Andrzej Calka

Synthesis and structural evolution of tungsten carbide prepared by ball milling

Tungsten carbide has been synthesized directly by ball-milling tungsten powder and activated carbon in vacuum. The structural development of the WC phase with milling times up to 310 h has been followed using X-ray, neutron diffraction and scanning electron microscopy. Subsequent annealing (at 1000 °C for 1 and 20 h) of material milled for 90 h or longer, results in samples comprising almost entirely crystalline WC. The production of WC itself during milling results in enhanced iron contamination from the steel mill and balls on extended milling which were monitored by energy-dispersive X-ray and Mossbauer spectroscopies.

Ball milling of Fe75-C25 : formation of Fe3C and Fe7C3

Abstract Powder mixtures of Fe 75 -C 25 (both graphite and activated carbon) have been ball-milled in vacuum for periods of up to 285 h. X-ray diffraction, Mossbauer spectroscopy and thermal analysis measurements indicate that an amorphous Fe 3 C-type phase is produced on short-term milling (less than 70 h), with a crystalline Fe 3 C being obtained on further milling to 140 h. This Fe 3 C-type phase was found to undergo partial carbon oxidation between 500 and 1000 °C during thermogravimetric measurement, indicating the metastable state of this phase. The carbon-rich Fe 7 C 3 phase was observed on extended milling of Fe 75 C 25 (graphite) to 285 h, in agreement with earlier findings.

Ti–TiN hardmetals prepared by in situ formation of TiN during reactive ball milling of Ti in ammonia

Abstract Vapour deposition of titanium nitride on WC/Co or hard ferrous-based cutting tips generally results in significant increases in cutting tool life. However, a major limitation of such nitrided tips is that they cannot be resharpened for re-use. Although monolithic TiN may be too brittle for cutting tool applications, with appropriate microstructural design, Ti–TiN composites should have the required combinations of toughness, ductility, hardness, wear resistance and thermal conductivity to replace coated tips for a range of machining applications. We report the synthesis of monolithic Ti–TiN composites from nanostructural precursor powders. Reactive ball milling of Ti in nitrogen or ammonia under controlled conditions eventually results in the formation of nanostructural TiN. Furthermore, by ending the reaction after an appropriate period a homogeneous and uniform mixture of Ti and TiN phases can easily be produced. Due to the highly reactive, nanostructural nature of the powder product this synthesis route has the potential to eliminate wetting problems generally associated with the current technology of conventional liquid-phase sintering. Moreover, by controlling nitriding gas pressure changes during milling good control of both the Ti to TiN ratio and final crystallite size distributions can be achieved. It was found that precursor Ti–TiN nanostructural powders synthesised in this way can be successfully compacted and liquid phase sintered without sintering aids. Such compacts show high densities and nanoindentation hardnesses in the range of 18–23 GPa. Structural characterization was performed using X-ray analysis, transmission and scanning electron microscopy as well as optical microscopy. The mechanical properties were characterised using micro- and macroindentation techniques.

Formation of nanocrystalline cubic (L12) titanium trialuminide by controlled ball milling

Abstract Pre-alloyed, as-cast ingots of the Mn-modified, cubic (L1 2 ) titanium trialuminide (65 at% Al, 25.6 at% Ti and 94 at% Mn) were homogenized (1000 °C/100), crushed into a coarse-particle powdered material and subsequently ball milled for up to 386 h under shearing mode in a controlled ball movement mill. X-ray spectra of milled powders showed line broadening and decrease in intensity of Bragg peaks with increasing milling time. This is associated with the formation of nanocrystalline grains and lattice strains upon milling. Crystallite size calculated from peak broadening, remains relatively unchanged from 19 up to 100 h of milling (20–30 nm) and then drastically decreases reaching a saturation size of about 3 nm after 200 h of milling. Lattice strains are on the average less than 1%. Simultaneously, the ordered L1 2 crystal structure undergoes disordering which commences after approximately 40 h and terminates after 160 h of milling. The microstructure of powder particles undergoes a complex evolution. With increasing milling time the particles are formed which appear to contain a work-hardened core. Each such a particle is surrounded by a heavily deformed, hard outer layer containing nanometer grains. After 386 h of milling all the core/outer layer particles are transformed into uniform ‘no core’ ones, characterized by approximately 3 nm crystallite size (XRD measurements). The microhardness data for both outer layer in the powder particles with a core, and the ‘no core’ particles can be fitted by a Hall-Petch dependence on the inverse root of crystallite size: HV 0.01 = 431.7 + 387.5 d −0.5 (kg mm −2 ) where HV 0.01 is Vickers microhardness at 0.01 kg and d is crystallite size in nm. These results are discussed in view of the existing models of hardening of nanosized materials.

