Characterization of the Phase Evolution of Mosi2 - TiB2 Composites Produced by In-Situ Reactions Using Scanning Electron Microscopy (SEM), Electron Probe Microanalysis (EPMA), and X-Ray Diffraction (XRD)

2000 ◽  
Vol 6 (S2) ◽  
pp. 374-375
Author(s):  
L. A. Dempere ◽  
M. J. Kaufman
Keyword(s):  
High Temperature ◽  
Elevated Temperatures ◽  
Mechanical Performance ◽  
Molybdenum Disilicide ◽  
Phase Evolution ◽  
Secondary Phases ◽  
Temperature Oxidation ◽  
Structural Applications ◽  
In Situ Reactions

Intermetallics are playing an important role in the development of new materials able of sustaining the escalating demands of the aerospace industry. A significant improvement in weight, operating temperatures or mechanical performance is required for materials to be considered as replacements in the most demanding applications. Molybdenum disilicide is one such compound that has potential for high temperature applications. Its most attractive properties are its high melting point (2020°C), reasonable density (6.24 g/cm3), and excellent high temperature oxidation and corrosion resistance. However, low ambient fracture toughness and loss of strength at elevated temperatures have been the most significant limitations to the use of MoSi2 in structural applications.The more promising solutions for improving the mechanical properties of brittle intermetallics such as MoSi2 are based on the incorporation and control of secondary phases. To date, the artificial introduction of reinforcing phases or their generation via in-situ reactions have been explored.

Phase transformation and layer evolution in molybdenum disilicide-silicon carbide nanolayered coatings

1994 ◽  
Vol 52 ◽  
pp. 538-539
Author(s):  
H. Kung ◽  
T. R. Jervis ◽  
J.-P. Hirvonen ◽  
M. Nastasi ◽  
T. E. Mitchell ◽  
...  
Keyword(s):  
Phase Transformation ◽  
High Temperature ◽  
Oxidation Resistance ◽  
Elevated Temperatures ◽  
Matrix Material ◽  
Molybdenum Disilicide ◽  
High Temperature Strength ◽  
Cross Sectional ◽  
Good Oxidation Resistance ◽  
Structural Applications

MoSi2 is a potential matrix material for high temperature structural composites due to its high melting temperature and good oxidation resistance at elevated temperatures. The two major drawbacksfor structural applications are inadequate high temperature strength and poor low temperature ductility. The search for appropriate composite additions has been the focus of extensive investigations in recent years. The addition of SiC in a nanolayered configuration was shown to exhibit superior oxidation resistance and significant hardness increase through annealing at 500°C. One potential application of MoSi2- SiC multilayers is for high temperature coatings, where structural stability ofthe layering is of major concern. In this study, we have systematically investigated both the evolution of phases and the stability of layers by varying the heat treating conditions.Alternating layers of MoSi2 and SiC were synthesized by DC-magnetron and rf-diode sputtering respectively. Cross-sectional transmission electron microscopy (XTEM) was used to examine three distinct reactions in the specimens when exposed to different annealing conditions: crystallization and phase transformation of MoSi2, crystallization of SiC, and spheroidization of the layer structures.

scholarly journals Molybdenum Disilicide Composites Produced by Plasma Spraying

1998 ◽  
Author(s):  
R.G. Castro ◽  
H. Kung ◽  
K.J. Hollis ◽  
A.H. Bartlett
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Plasma Spraying ◽  
Creep Resistance ◽  
Elevated Temperatures ◽  
Powder Processing ◽  
National Laboratory ◽  
Molybdenum Disilicide ◽  
High Temperature Strength ◽  
Structural Applications

Abstract The intermetallic compound, molybdenum disilicide (MoSi2) is being considered for high temperature structural applications because of its high melting point and superior oxidation resistance at elevated temperatures. The lack of high temperature strength, creep resistance and low temperature ductility has hindered its progress for structural applications. Plasma spraying of coatings and structural components of MoSi2-based composites offers an exciting processing alternative to conventional powder processing methods due to superior flexibility and the ability to tailor properties. Laminate, discontinuous and in situ reinforced composites have been produced with secondary reinforcements of Ta, A1203, SiC, Si3N4 and Mo5Si3. Laminate composites, in particular, have been shown to improve the damage tolerance of MoSi2 during high temperature melting operations. A review of research which as been performed at Los Alamos National Laboratory on plasma spraying of MoSi2-based composites to improve low temperature fracture toughness, thermal shock resistance, high temperature strength and creep resistance will be discussed.

