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Ru is a common addition to the Co-binder in WC Co hardmetals for advanced cutting tool applications. Internal Co-Co interfaces control many properties such as hot-hardness, toughness, and creep resistance. Hence, phase transformations determining the internal interface character can be harnessed to achieve superior properties. We investigate the γ (face-centred cubic) to α (hexagonally closed packed) martensitic phase transformation of a model Co-Ru binder alloy. We describe the crystallography of γ to α phase transformations and the resulting α/γ and α/α interface character distributions. The stabilisation of the γ-phase results in the formation of low energy (0 0 0 1) α/γ interphase boundary planes. Preferred formation of α/α intervariant and twin-related boundaries with symmetrical tilt configurations are also observed. We discuss how crystallographic constraints of the transformation promote the formation of grain boundary planes other than those of the lowest energy configurations. [Display omitted] [ABSTRACT FROM AUTHOR]

The ferrite-to-austenite phase transformation during water quenching of a duplex stainless steel was studied, where a duplex stainless steel was heated to 1370 °C (delta ferrite region) and quenched to room temperature. The microstructure consisted of coarse ferrite grains and fine needle-like austenite particles. The phase transformation mechanism appeared to be "diffusion-limited" displacive where shear was dominant, but also accompanied by prior or simultaneous diffusional elemental redistribution. A small fraction of the interfaces followed Kurdjumov–Sachs and Nishiyama–Wassermann orientation relationships (ORs) where austenite/ferrite interfaces terminated on {111}A//{110}F planes. The high undercooling associated with the fast cooling rates resulted in a considerable deviation from rational ORs. This was mostly due to the formation of intragranular austenite on Cr2N particles, which most likely caused a random OR with respect to the ferrite matrix. A detailed transmission electron microscopy (TEM) analysis revealed that the planar interphase boundaries characterised by the rational ORs typically contained one dominant set of parallel, regularly spaced dislocations. TEM analysis also showed the occurrence of small protrusions appearing on the edge/face of some austenite particles. Some of these did not leave a sub-boundary behind and formed a finger-like austenite morphology resulting from the instability mechanism. In some other cases, the protruded austenite possessed a low-angle grain boundary with the substrate austenite grain, which was the result of sympathetic nucleation of austenite on a pre-existing austenite particle. [ABSTRACT FROM AUTHOR]

Hot deformation mechanisms and restoration behavior of a single-phase face-centered cubic high entropy alloy was studied. The hot-deformed microstructure consisted of complex shaped deformation/transition bands. For the first time in these alloys, self-screening microband arrays were detected. The microbands were of highly crystallographic nature, i.e., they were aligned with traces of {111} slip planes containing slip systems having the highest or second highest possible Schmid factors. Dynamic recrystallization initiated mainly at the initial grain boundaries through strain-induced boundary migration generally followed by the formation of multiple twinning chains. Also, micro-shear-band induced recrystallization was observed locally, mainly due to strain localization. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Correction of γ/γ' interface artefacts in combined EBSD-EDS measurements. • Classification of recrystallised grains based on their location. • Identification of the recrystallisation mechanism at distorted twins. • Evolution of twin boundaries during hot compression. • Visualisation of the boundary planes of Σ3 twins based on two dimensional EBSD. Electron backscatter diffraction (EBSD) generally links crystallographic orientation to the microstructure of crystalline materials. EBSD datasets are now commonly used to identify phases, grains, and their orientations using off-the-shelf software, although substantial additional information may be extracted. Due to the lack of commercially available software, advanced analyses are often done manually and provide only localised information, lacking statistical significance. Here we introduce novel automated methodologies for advanced analyses of microstructural features in Ni-based superalloys. Our methodologies provide additional insights into the characteristics of these features and their underlying physical phenomena. We showcase how to correct wrongly indexed γ/γ' interface artefacts in combined EBSD and energy-dispersive X-ray spectroscopy (EDS) measurements, how to classify recrystallised grains based on their location, how to assess and visualise grain boundary planes, and how to study the evolution of Σ3 twins during hot deformation. We further demonstrate how phase fractions and grain sizes are more accurately determined in combined EBSD-EDS measurements. The classification of recrystallised grains into different groups enables individual analyses, facilitating the straightforward identification of the underlying recrystallisation mechanism. Our grain boundary plane analysis provides insights into the coherence of Σ3 twins and the potential boundary planes of incoherent Σ3 boundaries. The current paper is a tutorial-style guide for these methodologies. The algorithms are made freely available and, although demonstrated here on Ni-based superalloys, can also be applied to other systems. [ABSTRACT FROM AUTHOR]

