Formation of gas folds during pressing of Y1Ba2Cu3O7−δ powder

Conditions for the formation of density discontinuities during pressing of Y₁Ba₂Cu₃O_7−δ powder are described. The experimental results are explained using catastrophe theory. It is hypothesized that the density discontinuities are gas folds formed during pressing. Pis’ma Zh. Tekh. Fiz. 25, 74–78 (August 12, 1999)     

Influence of the shock wave pressure gradient in α-Fe irradiated by a high-power ion beam on the occurence of a microhardness maximum at large depths

The defect structure and microhardness of α-Fe irradiated by a high-power pulsed beam have been studied using positron annihilation, transmission electron spectroscopy, and a nanoindenter, and results are presented. It is shown that in α-Fe exposed to a high-power pulsed beam, the pressure gradient and depth of formation of the shock wave front influence the formation and location of the dislocation density maximum and the microhardness maximum in the modified layer. Pis’ma Zh. Tekh. Fiz. 24, 47–53 (February 12, 1998)     

Production of Si55 Al20 Fe10 Ni5 Cr5 Zr5 bulk amorphous alloy by hot pressing

Abstract

The temperature dependence of the relative density was examined for a Si₅₅ Al₂₀ Fe₁₀ Ni₅ Cr₅ Zr₅ alloy which was produced by hot pressing of the amorphous powder during heating up to various temperatures under a critical pressure of 1·5 GPa through a heating/pressing cycle. The density of the compacts increased with increasing temperature and reached a maximum near the crystallisation temperature of 698 K. The relative density of the compacts reached 98·3% at the critical condition of 1·5 GPa and 687 K. The hardness value of the bulk specimen was 940 HV(10 gf, 0·098 N), corresponding to that of the amorphous ribbon. Exposure to higher temperatures resulted in the precipitation of the crystalline phase. The present results indicate that Si based alloys can be produced in a compact form with a low fraction of voids by hot pressing the amorphous powder.     

Physical vapour deposition coating processes for encapsulation of powder metal components before hot isostatic pressing

Abstract

Application of coatings by plasma vapour deposition involving electron beam evaporation and ion plating onto green powder metal compacts has been studied as a potential method for encapsulating powder metal products before hot isostatic pressing. The deposition of defect free coatings is essential if this concept is to provide a reliable encapsulation technique. Coating structures are therefore discussed in terms of the plasma processing conditions and surface roughness of the powder substrate. It is shown that the most promising approach is a combined coating and sinter–hot isostatic pressing cycle, which enables defects within the coating to be removed by the formation of a transient liquid phase.

MST/1455     

Some physical properties of an Al2O3-SiC-TiC composite

Ceramic samples obtained by hot pressing from a mixture of Al₂O₃ with admixtures of 23 vol.% TiC powder and 30.9 vol.% SiC whisker crystals are investigated experimentally. The resistivity of the material is measured at temperatures of 4.2–300 K, the infrared reflection spectra are recorded in the region 400–4200 cm^–1, and the temperature dependence of the Young’s modulus is investigated at temperatures up to 1300 K. As a result it is it is shown that the conductivity and optical reflection of the high-strength composite have a semimetallic character, which is due to the titanium carbide particles contained in it. Pis’ma Zh. Tekh. Fiz. 23, 52–58 (December 12, 1997)     

Pulsed electric current sintering of electrically conductive ceramics

The processing of yttria-stabilised zirconia (Y-ZrO₂)-based ceramic nanocomposites by means of pulsed electric current sintering (PECS) is described. A nanometer-sized electrically conductive secondary TiCN phase was added to the insulating zirconia matrix in order to make the composite electrically conductive. The paper focuses on the importance of processing conditions and highlights the benefits of the PECS method as compared to more traditional hot pressing. The mechanical and microstructural properties of the ZrO₂–TiCN composites have been determined, and the benefits of using an electrical current to densify these composites were explained in terms of the evolution of the electrical properties of the densifying powder compact.     

