Producing robust recrystallization models which can assist metallic microstructural design requires effectively understanding recrystallization nucleation. When the nucleation of static recrystallization (SRX) occurs at deformed grain boundaries, strain-induced boundary migration (bulging) is generally accepted as the nucleation mechanism. However, the present study challenges that view, showing, for a Ni-30%Fe alloy, that nucleation at deformed grain boundaries is not solely determined by bulging: results indicate that the number of bulges developed in the deformed microstructure is over four times larger than the number of SRX grains. On the other hand, SRX nucleation is shown to occur only when the low-angle boundary (LAB) between a pre-existing bulge and its parent grain transforms into a high-angle boundary (HAB). Based on this, a novel nucleation criterion is proposed, which may apply to SRX irrespective of the nucleation site (and to dynamic/metadynamic recrystallization): nucleation occurs whenever the misorientation of the LAB surrounding a bulge reaches the minimum HAB misorientation (e.g., 15°). Besides, correlation exists between the dislocation density accumulated around the various triple junction and grain boundary types in the microstructure, and their nucleation efficiency. This has been attributed to the higher fraction of relatively large initial subgrain misorientations measured for higher boundary dislocation density.

The nucleation of recrystallization is determined by the properties of the underlying subgrains: namely, subgrain size and misorientation. Under defined conditions, recrystallization nucleation occurs at the boundaries between deformed grains, and their junctions. Nevertheless, previous characterization studies have dedicated no specific attention to the subgrains located around deformed grain boundaries. In this view, the substructure at such locations is here characterized for a hot-deformed Ni-30%Fe austenite alloy, employing electron backscatter diffraction. Firstly, results show that the boundary length fraction not surrounded by subgrains decreases with higher dislocation density around the boundary. Higher boundary dislocation density also results in smaller subgrain sizes and higher subgrain misorientations. The subgrain size reduction is appropriately described by a Staker-Holt-type relationship. This is the case despite the spatial gradient and relatively high values of the dislocation densities surrounding the deformed grain boundaries. To describe the increase in subgrain misorientation, a novel equation is proposed, showing good agreement with experimental data. Apart from this, subgrains neighboring triple grain junctions exhibit higher misorientations than those located around the boundaries, away from the junctions. This explains the preferential recrystallization nucleation at triple-junction sites. Finally, unlike suggested in literature, twin boundaries accumulate lower dislocation densities than general high-angle boundaries. This explains their lower recrystallization nucleation efficiencies. Nevertheless, for equal boundary dislocation density, the substructure developed around both boundary types does not differ significantly.

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.

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.

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.

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.

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.