Production of NiAl–matrix composites by reactive sintering

Abstract

This work was devoted to the development of NiAl–matrix composite and its production by reactive sintering powder metallurgy. Various types of reinforcement (aluminium oxide, silicon and tungsten carbides, titanium silicide) were tested. The best chemical compatibility and the highest hardness and wear resistance were achieved by Al₂O₃ fibres. Electroless nickel plating pretreatment of Al₂O₃ fibres improves both distribution of fibres and hardness of the composite. However, it strongly reduces the wear resistance, probably due to phosphorus content in the nickel coating. In situ formation of NiAl–Al₂O₃ composites by reactive sintering of a pressed powder mixture of Ni, Al and NiO was unsuccessful. Only a small amount of cubic γ-Al₂O₃ was detected after reactive sintering and hence no significant hardness increase was observed.     

Fabrication of in situ TiB2–TiC reinforced steel matrix composites by spark plasma sintering

Abstract

In situ TiB₂ and TiC particulates reinforced steel matrix composites have been fabricated using cheap ferrotitanium and boron carbide powders by spark plasma sintering (SPS) technique. The sintering behaviour and the formation mechanism of the composite were studied. The results show that when the composite was sintered at 1050°C for 5 min, the maximum relative density and hardness of the composite are 99·2% and 83·8 HRA respectively. The phase evolution of the composite during sintering indicates that the TiB₂ and TiC reinforcements were formed in situ as follows: first, the solid/solid interface reaction between Fe₂Ti and B₄C, resulting in the formation of a small amount of TiB₂ and TiC below 950°C; second, the solid–liquid solution precipitation reaction in the Fe–Ti–B–C system, resulting in the formation of the main TiB₂ and TiC reinforcements at ～1000°C.     

Compaction and sintering behaviour of A356–fly ash composites: a preliminary investigation

Abstract

In the present study, A356–fly ash metal matrix composites were developed through powder metallurgy route. The composites were mixed by using the ball milling technique, shaped through uniaxial and cold isostatic compaction, and then sintered at 520°C. Scanning electron microscopy and X-ray diffraction were used for microstructure and phase characterisation. The density and microhardness of the composites were evaluated as a function of fly ash content, compaction pressure, sintering time and age hardening time. Uniaxial cold compaction of the composites increased their green density and cold isostatic compaction of the compacts led to a further increase in the density. At a constant compaction pressure, the density decreased with increasing fly ash content, resulting in light weight composites. The microhardness of the composites increased with the addition of 10 wt-% fly ash while it decreased with the addition of 20 and 30 wt-% fly ash. Sintering at 520°C increased the density of the composites and the grain size of the α-Al phase of the matrix. The matrix alloy and the composite containing 10 wt-% fly ash showed some response to age hardening at 160°C. However, no response to age hardening was observed at 200°C.     

Hot forging of Ti–6Al–4V alloy preforms produced by spark plasma sintering of powders

Abstract

Powder preform forging is a technology that comprises the preparation of near net shape preforms through powder metallurgy and a subsequent hot forging in order to obtain the desired final shape. In this work, two Ti–6Al–4V powder preforms were sintered through spark plasma sintering (SPS) and then hot compressed in a horizontal dilatometer. Varying the temperature of the process, two full density preforms having different microstructures were produced: sintering at 950°C, a plate-like α was obtained, whereas sintering at 1050°C, an acicular α was obtained. The behaviour of the preforms under hot forging has been studied through hot compression tests carried out in a quenching and deformation dilatometer in a range of temperature and strain rates typically used in hot forging this alloy (850–1050°C, 0·01–1 s^?1). Hot workability has been evaluated by measuring the stresses required for deformation and by analysing both the stress–strain curves recorded during testing and the microstructures after deformation. The main microstructural phenomena occurring during hot compression were individuated. The best conditions for the hot forging operation of SPS preform are temperatures above β transus, where the materials are deformed in a regime of dynamic recrystallisation, at every strain rate.     