Carbonate binders by “cold sintering” of calcium carbonate

The solubility of calcium carbonate (limestone) particles depends on the amount of CO₂ dissolved in the water, which is a function of temperature and the pressure of CO₂ that is in equilibrium with water. At a constant temperature, increasing CO₂ pressure increases the solubility of CaCO₃, and decreasing CO₂ pressure favours the crystallisation of CaCO₃. This dissolution–crystallisation behaviour of CaCO₃ can be used in the development of carbonate binders—a process called “cold sintering”—of limestone. This paper examines the effect of a range of parameters on the cold sintering process of limestone powder. The parameters studied are CO₂ gas pressure (atmospheric, 10 atm and 35 atm), exposure time (0–90 min), post-compaction pressure (10 and 15 MPa) and compact pressing time (10–60 min). The water/limestone powder ratio was kept constant at 0.2 (by weight). The compressive strength of the limestone compacts was used as a measure of the efficiency of the carbonate binder formation process, and scanning electron microscopy was used to study the microstructural developments. The results show that carbonate binders can be produced by cold sintering of limestone powder. Exposure of limestone compacts to high CO₂ pressure followed by post-compaction at high mechanical pressure, enhances the strength of the compact. From the microstructural data, it is evident that newly formed calcium carbonate crystal growths are responsible for the strength improvements observed. The amount of water that is used in the limestone powder mixture is one factor that controls the quantity of the cementing phase. Future work has to be focussed on the application of methods to further increase the solubility of calcium carbonate, as the amount of calcium carbonate available for recrystallisation is important in producing a strong binder.     

Investigation of some characteristics of electron emission from the surface of a PLZT ferroelectric

Results are presented of an investigation of some characteristics of electron emission from the surface of a PLZT ferroelectric (TsTSL in Russian) when a pulsed voltage is applied to the sample. It is shown that the emitted charge is compensated by migration of electric charge through the bulk of the ferroelectric. The energies of the emitted electrons were estimated for a specific ferroelectric sample, and it was shown that the residual gas pressure influences the emitted charge. The results were obtained using a new method for direct measurement of the average pulsed currents in the electrode circuits of the solid-state ferroelectric sample and in the collector circuit. Pis’ma Zh. Tekh. Fiz. 24, 19–22 (December 12, 1998)     

Influence of a shock-wave pressure gradient on the appearance of a microhardness maximum in α-Fe irradiated by a high-power ion beam

The results of research on the defect structure and microhardness in α-Fe irradiated by a pulsed high-power ion beam are presented. The results are obtained with the use of positron annihilation, electron microscopy, and a nanoindenter. It is shown that the pressure gradient and the depth of formation of the shock wave front in α-Fe irradiated by a high-power ion beam have an influence on the formation and location of the maximum of the dislocation density and microhardness in the modified layer. Pis’ma Zh. Tekh. Fiz. 24, 72–77 (October 26, 1998)     

High performance stainless steel via powder metallurgy hot isostatic pressing

Abstract

Argon atomised austenitic stainless steel (AISI 304) powder was characterized for its physical properties such as particle shape, microstructure, median particle size, particle size distribution, apparent density, tap density, and jlowrate. Subsequently, the tap density of the as received powder was improved to the desired level by adjusting the powder distribution followed by mixing and blending. This powder was subjected to hot isostatic pressing (hipping) at two different combinations of temperature and pressure to optimise the microstructure and mechanical properties. Mechanical properties of the stainless steel obtained by the powder metallurgy (PM) hipping route were found to be superior to those of conventionally processed wrought steel. The superior performance of PM hipped steel is attributed to its low oxygen content, fine grain size, and high degree of chemical homogeneity. Although the production of billets by the hipping route does not appear to be economical owing to the high capital cost of the hot isostatic press, the added advantage of obtaining a nearnet shape makes the process economically viable for production of intricate shapes.     

