High-intensity ultrasonic grain refinement of magnesium alloys: role of solute

Liquid metals that solidify in a high-intensity ultrasonic field are subjected to vibrations typically at a frequency of 20 kHz with an amplitude in the range 10-50 mu m. In addition, ultrasonic cavitation produces violent local disturbances. These dynamic factors are expected to diminish solute buildup ahead of the growing fronts, thereby diminishing the dependence of grain size on solute. An investigation has been made of high-intensity ultrasonication of pure magnesium, binary, and commercial magnesium alloys to determine the role of solute under such dynamic conditions. It was found that the average grain size ((d) over bar) achieved by high-intensity ultrasonication exhibited strong dependence on solute and the dependence can be described quantitatively by the same (d) over bar -1/Q linear relationship proposed for solidification under normal casting conditions. These observations suggest that high-intensity ultrasonication has not obviously changed the role of solute and ultrasonic grain refinement arises from the effects of solute as well as ultrasonication.