Characterisation of TRIP-assisted steel by atomic force microscopy and OIM

TRIP-assisted steel has a complicated microstructure, obtained through well designed heat treatments and consisting of different phases, mainly ferrite, retained austenite, bainite and martensite. The drastic influence of microstructure on physical and mechanical properties makes metallographic examination a necessity for property understanding and enhancement. A fairly simple tint etching procedure (LePera method) using light microscopy and SEM observation ensures the distinction of ferrite, bainite and retained austenite/martensite grains. Nevertheless, the etching is not selective enough to allow a distinction between retained austenite and martensite. Atomic force microscopy was used for the phase identification and characterisation of this kind of steel. A series of tests was performed using AFM-contact mode and magnetic force microscopy in C-Mn-Si and C-Mn-Al TRIP-assisted steel after intercritical annealing followed by ageing, After ageing, some samples were tempered in order to make metallographic observation easier. It has been possible to identify the different phases and their topographic characteristics and to study their morphology using AFM. Orientation imaging microscopy was used to calibrate the observations done with AFM, distinguishing between bcc- and fcc-zones and quantify the retained austenite.