Preparation and humidity-sensitive properties of nanocrystalline BaTiO3 thick film fabricated on Teflon substrate by using pressing method

Nanocrystalline BaTiO₃ precursor has been synthesized by using the stearic acid gel (SAG) method. After calcining the precursor at above 600 °C for 0.5 h, nanocrystalline BaTiO₃ powder with cubic perovskite structure is obtained. The powder is compacted further into solid material under a pressure of 0.1 to 2.0 GPa or fabricated into thick film by pressing on a Teflon substrate at room temperature and 200 °C. The powder and film samples are characterized by XRD, TEM and SEM. The results show that the average grain size of the powder is between 20 and 60 nm, and the thickness of the compacted film is about 56 μm. The investigation on the humidity-sensitive properties of nanocrystalline BaTiO₃ thick film shows that a compacted thick-film sample possesses higher humidity sensitivity, simpler preparation process, and enhanced adhesion strength on the Teflon substrate in comparison with a coated sensor, and the grain size and pressing pressure have obvious influence on its properties. In addition, the solid reaction mechanism and pressure effect on the material in the course of processing are also discussed.     

Effect of chemical degradation on superconducting properties of the HTSC ceramics

The effect of humid atmosphere (59 and 86%) on phase composition and properties of YBa₂Cu₃O_7−y powder and ceramics prepared by hot pressing has been studied. It has been found that with increase of water content in the powder to 1·6 mass.% the superconducting properties of the corresponding ceramics gradually improve; on further increase of the water content the ceramics degrade resulting in a complete loss of superconductivity. The observed phenomenon can be explained by a plastifying effect of water on the powder by hot pressing on the one hand and by a solid-phase reaction of the products of decomposition (1-2-3) at a high temperature annealing, on the other.     

Sinter and sinter-HIP of silicon nitride ceramics with yttria and alumina additions

The sintering behaviour of silicon nitride ceramics has been investigated by preparing powder mixtures containing controlled amounts of yttria and alumina. The fired density of the compacts, having a fixed quantity of yttria, has been observed to increase with increasing amounts of alumina added to the powder. There exists, however, a maximum limit to the beneficial effect of Al₂O₃, because an excess of alumina in the powder mixture does not produce a further increase in density. Sintering of green compacts, obtained by uniaxial pressing, has been carried out at various temperatures and pressures in order to determine the relationship between fired density and the processing variables. The results obtained, after subjecting a selection of specimens to different sinter-HIP cycles, clearly show that the final density is very sensitive to the applied pressure and the time at which it is applied. The range of microstructures produced by different processing routes has also been followed by electron microscopy. From the results obtained by X-ray diffractometry it is shown that the additions of yttria and alumina not only produce denser specimens than pure silicon nitride, but also promote the α-β transformation.     

Computer-modelled deformation mechanism maps for hot uniaxially pressed Bi-2223 superconductor

A deformation mechanism map represents the densification behaviour of a green powder body. A map is created by evaluating the rate-equations for five types of densification mechanisms: yield, boundary diffusion, volume diffusion, power-law creep, and Nabarro–Herring creep. These rates are summed where appropriate and integrated to give the density at a given pressure, temperature and time; this can then be plotted on a map. Such maps have been created for hot-pressed Bi-2223, using the HIP 6.0 computer software. Four temperature–density deformation maps were created at different pressing pressures and for two types of powder: a solid-state reacted powder and a pyrolysed organic precursor powder. The resulting maps are compared and discussed in relation to a set of experimental results. This revised version was published online in November 2006 with corrections to the Cover Date.     

