SUPERPLASTICITY IN AN AL-ZN-MG-CU-ZR ALLOY PREPARED BY POWDER-METALLURGY

A very fine-grained Al-Zn-Mg-Cu-Zr alloy prepared using high energy ga s atomization was studied. The alloy exhibits structural superplastici ty at temperatures between 700 and 800 K at strain rates of the order 10(-2) s(-1). Its deformation behaviour may be described by the concep t of a threshold stress for grain boundary sliding. A continuous recry stallization takes place during superplastic straining. The gradual in crease of the mean misorientation angle between neighbouring grains is believed to contribute to the development of grain boundary sliding a nd to the dynamic softening.