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.

Interface Related Strength Phenomena in Two-Phase Titanium Aluminides

1999 ◽  
Vol 586 ◽  
Author(s):  
U. Christoph ◽  
M. Oehring ◽  
F. Appel
Keyword(s):  
Crystal Structure ◽  
Electron Microscope ◽  
Microscope Study ◽  
Lamellar Structure ◽  
Electron Microscope Study ◽  
Titanium Aluminides ◽  
Intermetallic Phases ◽  
Dislocation Loops ◽  
Two Phase ◽  
Interfacial Dislocations

ABSTRACTPhase equilibria and transformations in near-equiatomic titanium aluminides lead to the formation of a lamellar structure comprising of the intermetallic phases α2(Ti3Al) and γ(TiAl). Due to the differences in lattice parameters and crystal structure, coherency stresses and mismatch structures occur at various types of semicoherent interfaces present in the material. The present paper reports an electron microscope study of the atomic structure of the interfaces. The residual coherency stresses present at the interfaces were determined by analysing the curvature of dislocation loops which were emitted from the network of interfacial dislocations. The implication of these stresses on creep will be discussed.

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.

scholarly journals AN ELECTRON MICROSCOPE STUDY OF YOLK FORMATION DURING OOGENESIS IN LEBISTES RETICULATUS GUPPYI

1966 ◽  
Vol 28 (2) ◽  
pp. 209-232 ◽  
Author(s):  
Michael J. Droller ◽  
Thomas F. Roth
Keyword(s):  
Endoplasmic Reticulum ◽  
Electron Microscope ◽  
Microscope Study ◽  
Electron Microscope Study ◽  
Structural Changes ◽  
Extracellular Material ◽  
Yolk Formation ◽  
Selective Uptake ◽  
Lebistes Reticulatus

The present investigation describes the fine structural changes that occur during proteid yolk formation in the developing oocytes of the guppy (Lebistes reticulatus), an ovoviviparous teleost. These changes suggest the operation of a number of different intra- and extraoocyte processes that may account for the synthesis and deposition of the proteid yolk. Early in oogenesis, the egg's Golgi systems proliferate and begin to disclose an electron-opaque content. Numerous 70-mµ diameter vesicles apparently pinch off from the Golgi systems, transport this material through the egg, and probably then fuse to form a crenate, membrane-limited yolk droplet. At the same time, the rough-surfaced endoplasmic reticulum accumulates a flocculent substance that differs in appearance from the Golgi content. Smooth vesicles, presumably derived from the ER, then coalesce to form a second type of intraoocyte yolk droplet. These dissimilar, separately derived droplets subsequently fuse, thus combining the materials that constitute the intraoocyte contribution to the proteid yolk. Somewhat later in development, the egg appears to ingest extracellular material via 75-mµ diameter bristle-coated micropinocytotic pits and vesicles. These structures apparently fuse to form tubules which then coalesce into large yolk droplets. At a later stage, bristle-coated micropinocytotic vesicles of 100 mµ diameter presumably take up a material that is then probably immediately deposited into a second type of proteid yolk droplet. It is postulated that these two different micropinocytotic structures are specifically involved with the selective uptake of dissimilar extracellular proteid materials.

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.

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