Room temperature creep and substructure formation in pure aluminum at ultra-low strain rates

The creep behavior in pure aluminum has been investigated by helicoid spring creep tests at strain rates, ε, lower than 10^-10 s ^-1 and room temperature, 298 K. It was found that the creep behavior at the very low ε depends strongly on grain sizes and impurity concentrations: in high-purity aluminum (5 N Al) with an average grain size of 24 μm, the stress exponent was n ∼ 1; while, when the average grain size was larger than 1600 μm, the stress exponent was n ∼ 5. Microstructural observation shows the formation of large dislocation cells, 10 μm. On the other hand, in commercial low-purity aluminum (2 N Al) with the average grain size of 25 μm, the stress exponent was n = 2. Microstructural observations revealed dislocations emitted from grain boundaries and the formation of lattice dislocation cells. To evaluate creep deformation mechanisms of the pure aluminum, stress change tests were conducted during creep tests. It was revealed that the deformation in the range of the stress exponent, n ∼ 5, was controlled by recovery driven by internal stress, ^σi, because instantaneous strains at stress increment were larger than that at stress reduction. While the deformation behaviors in the ranges of the stress exponents, n ∼ 1 and n = 2, were in viscous manner, because instantaneous strains at stress increment and reduction were in the same level. Based on those experimental results, the creep mechanisms have been discussed.