Effect of solidification cooling rate on kinetics of continuous/discontinuous Al3(Sc,Zr) precipitation and the subsequent age-hardening response in cold-rolled AlMgSc(Zr) sheets

Abstract

We explain the critical effect of casting processing route (i.e., the solidification cooling rate) on the kinetics of Sc–Zr precipitation upon solidification and, subsequently, on the age-hardening response of cold-rolled/aged Al–3Mg-0.36Sc-0.14Zr sheets. The evolution mechanisms of various types of Al3(Sc,Zr) precipitates are investigated in the as-cast specimens (fabricated via strip casting vs. mold casting techniques), as well as in the subsequently cold-rolled/aged sheet specimens. The Al3(Sc,Zr) precipitates appeared to have formed upon either discontinuous or continuous precipitation during cooling from solidification temperatures; the former leads to the formation of lamellar Al3(Sc,Zr) cells while the latter results in the formation of spherical precipitates, all of which possessing an L12 crystal structure and a coherent interface with the α-Al matrix. It is shown that slower cooling rates via mold casting lead to the occurrence of both continuous and discontinuous precipitation upon cooling from the solidification temperature. In contrast, higher cooling rates via strip casting results in the apparent prevention of all discontinuous precipitation reactions as well as in the suppression of continuous precipitation. The overall effect is the greater preservation of Sc supersaturation within the α-Al matrix of the as-cast specimens fabricated via strip casting which, subsequently, leads to higher precipitation kinetics and thus higher tensile properties upon age-hardening of the cold-rolled sheets.