Volume 14, Issue 6
December 1991, pages 1279-1374
pp 1279-1307 December 1991
The phenomenon of repeated yield drops is extensively studied using computer simulation. The basic input is the Cottrell-Bilby model for dislocation drag, used in conjunction with a suitable scenario for work hardening. Stochastic effects associated with dislocation velocity and density are built in via multiplicative noise. Using the above scheme for modelling the plastic response and assuming first that the yielding is homogeneous, the machine equation is solved for successive time steps to obtain complete stress-strain curves, similar to those obtained in laboratory experiments. Real-life patterns are well simulated, including several detailed features observed in diverse experiments. The role of noise in the region of marginal stability is explored. The simulation study is next extended to the case of inhomogeneous yielding where, in addition to the model earlier proposed by Penning, we explore one which takes some account of dislocation flow across the sample. Attention is then drawn to similarities observed in the flow of sand in a sand pile. Finally, some suggestions are made for further work.
pp 1309-1322 December 1991
The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.
pp 1323-1329 December 1991
A divergent beam X-ray diffraction technique has been described for study of single crystals. This technique not only helps in observing defects in single crystals but also in determining the crystal orientation. Two methods have been proposed for observing images of sections of single-crystal components.
pp 1331-1341 December 1991
The effect of grain boundary width has been accounted for and a modified simple model of average carrier concentration is presented considering the transport mechanism of charge carriers by thermionic emission only. It is found that the electrical properties of polysilicon are very sensitive to doping concentration when the grain size is small and the effect of grain boundary width on electrical properties increases as the grain size decreases. The inclusion of grain boundary width in resistivity and mobility formulae also gives better results near the critical doping concentration. The proposed model gives better agreement between experimental data and theoretical results.
pp 1343-1348 December 1991
Single crystals of Bi2Sr2CaCu2O8 (2212) have been grown by self-flux technique using stoichiometric and non-stoichiometric melts of excess CuO and Bi2O3. Single-crystal and powder X-ray diffraction studies have been made on the grown crystals to confirm their single crystallinity and structure respectively. Resistivity and susceptibility measurements provide information on the superconducting nature of the crystals. The effects of fluxing agents and starting composition on surface morphology and superconducting properties have been discussed.
pp 1349-1352 December 1991
Spontaneous polarization and coercivity of sodium and lithium vanadates were found to be remarkably dependent on the solid solutions formed between the respective vanadates and Fe2O3. The decreasing trend inPs and the depression ofTc observed for increasing concentrations of the offvalent impurity were attributable to the formation of defective grain boundary layers of low electric polarization.
pp 1353-1362 December 1991
Electroinitiated polymerization of urethane was carried out using electrolysis of MDI (methylene 4,4′-diphenyl diisocyanate) with polyoxypropylene glycol in dimethyl sulphoxide (DMSO) and dimethylformamide (DMF) in the presence of oxides and chlorides of Zn, Cd, Pb and Sn as supporting electrolytes. These salts also served as complexing agents. The polymer formation took place only in the cathodic compartment having good yields with highestRp of 16·25% per h and polymerization efficiency of 30 × 102 moles of monomer converted per Faraday in DMF solvent. The mechanism of polymerization has been suggested.
pp 1363-1374 December 1991
The heart of a normal human being beats about 38 million cycles every year. An artificial heart valve, to perform at this rate in the adverse conditions inside the heart for 20 years or more, should be highly wear-resistant with excellent fatigue strength. Thus, the study of mechanical and physical properties of the materials intended for use in artificial valves becomes an inseparable part of the valve development process itself. The physical and mechanical requirements of the materials used in the Chitra heart valve have been evaluated by studying their water absorption, adhesive wear and abrasive properties. The mechanical durability of the device has been assessed by accelerated life cycle testing. The test systems developed for the above are described here. The results show UHMW-PE to be a highly wear-resistant material suitable for the occluder. The accelerated wear tests show that the valve with Haynes-25 alloy cage and UHMW-PE disc has durability in excess of 50 years.
Volume 42 | Issue 6
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