Coherency between Al3Sc precipitate and the matrix in Al-Sc alloys

Coherency of the Al3Sc precipitates in Al-0.2Sc, Al-3.0Mg-0.2Sc and Al-2.5Cu-0.23Sc was investigated by quantitative analysis of spherically symmetrical strain contrasts around the precipitates in the TEM images. Measurement of coherency strain was made utilizing the equations developed by Ashby and Brown and assuming negligible solutions of Mg and Cu in the Al3Sc phase. Elastic strain at the Al/Al3SC interface in the binary alloy is almost independent of particle size. The strain in the ternary AlMg-Sc alloy tends to be smaller than that in the binary alloy, and shows negligible dependence on particle size. The critical radius for coherency-semicoherency transition as defined to be the critical radius where misfit dislocations appear, ranges from 20 nm to 40 nm in the binary alloy, while that in the Al-Mg-Sc ternary alloy ranges from 40 nm to 80 nm. For the Al-Cu-Sc alloy, coherency strain becomes slightly smaller with the growth of the Al3SC precipitates, and the critical radius falls on the same range as that for the binary alloy. The results above show that the addition of Mg or Cu enhances stability of the Al3SC precipitates.