Effects of ultrasonic treatment on the tensile properties and microstructure of twin roll casting Mg–3%Al–1%Zn–0.8%Ce–0.3%Mn (wt%) alloy strips

Twin roll casted Mg–3%Al–1%Zn–0.8%Ce–0.3%Mn alloy strips with thicknesses of approximately 3.6–4 mm are prepared, and the effects of ultrasonic treatment on their tensile properties and microstructure are investigated. The results show that, after treatment with an ultrasonic power of 800 W, the grain size of α-Mg decreased from 136.3 μm to 44.7 μm, and the morphology changed from dendritic to globular. Grain multiplication by fragmentation of dendrites and cavitation-induced heterogeneous nucleation are the mechanisms of refinement of the α-Mg grains by ultrasonic treatment. The needle-like shaped intermetallic MgAlCeMn is modified by ultrasonic treatment and obtains a more globular shape with finer particles. This change was a result of undercooling created by the cavitation of ultrasonic treatment. This improved microstructure contributes to an increase in both tensile strength and elongation of twin roll casting magnesium alloy. The yield strength of the experimental alloy strips subjected to 800 W ultrasonic treatment is approximate 30% higher than that of alloy without ultrasonic treatment. Furthermore, the elongation of experimental alloy is almost double that of alloy without ultrasonic treatment. The relationship between the yield strength and the refined grain size of the experimental alloy can be expressed by the Hall–Petch equation σy = 74.8 + 31.4 × d−1/2.

• Ultrasonic treatment promoted grain refinement of α-Mg and modification of intermetallic Mg17(Al,Zn)12 and MgAlCeMn phases.
• Both the yield strength and elongation of twin roll casting Mg alloy strip improve obviously with ultrasonic treatment.
• The grain multiplication and the cavitation-induced heterogeneous nucleation by ultrasonic treatment lead to the refinement of Mg alloy grains.
• The undercooling created by the cavitation of ultrasonic treatment cause the morphology modification of second phases.