Directional solidification of white cast iron

Several studies of the ledeburite eutectic (Fe-Fe₃C), in pure Fe-C alloys have shown that it has a lamellar morphology under plane front growth conditions. The structure of ledeburite in white cast irons, Fe-C-Si, consists of a rod morphology. It is generally not possible to produce plane front growth of Fe-C-Si eutectic alloys in the Fe-Fe₃C form, because at the slow growth rates required for plane front growth, the Fe₃C phase is replaced by graphite. By using small additions of Te, the growth of graphite was suppressed, and the plane front growth of the ledeburite eutectic in Fe-C-Si alloys was carried out with Si levels up to 1 wt pct. It was found that the growth morphology became a faceted rod morphology at 1 wt pct Si, but in contrast to the usual rod morphology of white cast irons, the rod phase was Fe₃C rather than iron. It was shown that the usual rod morphology only forms at the sides of the two-phase cellular or dendritic growth fronts in Fe-C-Si alloys. Possible reasons for the inability of plane front directional solidification to produce the usual rod morphology in Fe-C-Si alloys are discussed. Also, data are presented on the spacing of the lamellar eutectic in pure Fe-C ledeburite, which indicates that this system does not follow the usual λ² V = constant relation of regular eutectics. Formerly Graduate Student, Department of Materials Science and Engineering, Iowa State University.     

Properties of nanocrystalline refractory compounds (review)

Available information concerning the mechanical and physical properties of nanocrystalline refractory compounds (carbides, borides, nitrides and oxides) with grain size below 100 nm is analyzed. The absolute hardness values for these compounds in the nanocrystalline state are not as high as those of nanocrystalline metals and intermetallics. Information regarding the creep, superplasticity, and other physical properties is also reported. The effect of particle size on the eutectic temperature of nanocrystalline composites is estimated.     

The solidification of pure metals (and eutectics) under uni-directional heat flow conditions: II. Solidification in the presence of superheat

The generalized integral-profile method previously developed² to predict the solidification rates of pure metals and eutectic alloys under a range of cooling conditions has been extended to treat solidification rates in the presence of super heat. Heat transfer within the liquid metal has been characterized in terms of a single parameter for which a heat balance on the liquid yields an ordinary nonlinear equation. This equation is additional to the two equations derived previously for the growth of the solid metal, and the three equations have been solved simultaneously using a Runge-Kutta technique. Solutions have been obtained for heat transfer conditions that can be reproduced accurately in laboratory experiments. The results of a series of such experiments are also presented and shown to agree very well with the theoretical predictions. In carrying out the analysis, it has been shown that a number of different cooling and solidification modes can occur during the solidification of superheated liquid metal, the solidification process following one of two possible routes through these modes. This presents a useful approach to the analysis of this solidification problem and, indeed, of more complicated problems since the logic of the computer program used in the analysis is closely related to the logic of the solidification process. The work described in this paper was carried out while the authors were with the John Percy Research Group in Process Metallurgy at Imperial College, London.     

Directional solidification of aluminum-nickel eutectic alloys using electroslag remelting

Attempts were made to produce directionally solidified, specifically grain aligned Al-6 wt pct Ni eutectic alloy using a laboratory scale ESR unit. For this purpose sand cast alloy electrodes were electroslag remelted under different mold conditions. The grain structure of the ingots obtained from these meltings showed that insulated silica molds gave the best vertical alignment of grains along the length of the ingot. The NiAl₃ fibers within the grains tended to fan out and there was only a preferred alignment of fibers along the growth direction under the conditions of our experiments. The ESR parameters most suitable for vertical alignment of eutectic grains have been identified. In some electroslag remelting trials ingots were grown on a seed ingot. This resulted in a fewer vertical grains compared to the case when no seed ingot was used. The sand cast specimen of the eutectic exhibited a maximum tensile strength of around 88.2 MN/m² (9.0 kg/mm²) whereas conventional ESR using water cooled mold gave strength value of 98.0 MN/m² (10 kg/mm²). The directionally solidified ESR material showed longitudinal tensile strength as high as 213.7 MN/m² (21.8 kg/mm²) which could be further increased to 220.6 MN/m² (22.5 kg/mm²) by using the seed ingot. The average growth rate was varied between 5 to 25 mm/min during electroslag remelting in this study. The flow stresses, tangent modulus and ultimate tensile strength of directionally solidified eutectic increased with increasing growth rates. Formerly Research Fellow, Indian Institute of Science, Bangalore 560012 is now     

