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 Properties Comparison of Ti-Al-Si Alloys Produced by Various Metallurgy Methods

Materials ◽  
2019 ◽  
Vol 12 (19) ◽  
pp. 3084 ◽  
Author(s):  
Anna Knaislová ◽  
Pavel Novák ◽  
Jaromír Kopeček ◽  
Filip Průša
Keyword(s):  
Fracture Toughness ◽  
Phase Composition ◽  
Powder Metallurgy ◽  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Compression Tests ◽  
Large Area ◽  
Fine Grained ◽  
Central Porosity ◽  
Fine Grained Structure

Melting metallurgy is still the most frequently used and simplest method for the processing of metallic materials. Some of the materials (especially intermetallics) are very difficult to prepare by this method due to the high melting points, poor fluidity, or formation of cracks and pores after casting. This article describes the processing of Ti-Al-Si alloys by arc melting, and shows the microstructure, phase composition, hardness, fracture toughness, and compression tests of these alloys. These results are compared with the same alloys prepared by powder metallurgy by the means of a combination of mechanical alloying and spark plasma sintering. Ti-Al-Si alloys processed by melting metallurgy are characterized by a very coarse structure with central porosity. The phase composition is formed by titanium aluminides and titanium silicides, which are full of cracks. Ti-Al-Si alloys processed by the powder metallurgy route have a relatively homogeneous fine-grained structure with higher hardness. However, these alloys are very brittle. On the other hand, the fracture toughness of arc-melted samples is immeasurable using Palmqvist’s method because the crack is stopped by a large area of titanium aluminide matrix.

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.

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.

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.

Development of Platinum-Group-Metal Superalloys for High-Temperature Use

MRS Bulletin ◽  
2003 ◽  
Vol 28 (9) ◽  
pp. 632-638 ◽  
Author(s):  
L. A. Cornish ◽  
B. Fischer ◽  
R. Völkl
Keyword(s):  
Electron Microscopy ◽  
High Temperature ◽  
Oxide Dispersion Strengthened ◽  
Platinum Group Metal ◽  
Platinum Group Metals ◽  
Compression Tests ◽  
Two Phase ◽  
Transmission Electron ◽  
Oxide Particles ◽  
Platinum Group

AbstractSuperalloys based on platinum-group metals are being developed for high-temperature applications. These alloys have two-phase structures comprising either ordered precipitates in a matrix analogous to the nickel-based superalloys or a fine dispersion of oxide particles in a matrix analogous to oxide-dispersion-strengthened nickel-based alloys. Currently, alloys based on iridium, rhodium, and platinum have been obtained. This article reviews the rationale of developments and the progress made in this area. Oxidation and compression tests as well as characterization with scanning electron microscopy and transmission electron microscopy were undertaken. These tests showed encouraging results, and further work is being done on new alloying additions and tensile testing.

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.

Platinum Group Metals Base Refractory Superalloys

1996 ◽  
Vol 460 ◽  
Author(s):  
Y. Yamabe-Mitarai ◽  
Y. Koizumi ◽  
H. Murakami ◽  
Y. Ro ◽  
T. Maruko ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Platinum Group Metals ◽  
Compression Tests ◽  
Two Phase ◽  
X Ray ◽  
Melting Temperatures ◽  
High Temperature Materials ◽  
Transmission Electron ◽  
Two Phases

ABSTRACTIr- and Rh-base refractory superalloys with an fee and Lb two phase structure similar to Ni-base superalloys, yet with considerably higher melting temperatures have been proposed. Fee and Ll2 two phases were observed in these alloys by transmission electron microscopy and X-ray powder diffractometry. The compression tests of these alloys showed that the strengths of several alloys were about 200 MPa at 1800 °C and these alloys have potential to become ultra-high temperature materials for use in power engineering field.

Thermally Activated Processes and Dislocation Mobilities in Two-Phase γ-Titanium Aluminides

1994 ◽  
Vol 364 ◽  
Author(s):  
Fritz Appel ◽  
Ulf Sparka ◽  
Richard Wagner
Keyword(s):  
Room Temperature ◽  
Titanium Aluminides ◽  
Dislocation Climb ◽  
Ductile Material ◽  
Material Behaviour ◽  
Two Phase ◽  
Dislocation Glide ◽  
Thermally Activated ◽  
Deformation Tests ◽  
Lattice Friction

AbstractThe processes controlling the dislocation mobility in two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining the activation volumes and activation energies of thermally activated dislocation glide processes. The deformation tests are supplemented by electron microscope observations. Accordingly, at room temperature the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from dislocation dipoles and debris defects, which are trailed and terminated at jogs in 1/2 <110] screw dislocations. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behaviour.

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