Twinning development in the annealing of hot-deformed austenite in steels has often been suggested to play a relevant role in e.g. the evolution of grain size and texture across the process. Nevertheless, the phenomenon has not been systematically studied. In this view, a detailed assessment of annealing twin boundary evolution in austenite after hot deformation is carried out for the first time. Particularly, three materials are examined via electron backscatter diffraction (EBSD): a stainless steel, a carbon steel, and a Ni-30Fe alloy. Results demonstrate that twin boundaries form via recrystallization, and disappear by grain growth. However, unlike previously reported for lower annealing temperature in nickel, the number of twins per recrystallized grain does not increase throughout recrystallization. On the contrary, it stagnates before its end, upon activation of concomitant grain growth. Additionally, twin density increases with lower deformation/annealing temperature, higher strain rate, and higher applied strain. This has been rationalized via the higher resultant stored energy, which increases the density of microstructural discontinuities inside the deformed matrix (and, thereby, the rate of growth accidents). By contrast, no correlation has been observed between the measured boundary tortuosity and twin density. While Σ3 and Σ9 boundaries appear at the same rate during recrystallization, Σ9 ones disappear considerably more quickly with grain growth. Finally, the twin density trends examined after EBSD parent austenite reconstruction on the carbon steel have all been consistent. Consequently, that method represents a promising approach to analyze annealing twinning in steels that undergo phase transformations upon cooling.

Previous research has consistently found that introducing metastable retained austenite (RA) as a second phase retards the failure of steel under fatigue. However, the reasons for this benefit are not understood. Accordingly, the properties of RA most advantageous to resist fatigue are not known. Within this context, this paper examines the interaction between second-phase RA and short fatigue crack growth in a steel processed via quenching and partitioning, using quasi in situ electron backscatter diffraction experiments. Results show that most RA transforms into martensite under the plastic strain surrounding the crack. They also reveal various mechanisms whereby RA transformation delays short fatigue crack propagation; transformation-induced crack closure (TICC), crack deflection and branching, and roughness-induced crack closure (RICC). Crack deflection and branching are driven by a tendency of cracks to propagate towards transformed RA, which is against the previous assumptions in the literature. Furthermore, the impact of crack deflection/branching on retardation is more powerful than that of TICC acting alone. Microstructures including second-phase RA should avoid RA-lean areas and promote elongated RA grains, with relatively large size, and major axis normal to the preferential crack growth direction. Untransformed RA within the plastic zone (i.e. overstabilized) does not contribute to crack retardation.

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.