Yield Stress Anomaly for 1/2〈112] {1111} Slip in y-Titanium Aluminide

1998 ◽  
Vol 552 ◽  
Author(s):  
S. Jiao ◽  
N. Bird ◽  
P. B. ◽  
G. Taylor
Keyword(s):  
Yield Stress ◽  
High Temperatures ◽  
Titanium Aluminide ◽  
Room Temperature ◽  
Elastic Strain Energy ◽  
Weak Beam ◽  
Transmission Electron ◽  
Different Temperatures ◽  
Edge Dislocations ◽  
Dislocation Dissociation

ABSTRACTTransmission electron microscopy (TEM) studies of dislocation structures in single crystals of TiAl containing 54.5 or 54.7 at% Al deformed at different temperatures revealed the occurrence of slip on ½〈112]{ {111} at room temperature and near the peak of the yield stress anomaly (YSA). Measurements of the corresponding yield stresses revealed the existence of a YSA for this type of slip. Weak-beam TEM showed the presence of locks at room temperature for 30° dislocations and at high temperatures for edge dislocations. Both types of locks involve dissociation on two intersecting { 1111} type planes, driven by reduction in elastic strain energy. The edge dislocation dissociation at high temperatures involves both climb and glide.

Nature of Slip in γ-Titanium Aluminide Above the Yield Stress Anomaly Temperature

1998 ◽  
Vol 552 ◽  
Author(s):  
S. Jiao ◽  
N. Bird ◽  
P. B. Hirsch ◽  
G. Taylor
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Shear Stress ◽  
Yield Stress ◽  
Titanium Aluminide ◽  
Transmission Electron ◽  
Different Temperatures ◽  
Critical Resolved Shear Stress

ABSTRACTCrystals of γ-TiAl, containing ∼54.5 at% Al, with various orientations were deformed at different temperatures and examined by transmission electron microscopy (TEM). It was found that while in many cases slip and climb of ordinary dislocations tend to predominate, slip by [001] dislocations can be important also. The occurrence of [001] slip has not been reported before. The values of critical resolved shear stress were determined and strong locks on [001] dislocations have been identified as local segments of ½ ⟨112] dislocations arising from interactions between [001] and ½⟨110] dislocations.

Deformation and its Structure in Llo Ti-Al-Nb Compound Alloys

1988 ◽  
Vol 133 ◽  
Author(s):  
Yoo-Dong Hahn ◽  
Sung H. Whang
Keyword(s):  
Temperature Dependence ◽  
Yield Stress ◽  
Uniaxial Compression ◽  
High Temperatures ◽  
Room Temperature ◽  
Fracture Strain ◽  
Positive Temperature ◽  
Deformation Structures ◽  
Miniature Specimens ◽  
Deformation Tests

ABSTRACTThe ternary TiAl-Nb (Llo) alloys stabilized at 1000°C for a week were prepared into miniature specimens for compressive deformation tests. The specimens were deformed in uniaxial compression at room temperature as well as various high temperatures. The yield stress and fracture strain were determined with respect to Nb concentration, and as a function of temperature to investigate positive temperature dependence.Dislocation structures and other deformation structures of the deformed alloys were studied by TEM.

Experimental Tem and Image Simulation of Dislocations in Ti3A1

1994 ◽  
Vol 364 ◽  
Author(s):  
JÖrg M.K. Wiezorek ◽  
S. A. Court ◽  
C.J. Humphreys
Keyword(s):  
Fine Structure ◽  
Room Temperature ◽  
Stacking Faults ◽  
Antiphase Boundary ◽  
Computational Techniques ◽  
Image Simulation ◽  
Prism Plane ◽  
Weak Beam ◽  
Transmission Electron ◽  
Beam Technique

AbstractThe fine structure and the character of a dislocations on prism planes have been determined for room-temperature deformed polycrystalline Ti3Al using a combination of experimental and computational techniques of transmission electron microscopy (TEM). Within the resolution limits of the weak-beam technique the fine structure of prism plane a dislocations in Ti3Al is found to be planar and to comprise only a single ribbon of antiphase boundary and no experimentally observable stacking faults.

