ANOMALOUS STRESS RELAXATION BEHAVIOR OF POLYCRYSTALLINE ALUMINUM AT LOW TEMPERATURE

Abstract

The tensile deformation of polycrystalline aluminum (99.999%) was studied between 18 and 300 K. The stress-relaxation at constant strain was determined at strain intervals of about 0.5% with total strain exceeding about 3%. Stress-relaxation curves were logarithmic except at large "t" where they flatten. The relaxation rate "s" was determined by using equation s=d(Δσ)/d ln t, where Δσ(t)=σ0-σ(t) is the amount of stress relaxed at any instant of time "t" from the initial stress level σ0 at which relaxation was allowed to start. The slope ds/dσ0 was observed to attain maxima at about 30 K and minima at about 60 K. The undulation in the temperature dependence of stress relaxation rate in the range 18–60 K is an outcome of changes in the substructures of dislocations which have developed during the deformation process. These changes then require stresses higher than that applied in the basic equations which describe the kinetics of the mode of deformation. The average intrinsic height of potential barrier U0, estimated by means of the single barrier model of stress relaxation, was 1.2±0.3 eV. These values appeared to be compatible with the dislocation-dislocation intersection, controlling the rate processes in polycrystalline aluminum.