The kinetics of transformation in Zn-Ai superplastic alloys

Abstract

We report herein on the kinetics of transformation of a eutectoid Zn-AI alloy containing additions of Cu, Mg and Ca. The alloy possesses excellent superplasticity at elevated temperatures, and it has a relatively high strength at ambient temperature (∼345 MPa). TTT curves for the alloy are presented, and the corresponding microstructures obtained at the various transformation temperatures are reported. Also, the results of Jominy endquenched tests are reported and the corresponding continuous cooling kinetics are compared to the isothermal kinetics. The alloy was observed to decompose by two distinct mechanisms, depending upon the degree of supercooling. At temperatures just below the eutectoid, it decomposes into a lamellar microstructure, whereas at larger undercooling it decomposes into a coherent two-phase mixture. The interlamellar spacing and colony size are reported as a function of transformation temperature, and shown to follow expected trends. Since neither the lamellar nor coherent microstructure is superplastic, thermomechanical methods of producing a superplastic structure are discussed.