Enhanced heterogeneous nucleation in Al alloys by melt ultrasonic treatment

The mechanisms of grain refinement in the A1050 alloy through melt ultrasonic treatment (UT) and isothermal holding were investigated. The results showed that UT resulted in considerable grain refinement, with refining efficiency tending to stabilize after 15min of holdings. Meanwhile, it was found that UT breaks down natural oxide films into fine γ-Al2O3 particles with {111} or {110} exposed surfaces. The well-defined cube-on-cube orientation relationships between γ-Al2O3 and α-Al verify that γ-Al2O3 dispersoids can act as potent substrates for the nucleation of α-Al grains. Based on experimental observations and theoretic calculations, enhanced heterogeneous nucleation in A1050 alloys can be attributed to the high potency of γ-Al2O3 particles with atomic misfits of ~2% on all exposed surfaces and their increased number density induced by the synergistic effects of cavitation and acoustic streaming.