Microstructural development during liquid phase sintering of Fe and Fe–Mo alloys containing elemental boron additions

Abstract

The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm^?3 were obtained for these alloys.     

Research on the preparation and shielding properties of W–Ni–Fe alloy material by liquid phase sintering

Traditional and single shielding material has not satisfied the demand for radiation protection. Shielding materials with a good comprehensive performance have attracted attention. W–Ni–Fe alloys with different Ni/Fe ratios have been prepared through liquid phase sintering using W, Ni and Fe elementary powders. The microstructure, morphology and fracture appearance of such prepared W–Ni–Fe alloys with different Ni/Fe ratios were analysed with the SEM and metallographic test. The effects of the Ni/Fe ratio on the density, microhardness, tensile strength and shielding efficiency were investigated. The results show that for W–Ni–Fe alloys with different Ni/Fe ratios, the alloy surfaces are composed of ellipsoidal W particles on a Ni–Fe substrate. However, when the Ni/Fe ratio is 7:3, uniform and spherical W crystals are embedded in the Ni–Fe substrate and evidently form dense boundaries, which contribute to the good mechanical properties and shielding effect of the alloy.     

Synthesis and Characterization of Spark Plasma Sintered FeAl and In situ FeAl–Al2O3 Composite

In the present work, nanocrystalline FeAl and FeAl–Al₂O₃ composite were synthesized by high energy ball milling and subsequent compaction by spark plasma sintering. Microstructural changes during all stages of processing are studied using X-ray analysis. After 20 h of milling, the disordered FeAl and some amount of Fe rich solid solution was obtained in both of these compositions. Subsequent heat treatment results in formation of ordered FeAl. However, disordering of FeAl was observed in both compositions after spark plasma sintering. Nanocrystallinity is retained in both the compositions even after sintering at high temperature of 1,000 °C. Very high hardness of ~575 HV₁ and ~600 HV₁ was exhibited by FeAl and FeAl–Al₂O₃ composite.     

Densification kinetics ofα-AlB12 and Al3B48C2 powders during hot pressing

A study was made of the densification by hot pressing of ultrafine (< 2 m -AlB₁₂ and Al₃B₄₈C₂) powders synthesized from the elements, and of the same powders ground in a planetary mill. The optimum regimes for obtaining almost pore free specimens were determined, as well as the effects of iron and particle size on the kinetics of densification.Institute for the Problems of Materials Science, Ukrainian Academy of Sciences, Kiev. T. G. Shevchenko University of Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(383), pp. 28–35, January–February, 1996. Original article submitted January 19, 1990.     

Cu redistribution during sintering of Fe–2Cu and Fe–2Cu–0·5C compacts

Abstract

The effective use of alloying elements in powder metallurgical steels requires a deep understanding of their redistribution kinetics during sintering. In this work, interrupted sintering trials of Fe–2Cu and Fe–2Cu–0·5C compacts were performed. Moreover, diffusion simulations of Cu in γ-Fe using Dictra were performed. It is found that transient liquid phase penetrates the Fe interparticle and grain boundaries in less than 3 min of holding time. However, C addition limits the penetration of liquid Cu, particularly into grain boundaries of large Fe particles. The results also show that the mean diffusion distance of Cu in γ-Fe in the C added system is slightly lower than that in the C-free system at 3 min of holding time; however, after 33 min, the mean diffusion distance is similar in both systems. The diffusion distances of Cu in γ-Fe, predicted by Dictra, are in good agreement with the measured values.     

Combustion synthesis of Fe–Al/TiC composite powders and effect of Al content on its characteristics

Abstract

In this work, combustion synthesis of ferrotitanium–Al–C powder mixtures with different compositions was carried out to synthesise Fe–Al/TiC composites. Differential thermal analysis was performed on the precursor powder from ambient temperature to 1673 K at a heating rate of 30 K min^?1. Phase development and structural changes were investigated by X-ray diffraction technique and scanning electron microscopy. The results showed that no trace of TiAl_x (x?=?1, 3) was formed in all samples, and the reaction of (Ti–Fe)–Al–C system took place in the following two steps: first, molten Al and Fe reacted exothermically to form Fe–Al intermetallic compound. Second, the produced heat melted the ferrotitanium with lower Fe content and resulted in a liquid containing Ti, Fe, Al and C. TiC formed in all samples, but depending on the Al content, different phases containing FeAl₂, FeAl, Fe₃Al, Fe₃AlC_x and α-Fe formed as phases of matrix. The mixture with the lower Al content gave out a higher combustion temperature.     