On constrained sintering—II. Comparison of constitutive models

Several authors have proposed constitutive equations to describe bodies that sinter under constraint. We provide a critical examination of these models and show that some of them imply a negative Poisson's ratio, in contradiction to experimental evidence.     

Influence of thermal storage on microstructure and properties of Sn–9Zn–xIn alloys

Abstract

This study investigated the solidification behaviour and microstructural properties of Sn–9Zn–xIn alloys cooled under various crucible environments (i.e. gypsum or graphite moulds). Sn–9Zn–xIn solder specimens were prepared with the addition of In ranging from 0 to 0·5 wt-%. The specimens were melted at 200°C, poured into gypsum or graphite moulds and then allowed to cool in air. The microstructure, chemical composition and hardness properties of various specimens were then systematically examined. It was found that the microstructure (and hence the mechanical properties) of the solder samples can be controlled by manipulating the variables of the solidification process, such as cooling rate, element diffusion rate, solubility among elements and the crucible environment.

Cette étude a examiné le comportement de solidification et les propriétés de la microstructure des alliages Sn–9Zn–xIn refroidis dans divers creusets (c’est-à-dire des moules en plâtre ou en graphite). On a préparé des spécimens de soudure de Sn–9Zn–xIn avec des additions d’In variant de 0 à 0·5% en poids. Les spécimens étaient fondus à 200°C, coulés dans des moules en plâtre ou en graphite, et ensuite refroidis à l’air. Ensuite, on a systématiquement examiné la microstructure, la composition chimique et la dureté des divers spécimens. On a trouvé que l’on pouvait contrôler la microstructure (et ainsi les propriétés mécaniques) des échantillons de soudure en manipulant les variables du procédé de solidification, comme la vitesse de refroidissement, la vitesse de diffusion de l’élément et la solubilité entre les éléments ainsi que la nature du creuset.     

Cause of gas emission during sintering of Nd–Fe–B magnets

The degassing of an Nd_14.5Fe_76.3Ti_1.4Al_1.1B_6.7 alloy (at %) has been studied. The cause for the degassing of the Nd–Fe–B material during its sintering is shown to be hydrogen evolution. At 250–550°C, hydrogen escapes from the Nd₂Fe₁₄B phase; at higher temperatures (700–900°C), hydrogen evolution from a Nd-rich phase takes place. The cause for hydrogen evolution consists in the decomposition of the Nd(OH)₃ neodymium hydroxide during fine milling. The decomposition of hydroxide results in the formation of the Nd₂O₃ neodymium oxide, which deteriorates the magnetic isolation of Nd₂Fe₁₄B-phase grains; this leads to an abrupt increase in the irreversible magnetization losses.     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

Volumetric thermal expansion coefficients of the equiatomic Na–K and K–Rb melts

The volumetric thermal expansion coefficients of the equiatomic Na–K and K–Rb melts at 373 K are calculated using variational method with an additive hard-sphere reference system. It is found that the Krasko–Gurskii model pseudopotential with the exchange-correlation Geldart–Vosko function yields better agreement with the experimental data as compared to the local Animalu–Heine pseudopotential with the exchange-correlation Toigo–Woodruff function.     

Effects of wet and dry milling conditions on properties of mechanically alloyed and sintered W–C and W–B4C–C composites

Abstract

Tungsten based W–1C and W–2B₄C–1C (wt-%) powders synthesised by mechanical alloying (MA) for milling durations of 10, 20 and 30 h, in wet (ethanol) and dry conditions, were characterised. X-ray fluorescence spectroscopy investigations revealed Co contamination which increased with increasing milling time during wet milling. X-ray diffraction investigations revealed the presence of W and WC phases in all powders, Co₃C intermetallic in the wet milled W–1C powders and W₂B intermetallic phase in both wet and dry milled W–2B₄C–1C powders. As blended and MA processed powders were consolidated into green compacts by uniaxial cold pressing at 500 MPa and solid phase sintered at 1680°C under hydrogen and argon atmospheres for 1 h. X-ray diffraction investigations revealed the presence of W₂C intermetallic phase in sintered composites produced from both wet and dry milled W–1C powders and the W₂B intermetallic phase in sintered material from the wet milled W–2B₄C–1C powder. Sintered composites from wet milled powders showed relative densities >91%, with the maximum density of 99·5% measured for the sintered 30 h wet milled W–2B₄C–1C composites. Microhardness values for the wet milled W–1C and W–2B₄C–1C composites were 2–2·5 times higher than those for dry milled composite powders. A maximum hardness value of 23·7±2·1 GPa was measured for the sintered W–2B₄C–1C composite wet milled for 20 h.     

