On the mechanism of grain refinement in Al–Zr–Ti alloys

In high-strength aluminum alloys Ti and Zr are commonly present as alloying elements, mostly as anti-recrystallization agents. Grain refinement during solidification is also achieved using Ti but in the form of titanium borides. Our previous investigations showed that a combined addition of Zr and Ti enables considerable grain refinement in aluminum alloys upon cavitation treatment, much stronger than that of Zr alone. The role of titanium and ultrasonic processing remained unclear. In this paper we propose a mechanism of the grain refinement that includes structural changes in solidification sites, their refinement and initiation of heterogeneous nucleation at lower undercooling.

Research highlights▶ Cavitation melt treatment applied above the temperature of primary aluminum formation promotes fragmentation of Al3Zr particles, decreases the size of potential solidification sites, and increases their number. ▶ Ti substitutes Zr in Al3(Zr1−xTix) particles, increasing the interfacial energy between aluminum and Al3(Zr1−xTix) particles, which leads to the delay of nucleation to larger undercooling. ▶ In this way Ti delays the nucleation to the temperature when smaller Al3(Zr1−xTix) particles, previously refined by cavitation, can be activated.