Solid State Phenomena Vols. 101-102

Abstract: The phenomenon of low-temperature homogenization (LTH) during interdiffusion is studied under condition a t Dv £ 2 / 1 ) ( (Dv is the bulk diffusion coefficient, a is the lattice parameter) using nano-objects of binary Cu-Ni and Cr-Ni systems compacted from nano-powders and produced by mechanical alloying. Two stages of LTH were detected: at the first stage (t £ 103 s) the volume fraction of solution rapidly grows; at the second stage (t > 103 s) the volume fraction of solutions grows slowly with practically constant average solution concentration. The first stage of LTH correlates with active grain growth caused by small size (l) of structural element and nonequilibrium structure of nano-objects. Obtained results are analyzed theoretically in terms of interdiffusion along migrating GBs due to grain growth at the first stage and DIGM mechanism at the second stage. It is shown that the GB concentration distribution during interdiffusion along migrating GBs and the kinetics of LTH depend on a parameter l/l where 2 / 1 ) / ( b b V sD d l= is the characteristic diffusion length. The mechanisms and criteria of LTH are proposed.

Abstract: The paper presents results of mechanical tests and grain structure investigation of aluminium wires obtained by plastic consolidation of aluminium powders produced by atomisation from liquid state. The powders produced by this method could be potentially adopted to obtain ultra fine/nano structured materials by rapid solidification. Conversion of material from powder to bulk state with retaining of powders fine structure and minimizing of porosity are crucial for technological route of sub-micron grained materials production. An influence of condition of plastic consolidation by hot extrusion on mechanical properties of consolidated products is a main target of presented investigation. The standard mechanical properties of extrusion products have been determined and grain structure has been inspected. The results have been compared to extrusion products of conventional bulk aluminium. It has been stated substantial increase of properties and refining of grain structure in materials consolidated from powders.

Abstract: The article illustrates the influence of ball milling of the 316L and 434L stainless steel powders as well as their mixture (50 wt. % of 434L + 50 wt. % of 316L) on their structure. Medium size of the grains (about 30 nm) was obtained after 110 hours of milling. The powders that obtained were pressed isostatically and sintered by impulse plasma method in vacuum. The samples were then characterized using an optical microscope equipped with a computer image analyzer, scanning electron microscope and X-ray diffractometer.

Abstract: The present paper is focused on ceramic–metal composites obtained via different technologies which leads to different microstructures in terms of size and distribution of metal phase. Composites analysed in paper were produced by the following methods:(a) infiltration of porous ceramics by metal, (b) consolidation under high pressure and (c) sintering of ceramic powder coated by metal. Their microstructures were investigated by scanning and transmission electron microscopy methods. The three methods of composite fabrication employed in the present study result in specific spatial distribution and dispersion of metal phase. Presureless infiltration of porous ceramics by liquid metal is driven by capillary force and make it possible to produce microstructure with percolation of metal phase in ceramic matrix. The volume fraction of metal phase in this case depends on the size of pores. The size of pores influence also the kinetics and extent of infiltration. Ceramic preforms with small size of pore are not fully infiltrated. This method is useful for composite with size of metal phase in the range of micrometers. Hot pressing under high pressure produces microstructures of composites with metal phase grain size in the range from nano to micrometers. Moreover, it allows to achieve the nanometric size of ceramic grains. In the case of ceramic powders covered by metal, compression and hot pressing preserves nanometric size of metal. The grain growth of ceramic grains is suppressed.

Abstract: This paper describes technology, which can be used to obtain ceramic-metal composites with a gradient of metal particles concentration. Graded composites, have been obtained by slip casting. The gradient of iron concentration was induced by magnetic field. Microstructures of the specimens have been investigated using a light and scanning electron microscopy. Quantitative analysis of microstructures has been carried out with the help of image analyzer. The obtained results prove the possibility to produce Al2O3-Fe functionally graded materials under the magnetic field.

Abstract: The influence of sintering temperature and high pressure on Ni-P nanoparticles in Al2O3/Ni-P nanocomposites was investigated in this paper. The fine-grained alumina powder was covered with Ni-P nanoparticles of 20 – 50 nm size by electroless nickel plating. The material was sintered in temperatures from 900 to 1400oC using pressures above 5 GPa. It was found that sintering in such conditions give a possibility to maintain nanometrical size of Ni-P particles. In the case of 1400oC the metallic phase melts and non-uniform grain growth of ceramic was observed. Hot pressing at 900oC allowed for the metal to remain in solid state and ensured uniform microstructures of the nanocomposites, with uniform distribution of nanometrical Ni-P grains in the ceramic matrix. In this temperature the grain growth of the ceramic was not observed. The results are discussed in terms of technology for production of ceramic-metal composites for various applications.

Abstract: SiC-Zn nanocomposites with about 20% volume fraction of metal were fabricated by infiltration process under the pressure of 2-8 GPa and at the temperature of 400_1000oC. SiC nanopowders used in the experiments formed loosely agglomerated chains of single crystal nanoparticles. The dimension of the agglomerates was in the micrometer range, the mean grain size was up to tens of nanometers. Microstructural investigations of the nanocomposites were performed at a different resolution levels using scanning, transmission electron microscopy and atomic force microscopy techniques (SEM, TEM, AFM, respectively). SEM observations indicate a presence of nano-dispersed, uniform (on the micrometer scale) mixture of two phases. TEM observations show that distribution of SiC and Zn nanocrystallites is uniform on the nanometer scale. High-resolution TEM images demonstrate an existence of thin (on the order of tens of Angstroms) Zn layers separating SiC grains. AFM images of the mechanically polished samples show a smooth surface with the roughness on the order of the SiC grain size (10-30 nm). After ion etching of some samples the AFM topographs show surface irregularities: periodically spaced hillocks 50-100 nm in height. The size of the SiC grains remains equal to that of the initial powder crystallites. The size of the Zn grains varies in the range of 20-100 nm depending on the initial SiC grain size and the composite fabrication conditions.

Abstract: A technique of preparation of novel nanocrystalline composites by infiltration of a liquid metal under high-pressure is presented. A porous nanocrystalline body is obtained by compacting nanosized powder of a high-hardness ceramic material under the pressure of 2-8 GPa. The molten metal penetrates into the open pores and crystallizes there upon cooling. As a result the second nano-phase is obtained. Practical aspects of the technique and some properties of the composites are discussed.

Abstract: The tool carbon steel powder, containing 1.1 % C, was subjected to heavy cold working by ball milling in a Fritsch P5 planetary ball mill. XRD studies showed that ball milling results in a dissolution of cementite and formation of nanoferrite. The crystallite size and lattice strain of ferrite, calculated by applying Williamson-Hall method, were 10 nm and 1%, respectively. Mössbauer spectroscopy measurements confirmed the formation of a phase called “distorted ferrite”, characterized by the values of hyperfine field of 28.5 T and isomer shift of 0.15 mm/s, different from ones of ferrite (32.9 T and 0.00 mm/s, respectively). DSC investigations revealed two heat effects recorded during heating the sample after 100 h of ball milling: exothermic effect at 360oC and endothermic one at 580oC. The first one was attributed to the dramatic decreasing of lattice strain (from 1% after milling down to 0.1%, as showed XRD studies) and slightly increasing of crystallite size (from 10 to 25 nm).The formation of Fe3C was not observed in this temperature and the structure of nanoferrite was preserved. The second observed heat effect was reversible and probably related to the eutectoid transformation, shifted by ball milling to lower temperature range, comparing to equilibrium conditions.