Solute-induced strengthening during creep of an aged-hardened Al-Mn-Zr alloy

We examine the precipitation and creep behavior of Al-0.5Mn-0.02Si (at.%) alloys, with and without the L1(2)-forming elements Zr and Er (0.09 and 0.05 at.%, respectively), utilizing isochronal aging experiments as well as compressive and tensile creep tests performed between 275 and 400 degrees C. The Al-0.5Mn-0.09Zr-0.05Er-0.05Si alloy exhibits an unusually high creep resistance in the peak-aged state, which is significantly better than that observed generally in its Mn-free L1 2-strengthened counterparts; for example, the creep threshold stresses at 300 degrees C are 34-37 MPa, about three times higher than those in a Mn-free Al0.11Zr-0.005Er-0.02Si alloy. Scanning transmission electron microscopy illustrates that nanoscale Al-3(Zr,Er) L1(2)-precipitates are formed in the dendritic cores and micron-sized Al(Mn,Fe)Si alpha- precipitates in the interdendritic channels. Moreover, the Al(f.c.c.)-matrix remains supersaturated with randomly distributed Mn solute atoms, as determined by atom-probe tomography and electrical conductivity measurements, for months at creep temperatures. Creep experiments on the Zr-and Er-free Al-0.5Mn-0.02Si solid-solution alloy reveal a small primary creep strain, a high apparent stress exponent, n(a) similar to 9-7, and a threshold-stress-type behavior. After ruling out other possible mechanisms, we provide evidence that the threshold stress in this precipitate-free alloy originates from dislocation/solute elastic interactions leading to a strong drag force exerted on edge dislocations, hindering their ability to climb. The relatively high creep resistance of Al-0.5Mn-0.09Zr-0.05Er-0.05Si is interpreted in terms of the synergy between this solute induced threshold stress (SITS , from Mn in solid-solution) and the known precipitate-bypass threshold stress (from the L1(2)-nanoprecipitates). (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

The precipitation and creep behavior of Al-Mn-Si alloys with L1(2)-forming elements Zr and Er were examined, showing significantly higher creep resistance in the Zr and Er-added alloy. Nanoscale and micron-sized precipitates were found to influence the alloy's performance.