Size-dependent creep of duralumin micro-pillars at room temperature

The strength of aluminum alloy 2025 (duralumin) micro-pillars is known to be significantly higher than that of pure Al micro-pillars of comparable sizes since the precipitates present act as obstacles to trap dislocations within the small sample volume. In this work, the creep behavior of precipitate-hardened duralumin micro-pillars of sizes ∼1 μm to ∼6.5 μm is investigated by compression experiments at room temperature. The effects of an internal grain boundary were also investigated by comparing the creep behavior between single crystalline and bi-crystalline micro-pillars. The results reveal that peak-aged duralumin pillars, in which the produced precipitates can efficiently block mobile dislocations, show increasingly significant creep with increasing pillar size, with a typical creep rate of ∼10−4 s−1 which is drastically larger than that of bulk at room temperature. The bi-crystalline pillars creep even faster than the single crystalline counterparts. TEM examination of the deformed microstructures reveals that the creep rate depends on the residual dislocation density, indicating that dislocations are the agents for creep. Theoretical modeling suggests that the steady-state creep rate is proportional to the lifetime of mobile dislocations, which rises with specimen size in the microns range due to the fact that the dislocations are not easily pinned in this range. As the pillar size increases in this range, the dislocations spend longer time in viscous motion across the specimen, hence the retained dislocation density is higher which leads to a higher strain rate according to the Orowan equation. It is expected that this trend of creep rate with specimen size will be reversed for larger specimens, probably in the tens of microns range, when dislocations experience a higher chance of being pinned and immobilized by the precipitates.