THE TECHNOLOGY OF UAl3–Al IRRADIATION TEST-PLATES

Abstract

The existing literature concerning UAl₃ and UAl₃-Al dispersions is reviewed. Calculations of the strength of the dispersions have also been made. The experimental work deals with the preparation of UAl₃ by induction melting, its chemical analysis, and crushing to the correct particle size for use in reactor-fuel elements. The blending of UAl₃-Al dispersions and their compaction by cold pressing to densities suitable for roll-bollding are described, and their properties have been determined. The picture-frame technique has been used for the aluminium cladding of the UAl₃-Al dispersions and to produce irradiation test-plates.     

THE HIGH-TEMPERATURE PROPERTIES OF CERAMICS AND CERMETS

Abstract

It has been recognized for some time that, with progressive increase in the working temperature of gas turbines, metallic alloys may no longer be adequate for rotor or stator blading. The use of more refractory but more brittle materials, i.e. ceramics and ceramic-metal mixtures (cermets) has been suggested. The paper is concerned with the evaluation of the major properties involved, viz. creep strength, fatigue strength, resistance to thermal fatigue (i.e. to repeated thermal shocks), oxidation-resistance, and impact-resistance. The materials evaluated include oxides, oxide–metal cermets, carbides, carbide-metal cermets, molybdenum disilicide, and silicon nitride. The equipment for determining the effects of alternating and steady mechanical stresses up to 1200° C. is described. The relative merits of the test materials are discussed. It is concluded that the resistance to thermal fatigue and to impact of the ceramics and cermets is inferior to that of metallic alloys in current use.     

UAl2-AI INSTEAD OF UAl3-AI IN FUEL-ELEMENT PLATES FOR ADVANCED TEST-REACTORS

Abstract

The existing literature on UAl₂-Al dispersions is surveyed and a comparison with similar data for UAl₃-Al dispersions is attempted. The experimental work included the preparation of a practically single-phase UAl₂ compound by induction-melting and its grinding to a predetermined particle size. The blending of UAl₂ and aluminium powders into suitable dispersions and their compaction by cold pressing to the densities required for roll-bonding are described. The picture-frame technique was used to clad the dispersions with aluminium to produce test-plates for irradiation experiments.

The kinetics of the UAl₂ and UAl₃ solid-state reactions with aluminium are dealt with on a qualitative basis. Throughout, comparison is made with the known technology of UAl₃-Al dispersions, taking into consideration uranium densities, production aspects, pyrophoricity, volume changes, &c.

It is concluded that UAl₂-Al dispersions have potential applications as fuels for advanced test reactors.     

A study of the effect of processing factors on the properties of alumina-molybdenum cermets

Conclusions The mathematical experiment planning method has been found to be an effective means of studying the influence of processing factors on the properties of cermets. A determination of the optimum properties of Al₂O₃-Mo cermets (with metal contents of up to 30 vol.%) by this method has shown that the most important processing variables are heat treatment conditions and pressing pressure.Translated from Poroshkovaya Metallurgiya, No. 5(185), pp. 93–96, May, 1978.     

THE FABRICATION AND PROPERTIES OF URANIUM OXIDE–IRON CERMETS

Abstract

Cermets of iron–uranium dioxide, containing up to 30 vol.-% UO₂ have been rolled to rod. Although the cores rolled centrally along the length, the core was distorted in section owing to stiffening of the core relative to the sheath as the temperature dropped during rolling. The UO₂ particles were fragmented during rolling and formed as “stringers” along the direction of rolling, seriously weakening the structure transversely.

By extrusion at 1200° C., cermets containing up to 50 vol.-% UO₂ were satisfactorily fabricated. The extrusion pressure increased sharply above 35 vol.-% of ceramic and when large reductions in area were taken. The extrusion characteristics do not conform to isotropic plastic extrusion nor to a pure viscosity effect; the influence of the dispersed particles on metal flow is explained by a strong interference effect between particles and the metal during flow. The properties of the extruded rods were good; the core density was 90–95% of theoretical, decreasing slightly with increasing ceramic content; there were no obvious directional properties in the core, nor was the ceramic fragmented as in rolling. The characteristic “tail-end defect” of the extrusions was partly corrected by using shaped sealing plugs.     

