Effect of Er additions on ambient and high-temperature strength of precipitation-strengthened Al-Zr-Sc-Si alloys

The effect of substituting 0.01 at.% Er for Sc in an Al-0.06Zr-0.06Sc-0.04Si (at.%) alloy subjected to a two-stage aging treatment (4 h/300 degrees C and 8 h/425 degrees C) is assessed to determine the viability of dilute Al-Si-Zr-Sc-Er alloys for creep applications. Upon aging, coherent, 2-3 nm radius, L1(2)-ordered, trialuminide precipitates are created, consisting of an Er- and Sc-enriched core and a Zr-enriched shell; Si partitions to the precipitates without preference for the core or the shell. The Er substitution significantly improves the resistance of the alloy to dislocation creep at 400 degrees C, increasing the threshold stress from 7 to 10 MPa. Upon further aging under an applied stress for 1045 h at 400 degrees C, the precipitates grow modestly to a radius of 5-10 nm, and the threshold stress increases further to 14 MPa. These chemical and size effects on the threshold stress are in qualitative agreement with the predictions of a recent model, which considers the attractive interaction force between mismatching, coherent precipitates and dislocations that climb over them. Micron-size, intra- and intergranular, blocky Al3Er precipitates are also present, indicating that the solid solubility of Er in Al is exceeded, leading to a finer-grained microstructure, which results in diffusional creep at low stresses. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.