Directional solidification and heat treatment of terfenol-D magnetostrictive materials

Directional solidification techniques have been developed to produce grain-aligned rods of the highly magnetostrictive material Terfenol-D, Tb_xDy_1-xFe_y The magnetostrictive properties have been compared to those of single-crystal, bicrystal, and polycrystal rod prepared by a float zone melting (ZM) technique. It was found that the properties of the grain-aligned material were comparable to three-and four-grain ZM material and the magnetostriction for both materials was less than the ZM single crystal and the better bicrystal rods by around 20 pct. It was demonstrated that a heat treatment consisting of a 1-hour hold at 950 °C was very effective at improving the magnetostrictive properties of all of the materials. Experiments have demonstrated that, to be effective, the heat-treatment temperature must be above somewhere between 850 °C and 900 °C. It is postulated that the mechanism for the improved properties involves a relief or redistribution of residual stresses produced by localized melting near second-phase particles at the predicted eutectic temperature of around 875 °C in the Terfenol-D material.     

Directional dendritic solidification of a composite slurry: Part II. Particle distribution

The present work attempts to form an understanding of particle distribution during dendritic solidification of a composite slurry. It is shown that the magnitude and nature of the forces involved between a foreign particle and a growing dendrite are distinctly different from those during plane front solidification. A particle distribution map, based on theoretically evaluated forces, is proposed to predict the conditions under which engulfment, entrapment, or pushing of particles occurs during dendritic solidification. Directional solidification experiments have been conducted to study particle distribution, and comparison of these results with theoretical predictions based on the map agree well with a large number of experimental observations reported to date.     

Directional dendritic solidification of a composite slurry: Part I. Dendrite morphology

A simple analytical model to describe the morphology of a growing dendrite in the presence of an inert particle has been presented. The presence of a particle changes the solute concentration gradient at the tip of a growing dendrite and this, in turn, affects the dendrite tip radius. Results of the analysis show that the dendrite tip radius decreases at a high growth velocity due to the presence of a particle, while there is no influence on the tip radius at both low and intermediate growth velocities. Lower thermal conductivity of the particle decreases the tip radius, while a higher thermal conductivity increases the radius. However, the effect of thermal conductivity on the tip radius is only significant in the cellular growth regime. The analysis shows that the presence of SiC particles in Al-Cu alloys reduces the cell to dendrite transition velocity. Results of directional solidification experiments carried out on an Al-4.5Cu-SiC composite system agree with our model.     

Directional solidification of lead-copper immiscible alloys in a cyclic gravity environment

Hypermonotectic copper-lead alloys were directionally solidified at unit gravity on earth and also in the cyclic gravitational environment attainable during flight of NASA's KC-135 aircraft. In both cases macrosegregation developed that consisted of an initial lead-rich phase above which an aligned composite structure of apparent monotectic composition grew. Differences within these regions are examined, and the suitability of the KC-135 environment for directional solidification of monotectic alloys is discussed. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–229, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Directional solidification of Al- Ni/SiC composites during parabolic trajectories

Aluminum-6.1 wt pct nickel-silicon carbide composites containing varying volume fractions and particle sizes of SiC were directionally solidified at different translation rates and temperature gradi-ents, under variable gravity levels. The gravity level was changed by solidifying the composites in a Bridgman type directional solidification furnace aboard NASA KC-135 aircraft, flying on parabolic trajectories. It was observed that high gravity, high volume fractions of the particles or high effec-tive viscosity of the liquid favors the engulfment of particles by the melt-interface. Solidification in low gravity seems to deflocculate the SiC particle agglomerates while opposite results are obtained when solidifying under high gravity. Intercellular spacings are found to be higher in low gravity so-lidification as compared to high gravity solidification. These results are discussed in terms of the influence of gravity on various physical phenomena involved in the solidification process of the above composite. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermo-dynamic Data Committee and the Material Processing Committee.     

Directional solidification of large superalloy castings with radiation and liquid-metal cooling: A comparative assessment

A columnar-grain variant of single-crystal RENé N4 has been directionally solidified (DS) over a range of conditions in order to assess the possible benefits of the use of liquid metal-enhanced cooling for large cross-sectional castings. Castings were solidified at a rate of 2.5 mm/min using conventional radiation cooling and at rates between 2.5 and 8.5 mm/min using liquid-metal cooling (LMC) with tin as a cooling medium. Thermocouples inserted in the casting directly measured thermal gradients during solidification. The LMC process exhibited higher gradients at all withdrawal rates. The higher thermal gradients resulted in a refined structure measurable by the finer dendrite-arm spacing. Additionally, the conventionally cast material exhibited several freckle-type defects, while none were observed in the liquid-metal-cooled castings.     

