Papers by Author: Dariusz Oleszak

Abstract: The influence of Sn addition on the amorphization of CuTiZrNi alloys processed by mechanical alloying of a mixture of pure elemental powder was studied. The thermal stability and crystallization behaviour of the amorphous mechanically alloyed powders was determined and compared with rapidly quenched ribbons with the same nominal chemical compositions. X-ray diffraction and differential scanning calorimetry were employed as the experimental techniques for samples characterization. Both applied samples preparation techniques resulted in the formation of fully amorphous Cu47Ti34Zr11Ni8 and Cu37Sn10Ti34Zr11Ni8 alloys. However, significant differences in thermal stability and crystallization behaviour have been found, depending not only on the alloy composition, but on the fabrication method as well. The observed influence of Sn addition was more evident for the ribbons, resulting in the change of the number of crystallization effects, their temperatures and activation energy of crystallization. For mechanically alloyed powders these changes were not so dramatic.

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.

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.