Annealing twins of Σ3 character are common special grain boundaries in face centred cubic metals with low-to-medium stacking fault energy such as Ni-based superalloys. Despite their prevalence, little is known about the detailed evolution of these interfaces during hot deformation, then often referred to as distorted twins, and how their presence contributes to nucleation during dynamic recrystallisation. Here, we developed an automated methodology to systematically analyse large numbers of distorted Σ3 twins in microstructures after hot compression tests using electron backscatter diffraction. We show that annealing twins lose their Σ3 character via a deviation in misorientation angle from the ideal 60°. This is accompanied by a rotation axis shift from 〈111〉 towards 〈110〉. These distorted twins then facilitate discontinuous dynamic recrystallisation. Newly recrystallised grains generally form via twinning phenomena and maintain a special orientation relationship to their neighbouring deformed grains by sharing a close to common 〈111〉 direction. At high deformation temperatures above 1130 °C, most pre-existing twin boundaries are fully transformed into recrystallised grains. This highlights the importance of twin boundaries as nucleation sites for recrystallisation. Furthermore, we demonstrate that automated methodologies are required to enable systematic studies of the interplay between recrystallisation and microstructural features such as Σ3 twins. [Display omitted] • Σ3 twin character loss via rotation axis shift from 〈111〉 to 〈110〉 during deformation. • Recrystallised grains nucleate at distorted twins via twinning phenomena. • Deformed twins and recrystallised grains often share a close to common 〈111〉 direction. • An automated methodology is developed to analyse large datasets. [ABSTRACT FROM AUTHOR]

From the two possible intergranular phases in nickel alloy 725 (UNS N07725, Inconel 725), i.e., M 23 C 6 and the σ-related F phase, the abundant formation of F phase was found to critically compromise the hydrogen embrittlement resistance of the alloy. Nevertheless, the processing conditions and microstructural features that favor the formation of one phase over the other are yet to be investigated. Herein, intergranular phases in commercial batches in their as-received condition and after selective thermal treatments were studied to better understand the precipitation behavior. The distribution of grain boundary planes of the nickel matrix was assessed using stereological analysis of electron backscatter diffraction data. Results showed a direct correlation between the extent of F phase precipitation and the population of grain boundaries terminated on (111) planes, which provide low-energy nucleation sites for F phase. Our results suggest that while intergranular precipitates formed during aging, the preceding thermomechanical and thermal treatments defined the relative abundance of nucleation sites for each phase by affecting the grain boundary character distribution. Thus, targeted grain boundary engineering approaches could be developed to avoid susceptible microstructures and improve the hydrogen embrittlement resistance of the alloy. Furthermore, the results may help produce samples with different extents of intergranular M 23 C 6 and F phase to evaluate, in future studies, the individual effect of the phases on the properties of the alloy. [Display omitted] [ABSTRACT FROM AUTHOR]

