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, _ , lower than 10 � 10 s � 1 and low temperature ranging from 0:32Tm to 0:43Tm. 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, dg > 1600 mm, 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 Qc ¼ 32 kJ/mol. Microstructural observation showed the formation of large dislocation cells (� 10 mm) and tangled dislocations at the cell walls. For high-purity aluminum (5 N Al) with dg ¼ 24 mm, the stress exponent was n � 1 and the activation energy was Qc ¼ 15 kJ/mol. On the other hand, for commercial low-purity aluminum (2 N Al) with dg ¼ 25 mm, the stress exponent was n ¼ 2 and the activation energy was Qc ¼ 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 _ 10 � 10 s � 1 have been discussed. [doi:10.2320/matertrans.M2010405]