Linear friction welding of Al–Cu Part 2 – Interfacial characteristics

Abstract

Metallurgically bonded aluminium–copper assemblies are used as electrical transition pieces in high direct current bus systems to transmit electricity. The brittle, high electrical resistance intermetallics that tend to form at the interface between these two materials are problematic for the electrical conductivity and efficiency of the connector. The manufacture of such assemblies therefore requires low heat input during joining so as to minimise the extent of formation of the intermetallic phases. The application of linear friction welding (LFW), an emerging high energy density solid state joining technology, for this purpose limits the formation of the intermetallic phases and produces more promising interfacial characteristics. In this work, aluminium–copper assemblies with two different geometries, namely, laboratory scale and full scale (customised to actual industrial requirements) were produced by LFW. These were characterised using scanning electron microscopy (SEM), electron probe microanalysis (EPMA) and energy dispersive spectrometry (EDS) to identify the phases at the interface. In order to simulate the in-service behaviour of the aluminium–copper assemblies manufactured by LFW, the laboratory scale samples were heat treated at 300°C. The evolution in the interfacial region was compared with that of post-service connectors produced by explosive welding (EW) and LFW.

Les assemblages d’aluminium–cuivre à joint métallurgique sont utilisés comme pièces de transition électrique dans les systèmes de barre omnibus à courant continu élevé pour transmettre l’électricité. Les intermétalliques fragiles, à haute résistance électrique, qui tendent à se former à l’interface entre ces deux matériaux sont problématiques pour la conductivité électrique et pour le rendement du connecteur. La fabrication de tels assemblages requiert donc un faible apport de chaleur lors de l’assemblage afin de minimiser le degré de formation des phases intermétalliques. L’application du soudage linéaire par friction (LFW), une technologie émergeante d’assemblage par diffusion à haute densité d’énergie, pour cet objectif, limite la formation des phases intermétalliques et produit des caractéristiques interfaciales plus prometteuses. Dans ce travail, des assemblages d’aluminium–cuivre à deux géométries différentes, soit à l’échelle du laboratoire et à pleine échelle (adaptés pour des besoins industriels réels), ont été produits par LFW. On les a caractérisés en utilisant la microscopie électronique à balayage (SEM), la microanalyse par électrons (EPMA) et la spectroscopie à dispersion d’énergie (EDS) afin d’identifier les phases interfaciales. Afin de simuler le comportement en service des assemblages d’aluminium–cuivre fabriqués par LFW, on a traité thermiquement à 300°C les échantillons à l’échelle du laboratoire. On a comparé l’évolution de la région interfaciale à celle de connecteurs d’après service produits par soudage par explosion (EW) et par LFW.     

Processing and performance of Cr2O3–Fe2O3 reference electrode powders prepared by oxide coprecipitation

Abstract

Cr₂O₃–Fe₂O₃ based oxide mixtures for reference electrode powders of oxygen sensors were processed using oxide coprecipitation route. A special method for preparing reference electrode powders has been developed by mixing coarse Cr particles with the oxide mixture in the form of Cr–Fe hydroxide. Morphology and size of the mixed oxide powders were characterised by means of scanning electron microscopy and laser diffraction method. With the coprecipitation process, chemically homogeneous and very fine powders with a mean particle size of 1.53 μm were prepared. This powder mixture adhered and loosely coated to Cr particles. The processed reference electrode powders were tested in low level oxygen concentration measurements of steelmaking process under industrial scale. The reference electrode powders showed excellent results in terms of electromotive force reproducibility, response time and accuracy in soluble aluminium predictions at the oxygen concentration measurements. Most of the particles of the reference electrode powder remained separated after dipping to molten steel.     

