Transformation Superplasticity in Ductile Cast Iron

Abstract

The influence of the concurrent phase transformation on the deformation of ferritic and pearlitic ductile cast irons was studied under tensile stresses at high and low thermal cycling rates. Transformation plasticity, characterized by the linear relationship between the transformation strain per cycle and the externally applied stress (i.e., strain rate sensitivity exponent, m is 0.8), was observed under all conditions. The transformation strain is greater on heating from pearlite to austenite than on cooling from austenite to pearlite. The transformation strain at the high heating and cooling rate results in a smaller transformation strain. Large uniform elongations (above 200%) can be obtained by repeated thermal cycling under a constant stress. It is suggested that the plasticity is due to the creep enhanced by excess vacancies generated during the volume change. In the early thermal cycling, the voids are formed at both sides of the spheroidal graphite. In the later stage, the voids are filled up with the matrix strnchire snrressively.