Dislocation Structures in Ni3(Al, Hf)

1996 ◽  
Vol 460 ◽  
Author(s):  
T. Kruml ◽  
B. Viguier ◽  
J. Bonneville ◽  
P. Spätig ◽  
J. L. Martin
Keyword(s):  
Electron Microscopy ◽  
Antiphase Boundary ◽  
Image Simulation ◽  
Compression Tests ◽  
Simulation Techniques ◽  
Weak Beam ◽  
Thin Foils ◽  
Transmission Electron ◽  
Different Temperatures

ABSTRACTSingle crystalline specimens of Ni74.8Al21.9Hf3.3 were subjected to compression tests at different temperatures. Thin foils for transmission electron microscopy observations were prepared from several specimens deformed within and above the yield stress anomaly domain. The dislocation microstructure was studied. The weak beam imaging and image simulation techniques followed by anisotropie elasticity calculations were used for the determination of antiphase boundary energies in both cube and octahedral planes, resulting in values of 237 mJm-2 and 252 mJm-2 respectively. The comparison of the present results with data taken from literature shows the influence of Hf on mechanical properties, dislocation microstructures and APB energies.

A Weak-Beam Study of the Microstructure of β-CUZN Deformed in the Domain of Flow Stress Anomaly

1988 ◽  
Vol 133 ◽  
Author(s):  
G. Dirras ◽  
P. Beauchamp ◽  
P. Veyssière
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Flow Stress ◽  
Single Crystals ◽  
Room Temperature ◽  
Burgers Vector ◽  
Weak Beam ◽  
Transmission Electron ◽  
Edge Component ◽  
Imaging Conditions

ABSTRACTβ-brass single crystals oriented along <001> were deformed between room temperature and 300°C. The deformation microstructure and dissociation properties were studied by transmission electron microscopy under weak-beam imaging conditions.Whatever the deformation temperature, superdislocations with <111> Burgers vector and strong edge component dominate within the microstructure. In addition, below the temperature of the flow stress peak (≈ 250°C), the density of screw relative to mixed superdislocations decreases as straining temperature increases. Dissociation does not always occur on the slip plane neither does it proceed exclusively by glide, even in samples deformed at 100°C.

Dislocation dissociation in Ll2 ordered alloy Ni3Ge

1994 ◽  
Vol 52 ◽  
pp. 688-689
Author(s):  
J. Fang ◽  
E. M. Schulson ◽  
I. Baker
Keyword(s):  
Yield Stress ◽  
Room Temperature ◽  
Complete Analysis ◽  
Deformation Structure ◽  
Burgers Vector ◽  
Tem Characterization ◽  
Perfect Dislocation ◽  
Increasing Temperature ◽  
Dislocation Dissociation

It is well established that the yield stress of Ni3Ge anomalously increases with increasing temperature from 77 K to 673 K. Above room temperature, the anomaly is generally attributed to the Kear-Wilsdorf (K-W) mechanism. Since no parallel systematic TEM investigation has been performed to characterize the dislocation structures in Ni3Ge, it is not clear whether the yield stress anomaly below room temperature is governed mainly by the K-W mechanism or by other mechanism(s). Thus, detailed TEM characterization of dislocation dissociation has been carried out in polycrystalline Ni3Ge deformed at room temperature. In this report, we briefly describe the main results obtained. The complete analysis can be found elsewhere.The deformation structure mainly consists of apparently undissociated dislocations with Burgers’ vector parallel to a<110> and of SISF (super intrinsic stacking fault)-coupled partials. Fig. 1 shows an example of SISF dissociation. First, consider dissociation of a perfect dislocation into partials 1 and 2.

Dislocations Produced by Indentation Deformation of HPVE GaN Films

2000 ◽  
Vol 622 ◽  
Author(s):  
M. H. Hong ◽  
P. Pirouz ◽  
P. M. Tavernier ◽  
D. R. Clarke
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Plastic Zone ◽  
Room Temperature ◽  
Prism Plane ◽  
Weak Beam ◽  
Transmission Electron ◽  
Hardness Tests ◽  
Increasing Temperature ◽  
Gan Film

ABSTRACTVickers hardness tests on {0001} and (1120) faces of a relatively thick GaN film grown on a (0001) sapphire substrate have been performed in the temperature range 25-1200°C. The microstructure of the plastic zone around the indentation sites has been investigated by transmission electron microscopy (TEM). At room temperature, the hardness was measured to be 12.3 GPa on the basal plane, and 11.1 GPa on the prism plane. The hardness decreases gradually with increasing temperature up to ∼800°C and then shows a plateau between ∼800 and ∼1050°C after which it decreases again above ∼1100°C. In contrast to the rather straight dislocations produced by room-temperature indentation of the (0001) face, the dislocations generated by indenting the (1120) face at room temperature were curved and in the shape of half-loops emanating from the indentation sites on the prism planes. Such dislocations were not dissociated within the resolution of weak-beam TEM.

