In-situ characterization of the microstructure evolution during cyclic deformation of novel cast TRIP steel

Abstract

In-situ investigations of the cyclic deformation behavior of a metastable high-alloyed austenitic stainless cast TRIP-steel (TRIP–effect; TRansformation Induced Plasticity) in a scanning electron microscope (SEM) are presented. Low cycle fatigue (LCF) tests of the metastable cast steel alloy at different strain amplitudes showed that three different kinds of cyclic softening/hardening behaviour can be distinguished caused by different microstructures. In-situ push-pull tests in the SEM at two different applied total strain amplitudes were carried out showing the evolution of the microstructure in dependence on the number of cycles. The phase transformation of the metastable austenite to the α′-martensite and changes in the microstructure (deformation bands, dislocation arrangements) were investigated applying different SEM techniques as electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI). It is shown that the formation of deformation bands starts just from the beginning of the cyclic deformation and almost as multiple slip. ECC images show that these bands consist of very fine, elongated lamellas with increasing density and length at increasing number of cycles. Many of these fine lamellas grow together and form the deformation bands. The lamellas can be correlated with stacking faults. At a certain thickness the deformation bands were identified by EBSD as a hexagonal crystal structure. For the two applied total strain amplitudes different microstructures were observed regarding both the amount of martensite phase transformation as well as the dislocation arrangements.