Effect of ultrasonic treatment on segregation and mechanical properties of as-cast Mg-Gd binary alloys

The current study investigates the effects of ultrasonic treatment on the gravity segregation and dendritic segregation of the Mg-8Gd alloys. Corresponding microstructures and mechanical properties at different heights of the ingots were analyzed. Without ultrasonic treatment, the microstructures and mechanical properties at different heights of the ingot were significantly different. The content of Gd in the bottom sample reached up to 11.4% and a large number of network eutectic structures could be observed along the coarse dendrites, which was confirmed to be the main reason for the brittle fracture with a poor elongation of 2.75%. In addition, there were also some cubic phases proved to be alpha-Gd, MgGd and Mg2Gd. When the ultrasonic was introduced into the melt, the cavitation effect and acoustic streaming occurred, resulting in the uniformities of temperature and Gd content in the alloy melt. As a consequence, the microstructures at different heights tend to be similar with the equiaxed alpha-Mg matrix containing Mg5Gd in homogeneous dispersion state and both the strength and the elongation of the samples were improved obviously.