A computational 3D model that accounts for both nucleation and interface migration is a very useful tool to monitor and grasp the complexity of microstructure formation in low-alloyed steels. In the present study we have developed a 3D mixed-mode multigrain model for the austenite-ferrite and the austenite-ferrite-austenite formation capable of following diffusional phase transformations under arbitrary thermal routes. This new model incorporates the solute drag effect of a substitutional element (in this case Mn) and ensures an automatic change in transformation direction when changing from heating to cooling and vice-versa. An analytical solution for calculating the energy dissipation of solute drag together with multiple regression approximations for chemical potentials are proposed which significantly accelerate the computation. The modelling results are first benchmarked for an Fe-0.1C-0.5Mn (wt.%) alloy under different continuous cooling and isothermal holding conditions. The model revealed relatively large variations in transformation kinetics of individual grains as a result of interactions with neighboring grains. Then the model is applied to predict the transformation kinetics of a series of Fe-C-Mn alloys during cyclic partial phase transformations. The comparison with experimental dilatometer results nicely validates the predictions of this model regarding the change in overall transformation kinetics of the ferrite transformation as a function of the Mn content. New features of this model are its efficient algorithm to compute energy dissipation by solute drag, its capabilities of predicting the microstructural state for spatially resolved grains and the minimal fine tuning of modelling parameters. The code to implement this model is publicly available.

It is still a big challenge to obtain excellent low-temperature toughness for bulk steel materials. Delamination is an effective method to improve low-temperature toughness. In the present study, delamination toughening in a low carbon microalloyed steel plate with elongated and ultrafine-grained microstructure rolled in the dual-phase region has been investigated in detail. When toughness was measured along normal direction, the steel plate had a high upper shelf energy and no delamination occurred in the upper shelf region. A large delaminated crack parallel to rolling plane started to appear and changed the propagation path of main crack when testing temperature was lower than −60 °C. We find this kind of delamination induces a second upper shelf in the Charpy transition–temperature curve. The second upper shelf, reaching up to 300 J in the temperature range of −60 °C to −140 °C, results in excellent low-temperature toughness for the steel plate, and the ductile-brittle transition temperature is lowered to −157 °C. The developed steel plate also has high low-temperature toughness measured along transverse direction due to delamination. The effect factors on upper shelf energy, delamination mechanism and delamination toughening are discussed.

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.

We have analyzed the evolution of the ferrite fraction and average ferrite grain size during partial cyclic austenite-to-ferrite and ferrite-to-austenite phase transformations in an Fe-0.25C-2.1Mn (wt pct) steel using three-dimensional neutron depolarization (3DND). In the 3DND experiments, the ferrite fraction is derived from the rotation angle of the neutron polarization vector, and the average grain size is determined from the shortening of the polarization vector. From these, the number density of ferrite grains is derived, which indicates that grain nucleation is negligible during partial cycling in the intercritical regime and that all transformation kinetics can be attributed to growth processes only. In the multiple successive cyclic partial transformations, the interfacial migration rate was found to be sluggish due to Mn partitioning. The transformation kinetics determined with 3DND was compared to the predicted behaviors for diffusion-controlled simulations under local equilibrium and para-equilibrium interfacial conditions. It was found that the simulation predictions under local equilibrium only qualitatively capture the transformation kinetic with a difference of one order of magnitude in the variation in the ferrite fraction during cycling. The cyclic behavior of this Fe-0.25C-2.1Mn (wt pct) steel shows that the austenite-ferrite interface indeed migrates back and forth during cycling, while at the same time, there is a gradual increase in both the ferrite fraction and the average ferrite grain size over subsequent cycles. The intrinsic cyclic behavior is only visible after subtracting the effect of the progressive interfacial migration into austenite. The present study demonstrates the advantage of 3DND in studying partial cyclic phase transformations over conventional experimental approaches.

The precipitation of Au-rich precipitates on the external surfaces of Fe-Au alloys has been studied by scanning and transmission electron microscopy. The surface precipitates formed at elevated temperatures are found to self-organize in regular patterns and their growth rate is determined quantitatively. The observed surface precipitation at a free surface is compared to the precipitation at the internal surface of grain boundary cavities induced by creep loading. A close agreement between both processes is observed.

The autonomous filling of creep-loading induced grain-boundary cavities by gold-rich precipitates at a temperature of 550 °C has been studied as a function of the applied load for Fe-Au alloys using synchrotron X-ray nano-tomography. The alloy serves as a model alloy for future self-healing creep resistant steels. The size, shape and spatial distribution of cavities and precipitates are analyzed quantitatively in 3D at a nanometer resolution scale. The filling ratios for individual cavities are determined and thus a map of the filling ratio evolution is obtained. It is found that the gold-rich precipitates only form at cavity surfaces and thereby repair the creep cavity. The shape of the cavities changes from equiaxed to planar crack like morphologies as the cavities grow. The time evolution of the filling ratio is explained by a simple model considering isolated cavities as well as linked cavities. The model predictions are in good agreement with the measurements.

We have investigated the autonomous repair of creep damage by site-selective precipitation in a binary Fe-Mo alloy (6.2 wt pct Mo) during constant-stress creep tests at temperatures of 813 K, 823 K, and 838 K (540 °C, 550 °C, and 565 °C). Scanning electron microscopy studies on the morphology of the creep-failed samples reveal irregularly formed deposits that show a close spatial correlation with the creep cavities, indicating the filling of creep cavities at grain boundaries by precipitation of the Fe₂Mo Laves phase. Complementary transmission electron microscopy and atom probe tomography have been used to characterize the precipitation mechanism and the segregation at grain boundaries in detail.