Disilicide Powders Research Articles

Effects of nickel on sintering of molybdenum disilicide powder : I.H.Moon et al. (Hanyang University, Korea.)

Effects of nickel on sintering of molybdenum disilicide powder : I.H.Moon et al. (Hanyang University, Korea.)

Phase formation in molybdenum disilicide powders during in-flight induction plasma treatment

In-flight modification of MoSi2 powders has been carried out by using an Ar–H2 induction plasma. Reactor pressure, powder feed rate, and plate power level were taken as the experimental parameters to alter the thermal history of the injected powder particles. Metastable hexagonal structure of β–MoSi2 is the major phase observed in the Ar–H2 induction plasma-treated molybdenum disilicide powders, while the stable phase of tetragonal structure of α–MoSi2 usually retains no less than 30 wt. %. Depending on the experimental condition and the deviation from stoichiometry in raw materials, low silicides, Mo5Si3 and Mo3Si, and free Si were observed,

Formation of Nanostructural Materials Induced by Mechanical Processings (<I>Overview</I>)

Mechanical alloying (MA) was firstly developed to synthesize metallic matrix composite by mechanically incorporating preformed oxide and or carbide particles into a metallic matrix. A consecutive compaction process is applied to obtain bulk materials. During MA, powders are repeatedly welded, fractured and rewelded in a high energy mill leading to an intimate mixing on a nano/micro-scale with the possible formation of far from equilibrium phases. The versatility of MA is well known ; high volume, low energy mills can be used to commercially produced dispersion strengthened Al, Ni and other transition metal alloys. Various intermetallics and inorganic compounds (amorphous and/or nanocrystalline) have been synthesized by using higher energy mills which have been specially developed in some cases. Mechanical alloying, it appears, as suggested by T. H. Courtney et al., is the Alladin's lamp of powder processing. All the published works have shown that the reaction and end products of the MA process strongly depend on the milling conditions. As a consequence, it is obvious that an improved understanding of the dynamics of MA process is required to gain a full appreciation of the industrial potential of the technique for synthesizing materials. Recently, M. Abdellaoui and E. Gaffet have shown that the crystal to amorphous phase transition (at least in the case of the model Ni 10 Zr 7 ) only depends on the injected mechanically power, allowing a direct comparison among experiments performed using distinct type of milling apparatus (planetary milling machine, horizontal apparatus). An alternative method has been recently proposed by N. Malhouroux-Gaffet and E. Gaffet, for the solid state synthesis of disilicide powders exhibiting a wide contamination during the direct MA preparation : the mechanically activated annealing process (M2AP). Such a M2AP method has been applied to the synthesis of FeSi 2 , MoSi 2 , WSi 2 compounds. Such a method appears as being a well suitable one for the low temperature synthesis of refractory nanomaterials. Recent applications have been successfully performed to mechanically activated sintering (MAS).

Consolidation of molybdenum disilicide based materials by hot isostatic pressing (HIP): comparison with models

Monolithic molybdenum disilicide (MoSi 2) powder and MoSi 2 powders blended with ductile and brittle reinforcements were consolidated by hot isostatic pressing (HIP). The extent of densification of the consolidated samples as a function of temperature, pressure and time was determined by precision density measurements. HIP diagrams were constructed based on theoretical models. Material properties, required as input for the HIP map software, were compiled or extrapolated from published literature, experimentally determined and in some cases were estimated from the rule of mixtures. The model predictions were compared with experimental data and dominant mechanisms of densification were identified.

Determination of impurities in zirconium disilicide and zirconium diboride by inductively coupled plasma atomic emission spectrometry

A sensitive method is described for the determination of impurities (Al, Ca, Co, Cr, Fe, Hf, Mg, Mn, Ni, Sr, Ti, V, W, and Y) in zirconium disilicide (ZrSi2) and zirconium diboride (ZrB2) by inductively coupled plasma atomic emission spectrometry after acid decomposition under pressure. A zirconium disilicide powder sample was decomposed with sulfuric acid in a poly(tetrafluoroethylene)(PTFE) pressure vessel after hydrofluoric acid and sulfuric acid treatment. A zirconium diboride powder sample was also decomposed with sulfuric acid, nitric acid, and hydrofluoric acid in a PTFE pressure vessel after predecomposition with the sulfuric and nitric acids. The matrix effects on the background levels and the emission intensities of the elements were compensated by using matrix-matched standard solutions for calibration. Fourteen elements in commercial ZrSi2 and ZrB2 powder samples were determined with good precision and the results obtained by the proposed methods were found to be in good agreement with those obtained by a fusion method with sodium carbonate.