Cold-work induced phenomena in B2 FeAl intermetallics

For the last several years we have been conducting systematic studies of microstructural changes and mechanical behavior of B2 FeAl alloys, cold-worked by ball-milling and shock-wave loading. Fundamental physical phenomena induced in iron aluminides such as an accelerated chemical disordering, increase in the lattice parameter, non-magnetic/magnetic transformation and formation of nanocrystals in ball-milled powder particles are presented and discussed.

The role of hydrogen and iron in silicon nitridation by ball milling

We have studied in detail the α-Si3N4 formation during ball milling of Si in NH3 and N2 environments and subsequent annealing. α-Si3N4 can be formed upon annealing after Si is milled in NH3, but no such phase is found for milling in N2. This difference is attributed to the role of H in embrittlement of the powder particles and to the subsequent introduction of Fe contamination into the Si powder. Our studies also show that the presence of Fe greatly enhances the nitridation reaction of as-milled Si powder. Fe silicide phases can be found after milling in NH3 and annealing, and the extent of silicide depends on the N content in the powder. In favorable cases, Fe can be removed from the α-Si3N4 powder by an acid leach.

Mechanical Milling Assisted by Electrical Discharge - Recent Developments

A novel device for milling, incorporating high voltage, low current electrical discharges was constructed and its application for materials processing investigated [1, 2]. This type of milling has been found to result in rapid fracture rates, enhanced mechano-chemical reactions and novel reaction paths. We present recent studies of fracturing, agglomeration and phase formation using this method applied to a vibrational rod mill. The effect of spark discharge milling condition on particle size and surface morphology was investigated for a number of different materials including; alumina, NiZr, Ni plus Si, Mg-Zn alloy of eutectic composition, and FeSiB metallic glass ribbons. In these experiments, variations in milling vibrational amplitude resulted in variations in nominal average spark length for samples discharged milled under repeated impact. It was confirmed that during discharge milling, rapid fracturing occurs over very short milling times and is accompanied by the formation of both fine particles and agglomerates. Large vibrational amplitudes tended to promote increased particle agglomeration in both ceramics and metals, while discharge milling with lower vibrational amplitudes promoted the formation of finer particles and smaller agglomerates. In the case of alumina, particle coarsening and spheroidisation was believed to result from repeated sintering of individual particles. For metals, alloys and metallic glasses, the tendency for coarsening and formation of spherical particles resulted from some combination of partial melting and deformation. Spark milling of Mg-Zn eutectic decomposition product was found to result in formation of the metastable eutectic phase, Mg7Zn3, while spark milling of FeSiB metallic glass resulted in formation of extremely fine particles of metallic glass powder, and remelted particles still in the amorphous state. Such amorphous particle distributions are difficult to produce using conventional milling techniques. The results were taken as evidence that extremely high heating and cooling rates are associated with discharge milling of metals. Introduction Electric discharge assisted milling combines mechanical milling with electrical discharge materials processing. This new technique involves application of high voltage, low current electrical discharges during materials synthesis and processing. Within the mill, the electrical discharges cause molecular breakdown of the controlled atmosphere, the formation of monatomic gasses and, depending on the species present and discharge conditions, formation of specific types of plasma in the proximity of powder particles. Reactive milling experiments were performed under different discharge conditions and were found to result in completely different reaction paths for the same reacting species [1-3]. Conditions could be optimised to promote formation of a range of metastable and nanostructural products, enhancement of reactions, such as nitration of solids, the direct formation of new phases from elemental ingredients, and reduction reactions. Arc discharge milling (described as spark milling) was found to result in orders of magnitude increases in fracture rates. For brittle, low electrical conductivity materials it was found that the electrical discharges associated with this milling method significantly speeded up fracturing, the fracture mechanism involving the bulk breakdown of individual powder particles. For conductive metals, fracturing was found to occur via chipping and shaving from the surface of particles. The mechanisms associated with arc discharge milling are complex and not understood. and is little literature on materials processing under conditions which predominantly involved arc Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 111-117 doi:10.4028/www.scientific.net/JMNM.20-21.111

