Papers by Author: Andrzej Calka

Abstract: Aluminium reinforced with different volume fractions of milled carbon fibres (MCFs) were manufactured via advanced powder metallurgy processing method. Composites containing 5, 10, 15 and 20 % volume fraction of MCFs were prepared using the Uniball magneto milling technique. Full density cylindrical compacts were produced by uniaxial hot pressing (UHP) at 600°C for a dwelling time close to 15 minutes. Characterization was done using x-ray diffractometry (XRD) and Field Emission Scanning Electron Microscopy (FSEM). The mechanical and physical properties were determined by compression testing, Vickers hardness, and Archimedes density. Unniball milling of blends resulted in decrease of aspect ratio of the MCFs and refinement of the Al grain size. Results show slight uniform distribution of MCFs in Al matrix for higher volume fractions without discernible porosity. In samples containing low volume fractions of MCFs (5% and 10%) fibres tended to be pushed towards grain boundaries during grain growth. In samples with higher volume fraction of MCFs (15 and 20% a more uniform distribution of MCFs in the product was obtained. The compressive strength of the composites was enhanced by the fibres, attributable to good interface bonding and wetting between the matrix and the reinforcement fibres. The combination of controlled ball milling and UHP techniques has facilitated the development of Al-MCFs composites potentially to be used in automobile industries.

Abstract: The effect of phases and steel processing on hydrogen uptake (diffusible and residual), surface and internal damage were evaluated using optical and scanning electron microscopy. The results have shown the fastest formation of blisters in ferrite-pearlite microstructure of strip, followed by equaixed ferrite-pearlite microstructure in normalised condition, then by ferrite-bainite microstructure. No blistering was observed in heat affected zone samples for up to 24 hrs charging. Analysis of hydrogen-induced cracking using electron back scattering diffraction has revealed that crack propagation has predominantly intragranular character without a clear preference on {001}, {110}, {112} and {123} planes and is independent of the steel microstructure and prior processing.

Abstract: Advanced materials manufacturing methods require clean, non-polluting, high speed and precise processes, and should result in highly reliable final products. However, traditionally, many functional materials are synthesized by slow reaction techniques that are both energy and time consuming. In such cases there is strong demand for more appropriate materials processing methods that could offer increased rapid reaction rates and energy efficiencies, and be environmentally safe. Electric discharge assisted mechanical milling (EDAMM) is a new and exciting materials processing technique which combines the attributes of conventional mechanical and mechanochemical milling with the several additional processing effects which can be generated by the simultaneous application of electric discharges. It is shown that EDAMM can be applied to synthesize a range of functional materials in a matter of minutes, rather than the hours or days required using traditional techniques. This presentation provides an overview of recent developments in the EDAMM method and its application to rapid materials processing, and the synthesis of certain functional materials. In this report, we demonstrate the versatility of EDAMM by; (i) synthesis of hard materials, (ii) synthesis of functional oxides used in electronic, magnetic and optical applications, (iii) rapid reduction reactions including extraction of metals from oxides and sulfides and (iv) synthesis of fine metallic and nonmetallic powders and (v) formation of nano- fragments, including carbon nanoribbons and iron oxide nanorods.

Abstract: The densification behavior of WC composites based on iron aluminide binder was investigated using laser scanning confocal mi¬croscopy (LSCM). Doped Fe60Al40 alloys with boron levels ranging from 0 to 0.1 wt% were used as the aluminide binders. The aluminide binders were prepared using controlled atmosphere ring grinding and then blended with WC powder. The composite powder compacted in an alumina crucible and held in a platinum holder in the confocal microscope. The temperature increased from ambient temperature up to 1500 °C under high purity argon. The presence of boron was found to facilitate compaction of the composites and improve the wetting between WC and FeAl binder during liquid phase sintering. Increasing the amount of boron in the binder resulted in the melting of binder at lower temperature and increasing of the compacting of the intermetallic tungsten carbide composites.

Abstract: In this work oxidized and oxide-free amorphous boron (a-B) powder and elemental Mg were used in an attempt to directly synthesize the Mg(BH4)2 complex hydride by controlled reactive mechanical alloying (CRMA) under hydrogen in a magneto-mill up to 200h. The particle size was refined to the 100-200nm range. Nanocrystalline MgH2 (~6nm crystallite size) was formed within the particles when an oxidized a-B is used. In contrast, a mixture of MgB2 and an amorphous hydride MgHx was formed when an oxide-free a-B was used. Differential scanning calorimetry (DSC) test up to 500°C produced a single endothermic heat event at 357.7°C due to hydrogen desorption. In addition, desorption conducted in a Sieverts-type apparatus revealed ~1.4wt.% of hydrogen release. The X-ray diffraction pattern after DSC test of the 200h milled sample made with oxide-free boron showed the presence of MgB2.

