Characterisation of Multiphase Steel Surface Topography by Atomic Force 
Microscopy 
Tanya Ros Yäfiez, Yvan Houbaert, Ghent University, Department of Metallurgy and Materials 
Science, Zwijnaarde (Gent), Belgium 
1. Introduction 
[n recent years considerable research effort has been carried out on the development of high strength 
steels for the automotive industry, but in general strength increase results in a decrease of the 
formability. TRIP-assisted steel (TRansformation Induced Plasticity) is a new family of steels 
offering a unique combination of high strength and ductility achieved through the transformation of 
metastable retained austenite to martensite under straining. The presence of austenite in the 
microstructure appears therefore to be critical for the achievement of the desired properties. Different 
metallurgical mechanisms can be used to control exactly the microstructure, in order to manipulate 
the mechanical behaviour of the material and to obtain the adequate combination of properties. TRIP 
assisted-steels have 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 understanding and enhancement of the behaviour. 
Microstructural observations are usually made with scanning (SEM), transmission electron 
microscopy (TEM) and colour etching methods in optical microscopy (OM). Observations with 
optical microscopy on samples etched with Nital (2%) do not give satisfactory results since they 
hardly exhibit contrast. Nital etches preferentially ferrite while leaving almost intact austenite and 
cementite. When the microstructure contains simultaneously austenite and martensite, Nital is not 
selective enough to permit a distinction between both phases. Etching by the LePera method (colour 
etching) allows phase identification according to the coloration. However, both retained austenite 
and martensite, if present, appear as yellow fine dispersed grains and consequently cannot be 
distinguished, so that martensite and austenite can not be quantified. Also in the SEM using 
secondary electrons, Nital-etching is not selective enough when martensite and retained austenite 
appear simultaneously. This indicates the need for additional techniques in order to allow a better 
identification of phases (1). 
Atomic Force Microscopy (AFM) for the study of TRIP-assisted steels can give interesting results 
concerning phase identification and better understanding of the distribution of phases, through 
topographic analysis of the observed region and additional techniques as Lateral Force Microscopy 
(LFM) and nanoindentation. 
2. Materials and experimental procedure 
TRIP-assisted steels with different chemical compositions have been studied in the lab, but in this 
paper only results are given obtained with one of these steels produced at the lab with following 
composition: 0.11 % C — 0.06 % Si — 1.50 % Al — 1.55 % Mn. In this steel the Si (normally used to 
provoke the TRIP effect) has been replaced by Al to avoid technological problems during industrial 
processing of the material (2). The slabs were initially hot rolled to a thickness of 3 mm following a 
classic processing route and samples were studied after different heat treatments. Intercritical 
annealing was done at 800°C during variable times in order to obtain a two-phase structure of ferrite 
and austenite. Dual phase samples were then obtained by quenching in water immediately after the 
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