Effects of powder processing on densification of titanium diboride based cermets

Abstract

The sinterability of TiB₂-Ni₃(Al,Ti) based cermets has been significantly improved by aggressive milling of the starting TiB₂-Ni-TiAl₃ powder mixtures. This technique improves not only liquid spreading by reducing TiAl₃ particle size but also eliminates alumina agglomerates and the associated porosity found after vacuum sintering. Liquid phase sintering of TiB₂-Ni-TiAl₃ powder mixtures involves the presence of Ni based secondary borides at low temperatures (1200°C), which react afterwards with TiAl₃ particles leading to the formation of the final TiB₂-Ni₃(Al,Ti) eutectic liquid. Apart from improving liquid spreading around TiB₂ grains, aggressive milling is also found to disperse alumina agglomerates, which reduces the porosity associated to these particles. By this refined procedure, the amount of binder phase required for full densification of TiB₂ cermets by sinter hipping has been reduced from a previous limit of 16 vol.-% to 10 vol.-%. The hardness of these TiB₂-10 vol.-%Ni₃(Al,Ti) cermets is in the range of ultrafine WC-Co hardmetals in spite of their much coarser microstructure.     

THE FABRICATION AND PROPERTIES OF CHROMIUM–ALUMINA AND MOLYBDENUM–CHROMIUM–ALUMINA CERMETS

Abstract

After a brief review of the literature, various methods for preparing chromium–alumina and molybdenum–chromium–alumina cermets are described. The properties of such materials are discussed and an indication is given of their possible applications.     

DIMENSIONAL AND MICROSTRUCTURAL ANALYSIS OF COEXTRUDED UO2-STAINLESS STEEL CERMET PINS

Abstract

Coextrusion has been used to prepare UO₂ (35 vol.-%)-stainless steel cermet fuel-pins (5·6 mm dia. × 1000 mm long), which could be used as fuel elements in fast-flux test reactors. The procedure required to obtain a nearly homogeneous distributionof the fissile phase in the matrix is described. The influence of billet design on the dimensions of the fuel core and cladding is analysed with a view to its optimization. The extent of particle deformation as a function of extrusion parameters has been measured quantitatively by means of image analysis on particles having different (12·7%; 5·8%) inner porosities. Thermal expansion and conductivity as well as tensile strength of the cermets are reported and compared with existing data.     

METAL–CERAMIC MIXTURES

Abstract

The present position regarding cermets based on either oxides or carbides as the brittle phase is reviewed. The outlook for oxide systems is to some extent disappointing, since little gain in strength has been found compared with the oxide itself. Oxide cermets do, however, show an intermediate improvement in thermal shock-resistance that is useful in a number of directions. Carbide cermets appear adequate for many high-duty applications so far as thermal shock-resistance is concerned, but improvements in their resistance to mechanical shock are still necessary.

Recent work on ductile ceramics is also reviewed, and the importance of carrying out experiments at high strain rates is emphasized.

Finally, suggestions are made for further work in the field of cermets.     

Microcomposite hard titanium carbonitride-titanium nickelide cermets

Microcomposite titanium carbonitride-titanium nickelide (TiNi)_0.3(TiC, N)_0.7 cermets are fabricated by liquid-phase sintering of compacted initial components, namely, titanium carbonitride and titanium nickelide. The microstructure and properties of the cermets are studied after heat treatment under various conditions.     

Hipping after sintering of titanium diboride cermets

Abstract

A new fabrication route, an alternative to glass encapsulated hipping (GEHIP), has been developed to produce dense TiB₂ cermets. Key points of this technique, based on hipping after vacuum sintering (VS + HIP), are the use of Ni₃ (Al,Ti) as binder phase and the selection of the proper amount of additions. The main advantage of VS + HIP with respect to GEHIP is the simplification of the sintering procedure which avoids the glass encapsulation step that makes it more adaptable for industrial use. Successful application of VS + HIP requires a minimum binder content about 10 vol.-% below which a significant hardness reduction is observed owing to the presence of residual porosity as compared with GEHIP. The materials produced by this technique combine low density and high stiffness with high hardness and toughness values, thus giving a set of properties especially attractive for applications where inertial loads are responsible for failure.     

