Point Defect Mechanisms in Deformation and Phase Transformation of Titanium Aluminide Alloys

2002 ◽  
Vol 753 ◽  
Author(s):  
Fritz Appel ◽  
Jonathan D.H. Paul ◽  
Ulrich Fröbel
Keyword(s):  
Phase Transformation ◽  
Point Defects ◽  
Titanium Aluminide ◽  
High Resolution Electron Microscopy ◽  
Transport Processes ◽  
Two Phase ◽  
Structural Characterisation ◽  
Resolution Electron ◽  
Term Creep ◽  
Titanium Aluminide Alloys

ABSTRACTPoint defects in intermetallic compounds are subjected to significant constraints due to the directional bonding, long-range order and off-stoichiometric deviation of the materials. This leads to a variety of defects with distinct differences in morphology, concentration and mobility. In the present study the implications of these defect characteristics on the mechanical properties of two-phase titanium aluminide alloys will be investigated. The major areas of the study are: (i) deformation induced point defects that contribute to work hardening; (ii) dislocation locking due to the formation of defect atmospheres; (iii) transport processes involved in phase transformation and recrystallization occurring during long-term creep. The applied methods include mechanical testing, static strain aging and structural characterisation by high resolution electron microscopy.

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.

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.

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.

Atomic Scale Structure-Property Relationships of Defects and Interfaces in Ceramics

1998 ◽  
Vol 4 (S2) ◽  
pp. 556-557
Author(s):  
S. Stemmer ◽  
G. Duscher ◽  
E. M. James ◽  
M. Ceh ◽  
N.D. Browning
Keyword(s):  
Point Defects ◽  
High Resolution Electron Microscopy ◽  
Complete Characterization ◽  
Atomic Scale ◽  
Structure Property ◽  
Defect Engineering ◽  
Impurity Atoms ◽  
Structure Property Relationships ◽  
Resolution Electron ◽  
Scale Composition

The evaluation of the two dimensional projected atom column positions around a defect or an interface in an electronic ceramic, as it has been performed in numerous examples by (quantitative) conventional high-resolution electron microscopy (HRTEM), is often not sufficient to relate the electronic properties of the material to the structure of the defect. Information about point defects (vacancies, impurity atoms), and chemistry or bonding changes associated with the defect or interface is also required. Such complete characterization is a necessity for atomic scale interfacial or defect engineering to be attained.One instructive example where more than an image is required to understand the structure property relationships, is that of grain boundaries in Fe-doped SrTi03. Here, the different formation energies of point defects cause a charged barrier at the boundary, and a compensating space charge region around it. The sign and magnitude of the barrier depend very sensitively on the atomic scale composition and chemistry of the boundary plane.

scholarly journals Protected Gold Nanoparticles with Thioethers and Amines As Surrogate Ligands

2013 ◽  
Vol 2013 ◽  
pp. 1-4 ◽  
Author(s):  
M. Rafiq H. Siddiqui
Keyword(s):  
Gold Nanoparticles ◽  
Variable Temperature ◽  
Size Control ◽  
Analytical Techniques ◽  
High Resolution Electron Microscopy ◽  
Phase Method ◽  
Two Phase ◽  
H Nmr ◽  
Resolution Electron ◽  
Vis Spectroscopy

Dodecyl sulfide, dodecyl amine, and hexylamine were shown to act as surrogate ligands (L) via metastable gold nanoparticles. By collating analytical and spectroscopic data obtained simultaneously, empirical formula Au24L was assigned. These impurity-free nanoparticles obtained in near quantitative yields showing exceptional gold assays (up to 98%Au) were prepared by a modification of the two-phase method. Replacement reactions on the Au24L showed that Au:L ratios may be increased (up to Au55:L (L= (H25C12)2S)) or decreased (Au12:L (L= H2NC12H25and H2NC6H13)) as desired. This work encompassing the role of analytical techniques used, that is, elemental analysis, variable temperature1H NMR, FAB mass spectrometry, UV-Vis spectroscopy, thin film X-ray diffraction, and high-resolution electron microscopy (HREM) has implications in the study of size control, purity, stability, and metal assays of gold nanoparticles.

The decagonal quasicrystal formed from an Al–Pd–Mn icosahedral quasicrystal

1995 ◽  
Vol 10 (5) ◽  
pp. 1146-1153 ◽  
Author(s):  
W. Sun ◽  
K. Hiraga
Keyword(s):  
Structural Characteristics ◽  
High Resolution Electron Microscopy ◽  
Icosahedral Quasicrystal ◽  
Two Phase ◽  
Growth Interface ◽  
Decagonal Quasicrystal ◽  
Resolution Electron ◽  
The Matrix ◽  
Solid State Phase Transformation ◽  
Phase Proceeds

We present a detailed investigation on the decagonal quasicrystal (D-phase) formed from an Al-Pd-Mn icosahedral quasicrystal (I-phase) through a solid-state phase transformation, including its formation, compositional and crystallographical relationships with the matrix I-phase, growth mode, and structural characteristics. The as-melt-spun Al70Pd20Mn10 alloy contains only I-phase. By annealing at 800 °C, the D-phase is found to grow cpitaxially from the I-phase to establish a D/I two-phase equilibrium with distinctly different composition between them. The D-phase exhibits a stepped growth interface, which consists of a facet plane, formed by sharing the tenfold plane with a fivefold plane of the matrix I-phase, and some ledges across it. The growth of the D-phase into the I-phase proceeds through lateral movement of the ledges along the tenfold plane. High-resolution electron microscopy reveals that the structure of the D-phase is constructed by an aperiodic arrangement of decagonal atom clusters with definite linkages and long-range quasiperiodic correlation.

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