A low temperature Weissenberg camera

Details of the design and operation of a Weissenberg camera suitable for x-ray investigations between -180°c and 200°c are presented. The camera employs a novel arrangement of spur and bevel gears to couple the goniometer spindle to the worm rod which controls the range of oscillation. The entire drive system and the goniometer assembly are mounted on a support which permits the insertion or removal of a cylindrical cassette from the gear-box side without disturbing the cooling assembly and the layer-line screen. The cassette can also be inserted from the opposite side. The specimen can be cooled either directly by a stream of liquid air or by the cold gas from its evaporation. Condensation of moisture at low temperatures is prevented by heating the layer-line tubes internally.

Room-temperature equal channel angular extrusion of pure magnesium

In this paper, we demonstrate a way to impart severe plastic deformation to magnesium at room temperature to produce ultrafine grain size of similar to 250 nm through equal channel angular extrusion (ECAE). The strategy to deform magnesium at lower temperature or to achieve such grain sizes has been proposed as: (i) to obtain a suitable initial orientation with high Schmid factor for basal slip and low Schmid factor for pyramidal/prismatic slip; (ii) to take advantage of low stacking fault energy of basal and high stacking fault energies of prismatic/pyramidal planes in order to relatively work-harden the basal plane with respect to the pyramidal/prismatic plane; and (iii) to lower the temperature of deformation in steps, leading to continual refinement of grains, resulting in finer grain size. The experimental as well as simulated texture of ECAE-processed samples indicate that the deformation mechanism leading to ultrafine grain size is slip-dominated. The recrystallization mechanism during ECAE has been found to be orientation-dependent. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Low-temperature deformation behaviour of polycrystalline copper

Low-temperature plastic flow in copper was investigated by studying its tensile and creep deformation characteristics. The dependence of the flow stress on temperature and strain rate was used to evaluate the thermal activation energy while the activation area was derived from the change-in-stress creep experiments. A value of 0.6 eV was obtained for the total obstacle energy both in electrolytic and commerical copper. The activation areas in copper of three selected purities fell in the range 1200 to 100 b2. A forest intersection mechanism seems to control the temperature dependent part of the flow stress. The increase in the athermal component of the flow stress with impurity content in copper is attributed to a change in the dislocation density. The investigation also revealed that thermal activation of some attractive junctions also takes place during low-temperature creep. The model of attractive junction formation on a stress decrement during creep, yields a value of 45±10 ergs cm-2 for the stacking fault energy in copper.

Free convection heat transfer from vertical cylinders part I: Power law surface temperature variation

This paper reports on the investigations of laminar free convection heat transfer from vertical cylinders and wires whose surface temperature varies along the height according to the relation TW - T∞ = Nxn. The set of boundary layer partial differential equations and the boundary conditions are transformed to a more amenable form and solved by the process of successive substitution. Numerical solutions of the first approximated equations (two-point nonlinear boundary value type of ordinary differential equations) bring about the major contribution to the problem (about 95%), as seen from the solutions of higher approximations. The results reduce to those for the isothermal case when n=0. Criteria for classifying the cylinders into three broad categories, viz., short cylinders, long cylinders and wires, have been developed. For all values of n the same criteria hold. Heat transfer correlations obtained for short cylinders (which coincide with those of flat plates) are checked with those available in the literature. Heat transfer and fluid flow correlations are developed for all the regimes.

Free convection heat transfer from vertical cylinders part II: Exponential surface temperature variation

Investigation on laminar free convection heat transfer from vertical cylinders and wires having a surface temperature variation of the form TW - T∞ = M emx are presented. As in Part I for power law surface temperature variation, the axisymmetric boundary layer equations of mass, momentum and energy are transformed to more convenient forms and solved numerically. The second approximation refines the results of the first upto a maximum of only 2%. Analysis of the results indicates that cylinders can be classified into the same three categories as in Part I, namely, short cylinders, long cylinders, and wires, heat transfer and fluid flow correlations being developed for each case.

An evaluation of rate-controlling obstacles for low-temperature deformation of zirconium

The low-temperature plastic flow of alpha-zirconium was studied by employing constantrate tensile tests and differential-stress creep experiments. The activation parameters, enthalpy and area, have been obtained as a function of stress for pure, as well as commercial zirconium. The activation area is independent of grain size and purity and falls to about 9b2 at high stresses. The deformation mechanism below about 700° K is found to be controlled by a single thermally activated process, and not a two-stage activation mechanism. Several dislocation mechanisms are examined and it is concluded that overcoming the Peierls energy humps by the formation of kink pairs in a length of dislocation is the rate-controlling mechanism. The total energy needed to nucleate a double kink is about 0.8 eV in pure zirconium and 1 eV in commercial zirconium

The effect of temperature on the magnetic susceptibility of vanadium pentoxide systems

The magnetic susceptibilities of certain vanadium pentoxide systems supported by kieselgur have been determined in the temperature interval 30° to 400° C. The plot of reciprocal susceptibility against temperature for all the systems studied indicates sudden deflections at temperatures which are about 150° lower than those of optimum catalytic activity. It has been suggested that these points may mark the temperatures of commencement of structural changes which may be responsible for the activity of these catalysts.

Investigation of activated transport in hole doped rare earth manganites in the high temperature paramagnetic regime

Electronic transport in the high temperature paramagnetic regime of the colossal magnetoresistive oxides, La(1-x)A(x)MnO(3), A=Ca, Sr, Ba, x similar or equal to 0.1-0.3, has been investigated using resistivity measurements. The main motivation for this work is to relook into the actual magnitude of the activation energy for transport in a number of manganites and study its variation as a function of hole doping (x), average A-site cation radius (< r(A)>), cationic disorder (sigma(2)) and strain (epsilon(zz)). We show that contrary to current practice, the description of a single activation energy in this phase is not entirely accurate. Our results clearly reveal a strong dependence of the activation energy on the hole doping as well as disorder. Comparing the results across different substituent species with different < r(A)> reveals the importance of sigma(2) as a metric to qualify any analysis based on (r(A)). (c) 2006 Elsevier Ltd. All rights reserved.