Hydrogen Storage Properties of Mg-BCC Composite

MgH2 with 10 wt% Ti0.4Mn0.22Cr0.1V0.28 (termed BCC for its body-centered cubic structure) nanocomposite was fabricated by ball milling using different ball-to-powder weight ratios. The X-ray diffraction patterns make it clear that pure Mg powder is partly transformed to MgH2, while by adding the BCC, its hydriding becomes complete. The scanning electron microscope images showed that the BCC particles were uniformly dispersed on the surface of the Mg particles. Differential scanning calorimetry traces of the samples showed that the addition of the BCC obviously decreases the desorption temperature, and an additional decrease is observed from increasing the ball-to-powder weight ratio. The hydriding/dehydriding and the pressure-composition isotherm curves indicate significant improvement in the absorption/desorption kinetics and the hydrogen storage capacity of MgH2 from both adding the BCC and increasing the ball-to-powder weight ratio. The results indicate that the BCC acts as a medium that facilitates hydrogen absorption during hydrogenation on Mg, thus improving hydrogen storage capacity and absorption/desorption kinetics.

Formation of Amorphous and Nanostructural Powder Particles from Amorphous Metallic Glass Ribbons Using Ball Milling and Electrical Discharge Milling

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.

Effect of Moisture on the Synthesis of TiC by Mechanically-induced Self-Propagating Reaction

Titanium and activated carbon powders, with a Ti60C40 starting composition, were milled under a helium atmosphere using a magneto ball mill. When using activated carbon that had been exposed to a humid atmosphere, milling for up to 68 hours failed to produce TiC. The activated carbon was subsequently dried and stored under helium. The experiments were repeated, using the dried activated carbon, and TiC was produced via a mechanicallyinduced self-propagating reaction (MSR), after milling for approximately 40 hours. It was concluded that moisture adsorbed by the activated carbon when exposed to the humid atmosphere prevented the reaction to form TiC. Introduction Titanium carbide (TiC) is a material of commercial interest because it possesses a range of desirable properties. It is extremely hard; being one of the hardest known metal carbides [1]. TiC exhibits excellent thermal stability and has a very high melting temperature of approximately 3100°C [2-6]. This combination of very high hardness, high melting temperature and excellent thermal and chemical stability makes TiC suited to a number of commercial applications. TiC is often used in applications such as abrasives, cutting tools, grinding wheels and coated cutting tips [2-6]. It is also used as a hardening phase in superalloys [4]. However, one of the disadvantages of using TiC for commercial applications is that it is difficult to produce. The production of TiC is currently energy intensive and requires expensive high temperature equipment. For instance, current production methods involve reactions carried out at temperatures well above the melting point of titanium (1670°C). These high temperature production processes include carbothermal reduction of titanium dioxide, carburisation of titanium by heating in the vapour of a suitable hydrocarbon and the direct reaction of titanium with carbon [1-4, 6-8]. It has been shown recently that TiC powder can be produced during the high-energy milling of titanium and carbon powders [1-4, 6, 9-14]. High-energy milling has a number of potential advantages compared to the existing processes used to commercially produce TiC. High-energy milling is performed at room temperature, so there is no need for expensive high temperature reaction equipment; which could result in significant capital expenditure savings. This synthesis method can also be easily scaled up to commercial capacities. Current highenergy milling production units, used for the production of oxide dispersion strengthened (ODS) superalloys, process 1 ton of powder in a 2m diameter mill which contains more than 1 million balls weighting approximately 10 tons [15]. Another advantage of high-energy milling is that the final product is a fine, homogeneous nanocrystalline powder. This powder can then be easily shaped and consolidated using conventional powder metallurgy processes. This work examines the effect of adsorbed moisture on the formation of TiC during the highenergy milling of titanium and carbon powders. Journal of Metastable and Nanocrystalline Materials Online: 2005-01-12 ISSN: 2297-6620, Vol. 26, pp 8-15 doi:10.4028/www.scientific.net/JMNM.26.8