Production of highly dispersed metal powders by electrical explosion in reduced-pressure nitrogen

It is suggested that nitrogen may be used as the ambient medium to obtain highly dispersed powders of pure metals. At reduced pressure nitrogen is chemically inert and has a higher dielectric strength than rare gases. A reduction in the gas density reduces the particle size of the pure metal powder down to the nanometer range. Pis’ma Zh. Tekh. Fiz. 25, 81–84 (July 26, 1999)     

Microstructural properties and hot pressing of rapidly solidified hypoeutectic low alloy white cast iron powders

Abstract

Hypoeutectic low alloy white cast iron powders were produced using a rapid solidification technique. The morphology and microstructural properties of these powders were investigated with respect to cooling rate and particle size. The density of hot pressed compacts as a function of parameters such as hot pressing time and pressure is described. It was found that retained austenite in the form of cells or dendrites was the main constituent of the powders. At 720°C the powder particles can be hot pressed into high density compacts that have a fine cementite–ferrite microstructure. These ultrafine grained compacts exhibited good superplasticity at elevated temperatures. An elongation to failure of 300% was observed.

MST/1682     

Possible method of reducing annealing temperatures of radiation defects in ion-implanted silicon carbide

It is shown that the temperatures of post-implantation annealing of radiation defects in silicon carbide may be reduced by pulsed photon treatment. With a correct choice of spectral composition and radiation energy, pulsed photon treatment is effective for annealing radiation defects through the selective absorption of photons at the corresponding levels. It is suggested that the annealing mechanism is ionizational (annealing under these experimental conditions cannot be explained by a thermal mechanism alone). Pis’ma Zh. Tekh. Fiz. 23, 26–29 (October 12, 1997)     

Curve of the dynamic compressibility of powder media

A technique for constructing the curves of dynamic compressibility of powder media from the results of an experiment on a plant of hydraulic explosive pressing is suggested which makes it possible to construct a certain portion of the compressibility curve with the aid of one experimental shot for any powder material in the pressure-density coordinates to the total exclusion of the apparatus that could register the dynamic parameters of the process of pressing. The technique is used for predicting the results of pressing concrete articles from powder materials, in particular, to determine the parameters of a charge and the coordinates of its disposition in a transmitting liquid medium to obtain a blank with prescribed properties. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 79, No. 4, pp. 179–185, July–August, 2006.     

In‐situ composite coating on powder metallurgy master alloy fabricated by vacuum hot‐pressing sintering technology

A material consisting of an in‐situ titanium carbide reinforced nickel‐aluminide (Ni₃Al) coating and a powder metallurgy master alloy was fabricated by vacuum hot‐pressing sintering technology. A metallurgical bonded, pores‐free interface between composite coating and powder metallurgy master alloy was formed at the sintering temperature of 1050 °C, pressure of 10^‐4 Pa and pressing pressure of 40 MPa. The phase, microstructure and wear behavior of composite coating were investigated. The results showed that polygonal titanium carbide particulates with various sizes were homogeneously distributed in nickel‐aluminide matrix. The sintering temperature, pressing pressure and heat from as‐reactions‐formed coating green compact facilitated the pore infiltration with transiently generated liquid phases and ensured the high‐intensity metallurgical bonding between composite coating and powder metallurgy master alloy. Due to the abnormal elevated‐temperature properties of nickel‐aluminide matrix, titanium carbide particulates reinforcement and the mechanically mixed layer protection, TiC/Ni₃Al‐coated parts demonstrated superior wear resistance and lower friction coefficient while compared with Ni₃Al‐coated parts and H13 steel.     

Repair and rejuvenation procedures for aero gas-turbine hot-section components

Abstract

The general procedures for repairing aero gas-turbine hot-section components are described. A number of examples are given in order to illustrate particular repair processes: (i) the welding of nozzle guide vanes made from high–strength, nickelbase alloy, demonstrating the effect of microstructural refinement and welding techniques; (ii) the brazing of similar nozzle guide vanes, with emphasis on the advantages of using a halide cleaning process and of modifying the braze alloy by adding a powder having a composition close to that of the parent metal; (iii) the use of an improved coating system to upgrade components; (iv) the possibility of using MCr AlY coatings applied by low–pressure plasma spraying; and (v) hot isostatic pressing as a method of restoring creep life.

MST/103