In situ characterisation of nanostructured multiphase thermoelectric materials at elevated temperatures

2016 ◽  
Vol 18 (48) ◽  
pp. 32814-32819 ◽  
Author(s):  
S. Aminorroaya Yamini ◽  
D. R. G. Mitchell ◽  
M. Avdeev
Keyword(s):  
High Temperature ◽  
Thermoelectric Materials ◽  
Single Phase ◽  
Elevated Temperatures ◽  
Chemical Interaction ◽  
Secondary Phases

Multiphase thermoelectric materials exhibit higher efficiencies than their single-phase counterparts. Here, we performed in situ high temperature structural characterisations and shown a strong chemical interaction between secondary phases and matrices at elevated temperatures.

Ionic Transport under Chemical Potential Gradients - Kinetic Demixing and Decomposition in Metal Oxides and Scale Growth in High-Temperature Oxidation

2011 ◽  
Vol 696 ◽  
pp. 28-33
Author(s):  
Manfred Martin
Keyword(s):  
High Temperature ◽  
High Temperature Oxidation ◽  
Elevated Temperatures ◽  
Chemical Potential ◽  
Transport Processes ◽  
Temperature Oxidation ◽  
X Ray ◽  
Kinetic Decomposition ◽  
Potential Gradients

In oxides which are exposed to thermodynamic potential gradients, transport processes of the mobile components occur. These transport processes and the coupling between different processes are not only of fundamental interest, but are also the origin of degradation processes, such as kinetic demixing, kinetic decomposition, and changes in the morphology of the material. The kinetics of high temperature oxidation processes of metals can be studiedin situby X-ray absorption spectroscopy (XAS), optical microscopy and X-ray diffraction (XRD) at elevated temperatures and defined oxygen partial pressures. As an example, thein situXAS investigation of the oxidation of cobalt, forming layers of CoO and Co3O4, will be discussed.

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.

High Thermo-Mechanical Stability in Polybenzoxazine Aerogels

2019 ◽  
Author(s):  
Sadeq Malakooti ◽  
Guoqiang Qin ◽  
Chandana Mandal ◽  
Chariklia Sotiriou-Leventis ◽  
Nicholas Leventis ◽  
...  
Keyword(s):  
Glass Transition ◽  
High Temperature ◽  
Elevated Temperatures ◽  
Mechanical Performance ◽  
Mechanical Stability ◽  
Three Dimensional ◽  
High Porosity ◽  
Transition Temperatures ◽  
Glass Transition Temperatures ◽  
Structural Applications

Abstract Aerogels are three-dimensional networks of nanoparticles with high specific surface area and high porosity. Following the significant improvement on the mechanical strengths and ductility of traditional aerogels with polymer cross-linking (i.e., X-aerogels), the emergence of pure polymeric aerogels has enabled unprecedented aerogel applications such as ballistic armor protection, which is quite surprising for such low-density materials. However, generally low glass transition temperatures (Tg) of polymeric aerogels hinder their structural applicability at service temperatures above their Tg temperatures. Thereby, developing novel polymeric aerogels with high Tg temperatures is crucial for high-temperature structural applications. As phenolic resins, polybenzoxazines are heat-resistant and mechanically strong with high glass transition temperatures. In this study, polybenzoxazine aerogels have been successfully synthesized, and their mechanical properties at different densities and elevated temperatures have been investigated. High thermo-mechanical stability has been observed over the entire temperature range of interest (i.e., below 250 °C) for their quasi-static compressive properties such as Young’s modulus and compressive strength. Moreover, the storage and loss moduli in shear of the aerogels have been studied at different temperatures and frequencies. The strong mechanical performance of these aerogels at elevated temperatures makes them an important, inexpensive, and advanced material for high-temperature applications, competitive with significantly more expensive polyimides.

The influence of nitrogen on the stability of nanophase molybdenum disilicide

1995 ◽  
Vol 53 ◽  
pp. 190-191
Author(s):  
H. Kung ◽  
T. R. Jervis ◽  
J-P Hirvonen ◽  
T. E. Mitchell ◽  
M. Nastasi
Keyword(s):  
High Temperature ◽  
Molybdenum Disilicide ◽  
Potential Candidate ◽  
Temperature Oxidation ◽  
Annealing Conditions ◽  
Dc Power ◽  
Structural Applications ◽  
The Stability ◽  
Nitrogen Contents

The intermetallic compound, MoSi2, has a combination of interesting properties, ranging from a high melting point to superior high temperature oxidation and corrosion resistance, which makes it a potential candidate for high temperature structural applications. It was shown previously that the addition of nitrogen into MoSi2 forms MoSi2Nx (x:3-4). The new phase has a very high crystallization temperature (> 1000°C) and is thermally compatible with both Mo and MoSi2. As suggested by these results, the introduction of nitrogen into MoSi2 may cause significant changes in its properties, yet the role of nitrogen in influencing the structure and phase stability of MoSi2 is still not known. In this study, we have systematically investigated the evolution of structure in MoSi2 in the presence of different nitrogen contents and annealing conditions.The system investigated is single phase MoSi2Nx. Films were prepared by sputter deposition using a planar magnetron MoSi2 target at a DC power of 100-200 W.

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