In situ formation of FeAl–Al2O3 nanocomposite at different conditions of milling and subsequent annealing

Abstract

FeAl–Al₂O₃ nanocomposite powder was synthesised under different conditions of milling and annealing. The structure, morphology and microstructure of the milled powders were monitored by the X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) respectively. Results showed that the formation of FeAl and Al₂O₃ took place in explosive mode during milling with the cup speed of 600 rev min^?1. However, at the cup speed of 500 rev min^?1, FeAl and Al₂O₃ were synthesised only during annealing. Formation of the FeAl and Al₂O₃ was completed after 120, 270 and 360 min of milling at the ball to powder weight ratios (BPRs) of 5∶1, 15∶1 and 10∶1 respectively. Maximum microhardness of 8·8 GPa was obtained in the 270 min milled sample with the BPR of 15∶1 and cup speed of 600 rev min^?1. Mean grain size of 30 nm was calculated in the annealed FeAl that was in consistent with TEM results.     

Development of Ti–Nb and Ti–Nb–Fe beta alloys from TiH2 powders

Ti–Nb β alloys are a promising alternative as an implant material due to their good properties and low Young’s modulus, compared to other Ti-alloys currently employed as biomaterials. In this study, three materials of the Ti–Nb and Ti–Nb–Fe systems were produced by powder metallurgy techniques starting from TiH₂ (TH) powder. Several sintering cycles were employed to evaluate the H₂ elimination and the effect of sintering temperature on densification and fraction of β-Ti phase. Also, the influence of alloying element size using two kinds of Fe powder was evaluated. The highest loss of H₂ was achieved by decreasing heating rate at the temperature range of hydride decomposition. SEM images and XRD results show mainly a β-Ti phase for TH–40Nb and TH–5Fe–25Nb samples. The TH–12Nb sample shows (α?+?β) microstructure. Fe addition with smaller particle size seems to improve the diffusion of Nb into Ti which promotes a higher β-phase fraction and sample homogeneity.     

Effect of carbon content,sintering temperature,density, and cooling rate upon properties of prealloyed Fe–1·5Mo powder

Abstract

Mixtures of prealloyed Fe–1·5Mo (Astaloy Mo) with and without additions of 0·5–1·2 wt-%C were prepared and their sintering, as well as their mechanical, properties investigated under different process conditions. It was found that carbon content, sintering temperature, and cooling rate had marked effect on physical and mechanical properties. Sintered density decreased with increase in carbon content and sintering temperature. On the other hand, UTS, TRS, and hardness values improved with up to 0·8 wt-%C addition, sintering temperature, and cooling rate. Percentage elongation decreased with increase in carbon content and cooling rate, but was higher for specimens sintered at higher temperatures. The as sintered microstructures consisted of either fine or coarse pearlite, upper or lower bainite, and their mixture depending on the carbon content and cooling rate. The heat treated mechanical properties showed some improvement for the specimens containing 0·5 and 0·8 wt-%C. It became evident that a variety of ternary low alloy steels consisting of Fe + 1·5Mo + 0·5–0·8 wt-%C can be produced and used in the as sintered or heat treated conditions for PM structural parts having good physical and mechanical properties as well as high dimensional accuracy with acceptable microstructures.     

High-temperature phase equilibria of Cu–O–Al2O3 system in air

Phase equilibria of Cu–O–Al₂O₃ system were experimentally investigated in a temperature range of 1100–1400°C under 0.21?atm oxygen pressure. The experiments were conducted employing a high-temperature equilibration and quenching method. Microstructures of the quenched samples were observed with scanning electron microscope. The phase compositions in the samples were analysed with electron probe microanalysis technique. Measured solubility of Al₂O₃ into the molten oxide ranged from 0.0 to 1.8?wt-%. A small solubility of Cu₂O into Al₂O₃ was also observed ranging from 1.20 to 1.58?wt-%.