Control of the homogenization process is important in obtaining high extrudability and desirable properties in 6xxx aluminum alloys. Three consecutive steps of the process chain were modeled. Microsegregation arising from solidification was described with the Scheil–Gulliver model. Dissolution of Mg2Si, Si (diamond) and β-AlFeSi (β-Al5FeSi) to α-AlFeSi (α-Al12(FeMn)3Si) transformation during homogenization have been described with a CALPHAD-based multicomponent diffusion Dual-Grain Model (DGM), accounting for grain size inhomogeneity. Mg2Si precipitation and associated strengthening during homogenization cooling were modeled with the Kampmann–Wagner Numerical (KWN) precipitation framework. The DGM model indicated that the fractions of β-AlFeSi and α-AlFeSi exhibit an exact spatial and temporal correspondence during transformation. The predictions are in good agreement with experimental data. The KWN model indicated the development of a bimodal particle size distribution during homogenization cooling, arising from corresponding nucleation events. The associated strengthening, arising from solid solution and precipitation strengthening, was in good agreement with experimental results. The proposed modeling approach is a valuable tool for the prediction of microstructure evolution during the homogenization of 6xxx aluminum alloys, including the often-neglected part of homogenization cooling.

Rolling contact fatigue (RCF) is one of the most important failure mechanisms in rails with significant cost‐ and safety‐related implications on the operation of railway systems. In this work, a metallurgical analysis of RCF crack initiation and propagation, including geometrical characteristics of RCF cracks – length, depth from surface, angle of propagation and spacing between cracks, is presented. The role of proeutectoid ferrite in crack initiation has been studied. Analysis of the fracture surface of an RCF crack revealed a ductile initiation zone followed by a quasi‐cleavage crack propagation. Iron oxide formed in the interior of all cracks in rails exposed to stagnant water with implications to crack propagation rate because of crack closure effects. Sequential sectioning parallel to the rolling surface revealed that RCF cracks possess convoluted surfaces. The crack trace expands with depth from the rolling surface. Subsurface crack initiation has also been documented.

The evolution of austenite fraction and the associated solute partitioning during the intercritical annealing of medium-Mn steels are of great importance for austenite stabilization and the mechanical performance of this class of steels. In the present work, a 4.5Mn steel is subjected to a cyclic treatment and the evolution of the austenite fraction is measured with dilatometry. The evolution of austenite fraction and solute partitioning are simulated for a case where the starting time of the cyclic treatment is well before the equilibrium fractions have been established in the respective isothermal intercritical treatment. The evolution of austenite during thermal cycling in the intercritical range comprises of forward, inverse, and stagnant stages. The fraction of austenite formed decreases in each successive cycle while the kinetics of the evolution of austenite is controlled by the Mn diffusion in ferrite. Partitioning of Mn and C takes place from ferrite to austenite during the cyclic transformation. Due to the low diffusivity in austenite, wells form in the composition profiles in austenite of both Mn and C. These wells are the locus of the interfacial compositions of austenite, corresponding to the variation of the local equilibrium conditions during the thermal cycle.

The mapping of Mg2Si and β-AlFeSi phase fractions in the as-cast microstructure of Al–Mg–Si–Fe–Mn (6xxx series) alloys has been performed over the useful composition range (0–1.2 mass%) of the principal alloying elements Mg and Si. The calculations were based on the Scheil–Gulliver assumption of infinite diffusion in the liquid and limited diffusion in the solid state. The computed phase fractions were validated with experimental measurements of phase fractions. The mapping procedure allows the control of intermetallic phases in the as-cast microstructure, the minimization of the β-AlFeSi phase in particular, which is a significant prerequisite in obtaining enhanced extrudability, combined with high strength in this alloy series. Construction of maps for different levels of Mn has shown that addition of Mn could allow for higher alloying with Mg and Si, in order to obtain higher amounts of Mg2Si, without at the same time increasing the β-AlFeSi phase in the as-cast microstructure.

Partial cyclic phase transformations α→γ and γ→α triggered by temperature cycling in the intercritical range (α+γ) have been simulated in a medium-Mn steel with composition Fe-0.2C-5Mn. Additional simulations have been performed in Fe-0.2C-0.2Mn and Fe-0.2C steels for comparison. The computational kinetics software DICTRA has been employed for the simulations. All simulations were carried out under local equilibrium conditions. A specially-designed thermal cycle was considered. The position and the velocity of the austenite-ferrite interface were monitored during temperature cycling. Cyclic behavior is characterized by hysteresis loop formation. No loop formation was observed for the plain carbon steel. The inverse transformation, where the interface proceeds in a direction opposite to the temperature change has been identified for the Fe-0.2C-5Mn steel. The duration of the inverse transformation stage is larger at the minimum temperature of the cycle while the phase fraction formed during the inverse transformation is larger at the maximum temperature of the cycle. When the cyclic transformation takes place at a time before the final phase equilibrium in the isothermal curve, the transformation is characterized as inverse during the whole cooling part of the cycle. A stagnant stage where the transformation is very sluggish was observed during the cyclic transformations for the steels investigated. The study of cyclic transformations provides insight into the kinetics of phase transformations in medium-Mn steels.

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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.

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.