High‐temperature strength of dispersion-hardened single crystals II. Theory

1977 ◽  
Vol 35 (4) ◽  
pp. 945-962 ◽  
Author(s):  
R. S. W. Shewfelt ◽  
L. M. Brown
Keyword(s):  
High Temperature ◽  
Single Crystals ◽  
High Temperature Strength

Precipitation of NiHfsi phase in NiAl single crystals containing Hf

1995 ◽  
Vol 53 ◽  
pp. 532-533
Author(s):  
A. Garg ◽  
R. D. Noebe ◽  
R. Darolia
Keyword(s):  
High Temperature ◽  
Single Crystals ◽  
High Temperature Strength ◽  
Bridgman Technique ◽  
Shell Mold ◽  
Third Phase ◽  
Unknown Phase ◽  
Transmission Electron ◽  
Two Phases ◽  
Nial Matrix

Small additions of Hf to NiAl produce a significant increase in the high-temperature strength of single crystals. Hf has a very limited solubility in NiAl and in the presence of Si, results in a high density of G-phase (Ni16Hf6Si7) cuboidal precipitates and some G-platelets in a NiAl matrix. These precipitates have a F.C.C structure and nucleate on {100}NiAl planes with almost perfect coherency and a cube-on-cube orientation-relationship (O.R.). However, G-phase is metastable and after prolonged aging at high temperature dissolves at the expense of a more stable Heusler (β'-Ni2AlHf) phase. In addition to these two phases, a third phase was shown to be present in a NiAl-0.3at. % Hf alloy, but was not previously identified (Fig. 4 of ref. 2 ). In this work, we report the morphology, crystal-structure, O.R., and stability of this unknown phase, which were determined using conventional and analytical transmission electron microscopy (TEM).Single crystals of NiAl containing 0.5at. % Hf were grown by a Bridgman technique. Chemical analysis indicated that these crystals also contained Si, which was not an intentional alloying addition but was picked up from the shell mold during directional solidification.

TEM studies of dislocations in deformed Ga1-xInxAs single crystals

1988 ◽  
Vol 46 ◽  
pp. 900-901
Author(s):  
R. S. Rai ◽  
S. Guruswamy ◽  
K. T. Faber ◽  
J. P. Hirth
Keyword(s):  
High Temperature ◽  
Dislocation Density ◽  
Single Crystals ◽  
Dark Field ◽  
High Temperature Deformation ◽  
Temperature Deformation ◽  
High Temperature Strength ◽  
Density Reduction ◽  
Transmission Electron

The perfection of GaAs single crystals can be controlled by doping the GaAs with In, at a level of about 5x1019-1x1020/cm3, in single crystals grown by the LEC process. It has been observed that In doping at this level reduces the grown in dislocation density from 104-l05 to ≤ 102/cm2 and results in a large increase in high temperature strength . However, the role of In in dislocation density reduction is not clearly understood. Therefore, a systematic study has been performed with the help of high temperature deformation of In-doped and undoped GaAs single crystals followed by dislocation structural characterization by transmission electron microscopy of the deformed specimens. Here, some results of dislocation studies performed by TEM are descri bed.Samples were examined in a JEOL JEM 200CX transmission electron microscope equipped with a double tilt goniometer stage. The standard g.b criterion was employed for characterization of dislocations. Dark-field weak beam pictures were taken for characterization of partial dislocations and dipoles.

Stacking faults in deformed α-silicon nitride single crystals

1993 ◽  
Vol 51 ◽  
pp. 916-917
Author(s):  
H. Suematsu ◽  
J. J. Petrovic ◽  
T. E. Mitchell
Keyword(s):  
High Temperature ◽  
Silicon Nitride ◽  
Single Crystals ◽  
Stacking Faults ◽  
High Strength ◽  
Secondary Phase ◽  
High Resolution Electron Microscopy ◽  
Diffusional Creep ◽  
Dislocation Slip ◽  
High Temperature Strength