Tem Study of Dislocations in Plastically Deformed AlN

1992 ◽  
Vol 242 ◽  
Author(s):  
V. Audurier ◽  
J. L. Demenet ◽  
J. Rabier
Keyword(s):  
Electron Microscopy ◽  
Room Temperature ◽  
Compound Semiconductors ◽  
Burgers Vector ◽  
Weak Beam ◽  
Deformation Substructure ◽  
Transmission Electron ◽  
Doping Effects ◽  
Electronic Doping ◽  
Beam Technique

ABSTRACTAlN ceramics were plastically deformed using uniaxial compression under hydrostatic pressure between room temperature (RT) and 800°C. Deformation microstructures have been studied by Transmission Electron Microscopy (TEM) using the weak beam technique. The deformation substructure at RT is characterized by perfect glide loops with 1/3<1120> Burgers vector in (0001) elongated in the screw direction. When deformation temperature increases, the screw character is associated to cross slip events and dislocation dipolesare found. In the investigated temperature range, slip of dislocations with 1/3<1120> Burgers vector is also evidenced on prismatic planes. Weak beam observations failed to evidence any dislocation splitting. Some of these dislocation properties, similar to those of III-V compound semiconductors, suggest that electronic doping effects could be used to control plastic behaviour of covalent ceramics.

Identification of partial dislocations and faults in cubic Al3Ti

1994 ◽  
Vol 9 (3) ◽  
pp. 553-562 ◽  
Author(s):  
S. Zhang ◽  
W.W. Milligan ◽  
D.E. Mikkola
Keyword(s):  
Computer Simulation ◽  
Room Temperature ◽  
Superlattice Reflection ◽  
Bright Field ◽  
Fringe Contrast ◽  
Weak Beam ◽  
Partial Dislocations ◽  
Ti Alloys ◽  
Transmission Electron ◽  
Imaging Conditions

Dislocation dissociations in Al3Ti alloys modified with Mn to stabilize the L12 cubic structure have been studied with transmission electron microscopy and computer simulation of images. Dissociations of a〈110〉 superdislocations into pairs of a/2〈110〉 partials bounding APB's were observed at all temperatures from room temperature to 700 °C. Asymmetrical image contrast, in which one of the a/2〈110〉 partials gives a much more intense image than the other, was observed at small separations of the partials. Although some investigators have taken such asymmetry to suggest SISF-type dissociations in similar alloys, the current work demonstrates that the asymmetry is fully consistent with APB-type dissociation. Further, the degree of image asymmetry decreases as the spacing of the partials increases. It is concluded that identification of the partial dislocations with “near-invisibility criteria” for fractional values of g · b is unreliable, and that computer simulation of images is useful for identification of the partials. However, as expected, the ability to distinguish simulated bright-field images of APB- and SISF-type dissociations also becomes difficult as the separation of the partials becomes very small. Under these conditions, both weak-beam imaging and simulations are necessary to identify the dissociations. Weak-beam simulations have shown that fringe contrast must be present under certain imaging conditions for SISF dissociations, and this contrast has never been observed in this study or in previously published studies of dissociated single superdislocations in cubic Al3Ti alloys. Finally, APB contrast formed with superlattice reflection imaging has been observed between partials on both {111} and {100} after deformation at 700 °C.

scholarly journals Characterizations of Lamellar Interfaces and Segregations in a PST-TiAl Intermetallic Alloy by an Analytical Scanning Transmission Electron Microscope

2000 ◽  
Vol 652 ◽  
Author(s):  
Wei Zhao ◽  
David E. Luzzi
Keyword(s):  
High Temperatures ◽  
Titanium Aluminide ◽  
Elevated Temperatures ◽  
High Strength ◽  
Scanning Transmission Electron Microscope ◽  
Zone Method ◽  
Specific Strength ◽  
Transmission Electron ◽  
Scanning Transmission ◽  
Lamellar Interfaces

ABSTRACTPolysynthetically-twinned titanium aluminide (PST-TiAl), a fully lamellar γTiAl + α2-Ti3Al dual-phase alloy, is under evaluation for applications in rotary components in aircraft and automobile industries due to its high specific strength, and a high strength-retention capability at elevated-temperatures. However, the low ductility at room- to mid-high temperatures of the material hinders its application. Additions of certain tertiary elements to the binary TiAl system appear to improve the ductility at room- to mid-high temperatures, thus a balance among strength, ductility, and fracture toughness can be expected. In this article, segregation of tertiary elements to the lamellar interfaces is investigated. Single crystals of a TiAl with 0.6% atomic percentage tertiary additions are grown by an optical float-zone method. Segregation to the lamellar interfaces and the microstructure of the interfaces are investigated. Structures of the lamellar interfaces are characterized, and microchemistry and distribution habits of these elements along the γ+α2 lamellar boundaries as well as the γ-γ lamellar and domain boundaries are analyzed.

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