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 15 min of holdings. Meanwhile, it was found that UT breaks down natural oxide films into fine gamma-Al2O3 particles with {111} or {110} exposed surfaces. The well-defined cube-on-cube orientation relationships between gamma-Al2O3 and alpha-Al verify that gamma-Al2O3 dispersoids can act as potent substrates for the nucleation of alpha-Al grains. Based on experimental observations and theoretic calculations, enhanced heterogeneous nucleation in A1050 alloys can be attributed to the high potency of gamma-Al2O3 particles with atomic misfits of similar to 2 % on all exposed surfaces and their increased number density induced by the synergistic effects of cavitation and acoustic streaming.