Authors: Praveen Kumar Michael E Kassner Terence G Langdon
Publish Date: 2007/01/04
Volume: 42, Issue: 2, Pages: 409-420
Abstract
Fifty years ago in a series of classic creep experiments conducted at the University of California in Berkeley Harper and Dorn obtained unique experimental data revealing the possibility of a new and heretofore unrecognized flow process occurring in pure aluminum when tested at low stresses and at temperatures very close to the melting temperature This flow mechanism subsequently designated Harper–Dorn creep has been the center of much argument and speculation in the ensuing years The present paper looks back over the last halfcentury and charts the various developments in attempts to obtain a more detailed understanding of whether Harper–Dorn creep is or is not a viable creep process Examples are presented for both metals and nonmetals It is concluded that although it appears Harper–Dorn creep may occur only under restricted conditions associated with high purity materials and low initial dislocation densities nevertheless there is good evidence supporting the validity of this creep mechanism as a viable and unique flow processWhen a stress is applied to a polycrystalline solid the material breaks if the stress is sufficiently high but when the stress is low the material gradually deforms plastically over a period of time leading ultimately to failure This extensive deformation with time is known as creep and it occurs more readily at high temperatures when diffusioncontrolled processes are reasonably rapid In practice the rate of creep in any crystalline solid is dependent upon the testing temperature the magnitude of the applied stress and the microstructural characteristics of the material Generally the variation of strain with time exhibits three distinct regions there is an initial or primary region where the rate of strain decreases with increasing strain there is often an extended secondary or steadystate region where the strain rate remains essentially constant and there is a third or tertiary region where the strain rate accelerates to final fracture Many of the theoretical creep mechanisms developed to date are concerned with predicting the rate of flow within the secondary or steadystate regionWhen polycrystalline metals deform under steadystate conditions it is well established that the creep rate dot varepsilon varies with the applied stress σ the absolute testing temperature T and the grain size d through a relationship of the formFor polycrystalline materials tested under creep conditions over a wide range of intermediate stresses the steadystate creep rate usually varies with the applied stress raised to a power lying typically within the range of ∼3–5 and the behavior is interpreted in terms of dislocation flow processes occurring within the grains In practice essentially similar powerlaw creep is observed in a very wide range of crystalline materials including metals 1 2 ceramics 3 4 geological minerals 5 and ice 6 At very high stresses the creep rate usually increases rapidly with stress in the region of powerlaw breakdown whereas at very low stresses there is another transition to a region where the stress exponent is very low and typically close to 1 This paper is concerned specifically with the flow characteristics in this low stress region where the behavior approximates to Newtonian viscous flow with a stress exponent of n ≈ 1 To place this report in perspective the following section examines the potential flow mechanisms occurring at low stresses the next section presents some of the arguments for and against the advent of Harper–Dorn creep as a separate flow process and the subsequent sections provide a comprehensive appraisal of the reported creep behavior occurring in this low stress region
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