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, ε̇, 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 (5 N Al) with an average grain size, d_g > 1600 μm, 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 ∼ 5 and the activation energy was Q_c = 32 kJ/mol. Microstructural observation showed the formation of large dislocation cells (∼10μm) and tangled dislocations at the cell walls. For high-purity aluminum (5N Al) with d _g = 24 μm, the stress exponent was n ∼ 1 and the activation energy was Q_c = 15kJ/mol. On the other hand, for commercial low-purity aluminum (2 N Al) with d_g = 25 μm, 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 ε̇ < 10^-10 s^-l have been discussed.