Silicon nitride(Si3N4) is well known for its high toughness and strength. This is the reason why it is selected for ceramic turbo charger rotors in automobile engines. However, the high strength of most sintered Si3N4 products drops above 1200°C because sintering aids like Y2O3 and MgO are required which form glassy phases with low melting points on the grain boundaries. This secondary phase degrades the high temperature characteristics of Si3N4. In order to overcome this deficiency, much work has been reported which aims at crystallizing or removing the glassy phase. If this aim could be successful, resulting in an increase in high temperature strength, other processes would determine the high temperature performance of Si3N4, such as diffusional creep and dislocation slip. Line and planar defects in Si3N4 play an important role in such the processes particularly in slip, however, available knowledge about them is limited. In the present work, stacking faults in deformed Si3N4 single crystals are investigated using high resolution electron microscopy(HREM).

Influence of Thermo-Mechanical Processing on the Microstructure of Beryllia Dispersed Nickel Alloys

1973 ◽  
Vol 31 ◽  
pp. 184-185
Author(s):  
M.S. Grewal ◽  
S.A. Sastri ◽  
N.J. Grant
Keyword(s):  
High Temperature ◽  
Nickel Alloys ◽  
Thermomechanical Processing ◽  
Cold Work ◽  
High Temperature Strength ◽  
Strengthening Effect ◽  
Mechanical Processing ◽  
Uniform Dispersion ◽  
Oxide Particles ◽  
Nickel Base

Currently there is a great interest in developing nickel base alloys with fine and uniform dispersion of stable oxide particles, for high temperature applications. It is well known that the high temperature strength and stability of an oxide dispersed alloy can be greatly improved by appropriate thermomechanical processing, but the mechanism of this strengthening effect is not well understood. This investigation was undertaken to study the dislocation substructures formed in beryllia dispersed nickel alloys as a function of cold work both with and without intermediate anneals. Two alloys, one Ni-lv/oBeo and other Ni-4.5Mo-30Co-2v/oBeo were investigated. The influence of the substructures produced by Thermo-Mechanical Processing (TMP) on the high temperature creep properties of these alloys was also evaluated.

Application of TEM to Deformation, Wear, and Fracture of Ceramics

1974 ◽  
Vol 32 ◽  
pp. 472-473
Author(s):  
B. J. Hockey
Keyword(s):  
Wear Resistance ◽  
High Temperature ◽  
Mechanical Behavior ◽  
Brittle Materials ◽  
High Temperature Strength ◽  
Wide Range ◽  
Corrosive Environments ◽  
Further Development

Ceramics, such as Al2O3 and SiC have numerous current and potential uses in applications where high temperature strength, hardness, and wear resistance are required often in corrosive environments. These materials are, however, highly anisotropic and brittle, so that their mechanical behavior is often unpredictable. The further development of these materials will require a better understanding of the basic mechanisms controlling deformation, wear, and fracture.The purpose of this talk is to describe applications of TEM to the study of the deformation, wear, and fracture of Al2O3. Similar studies are currently being conducted on SiC and the techniques involved should be applicable to a wide range of hard, brittle materials.

Lattice Imaging of Grain Boundaries in Ceramics

1977 ◽  
Vol 35 ◽  
pp. 114-115
Author(s):  
D. R. Clarke ◽  
G. Thomas
Keyword(s):  
Mechanical Properties ◽  
High Temperature ◽  
Silicon Nitride ◽  
Grain Boundaries ◽  
High Temperature Strength ◽  
Current Interest ◽  
Lattice Imaging ◽  
Special Significance ◽  
Structural Applications ◽  
Refractory Ceramics

Grain boundaries have long held a special significance to ceramicists. In part, this has been because it has been impossible until now to actually observe the boundaries themselves. Just as important, however, is the fact that the grain boundaries and their environs have a determing influence on both the mechanisms by which powder compaction occurs during fabrication, and on the overall mechanical properties of the material. One area where the grain boundary plays a particularly important role is in the high temperature strength of hot-pressed ceramics. This is a subject of current interest as extensive efforts are being made to develop ceramics, such as silicon nitride alloys, for high temperature structural applications. In this presentation we describe how the techniques of lattice fringe imaging have made it possible to study the grain boundaries in a number of refractory ceramics, and illustrate some of the findings.

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