Jozef Ivan

Influence of Al2O3 particles volume fraction on fracture mechanism in the Cu–Al2O3 system

Abstract The influence of Al 2 O 3 particles volume fraction on the fracture mechanism in the Cu–Al 2 O 3 system is analyzed by “in situ tensile test in SEM”. It is shown that tensile strain first produces microcavities on matrix–particle interfaces due to decohesion of particles from the matrix. The cavities grow and coalesce first in the direction of rows of particles with growing strain. Cracks develop in both directions: in the rows, and in the direction perpendicular to the rows, leading to final fracture. The deformation value at fracture is dependent on Al 2 O 3 particle volume fraction. A model explaining the fracture mechanism is introduced.

Seasonal dynamics of antibiotic-resistantEnterobacteriaceae in the gastrointestinal tract of domestic sheep

Considerable variation in counts of antibiotic-resistant enterobacteria in the ovine gastrointestinal tract was observed. The occurrence of ruminal and fecal isolates resistant to ampicillin (Ap), kanamycin (Km) and tetracycline (Tc) culminated in summer months, followed by rapid decline in subsequent months. Using PCR thetem1bla (Apr),aphA1 (Kmr) andtetB (Tcr) genes were found to be predominant. Underin vitro conditions all resistance genes were transferable into laboratoryEscherichia coli strain with relatively high frequency (10−3 transconjugants per recipient).

Structural instability of small rolling circle replication plasmids from Selenomonas ruminantium.

The complete nucleotide sequence of pSRD192 plasmid from Selenomonas ruminantium 19D has been obtained and analyzed. The plasmid, 2334bp in length, was shown to replicate by rolling circle replication mechanism. By PCR method variability of pSRD192-like plasmids was investigated and another variant of pSRD192-like plasmid; the pSRM22 plasmid 2338bp in length; was detected and characterized. Both pSRD192 and pSRM22 plasmids share an identical rep gene and origins of replication to that of another S. ruminantium pONE429 plasmid. Other than that there are additional regions of sequence similarity between the three plasmids, interspersed with divergent regions. The sequence comparisons suggest structural instability of pSRD192-like rolling circle replication plasmids.

Spreading and mutability ofSelenomonas ruminantium plasmids

Two small plasmids fromSelenomonas ruminantium strain 19D were cloned inEscherichia coli and completely characterized. Sequence comparison indicated that the plasmids are similar to those reported in genetically vaguely relatedS. ruminantium strain S20. Small 1.4-kb plasmids pSRD191 and pONE430 are only distantly related (≈30 % for deduced Rep protein amino acid sequence) but possess a short highly conserved region outsiderep gene. Larger plasmids pSRD192 and pONE429 possess large identical DNA regions in an otherwise dissimilar background. Recombination is proposed as an important mechanism of evolution and spreading ofS. ruminantium plasmids.

Influence of oxygen content on some properties of Nb3Sn tape used for superconducting transmission lines

The influence of the presence of oxygen in the Nb-Zr substrate upon some physical and mechanical properties of the Nb-ZrO2 substrate and/or Nb3Sn tape superconductor prepared from such a substrate has been studied. The results have shown that the Nb3Sn grains are much finer in the case of Nb-ZrO2, than those obtained on Nb-Zr substrate. As a result, both the electrical and mechanical properties have been substantially improved. The critical current at a field of 5 T increased by between 100 and 200 %, the alternating current losses decreased by about one order of magnitude, the tensile strength increased by about a factor of three and the microhardness value also increased by about a factor of three for oxygen contents in the range 0.03 to 0.70 wt%. The specific resistance of this substrate increased at the same time from 17 to 22 μΩ cm and the relative elongation decreased from 5 to 3 %.

Influence of technological factors on dispersion strengthened materials deformation mechanism studied by ‘in-situ tensile test in SEM’

Direct observation of deformation process until formation of fracture on extreme small samples of composite in SEM is interesting from practical and interpretation aspects. In the present work the effects of composite composition, volume fractions of secondary phases as well as the matrix grain sizes (micro and nano size) on deformation and failure processes of Al and Cu-based composites were evaluated. Experimental materials were prepared by the different powder metallurgy methods. Interpretation of results was based on the quantification of the physical parameters of material phases depending on the preparation methods and on the analysis of deformation processes using the ‘in-situ tensile test in SEM’. The mechanism of deformation and fracture for each composite system was estimated and models of fracture were proposed.

Microstructure, Mechanical Properties and Fracture of Pt -Y2O3 Composites

The composite prepared by mechanical alloying of ine Pt powder obtained by waste recovery from linings >f glass furnaces with Y 2 0 3 dispersed particles (0.5 nass %) has shown better mechanical properties at room temperature as well as at higher temperatures in comparison with those of Pt alloys commercially produced. The qualitative factor, which is known to be a function of starting powders parameters, technology of preparation and compacting operations, shows some correlation particularly with size and space distribution of Y 2 0 3 phase in the Pt matrix. K e y w o r d s : Pt-system, Fracture mechanism, Mechanical properties, Qualitative factor

Superplastic Deformation of Al-Al4C3 Composites

Deformation of the Al-Al 4 C 3 composites with different volume fraction of Al 4 C 3 phase was tested at different temperatures and different strain rates. As shown, at temperatures 400-450°C and the highest strain rate applied 10 -1 s -1 , a significant ductility growth was observed. According to the results of TEM analysis, it is supposed to be the result of dynamic grain polygonization, grain slip and rotation, partial recrystallization and dislocation creep in the tested system, known as strain induced dynamic recovery. Increase of the volume fraction of secondary phase in the investigated composite changed the deformation mechanism (from more slip on grain boundaries to more grain rotation).

Influence of particles in Cu-Al2O3 system on fracture mechanism

Abstract The influence of Al2O3 particles on fracture of the Cu–Al2O3 system is analyzed by ‘in situ tensile test in SEM’. It has been shown, that tensile strain in the extrusion direction produced first micro cavities on matrix–particle interfaces due to decohesion of particles from the matrix. The cavities grow and coalesce first in the direction of rows of particles with growing strain, and then in direction perpendicular to the rows, leading to final fracture. The strain at fracture is dependent on Al2O3 particle volume fraction. The fracture mechanism is not dependent on Al2O3 particle volume fraction in the tested range. Model explaining the fracture mechanism was introduced.

Variability of putative rep gene cassettes in Selenomonas ruminantium plasmids

Characteristic feature of the most of Selenomonas ruminantium cryptic plasmids is the presence of short, conserved sequences encompassing the gene for replication protein creating a potential rep gene cassette. PCR-based experiment was designed to analyse the genetic organization of putative plasmid rep modules and to assess S. ruminantium plasmid biodiversity. Analysed PCR amplicons contained single open reading frames encoding for putative replication proteins. While most of the derived protein sequences were often found to be conserved among putative plasmid molecules, at noncoding regions, genetic variability was observed to various extents. Complete nucleotide sequence of a plasmid was determined that contained probably a new rep gene only distantly related to known selenomonas Rep proteins but at noncoding regions shared high homology with already known plasmids. Our results document considerable structural instability and sequence variability of analysed rep gene cassettes and suggest a modular structure of S. ruminantium plasmids potentially accessible for rep gene module exchanges.