Low-Temperature Creep at Ultra-Low Strain Rates in Pure Aluminum Studied by a Helicoid Spring Specimen Technique

The creep behavior in pure aluminum has been investigated by helicoid spring creep tests at strain rates, (epsilon) over dot, lower than 10(-10) s(-1) and low temperature ranging from 0.32T(m) to 0.43T(m). It was found that the creep behavior in this region depends strongly on grain sizes and impurity concentrations. For high-purity aluminum (5N Al) with an average grain size, d(g) > 1600 mu m, nearly the wire diameter of the spring sample, where the role of grain boundary during creep deformation can be negligible, the stress exponent was n similar to 5 and the activation energy was Q(c) = 32 kJ/mol. Microstructural observation showed the formation of large dislocation cells (similar to 10 mu m) and tangled dislocations at the cell walls. For high-purity aluminum (5N Al) with d(g) = 24 mu m, the stress exponent was n similar to 1 and the activation energy was Q(c) = 15 kJ/mol. On the other hand, for commercial low-purity aluminum (2N Al) with d(g) = 25 mu m, the stress exponent was n = 2 and the activation energy was Q(c)= 25 kJ/mol. Microstructural observations revealed dislocations emitted from grain boundaries, those dislocations interacting with intragranular dislocations and the formation of dislocation cells in the grains. Based on those experimental results, the low-temperature creep mechanisms in pure aluminum at (epsilon) over dot < 10(-10) s(-1) have been discussed. [doi:10.2320/matertrans.M2010405]