Threshold stress during high temperature creep of a commercial purity aluminium

The natural law of creep with n=3 stress exponent can only be observed in solid solution alloys with large size effect of the solute atoms and it is interpreted by viscous glide of dislocations (1,2). Generally a higher stress exponent is observed during high temperature creep of pure metals and dispersion hardened alloys (3-S). In pure aluminium two hinds of stress exponents, n=4-5 and n=7-8 were found. The higher values are attributed to stable subgrain structure developing either before or at the beginning of the deformation and it does not change during the test The obstacles against the migration of dislocations can be other dislocations, grain (subgrain) boundaries as well as second phase particles. This is the reason that the same n value is observed in some oxide dispersion strengthened materials as in pure metals and the creep is explained by dislocation slip controlled by lattice diffusion (6,7). Thmshold stress has been observed in several alloys comaining dispersed particles resulting in a high apparent activation energy and stress exponent which were explained by local climb around particles (8). Recently, the local climb concept was criticized and a theory based on the attractive interaction between particles andmalrixorin aneutectic systembetweenthepatticles of the two metals determming the strength of pinning was developed (9). This theory can satisfactoiy explain the extreme high stress exponent and activation energy found in alloys strengthened by oxide, carbide, silicide, etc. (9,lO). In ahrminium alloys containing transition metal alloying elements (Fe, Mn, Cr, Co) the activation energy of the high temperature defmnation was found to be also higher than that of the lattice diffusion along with a stress exponent between 7 and 10 (1 1 14). These results were attributed to the ef%kct of the large amount of constituent particles formed during solidification and further processing because of the low solid solubility ofthe transition elements in aluminium. However, there was no explanation of this observation until now. The concept of threshold stress was applied mostly in the case of mperplastic alloys (15 18). In this work the high temperature creep of commercial purity ahaninium having special microstructure is studiedItisshownthatthere&scanbei.nkpmMbythepresence of a threshold stress. A comparison with the behaviour of a 99.99% purity almninium tested under similar experimental conditions is also presents