The Effects of Fe, Cu And Al Impurities on the Oxidation Resistance of TiSi2—III. Al

AbstractThe effect of aluminum on the oxidation resistance of titanium disilicide (TiSi2) was investigated. The aluminum was mixed with titanium disilicide powder, compacted and then sintered. The compacts were oxidized at elevated temperatures for various times. The oxide coatings were examined by thin film diffractometry and also in a scanning electron microscope equipped with energy dispersive spectroscopy.The oxidation of TiSi2 containing aluminum resulted in a porous, non-protective coating of mullite at temperatures of 1000°C and below. However, pre-oxidation at higher temperatures produced a more dense, protective mullite which showed adequate resistance to oxidation at the lower temperatures. Parabolic kinetics with an activation energy of 20 kcal mol−1 were observed. However, intergranular oxidation of an aluminum-rich phase resulted in the severe degradation of the material's strength. Resume L'effect de l'aluminium sur la resistance a l'oxydation du TiSi2 a ete etudie. L'aluminium est melange d...

Electrochemical properties of composite materials based on graphite and vanadium disilicide

Electrodes with different composition were prepared by the method of pressing followed by sintering of the graphite and vanadium disilicide powders, and their electrochemical and corrosion properties in water solutions of electrolytes were studied.

Creep in the hot pressing of powders of titanium nitride, molybdenum disilicide, and zirconium and chromium diborides

1. An analysis is made, using an equation of the densification of a porous solid incorporating a steadystate creep equation, of data yielded by an investigation into the effects of time, temperature, and applied pressure upon the relative density of titanium nitride, molybdenum disilicide, and zirconium and chromium diborides. It is shown that the equation can be employed for describing the densification kinetics of refractory compound powders during hot pressing. 2. It is demonstrated that the densification of molybdenum disilicide powder during hot pressing is effected throughα-creep at temperatures of 1600°C and lower, while at higher temperatures a short (∼3 min) stage of transient creep is followed by a stage of steady-state creep. 3. The energies of activation for the steady-state creep of the compounds investigated at sintering temperatures and the values of effective Laplacian pressure have been determined. The values of energy of activation for creep obtained correspond to the energies of activation for the self-diffusion of atoms in the metallic sublattices of the compounds under investigation.

Transportreaktionen und einkristallzüchtung von siliciden der IVA-VIA metalle

The chemical transport of the suicides TiSi 2, VSi 2 and CrSi 2 was achieved from the higher temperature T 2 (1100 °-900 °C) to the lower temperature T 1 (1000 °-700 °C) in a halogen atmosphere (bromine or chlorine) producing single crystals of the disilicides having a size of about 1 mm 3. The obvious effect of grain enlargement of the disilicide powders at T 2 leads to a separation of the oxide impurities in quite a short time. During the isothermic transport of T-metal + silicon (or germanium) + bromine, the metals Ti, V, Cr, U are transported towards silicon and, silicon towards the metals Ta, W, Mo (one-directional transport). In the system Ti + Ge + Br titanium is transported towards germanium and germanium towards titanium (bi-directional or coupled transport). The binary compounds (Ti xSi y or Ti xGe y) are deposited as layers corresponding to the sequence in the equivalent phase diagrams. Thereby the phase analysis of high melting systems is faciliated. The parabolic growth rate of TiSi 2 at 750 °C was determined.

Preparation of iron, cobalt, and nickel disilicide powders

Preparation of iron, cobalt, and nickel disilicide powders

Slip casting of molybdenum disilicide

1. A study of slip casting as a method of formation of parts from MoSi2 has demonstrated the feasibility of production of shaped components by this process. 2. Distilled water was found to be the best dispersion medium. 3. It is shown that, when it is required to slip cast a coarse powder which settles in water, the viscosity of the slip may be increased by using bentonite or sodium alginate additions. In such a case, the thixotropic slip structure must be eliminated by an appropriate method. 4. For slip casting using aqueous suspensions, molybdenum disilicide powder should be ground to 75% of fractions with particles less than 2 μ in diameter. 5. Aqueous molybdenum disilicide slips exhibit two casting ranges, with pH values of 4–5 and 9–11. Slip casting using suspensions with pH values in the 9–11 casting range secures castings of higher quality, which resist cracking during casting and attain the maximum density. 6. The density of parts produced by slip casting lies in the range of values attained by other methods of formation. Uniform particle packing throughout the whole section of a casting is an advantage of slip casting. 7. During sintering, parts produced by the above-described slip casting procedure exihibit a high but uniform shrinkage and attain an adequate level of density with a porosity of about 8%.