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.

Microstructural Instability in Mosi2/Sic Nanolayers

1997 ◽  
Vol 3 (S2) ◽  
pp. 399-400
Author(s):  
Y.C. Lu ◽  
H. Kung ◽  
J-P Hirvonen ◽  
T.R. Jervis ◽  
M. Nastasi ◽  
...  
Keyword(s):  
High Temperature ◽  
Ion Irradiation ◽  
High Temperature Strength ◽  
Second Phase ◽  
Microstructural Stability ◽  
Factors Affecting ◽  
Structural Applications ◽  
Thermal Compatibility ◽  
Two Phases ◽  
The Stability

Thin film multilayers have been the focus of extensive studies recently due to the interesting properties they exhibit. Since the improvement in properties can be attributed directly to the unique nanoscale microstructures, it is essential to understand the factors affecting the microstructural stability in these nanolayer structures. The intermetallic compound, MoSi2, despite its superior oxidation resistance and high melting point, suffers from inadequate high temperature strength and low temperature ductility, properties which hinder its high temperature structural applications [1]. SiC is a potential second phase reinforcement due to its high temperature strength and thermal compatibility with MoSi2. The addition of SiC in a nanolayered configuration has been shown to exhibit significant increase in hardness after annealing [2]. It has also been shown that when annealed above 900°C, the layers break down and grain growth sets in, with a significant decrease in hardness and. Due to the lack of a thermochemical driving force, the two phases remain separate at all temperatures investigated. In this study, the stability of the MoSi2/SiC nanolayers structure under ion irradiation has been investigated.

Microstructure and Properties of Directionally Solidified NbSi2/ Nb5Si3 Composites

2005 ◽  
Vol 475-479 ◽  
pp. 733-736 ◽  
Author(s):  
Wei Li ◽  
Hai Bo Yang ◽  
Ai Dang Shan ◽  
Lan Ting Zhang ◽  
Jian Sheng Wu
Keyword(s):  
High Temperature ◽  
High Temperature Strength ◽  
Directionally Solidified ◽  
Zone Method ◽  
Phase Constitution ◽  
Good Oxidation Resistance ◽  
Transmission Electron ◽  
Complex Crystal Structure ◽  
Electron Microscopes ◽  
Scanning Electron Microscopes

NbSi2 is an attractive material for high temperature applications due to its high melting point, low density and good oxidation resistance. The high-temperature strength of NbSi2 is expected to be further improved by incorporation with Nb5Si3, which performs a high creep resistance and strength at high temperature due to its complex crystal structure. In this paper, directionally solidified NbSi2/ Nb5Si3 in-situ composites have been prepared using an optical floating zone method. Scanning Electron Microscopes (SEM) and X-ray diffraction (XRD) have been used to investigate the phase constitution and microstructure. The orientation relationship between Nb5Si3 and NbSi2 is investigated by transmission electron microscopy (TEM). High-temperature properties of alloys are tested by compression at the strain rate of 1×10-4/s at 1673K and 1773K. It was found that high temperature strength and phase constitution of directionally solidified alloys depended on the addition of Mo.

The Structure and Properties of MoSi2-Mo5Si3 Composites

1992 ◽  
Vol 273 ◽  
Author(s):  
H. Kung ◽  
D. P. Mason ◽  
A. Basu ◽  
H. Chang ◽  
D. C. Van Aken ◽  
...  
Keyword(s):  
Electron Microscopy ◽  
Mechanical Properties ◽  
Transmission Electron Microscopy ◽  
High Temperature ◽  
Second Phase ◽  
Structure And Properties ◽  
Transmission Electron ◽  
Interfacial Structures ◽  
Two Phases ◽  
Hardness Values

ABSTRACTThe addition of Mo5Si3 as a reinforcing second phase in a MoSi2 matrix has been investigated for possible high temperature strengthening effects. MoSi2 with up to 45 vol % Mo5Si3 was fabricated using powder metallurgy (PM) and arc-casting (AC) techniques. Effects of processing routes, which result in different microstructures, on their mechanical properties are given. PM composites, which have an equiaxed microstructure, exhibit a limited increase in hardness. Higher hardnesses are observed in script-structured AC eutectics and Er-modifiedeutectics throughout the temperatures studied (25–1300°C). Crack propagation paths induced by indentation show long transphase cracks in the AC materials vs short intergranular and interphase cracks in the PM composites at high temperatures.Transmission electron microscopy discloses that the interface in the AC composites has a low-index orientation relationship between the two phases and shows regularly faceted interfacial structures, while planar interfaces are found in the PM composites. These observations suggest the interface is stronger and lower in energy in the AC composites, which is consistent with the higher hardness values and long transphase cracks observed.Dislocation analysis shows the presence of ordinary dislocations (<100>, <110> and 1/2<111>) in MoSi2 in the as-fabricated composites. These types of dislocation are also responsible for the high temperature plastic deformation in compression in both the monolithic MoSi2 and the composites. <331> types of dislocation are only found in MoSi2 either near the interface of the AC composites or in materials deformed below 1000°C.

Phase Decomposition and Deformation of L12-Ordered Co-Rich Co3Ti

1994 ◽  
Vol 364 ◽  
Author(s):  
M. Nemoto ◽  
W. H. Tian ◽  
K. Hayashi
Keyword(s):  
Electron Microscope ◽  
High Temperature ◽  
Annealing Temperature ◽  
High Temperature Strength ◽  
Phase Decomposition ◽  
Solution Annealing ◽  
Rich Phase ◽  
Transmission Electron ◽  
The Matrix ◽  
Matrix Lattice

AbstractThe Co3Ti phase hardens appreciably by the fine precipitation of disordered fee Co-rich phase upon aging after quenching from solution annealing temperature. Transmission electron microscope(TEM) observations revealed that the precipitates are platelet in shape, lying nearly parallel to the {100} planes of the L12-ordered matrix, and perfectly coherent with the matrix lattice at the beginning of aging. The high temperature strength increases appreciably with the fine precipitation of disordered Co-rich phase over the whole temperature range investigated. TEM observations of the underaged and deformed alloys revealed that superdislocations are pinned by precipitates indicating an attractive interaction between dislocations and precipitates. At the overaged state, thin twins are introduced in the fee Co-rich precipitates during deformation.

Dislocation Morphologies in TiB2/NiAl

1990 ◽  
Vol 194 ◽  
Author(s):  
L. Wang ◽  
R. J. Arsenault
Keyword(s):  
High Temperature ◽  
Dislocation Density ◽  
Nickel Aluminide ◽  
Volume Fraction ◽  
High Temperature Strength ◽  
Matrix Composites ◽  
Annealed Condition ◽  
Low Dislocation Density ◽  
Particulate Form ◽  
Nial Matrix

AbstractThe addition of 20 volume percent titanium diboride in particulate form (1-3 μm) to nickel aluminide (TiB2/NiAl) results in a twofold increase in the high temperature strength of NiAl. Theories that have been proposed to account for the high temperature strength of discontinuous reinforced metal matrix composites can not be adequately used as a basis to explain the observed strengthening.An investigation was undertaken of NiAl, 10 V% TiB2/NiAl and 20 V% TiB2/NiAl in the annealed condition and after deformation, allowed to cool slowly. There is a low dislocation density in the annealed samples and the dislocation density did increase slightly as a result of deformation. However, deformation did produce some intriguing dislocation arrangements; for example, it was found that there was a high dislocation density within the TiB2 in the deformed higher volume fraction composites and the dislocation density within NiAl matrix was not uniform.

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).

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