase TiAl-Ti3Al alloys is analyzed on the basis of the specific interfacial and surface energies determined from ab initio calculations. The propensity of twinning observed in these alloys is consistent with the low true-twin boundary energy. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed partly to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Interfacial fracture energies are estimated to be the highest for the α2/γ lamellar boundary and the lowest for the 120° rotational γ/γ boundary. The fracture mode mixity plays an important role in the crack-tip plasticity by ordinary slip and true-twinning, leading to translamellar and interfacial fracture.

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