Effect of te on morphological transitions in Fe-C-Si alloys: Part i. Directional solidification

Directional solidification experiments have been carried out on Fe-3.4 wt pct C-2 wt pct Si alloys with and without the addition of 0.06 wt pct Te. The effect of the Te addition upon the gray-to-white transition velocity and the white-to-gray transition velocity has been determined. Quenched solid/liquid interface studies on mixed structures, white + type D gray and type D gray + type A gray, have determined the temperature differences at the growth front for these structures.     

Heat of solidification in refractory metals studied by drop-tube experiments

A number of pure refractory metals have been investigated by drop-tube experiments. The cooling curves have been registered and theoretically analysed. A model for analysing the heat of fusion has been developed. It was found that the heat of fusion was approximately 50% of the tabulated value.     

The breakdown of single-crystal solidification in high refractory nickel-base alloys

The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.     

Management of hormone refractory prostate cancer: current standards and future prospects

PURPOSE: Recent advances in the biology and treatment of hormone refractory prostate cancer are reviewed. MATERIALS AND METHODS: A MEDLINE literature search of secondary hormonal therapy and chemotherapy for hormone refractory prostate cancer was performed. Recent advances in the biology of hormone refractory prostate cancer, changes in the measurement of response to therapy, and testing of new drugs and combinations of drugs were reviewed. RESULTS: Historically the treatment of hormone refractory prostate cancer has been disappointing. Useful parameters to monitor clinical response have been lacking but perhaps more importantly a scarcity of apparently active drugs has contributed to these results. Recently several developments have improved the outlook for treatment of hormone refractory prostate cancer. Recognition of antiandrogen withdrawal responses has had important ramifications for clinical trial interpretation and patient care. Secondary hormonal therapies, such as alternative antiandrogens and anti-adrenal agents, are well tolerated and can provide significant clinical benefits. Combining prostate specific antigen values with quality of life and measurable disease responses has made clinical trial end points more objective and more clinically relevant for the patient. Furthermore, a better understanding of the biology of hormone refractory prostate cancer, refinements in measuring response to treatment and availability of agents with proved palliative capabilities and/or generating greater than 50% response have all lead to improvements in treatment management. In 2 randomized studies mitoxantrone in combination with steroids has demonstrated significant palliative benefit compared with steroids alone. In phase II studies more than half of patients respond to estramustine combinations with vinblastine, etoposide or paclitaxel. Other novel combinations and new drugs currently are being tested. CONCLUSIONS: Recent advances suggest that available therapies for hormone refractory prostate cancer can have a meaningful impact on the disease. Improving treatment of hormone refractory prostate cancer remains an area of active investigation.     

Acute respiratory infections (ARI) in children: prospects for prevention

Acute respiratory infections (ARI) are a major cause of morbidity in children worldwide, and are estimated to cause four million deaths per year, mainly in the developing world. In those countries, bacterial infection with high case fatality is common, apparently following a primary viral infection. The case management strategy has had success in controlling severe outcomes. However, its dependence on the use of antibiotics and the advantage of primary prevention support the need for vaccines. Vaccines against viruses such as respiratory syncytial and parainfluenza would prevent what is often the initial infection and vaccines against Haemophilus influenzae and S. pneumoniae the major bacterial causes of mortality. Maternal immunization may have special relevance in developing countries where protection early in life is required. The development of combination vaccines would also be especially useful, since contacts with the medical system are often difficult. The introduction and use of new vaccines in those regions will require demonstration of cost effectiveness and acceptance by policy makers.     

Physiologo-biochemical characteristics if Gluconobacter oxydans and prospects for its use in biotechnology and biosensor systems (review)

Gluconobacter oxydans possesses a unique organization of metabolic systems, which are characterized by reduction of major dissimilation pathways, surface localization of main oxidative enzymes responsible for partial oxidation of carbon substrates, high performance of electron-transport chains, and accumulation of partially oxidized metabolites in the medium. These features allow us to use the cells of these microorganisms in biotechnology for production of several food products and medicines. The use of G. oxydans in biosensors for estimation of concentrations of sugars, aldoses and polyalcohols is promising. Physiological and biochemical features of these microorganisms enabling their use in biotechnology and receptor elements of biosensors are reviewed.