Additive manufacturing has emerged as a promising route to fabricate complex-shaped Ti-6Al-4V parts. The microstructural evolution and variant selection across builds in response to different printing strategies processed by electron beam powder bed fusion has been previously clarified. However, a detailed knowledge of the grain boundary plane characteristics of the α-α intervariant interfaces is still missing. The aim of this study was to reveal the full 'five-parameter' crystallographic characteristics of the intervariant boundaries. The most common α-α intervariant for colony and basketweave microstructures was 60°/[1 1 2 ¯ 0], while in the acicular microstructure, the maximum was at 63.26°/ 10 ¯ 5 5 3 ¯. This is discussed in terms of self-accommodation during the β to α phase transformation, and the degree of coherence of the α laths in the as-deposited condition and during further growth. The grain boundary plane distributions reveal a high tendency for intervariant boundaries to terminate on prismatic and pyramidal planes rather than on low-energy basal planes. This suggests that, during additive manufacturing of Ti-6Al-4V and irrespective of the α morphology, the crystallographic constraints imposed by the Burgers orientation relationship determine the boundary plane distribution characteristics. Unlabelled Image • 5-parameter crystallographic characteristics of boundaries in AM Ti-6Al-4 V were studied. • Irrespective of the morphology, there is a strict Burgers OR between α and β. • AM Ti-6Al-4V microstructures show a maximum population of prismatic planes. • The crystallographic constraints imposed by the BOR determine the plane characteristics. [ABSTRACT FROM AUTHOR]

A fundamental understanding of the δ-ferrite to austenite phase transformation and characteristics of the interfaces formed is currently lacking due to challenges in achieving fully ferritic starting microstructure during conventional processing. Here, a 2205 duplex stainless steel manufactured by laser powder bed fusion (LPBF) is used as a model system to reveal the fundamentals of the δ-ferrite to austenite phase transformation with the aid of three-dimensional electron backscattered diffraction (EBSD). A predominantly δ-ferritic non-equilibrium microstructure is obtained through the high cooling rate during LPBF. During a short thermal treatment of this starting microstructure, four distinct types of austenite (intergranular, instability-induced, sympathetic, and intragranular) are formed. The sympathetic and intragranular austenite present significantly higher area fractions of interfaces following the Kurdjumov-Sachs (K S) or Nishiyama-Wassermann (N W) orientation relationships (ORs) compared to intergranular austenite, owing to their different nucleation and growth mechanisms. The habit plane distributions of various interfaces reveal that ferrite and austenite terminate on (110) and (111) planes, respectively. Interestingly, the plane and curvature distributions do not always exhibit an inverse correlation in the sympathetic and intragranular transformation paths, while the non-K-S/N-W interfaces exhibit lower grain boundary curvatures compared to the K-S/N-W ones. This could be because the total energy minimization associated with phase transformation involves contributions from both the surface energy at grain boundaries and the elastic bulk energy. These new insights into the δ-ferrite to austenite transformation enable duplex microstructure design via additive manufacturing and subsequent post-processing to achieve superior properties. [Display omitted] • Austenite and ferrite terminate on (111)//((110) in the majority of transformation paths. • Higher fraction of K-S/N-W interfaces associated with heterogeneous nucleation. • Austenite heterogeneously nucleates on existing austenite and dislocation structures. • Low curvatures on high-energy interfaces in sympathetic and intragranular austenite. • Strain energy minimization dominates sympathetic and intragranular austenite growth. [ABSTRACT FROM AUTHOR]

The high temperature deformation behavior of a duplex low density steel (Fe-17.5Mn-8.3Al-0.74C-0.14Si, wt. %) was investigated. This was performed through utilizing the hot tensile deformation scheme with a particular emphasis on the austenite grain refinement mechanisms. The extended dynamic recovery under the lowest applied strain rate (0.0001 s − 1 ) was considered as the main restoration mechanism. In the latter, the misorientation was largely positioned below 15° and this was attributed to the progressive formation of subgrain boundaries. Therefore, a positive effect on the material ductility behavior was provided through stress relieving phenomena. The occurrence of partial recrystallization was also traced in the microstructure during tensile deformation in the low strain rate regime. Increasing the applied strain rate to 0.1 s − 1 provided an appropriate condition for the occurrence of continuous dynamic recrystallization, where the number of subgrain boundaries was decreased and the continuous transformation of subgrains into the recrystallized grain was occurred. The recrystallized grains generally acquired a different microtexture with respect to the parent austenite grains. It was found that the presence of fine grains associated with continuous recrystallization could favour the grain boundary sliding thereby resulting in wedge-type cracking which would deteriorate the ductility behavior of the material. [ABSTRACT FROM AUTHOR]