Formation Of Large Fragments Research Articles

Trying out spraying modes and properties of wear-resistant flame Al2O3–TiO2 coatings fabricated using a flexible cord material

Samples of Al2O3–TiO2 coatings are fabricated by the flame spraying of a flexible cord. The influence of process parameters and composition of the sprayed material on the structure, composition, and mechanical properties of coatings is investigated. It is shown that an increase in the spraying distance and feed rate of the sprayed material leads to a decrease in their density. An increase in the concentration of the low-melting TiO2 component predetermines a decrease in the coating porosity and has no significant effect on the coating hardness. Being subjected to measuring scratching, Al2O3–TiO2 flame coatings formed with minimal porosity (Π = 3.2%) are characterized by cohesion fracture behavior and no substrate opening under an indenter load of up to 90 N. The friction factor of coatings under study varies from 0.2 to 0.78 after 2800 counterbody revolutions (44 m of the friction path). This is associated with the accumulation of fatigue cracks in the coating material and its subsequent cohesive fracture by the formation of large fragments serving as an abrasive.

TRY-OUT OF SPRAYING MODES AND PROPERTIES OF WEAR RESISTANT Al2O3–TiO2 FLAME COATINGS OBTAINED USING FLEXIBLE CORD MATERIAL

Al 2 O 3 –TiO 2 -based coating specimens were obtained using the method of oxyfuel gas spraying of a flexible cord. The paper studies the influence of process parameters and composition of the sprayed material on the structure, composition and mechanical properties of coatings. It was shown that the increase in spraying distance and feed rate of the sprayed material leads to reduction in their density. An increased concentration of the low-melting TiO 2 component preconditions a decrease in coating porosity and has no significant effect on its hardness. During measuring scratching, the Al 2 O 3 –TiO 2 flame coatings formed with minimal porosity (porosity ~ 3,2 %) are characterized by cohesive fracture behavior and no substrate break up at the 90 N load applied to the indenter. The studied coatings show changes in their friction factor from 0,2 to 0,78 after 2800 counterbody revolutions (44 m of rubbing path). This is due to accumulated fatigue cracks in the coating material and its subsequent cohesive fracture through formation of large fragments that serve as an abrasive.

Образование фрагментированной структуры в низкоуглеродистых мартенситных сталях при теплой деформации

The structure of low-carbon martensite steels VKS-7 and VKS-10 subjected to warm rolling or compression in alpha state at 600 and 700 °C is studied by metallography, scanning and transmission electron microscopy. The role of dissolution and precipitation of carbide phases, sinking of dislocations to high-angle lath boundaries, polygonization and recrystallization in the process of formation of fragmented structure during warm deformation is considered. It is shown that the rolling to 40 % reduction at 600 °C does not produce fragmented structure, and laths divided into cells are retained in martensite. When deformation increases up to 60 %, separate fragments with high-angle boundaries appear. It is established that the fraction of fragmented structure increases significantly as deformation increases up to 80 %, and at the same time coalescence of subgrains having similar orientation occurs, resulting in the formation of large fragments of alpha phase. The increase of rolling temperature up to 700 °C and deformation up to 80 % produces effective fragmentation of structure in VKS-7 steel, though in the VKS-10 with higher alloy content some traces of lath structure remain. It is shown that the change of deformation mode of VKS-7 steel from rolling to overall compression by 50 % at 600 °C initiates in situ recrystallization of alpha phase and formation of large fraction of ultrafine-grained structure (grain size 0.2–0.5 m). In the VKS-10 steel these phenomena are observed at higher temperature of warm compression, in particular, at 700 °C.

Mg(2+)-dependent cleavage of DNA into kilobase pair fragments is responsible for the initial degradation of DNA in apoptosis.

Cleavage of DNA into oligonucleosomal fragments, recognizable as a DNA ladder on agarose gel electrophoresis, is usually considered as the biochemical hallmark of apoptosis. Recently, it has been shown that this internucleosomal cleavage is preceded by the formation of large fragments of DNA of > or = 700, 200-250, and 30-50 kilobase pairs (kbp) in length. Using isolated thymocyte nuclei, we now demonstrate that the formation of these large fragments is Mg(2+)- but not Ca(2+)-dependent. Further degradation of > or = 460- and 200-250-kbp fragments to 30-50-kbp fragments but not oligonucleosomal cleavage is also Mg(2+)-dependent but is facilitated by Ca2+. In contrast, formation of oligonucleosomal fragments does not occur in the presence of either Ca2+ or Mg2+ alone but requires the presence of both cations. These results support the hypothesis that the formation of these large fragments and the subsequent internucleosomal cleavage are distinct steps in the degradation of DNA in apoptosis in thymocytes.