Viscous Creep in Metallic Wires at Elevated Temperatures and Low Stresses
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Viscous Creep in Metallic Wires at Elevated Temperatures and Low Stresses. Jaroslav Fiala, Lubos Kloc1, Vaclav Sklenicka1 Brno University of Technology, Faculty of Chemistry, Institute of Material Chemistry, Purkynova 118, CZ-612 00 Brno, Czech Republic 1 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, CZ616 62 Brno, Czech Republic. ABSTRACT Viscous creep behaviour of several metals and alloys was investigated at the temperatures close to one half of the absolute melting point and at very low stresses using the technique of helicoid specimens. Due to extremely high sensitivity the technique represents a unique tool for measurement of very low creep strains in reasonable time. Helicoid spring specimens were made of wires of either circular or square cross section. The stress distribution along the wire radius (caused by shear stress loading) and threshold stresses were taken into account, as well as the influence of the surface layer loaded by maximum stress. The experimental results were interpreted as Coble diffusional creep and/or Harper-Dorn dislocation creep. Some data are in a very good agreement with Coble theory especially those obtained on some fine grained materials. For the coarse grained materials is this dependence replaced by large data scattering. Some authors dispute about the role or even the very existence of diffusional creep and offer other explanations. There are many theories trying to describe Harper-Dorn creep mechanism, but none of them is capable to explain all observed properties. The observed effects which cannot be explained by the current theories are discussed (large scatter of creep rates obtained for coarse grain materials, creep rates much higher than those predicted by the diffusional creep theory in some materials, transition stage duration independent of stress but dependent on temperature. Despite the problems in theoretical description, the experiment shows clearly that the viscous creep regime must be considered as an important behaviour of structural materials at conditions of engineering practice. INTRODUCTION The contemporary demands of the creep life of parts in power-generating and chemical industry lead to the very low creep rates in the order of 10-13 s-1. Extrapolation of the creep behaviour from the higher creep rates is frequently used, since for practical reasons it is impossible to carry out the experiments under the conditions corresponding to the practical application of the material. Unfortunately the extrapolation can provide some misleading conclusions [1]. The helicoid spring specimen technique of creep tests, developed by Burton and Greenwood [2] and later modified considerably in our laboratory [3] is a suitable method to verify the conclusions obtained by extrapolation because of its high strain sensitivity. Under the low stress creep regime, the main differences in the creep behaviour are: - stress dependence of the creep rate is weaker and is characterized by the stress exponent of 2 or even of 1(so called viscous
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