Structure and thermal properties of the matte of the autogenous smelting of copper–zinc concentrates

The phase composition and the structure of the solidified mattes of the autogenous smelting of copper–zinc concentrates containing 49.8 and 61.0% Cu in Vanyukov’s furnace are studied. The forms of the main elements (Cu, Ni, Fe, S, O) and the accompanying impurities (Zn, Pb, As, Sb, Co, Sn, Au, Ag) in the rich matte are determined. The phase-transformation temperatures in cooling of the mattes in an inert or oxidizing gas atmosphere are found.     

Synthesis of cast graphite-containing Cu–Cr–C composites

Casting with the use of low-frequency vibrations is applied to fabricate copper-based materials containing graphite and chromium. The conditions of formation of core (graphite)–shell (chromium carbide) complexes and the influence of low-frequency mechanical activation of molten copper on them are studied.     

Synthesis of pewter alloy from tin–copper–antimony powder mixtures by microwave and conventional sintering

Abstract

Two compositions of pewter alloy were sintered using both microwave and conventional vacuum sintering, and the effects of sintering time, temperature and weight percentage of copper and antimony on the mechanical and structural properties were examined for both sintering methods. Microwave sintered samples had finer microstructures, higher densities, higher hardness and tensile strength compared to the conventionally sintered samples and traditionally cast pewter. By increasing the copper and antimony contents, higher hardness was achieved. Better mechanical properties were found after microwave sintering after shorter sintering times compared with conventional sintering, but longer sintering times resulted in better diffusion for both sintering methods. The microwave sintered samples in general were capable of achieving similar amounts of diffusion to those conventionally sintered for the same time. But the total sintering process is much faster in microwave heating than in conventional heating due to the rapid heating effect.     

XRD and XANES study of reactions in particulate Al–Mg alloy composites

Abstract

Reactivity in stir cast Al Uju Mg alloy composites reinforced with fly ash was investigated by means of X‐ray diffraction (XRD) and X‐ray absorption near edge structure (XANES) spectroscopy at Al and Si K‐edges. High purity Al, α‐Al₂O₃, MgAl₂O₄ (spinel), MgO and SiO₂ powders were used as reference materials in the XANES investigation. The XRD and XANES data acquired from the raw fly ash used in making the composites were compared with fly ash particles extracted from the as cast composites. The major reaction product formed during fabrication of the composites was spinel. The XANES technique was useful in verifying the presence of various oxides in raw and extracted fly ash particles.

On a investigué la réactivité de composites de l’alliage Al–Mg renforcé avec des cendres volantes et moulé par agitation, au moyen de la diffraction des rayons X (XRD) et de la spectroscopie de structure près du front d’absorption des rayons X (XANES) aux seuils K de l’Al et du Si. On a utilisé des poudres d’Al, d’Al₂O₃‐α, de MgAl₂O₄ (spinelle), de MgO et de SiO₂ de haute pureté comme matériaux de référence dans l’investigation par XANES. On a comparé les données de XRD et de XANES acquises à partir des cendres volantes brutes utilisées dans la fabrication des composites, avec les particules de cendres volantes extraites des composites de brut de coulée. Le produit de réaction majeur formé lors de la fabrication des composites était le spinelle. La technique de XANES était utile dans la vérification de la présence d’oxydes variés dans les particules de cendres volantes brutes et extraites.     

China''''s imports and exports of nickel and cobalt by category

~~China's imports and exports of nickel and cobalt by category~~