Structural Stability of Nanocrystalline NiAl

1998 ◽  
Vol 4 (S2) ◽  
pp. 720-721
Author(s):  
T. Chen ◽  
J.M. Hampikian ◽  
N.N. Thadhani ◽  
Z.L. Wang
Keyword(s):  
High Temperature ◽  
Structural Stability ◽  
Carbon Film ◽  
Ion Milling ◽  
Plate Impact ◽  
High Melting Point ◽  
Temperature Oxidation ◽  
Transmission Electron ◽  
High Temperature Structural Material

NiAl is an important high temperature structural material, with a high melting point (1640°C), low density and excellent high temperature oxidation resistance. The room temperature ductility of NiAl may potentially be improved with the use of nanocrystalline grain size. However, a key question concerning the application of nanostructured NiAl is about its structural stability at high temperature. The current study is thus focused on the investigation of the structural stability of nanocrystalline NiAl using in-situ transmission electron microscopy (TEM) and differential thermal analysis (DTA).Nanocrystalline B2-NiAl was prepared by ball milling (24 hrs) from elemental Ni and Al powders. Subsequent consolidation into bulk form was performed using dynamic consolidation employing a 3-capsule plate-impact fixture at approximately 400 m/s [1-3]. Powder nanocrystalline NiAl was dispersed on a holey carbon film for TEM observation. TEM specimens of shock compacted bulk NiAl nanocrystals were prepared by cutting, polishing, dimpling and ion milling.

Grain Boundaries and Interfaces in MoSi2 and MoSi2-SiC Composites and Their Effect on High Temperature Strength

2000 ◽  
Vol 6 (S2) ◽  
pp. 424-425
Author(s):  
R. Mitra ◽  
W.-A. Chiou ◽  
A. Venugopal Rao
Keyword(s):  
High Temperature ◽  
Grain Boundaries ◽  
Weight Fraction ◽  
High Temperature Strength ◽  
High Melting Point ◽  
Temperature Oxidation ◽  
Structural Applications ◽  
Temperature Fracture ◽  
Sic Composites

Molybdenum di-silicides (MoSi2) based materials have a strong potential for high temperature structural applications due to high melting point of 2030°C, outstanding elevated temperature oxidation resistance and limited ductility above a temperature range of 1100-1300°C. The major shortcomings of MoSi2 for structural applications are its poor room temperature fracture toughness and low high temperature strength. Sustained efforts including reinforcing MoSi2 with ceramic reinforcements, alloying and in-situ processing, have been made to improve these properties. The purity of grain boundaries and interfaces, which in turn depends on the processing method plays a significant role in the high temperature properties and this paper aims to show that.Intimately mixed Mo and Si powders (Mo:Si = 63:37 by weight fraction) were reaction hot pressed (“RHP“) in vacuum at 1500°C for 1 h, using a pressure of 26 MPa. During the hot pressing process, Mo and Si reacted to form MoSi2.

High-temperature oxidation of CoGa: Influence of the crystallographic orientation on the oxidation rate

2002 ◽  
Vol 17 (10) ◽  
pp. 2489-2498 ◽  
Author(s):  
U. Koops ◽  
D. Hesse ◽  
M. Martin
Keyword(s):  
High Temperature ◽  
Crystallographic Orientation ◽  
High Temperature Oxidation ◽  
Elevated Temperatures ◽  
Electron Backscatter Diffraction ◽  
Ex Situ ◽  
Temperature Oxidation ◽  
Transmission Electron ◽  
Backscatter Diffraction

The crystallographic orientation plays an important role in high-temperature oxidation of the intermetallic compound CoGa. When CoGa is exposed to air at elevated temperatures, the oxide β–Ga2O3 is formed, and different scale growth rates are observed, depending on the crystallographic orientation of the CoGa grains. This dependence is a consequence of the anisotropy of the gallium diffusion rate through the β–Ga2O3 scale and of a topotaxial orientation relationship occurring between β–Ga2O3 and CoGa. The combination of ex situ techniques, such as transmission electron microscopy and electron backscatter diffraction with optical microscopy, applied in situ resulted in a thorough understanding of these relations and of the oxidation process in general.

Microstructural Study of Diamond Deformed Under High Pressure and Temperature

1985 ◽  
Vol 43 ◽  
pp. 230-231
Author(s):  
E. F. Koch
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Lattice Structure ◽  
Diamond Particle ◽  
Polycrystalline Diamond ◽  
Sintering Process ◽  
Ion Milling ◽  
Sintered Compact ◽  
Transmission Electron ◽  
High Pressure Sintering

Because of the extremely rigid lattice structure of diamond, generating new dislocations or moving existing dislocations in diamond by applying mechanical stress at ambient temperature is very difficult. Analysis of portions of diamonds deformed under bending stress at elevated temperature has shown that diamond deforms plastically under suitable conditions and that its primary slip systems are on the ﹛111﹜ planes. Plastic deformation in diamond is more commonly observed during the high temperature - high pressure sintering process used to make diamond compacts. The pressure and temperature conditions in the sintering presses are sufficiently high that many diamond grains in the sintered compact show deformed microtructures.In this report commercially available polycrystalline diamond discs for rock cutting applications were analyzed to study the deformation substructures in the diamond grains using transmission electron microscopy. An individual diamond particle can be plastically deformed in a high pressure apparatus at high temperature, but it is nearly impossible to prepare such a particle for TEM observation, since any medium in which the diamond is mounted wears away faster than the diamond during ion milling and the diamond is lost.