Some sintering characteristics of chromic oxide-chromium cermets

Conclusions A study was made of the preparation, under various conditions, of chromic oxide-chromium cermets, involving the extrusion of charge materials with a plasticizer and sintering of the resultant rod specimens in air or in argon, with or without a chromic oxide-alumina packing material. Some characteristic features of structure evolution during the sintering of these cermets in a carbon-containing atmosphere and in a vacuum were noted. In air, sintering proceeds slowly owing to the formation of a volatile oxide, but in a neutral atmosphere with oxide packing material it becomes activated, because the atmosphere generated by the packing material helps to maintain an equilibrium vapor pressure of Cr₂O₃ evaporation products within the specimen. The optimum chromium content of these cermets was found to be 15%. Some properties of such cermets were determined.Translated trom Poroshkovaya Metallurgiya, No. 10 (118), pp. 19–24, October, 1972.     

Reciprocating wear response of Ti(C,N)–Ni3Al cermets

Abstract

Ti(C,N) based cermets have received significant attention due to their enhanced mechanical properties at elevated temperatures, low mass and increased chemical stability, when compared to WC–Co hardmetals. These properties have been found to be superior in many cases to traditional hardmetal replacements, such as TiC–Ni. The current study focuses on Ti(C,N)–Ni₃Al cermets formed through melt infiltration (with binder contents ranging from 20 to 40 vol.-%Ni₃Al). Comparison is made with TiC–Ni₃Al cermets using an identical binder alloy. The reciprocating ball on flat wear properties of the cermets were evaluated as a function of applied load (using a WC–Co counterface), together with the composite hardness and indentation fracture resistance. It is shown that nitrogen content negatively affects infiltration, resulting in non-infiltrated areas within low binder content cermets, which decreases the indentation fracture resistance and hardness. This problem can be largely mitigated by increasing binder content. When comparing fully infiltrated cermets, increasing nitrogen content decreases hardness and increases toughness, while all Ti(C,N) cermets outperform TiC (at 40 vol.-% binder). Reciprocating wear increased with increasing load, and typically was the most severe for the lowest binder contents. A combination of wear mechanisms were apparent, including both abrasive and adhesive wear, with the formation of an oxide tribolayer containing components from both the tested cermets and the WC–Co counterface material.

Les cermets à base de Ti(C,N) ont reçu une attention importante grâce à leurs propriétés mécaniques améliorées aux températures élevées, à leur faible masse et à leur stabilité chimique augmentée, lorsque comparés aux carbures métalliques de WC–Co. Dans plusieurs cas, on a trouvé que ces propriétés étaient supérieures à celles des remplacements traditionnels de carbures métalliques, comme le TiC–Ni. La présente étude se concentre sur les cermets de Ti(C,N) –Ni₃Al formés par infiltration du bain (la teneur du liant variant de 20 à 40% en volume de Ni₃Al). On les compare aux cermets de TiC–Ni₃Al en utilisant un alliage de liaison identique. On a évalué les propriétés d’usure par déplacement alternatif de bille sur disque des cermets en fonction de la charge appliquée (utilisant un antagoniste en WC–Co), ainsi que de la dureté du composite et de la résistance à la fracture d’indentation. On montre que la teneur en azote affecte négativement l’infiltration, ayant pour résultat des régions non infiltrées à l’intérieur des cermets à faible teneur en liant, ce qui diminue la résistance à la fracture d’indentation et la dureté. On peut largement atténuer ce problème en augmentant la teneur en liant. Lorsque l’on compare des cermets entièrement infiltrés, l’augmentation de la teneur en azote diminue la dureté et augmente la ténacité, alors que tous les cermets de Ti(C,N) surpassent le TiC (à 40% en volume de liant). L’usure par déplacement alternatif augmentait avec une augmentation de la charge, et était typiquement plus sévère pour les plus faibles teneurs en liant. Une combinaison de mécanismes d’usure était apparente, incluant tant l’usure par abrasion que l’usure d’adhérence, avec formation d’une couche tribologique oxyde contenant des composantes tant des cermets évalués que du matériau antagoniste en WC–Co.     

PRELIMINARY STUDIES OF THE FABRICATION OF UAl4–Al TEST-PLATES BY POWDER-METALLURGICAL TECHNIQUES

Abstract

The preparation of UAl₄ by induction melting and annealing (720°C, 993 K) yielded a product of restricted UAl₄ content (81 wt.-%). The UAl₄ phase has a variable stoichiometry (UAl_4.0–UAl_4.8), which leads to a slightly modified U–Al phase diagram. UAl₄–Al dispersions can be prepared by powder-metallurgical techniques, resulting in structures with higher UAl₄ content, improved phase-distribution rates, and specific particle sizes, compared with melting techniques. The thermal conductivity and thermal expansion of UAl₄–Al dispersions have been measured over the whole concentration range. No technological difficulties, by comparison with UAl₃-Al plate fabrication, were encountered. The production of UAl₄-Al test plates (33 vol.-% UA1₄ ? 35 wt.-% U) is thus feasible on a laboratory scale. The work reported is the final stage in a series of studies of uranium aluminide-aluminium dispersion fuels obtained by powder routes.     

