A multiscale investigation of the microstructure and the mechanical behavior of TRIP steels is presented. A multi-phase field model is employed to predict the microstructure of a low-alloy TRIP700 steel during a two-stage heat treatment. The resulting stability of retained austenite is examined through the $M_s^{\sigma }$ temperature. The phase field results are experimentally validated and implemented into a model for the kinetics of retained austenite during strain-induced transformation. The kinetics model is calibrated by using experimental data for the evolution of the martensite volume fraction in uniaxial tension. The transformation kinetics model is used together with homogenization methods for non-linear composites to develop a constitutive model for the mechanical behavior of the TRIP steel. A methodology for the numerical integration of the constitutive equations is developed and the model is implemented in a general-purpose finite element program (ABAQUS). Necking of a bar in uniaxial tension is simulated and “forming limit diagrams” (FLDs) for sheets made of TRIP steels are calculated. The models developed provide an integrated simulation toolkit for the computer-assisted design of TRIP steels and can be used to translate mechanical property requirements into optimised microstructural characteristics and to identify the appropriate processing routes.

High silicon and molybdenum ductile cast irons (Si-Mo alloys) are commonly used as exhaust manifold materials suffering from high temperature-oxidation and thermal-mechanical fatigue. The structural integrity of cast Si-Mo alloys under these service conditions is attributed to their microstructure consisting of spheroidal graphite and Mo-rich carbide embedded in a ferritic matrix. However, the cast structure includes also pearlite structure having a detrimental effect on the mechanical properties, therefore the cast matrix needs to be heat treated. In this study, the solidification of a Si-Mo ductile iron was investigated using (i) thermodynamic and kinetic calculations by Thermo-Calc and DICTRA software and (ii) thermal analysis in order to reveal out the sequence of phase formation and the phase transformations during solidification and (iii) microanalysis by energy dispersive spectrometer in order to determine elemental segregation and compare with the calculated values. The solidified structure was also characterized and all microstructural features were specified.

Extrudability of aluminum alloys of the 6xxx series is highly dependent on the microstructure of the homogenized billets. It is therefore very important to characterize quantitatively the state of homogenization of the as-cast billets. The quantification of the homogenization state was based on the measurement of specific microstructural indices, which describe the size and shape of the intermetallics and indicate the state of homogenization. The indices evaluated were the following: aspect ratio (AR), which is the ratio of the maximum to the minimum diameter of the particles, feret (F), which is the maximum caliper length, and circularity (C), which is a measure of how closely a particle resembles a circle in a 2D metallographic section. The method included extensive metallographic work and the measurement of a large number of particles, including a statistical analysis, in order to investigate the effect of homogenization time. Among the indices examined, the circularity index exhibited the most consistent variation with homogenization time. The lowest value of the circularity index coincided with the metallographic observation for necklace formation. Shorter homogenization times resulted in intermediate homogenization stages involving rounding of edges or particle pinching. The results indicated that the index-based quantification of the homogenization state could provide a credible method for the selection of homogenization process parameters towards enhanced extrudability.