Transmission Electron Microscopy Studies of Aluminum Oxidation

1985 ◽  
Vol 43 ◽  
pp. 268-269
Author(s):  
J.Y. Lee
Keyword(s):  
Nucleation And Growth ◽  
Ion Milling ◽  
Cross Sectional ◽  
Polycrystalline Aluminum ◽  
Axis Orientation ◽  
Temperature Oxidation ◽  
Aluminum Oxidation ◽  
Transmission Electron ◽  
High Resolution Images ◽  
Argon Ion

In the oxidation of metals and alloys, microstructural features at the atomic level play an important role in the nucleation and growth of the oxide, but little is known about the atomic mechanisms of high temperature oxidation. The present paper describes current progress on crystallographic aspects of aluminum oxidation. The 99.999% pure, polycrystalline aluminum was chemically polished and oxidized in 1 atm air at either 550°C or 600°C for times from 0.5 hr to 4 weeks. Cross-sectional specimens were prepared by forming a sandwich with epoxy, followed by mechanical polishing and then argon ion milling. High resolution images were recorded in a <110>oxide zone-axis orientation with a JE0L JEM 200CX microscope operated at 200 keV.

In Situ High-Temperature Transmission Electron Microscopy Observations of the Formation of Nanocrystalline TiC from Nanocrystalline Anatase (TiO2)

1998 ◽  
Vol 4 (3) ◽  
pp. 269-277 ◽  
Author(s):  
A. Agrawal ◽  
J. Cizeron ◽  
V.L. Colvin
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
High Temperature ◽  
Solid Phase ◽  
Anatase Phase ◽  
High Resolution Electron Microscopy ◽  
Rutile Phase ◽  
Transmission Electron ◽  
New Phase

In this work, the high-temperature behavior of nanocrystalline TiO2 is studied using in situ transmission electron microscopy (TEM). These nanoparticles are made using wet chemical techniques that generate the anatase phase of TiO2 with average grain sizes of 6 nm. X-ray diffraction studies of nanophase TiO2 indicate the material undergoes a solid-solid phase transformation to the stable rutile phase between 600° and 900°C. This phase transition is not observed in the TEM samples, which remain anatase up to temperatures as high as 1000°C. Above 1000°C, nanoparticles become mobile on the amorphous carbon grid and by 1300°C, all anatase diffraction is lost and larger (50 nm) single crystals of a new phase are present. This new phase is identified as TiC both from high-resolution electron microscopy after heat treatment and electron diffraction collected during in situ heating experiments. Video images of the particle motion in situ show the nanoparticles diffusing and interacting with the underlying grid material as the reaction from TiO2 to TiC proceeds.

Motion of Crystal/Crystal and Crystal/Amorphous Interfaces

1990 ◽  
Vol 183 ◽  
Author(s):  
J. L. Batstone
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
High Temperature ◽  
Grain Boundary ◽  
Boundary Motion ◽  
High Temperature Annealing ◽  
Thermally Activated ◽  
Transmission Electron ◽  
Grain Boundary Motion

AbstractMotion of ordered twin/matrix interfaces in films of silicon on sapphire occurs during high temperature annealing. This process is shown to be thermally activated and is analogous to grain boundary motion. Motion of amorphous/crystalline interfaces occurs during recrystallization of CoSi2 and NiSi2 from the amorphous phase. In-situ transmission electron microscopy has revealed details of the growth kinetics and interfacial roughness.

Manganese Effect on Isothermal High Temperature Oxidation Behaviour of AISI 304 Stainless Steel

2008 ◽  
Vol 595-598 ◽  
pp. 1127-1134 ◽  
Author(s):  
Frédéric Riffard ◽  
Henri Buscail ◽  
F. Rabaste ◽  
Eric Caudron ◽  
Régis Cueff ◽  
...  
Keyword(s):  
Stainless Steel ◽  
High Temperature ◽  
Oxide Scale ◽  
304 Stainless Steel ◽  
Initial Growth ◽  
Aisi 304 Stainless Steel ◽  
Protective Oxide ◽  
Temperature Oxidation ◽  
Aisi 304

Chromia-forming steels are excellent candidates to resist to high temperature oxidizing atmospheres because they form protective oxide scales. The oxide scale growth mechanisms are studied by exposing AISI 304 stainless steel to high temperature conditions in air, and the analyses were carried out by means of thermogravimetry and in situ X-rays diffraction. The in situ XRD analyses carried out during high temperature AISI 304 steel oxidation in air reveals the accelerated growth of iron-containing oxides such as hematite Fe2O3 and iron-chromite FeCr2O4, when the initial germination of the oxide layer contains the presence of a manganese-containing spinel compound (1000°C). When the initial growth shows the only chromia formation (800°C), hematite formation appears differed in time. Protection against corrosion is thus increased when the initial germination of manganese-containing spinel oxide is inhibited in the oxide scale.

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