Carbide nano-precipitates are commonly used to improve mechanical properties of steel. It has been experimentally observed that TiC, NbC, and VC carbide precipitates initially form as ‘plate-like’ particles oriented in the {1 0 0} planes of the ferrite lattice. These platelets share similarities with Guinier-Preston zones in Al-Cu alloys. The clustering of group IV and V transition metal atoms (M = Ti, Zr, Hf, V, Nb, Ta) in ferrite is studied using density functional theory. It is deduced that the transition metal carbides all form in a similar way. Furthermore, the transition from an initial M–C cluster to a NaCl-structured platelet to a NaCl-structured precipitate is examined through atomistic simulations using Modified Embedded Atom Method potentials. A route is established along which transition metal carbides form and transform into precipitates that possess the Baker-Nutting orientation relation with the ferrite matrix.

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In this study possible routes from dissolved M and C atoms to a M-C (M = Ti, Nb) cluster are studied. Using atomistic modelling to perform relaxation simulations and molecular dynamics (MD) simulations for the Fe-M-C ternary system, the formation of clusters is studied for M. Additionally the stability of M-C clusters is assessed. The clustering of M and C atoms as observed in experiments is also found in simulations. The initial clusters found in this work have a (Fe,M)C composition with a large Fe fraction. Moreover, structurally relaxed clusters reveal that there are growth pathways with a monotone decrease in Gibbs energy, suggesting that the highest energy barrier in the formation of M-C clusters is the diffusion barrier for the atoms forming the cluster. The development of M-C clusters as found in this study suggests a formation mechanism for nano-precipitation of carbides consisting of several steps; first a C cluster forms, then M atoms attach to the C cluster forming a (Fe,M)C cluster, and in the final step the (Fe,M)C cluster transforms to a NaCl-structured carbide.

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In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

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A reference-free modified embedded atom method (RF-MEAM) potential for iron has been constructed. The new potential is made to predict both bcc and fcc (α-Fe and γ-Fe) lattice properties, with a special interest in modelling in the 800-1300 K temperature range. This is the range in which transformations and key processes in steel occur. RF-MEAM potentials can be used directly in commonly used molecular dynamics simulation software (e.g. LAMMPS). The new potential is compared to several other (M)EAM potentials which are commonly used. It is demonstrated that the new potential combines good characteristics for point defect energies with free surface and stacking fault energies. Also the Nishiyama-Wassermann and Kurdjumov-Sachs orientation relation ratios and interface energies are reproduced, allowing for simulations of α-Fe and γ-Fe interphases.

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A new approximation to the equations describing Classical Nucleation and Growth Theories, is proposed providing quick, and intuitive insight. It gives a prediction of the mean precipitate radius and number density development under quasi-isothermal conditions. Current “mean-radius”, and “multi-class” approaches to modelling classical nucleation and growth theory for precipitation, require considerable computation times. An analytical approximation is proposed to solve the equations, and its results are compared to numerical simulations for quasi-isothermal precipitation. From the approximation a start and end time for the nucleation stage is predicted, as well as a time at which growth occurs and when the coarsening stage starts. Ultimately, these times, outline the numerical solution to the precipitation trajectory, providing key insight before performing numerical simulations. This insight can be used to more efficiently simulate precipitate development, as time scales at which the various stages in precipitate development occur can be predicted for individual precipitates. When these time scales are known a numerical simulation can be used for a specific goal, for instance to only simulate nucleation and growth, thus saving computational time. Moreover, for a first indication of the precipitate development in a composition under a particular heat treatment a numerical simulation is no longer necessary. This is also useful for process control as consequences of changes in treatment can be assessed on-line. Using these approximate analytical results an estimate can be made for the matrix concentration of precipitate forming elements. Additionally some dimensionless parameters are established to provide intuitive details to the precipitation trajectory.@en

A cold-rolled Ti-V high strength low alloy (HSLA) steel was isothermally annealed at 650 °C and 700 °C for different times. A unique combination of techniques including visible light microscopy (VLM), transmission electron microscopy (TEM), matrix dissolution, small angle neutron scattering (SANS) and hardness measurement has been employed to investigate the evolution of microstructure, hardness and precipitate composition, size and volume fraction. Results show that recrystallization is completed after annealing 8 h at 650 °C and 30 min at 700 °C. Three types of precipitates were identified: large Ti(C,N), medium-size (Ti,V)(C,N) and small (Ti,V)C. The Ti/(Ti+V) atomic ratio in the (Ti,V)C precipitates decreases with increasing radius in the 1–15 nm range, which can be explained by the initial nucleation of a TiC-rich core. The average size of the (Ti,V)C precipitates increases, whereas the number density decreases during annealing. The volume fractions of the three types of precipitates were separately determined by the matrix dissolution method. The volume fractions of (Ti,V)C precipitates obtained by matrix dissolution are comparable even slightly more accurate than those obtained by SANS. The hardness first increases and then decreases when annealing at both temperatures, which can be correlated well with the observed microstructural and precipitate evolution.

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