Microstructural evolution in deformed austenitic TWinning Induced Plasticity steels

Abstract

This thesis studies the effect of plastic deformation on the stability of the austenitic microstructure against martensitic transformation and diffusional decomposition and its role in the phenomenon of delayed fracture in austenitic manganese (Mn)-based TWinning Induced Plasticity (TWIP) steels. The transformation to alpha’-martensite upon mechanical loading and diffusional decomposition into pearlite upon annealing at intermediate temperatures shows the austenite to be metastable. An increase in the austenite stability is expected to improve the resistance against delayed fracture. In the automotive industry, the requirements for fuel economy and safety are continuously increasing. Improvements in fuel economy require a lower weight of the vehicle whereas improvements in safety often result in additional weight. To resolve this contradiction, the requirements for strength and formability of steel increase continuously. To this purpose, the steel industry develops (Advanced) High Strength Steels and Press Hardening Steels. One of the latest developments is fully austenitic Mn-based TWIP steels that combine a high strength with a very high uniform elongation. These superior mechanical properties result from the high work-hardening of these austenitic Mn-based TWIP steels. The main reasons for this high work-hardening are deformation mechanisms combining slip of dislocations with strain induced microtwinning and martensite transformation. The deformation mechanisms relate to the austenite stability and form shear bands like slip bands, twins and/or epsilon-martensite laths, which are obstacles for further dislocation glide increasing work-hardening. In addition to usual application issues like formability and weldability, a problem encountered with austenitic Mn-based TWIP steels is delayed fracture. This is the phenomenon that even after successful forming, fracture may still occur. The time until fracture can range from seconds to weeks. Increased understanding of the phenomenon of delayed fracture would accelerate the introduction of austenitic Mn-based TWIP steels to the automotive industry, enabling further weight reduction and improved safety and fuel economy. The susceptibility to delayed fracture is a combination of (1) the austenite stability against microstructural defect formation, (2) the internal residual stress and (3) the presence of mobile hydrogen. Most research on delayed fracture concentrates on the role of hydrogen, leaving the austenite stability against defect formation and internal residual stress underexposed. Increasing the austenite stability against microstructural defect formation like strain-induced transformation improves the resistance against delayed fracture. This work discusses the effect of plastic deformation on the stability of the austenitic microstructure against martensitic transformation and diffusional decomposition and its role in the phenomenon of delayed fracture. The effect of deep drawing on the generation of structural defects in austenitic Mn-based TWIP steels is investigated experimentally using X-ray diffraction, positron annihilation Doppler broadening spectroscopy and magnetic measurements. To this purpose, the characteristics of defects were studied along the wall of deep-drawn cups, representing a gradually changing deformation state. Positron annihilation detects that two different defect types result from plastic deformation during deep drawing. The two defect types can be expected to be dislocations and partial dislocations. Magnetic field measurements reveal the formation of alpha’-martensite which correlates with the density of the defects identified as partial dislocations. The effect of strain on the defect and microstructure evolution in austenitic Mn-based TWIP steels is experimentally investigated using magnetic measurements, X-ray diffraction, positron beam Doppler Spectroscopy and Transmission Electron Microscopy techniques. The strain evolution during deep drawing is simulated by means of Finite Element Method simulations. The formation of alpha’-martensite is attributed to the accumulated equivalent strain and crystallographic texture. The presence of alpha’-martensite is observed at shear band and twin intersections and questions the sequential gamma -> epsilon -> alpha’ martensitic transformation. The results indicate that the formation of alpha’-martensite in a high Stacking Fault Energy (SFE) Face Centred Cubic alloy does not necessarily require the intermediate formation of epsilon-martensite laths. A model for alpha’-martensite volume fraction evolution upon straining is proposed and the estimated fraction of intersected shear bands - the preferred nucleation sites for alpha’-martensite formation - as a function of accumulated equivalent strain is in good agreement with the experimentally determined alpha’-martensite fraction. The role of alpha’-martensite in the phenomenon of delayed fracture is studied in austenitic Mn-based TWIP steels after deep drawing, observed by in-situ video recording. The formation of alpha’-martensite indicates the formation of crack initiation sites, which is discussed as a possible cause of delayed fracture. Delayed fracture occurs where the alpha’-martensite fraction is the highest. An intermittent crack propagation concept and model is proposed based on the coalescence of initial cracks into a macrocrack. A higher alpha’-martensite fraction indicates a higher density of shear-band intersections, resulting in more potential crack-initiation sites and easier coalescence. The SFE in the tested range of 22 to 52 mJ/m2 does not affect the formation of alpha’-martensite and does not relate to the delayed fracture susceptibility. The transformation of austenite by martensitic mechanisms upon plastic deformation shows the metastability of the austenite and indicates diffusional decomposition of austenite into pearlite in case the material is annealed at temperatures below the A1-temperature. This transformation and the effect of prior plastic deformation on the austenite decomposition into pearlite at intermediate temperatures is investigated. The transformation kinetics are governed by Mn-partitioning between ferrite and cementite within the pearlite. Mn-diffusion is too slow to allow partitioning between pearlite and austenite, and a mixed equilibrium condition is established of ortho-equilibrium between ferrite and pearlite and para-equilibrium between pearlite and austenite. Nucleation of pearlite takes place only in the initial stages of the transformation. Prior plastic deformation enhances the formation rate of pearlite from austenite and increases the number density of pearlite colonies, primarily through increased nucleation efficiency. Prior plastic deformation does not significantly affect the nucleation rate or growth rate in the observed timescale.