THE PREPARATION AND SOME PROPERTIES OF CHROMIC OXIDE-CHROMIUM CERMETS MADE BY PARTIAL REDUCTION OF THE OXIDE

Abstract

Compacts of chromic oxide/carbon mixtures have been sintered in vacuum to produce oxide-metal cermets. The effect of carbon addition, type of carbon, purity of oxide, compacting pressure, and sintering temperature on the green and sintered density has been studied, and this has been supplemented by tests of hardness and compressive strength.

Considerable densification can be attained by a small addition of carbon to the powder mixture, and this is accompanied by an increase in compressive strength to 20 tons/in², compared with 3 tons/in² for the pure oxide sintered to the same temperature.     

THE MECHANICAL PROPERTIES OF CERMETS WITH A METALLIC MATRIX

Abstract

The effective elastic moduli of cermets consisting of ceramic particles located in a metal are discussed and compared with the properties of sintered matrix materials. New experimental work shows that the properties depend significantly on the degree of bonding between the phases. If this is strong, the nominal Young’s modulus increases with concentration; if not (as in uranium oxide-stainless steel) it diminishes, but less rapidly than that of a porous steel. The rigidity modulus falls continuously with the proportion of the ceramic phase present, because local plastic deformation occurs rapidly in shear.

The decrease in tensile strength with concentration has previously been explained in terms of the packing arrangements of spheres, which reduce the load-bearing area of the matrix cross-section. A tentative equation is derived which shows that stress concentrations near the inclusions may also play an important role in determining the effective mechanicalstrength at high ceramic contents.     

Copper-base cermets produced by electrodeposition

Conclusions Copper-base cermets containing corundum, graphite, and MoS₂ have been prepared by electrolysis, and their properties and advantages over orthodox coatings have been examined.Translated from Poroshkovaya Metallurgiya, No. 6(90), pp. 75–81, June, 1970.     

ELEVATED-TEMPERATURE COMPACTION OF METALS AND CERAMICS BY GAS PRESSURES

Abstract

Techniques have been developed for the isostatic compaction of metals, cermets, and ceramics with no appreciable variation in density throughout the compacted structure. The temperatures needed are much lower than those normally utilized for sintering, so that a fine-grain, tough structural material can be produced. The starting material may be loose powder, cold-pressed preforms, explosively impacted preforms, coated particles, spherical particles, or vibratory-packed powder.

Toxic materials are easily handed. Since the process is quite adaptable to the preparation of complex shapes by direct compaction of powders, components can be produced from the more expensive materials, such as beryllium and tungsten, with a minimum loss of material during processing.     

THE PREPARATION AND SOME PROPERTIES OF CHROMIC OXIDE-CHROMIUM CERMETS.

Abstract

Compacts of chromium oxide/chromium mixtures have been sintered in vacuum at temperatures up to 1600°C to produce cermets. Two types of chromium powder were used—an electrolytic and an aluminothermic powder. The effect of chromium content and of sintering temperature on the sintered density and microstructure has been studied and supplemented by tests of hardness and compressive strength.

Considerable densification was achieved by additions of chromium in the range 1–50 wt.-%, with the optimum content at 5–10%. Compressive strengths < 10 tons/in² were attained over a wide range of composition.     

An X-ray structural investigation of the reactions of yttria with chromium and tungsten

Conclusions During the hot pressing of Y₂O₃-Cr cermets in an argon atmosphere the chromium experience partial oxidation, as a result of which yttrium chromite is formed at the yttrium/chromium interfaces. In Y₂O₃-Cr cermets being being pressed hot in a carbon-containing atmosphere (CO + CO₂) the chromium completely oxidizes, with the formation of yttrium chromite. In Y₂O₃-W cermets produced by hot pressing in argon and carbon-containing atmosphere at temperatures right up to 2100°C no new phases were detected.Translated from Poroshkovaya Metallurgiya, No. 11(203), pp. 75–77, November, 1979.