Resumen de Microstructure and mechanical response of nanostructured bainitic steels
Lucia Morales Rivas
Nanostructured bainite is a promising new steel concept presenting excellent values of rivalling mechanical properties, strength vs. toughness or ductility. These microstructures, as opposed to conventional bainite, contain retained austenite. This, together with the fact that the structure is refined down to the nanoscale, are responsible for the good mechanical properties. The microstructure is formed by a complete austenitization and a subsequent isothermal holding for the bainitic transformation, at temperatures about 200-350¿C. The microstructure consists basically of two phases: a hard matrix of bainitic ferrite and a carbon-enriched retained austenite, the second dispersed phase. The complex composite character of the microstructure determines its stress-strain response. Mechanically-induced martensitic transformation is thought to play an important role in the work-hardening behaviour, as in transformation-induced plasticity aided steels. Whereas extensive previous research has been carried out to understand the mechanisms and microstructural parameters that control the strength of these steels, no conclusive results exist for the ductility. It is the objective of this work to determine the factors that affect the ductility, deformation and fracture. For that purpose, tensile tests have been performed on different samples, and microstructural examinations have been carried out on both undeformed and cross-sections of the tensile deformed samples. First of all, the characterization of the microstructure and its relation with heat treatment parameters, transformation temperature and time, have been addressed. Special emphasis has been put on the crystallographic characteristics of nanostructured bainite, studied by means of electron backscatter diffraction, X-ray diffraction analysis and also transmission electron microscopy. Results show, in general, a refinement of the microstructure as the treatment temperature decreases. There is also a structural relaxation of bainitic ferrite and a favored carbon enrichment of austenite as the treatment temperature increases, which is now known to occur at the expenses of the carbon placed at defect-free solid solution in bainitic ferrite and at clusters, boundaries or dislocations. Noteworthy on their own it is the new advance on the characterization of bainitic ferrite crystal structure, which turns out to be tetragonal rather than cubic. The full understanding of the composite behavior of nanostructured bainite requires, at a first stage, the local characterization of its mechanical properties, which are expected to change from one phase, bainitic ferrite, to another, austenite. The combined use of atomic force microscopy-based techniques, such as nanoindentation and peak force quantitative nanomechanical measurements is devoted to that purpose. Limits and advantages of these challenging techniques have been critically addressed and some elastic-plastic parameters of both phases have been measured. Peak force quantitative nanomechanical results have been compared to those obtained from the analysis of the loading force curve of single indentation and have been discussed in terms of the nature and scale of the microstructures. Results point out that, on one hand, within the elastic region, differences in mechanical properties between phases are within the error bars. On the other hand, for high treatment temperatures, plastic behavior of retained austenite and bainitic ferrite might be quite similar. Understanding the deformation mechanisms is key to confirm the importance of the microstructure and its composite behavior in the control of ductility. They can be revealed by tracking the austenite fraction evolution and also the texture evolution of both austenite and ferrite while tensile testing. Martensitic transformation is found to contribute as a softening mechanism rather than as a strengthening one. Assuming stress-assisted martensitic transformation, stress partitioning between the different phases seems to decrease as both the heat treatment temperature and the carbon enrichment of austenite increase. In that case, martensitic transformation takes place more progressively as a function of the plastic strain or can even be inhibited. The texture evolution arises not only from the preferential martensitic transformation depending on the austenite crystal orientation, but also from the coordinated plastic crystal rotation of austenite and ferrite, which contributes to a positive latter work-hardening rise phenomenon. Finally, planar defects such as stacking faults and even nanotwins may develop during straining promoted by a strong stress shear component.
