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

Abstract

The creep behavior in pure aluminum has been investigated by helicoid spring creep tests at strain rates, \\dotε, 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 (∼10 μm) and tangled dislocations at the cell walls. For high-purity aluminum (5 N Al) with d_g=24 μm, the stress exponent was n∼1 and the activation energy was Q_c=15 kJ/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 \\dotε<10⁻¹⁰ s⁻¹ have been discussed.