Effect of Si micro-addition on creep resistance of a dilute Al-Sc-Zr-Er alloy

A dilute Al-0.06Sc-0.02Zr-0.005Er (at%) alloy, to which 0.09 at% Si was added, was peak-aged to create a high number density of (Al,Si)3(Sc,Er,Zr) precipitates, 3.6 nm in radius. The alloy shows a high resistance to dislocation creep, with a threshold stress of 18 MPa at 400 °C. After further aging under applied stress for ~ 1000 h at 400 °C, the threshold stress increases to 22 MPa, with the precipitates growing to a radius of 4-8 nm. This represents a very substantial improvement in creep resistance as compared to a similar alloy with one-third the Si content, 0.03 at%, whose threshold stress at 400 °C is 9-14 MPa. Atom probe tomography reveals that, for the new higher-Si alloy, the precipitates have an average Si concentration of 3.3 at% and show a broad core with uniform Sc-, Si- and Er concentrations and a thin Zr-enriched shell. By contrast, the low-Si alloy exhibits precipitates with half the average Si content, showing an Er-enriched core, a Sc-enriched inner-shell and a Zr-enriched outer-shell. A possible explanation for the higher creep resistance of the high-Si alloy is that the enhanced chemical homogeneity of Sc and Er in the core, as compared to the highly segregated core/shell/shell structure of the low-Si alloy, modifies the elastic strain field around precipitates so as to increase the repulsive force from the precipitate on the matrix dislocations climbing over them, thus enhancing the threshold creep stresses.