Abstract: The effect of boron on the WC morphology and on the grain size of binders in sub micron WC composites containing Fe60Al40 and Ni3Al binders was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composites were prepared under uniaxial hot pressing of milled powder samples at 1500 °C in inert argon atmosphere. Doped aluminides with boron levels ranging from 0 to 0.1 wt% were used as the binders. It was found that the microstructural characteristics of boron doped aluminide WC composites were similar to those of hot pressed WC-Co and commercial grade WC-10wt%Co (H10F) hardmetals. The contiguity of WC particles (WC/WC contact) and the grain sizes of aluminides decreased and the extent of faceting of tungsten carbide increased in the aluminide tungsten carbide composites in presence of boron.

Abstract: The hardness and indentation fracture toughness of sub micron WC composites based on aluminide and cobalt binders were investigated. Doped Fe60Al40 and Ni3Al alloys with boron levels ranging from 0 to 0.1 wt%, were used as the aluminide binders. The composite materials were processed by uniaxial hot pressing of milled powder samples at 1500 °C under argon atmosphere. The hardness of WC-40vol%(FeAl-B) was found to be higher than that of WC-40vol%(Ni3Al-B), and it approached to the hardness level of the commercial grade of WC-10wt%Co (H10F). The fracture toughness of both WC-40vol%(FeAl-B) and WC-40vol%(Ni3Al-B) cermets was higher than that of WC-40vol%Co and the toughness increased with increasing boron content. It is believed that boron addition to the aluminide binders leads to improvement in the fracture toughness of the intermetallic matrix composites as a result of increase in the ductility and toughness of the aluminides and also due to increase in WC solubility in the aluminide binders in presence of boron.

Abstract: Electric discharge assisted mechanical milling using a 50 Hz power supply has been used to produce a range of fine and nanostructural products, including nanocrystalline aglomerates and individual nano-particles and nano-fragments. Processing variables include; starting powder sample size; electric arc parameters such as arc length and arc voltage/current; mechanical milling parameters; gas atmosphere and ionized gas species present. We describe results of an experimental program underway to investigate phase transformations and/or particle fragmentation during discharge milling using a new pulsed power supply working at frequencies in the kHz range. The aims of this preliminary investigation were to determine processing parameters required for the synthesis of potentially useful high surface area particles, nanostructural powders and nanoparticles, and to compare products with those synthesised by Hz frequency discharge milling. Microstructural, morphological, and phase changes induced by kHz discharge milling were characterised by x-ray diffractommetry and transmission electron microscopy. Results were found to depend on the often competing processes of fragmentation into nano-particles, agglomeration of powder particles, particle melting and/or sintering, and chemical reaction induced by mechanoprocessing in the presence of a particular type of plasma. Discharge milling of graphite under Ar/4%H2 resulted in a range of products including; graphite nanostructures, carbon nanotubes and other exotic nanofragments. It was found that, compared with processing at 50 Hz, high frequency (kHz) electric discharge assisted mechanical milling of graphite resulted in higher yields of carbon nanotubes. hematite resulted in partial reduction to magnetite and FeO,and the formation of nanostructural oxide nanorods and nanorod clusters. Discharge milling of Co-WC resulted in products including; micron and submicron fracture products, nanostructural regions of Co and WC, and carbon rich nanorods and nanotubes.

Abstract: In this paper both electric discharge assisted milling [1, 2] and conventional mechanosynthesis techniques were applied to investigate the effects of milling conditions on the fracture and agglomeration of amorphous CoSiB ribbons produced by planar flow casting. The effect of spark energy on particle shape and size produced by discharge milling was studied. Conventional milling in inert atmosphere for extended periods generally leads to the formation of porous powder particle aggregates, each particle comprised of small amorphous or, after extended milling times, nanocrystalline elements. The mechanism of agglomeration was believed to originate from repeated fracture, deformation and cold welding of individual ribbon elements. In contrast to conventional milling, spark discharge milling was found to induce the formation of predominantly sub-micron single particles of amorphous powder. The morphology of individual particles varied from sub-micron irregular shaped particles to remelted particles, depending on selection of vibrational amplitude during discharge. For high vibrational amplitudes and high energy input a wider range of particles as produced. These included sub-micron particles, remelted particles and welded agglomerates, and nano-sized particles produced as a fume and collected during discharge milling under flowing argon. These results combined with observations that most re-melted particles produced by discharge milling were also amorphous confirmed that extremely high heating and cooling rates are associated with discharge milling of metals. They also confirm the potential of electrical discharge milling as a new route for the synthesis of ultrafine and nanosized powder particles from amorphous ribbon, for possible processing into 3-D shapes.