Strain-induced martensite in ADI alloys

The remarkable mechanical properties of austempered ductile irons arise from their microstructure, which is composed of a high-carbon stabilized austenite matrix filled with small isles of ferrite platelets, appropriately called ausferrite. Cold work imposed on the material yields a partial transformation of retained austenite into martensite, which can strongly influence the mechanical behavior of the alloy. The present study deals with the determination of transformed austenite upon plastic deformation in the range from 0 to 25% thickness reduction of the samples. The analyzed material was a Cu–Mo–Ni-alloyed ADI, obtained by austempering at 400 °C for 90 min after austenization at 900 °C for 80 min under Ar-protecting atmosphere. The determination of retained austenite and induced martensite made use of the classical X-ray diffraction technique and Rietveld refinement of the intensity data, with all experimental profile modeled by non-linear least-squares fitting, using pseudo-Voigt functions. The overall results showed that the starting contents of retained austenite in three groups of samples, i.e. 43.20, 35.79 and 44.05 vol.%, gradually evolved to 18.23, 13.02 and 18.59%, respectively, after 20% cold work, while formation of martensite increased from nearly zero to 24.95, 24.01 and 25.52% under the same deformation process. These results are further analyzed on the basis of some classical transformation theories.