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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]

The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite-ferrite and austenite-austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite-ferrite interfaces associated with Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite-ferrite interfaces with orientation relationships which are neither K-S nor N-W, both austenite and ferrite habit planes had (1 1 1) orientations. Σ3 twin boundaries comprised the majority of austenite-austenite interfaces, mostly showing a pure twist character and terminating on (1 1 1) planes due to the minimum energy configuration. The second highest populated austenite-austenite boundary was Σ9, which tended to have grain boundary planes in the tilt zone due to the geometrical constraints. Furthermore, the intervariant crystallographic plane distribution associated with the K-S orientation relationship displayed a general tendency for the austenite habit planes to terminate with the (1 1 1) orientation, mainly due to the crystallographic preference associated with the phase transformation. [ABSTRACT FROM AUTHOR]

The austenite microstructure evolution and softening processes have been studied in a 23Cr–6Ni–3Mo duplex stainless steel, comprising equal fractions of austenite and ferrite, deformed in uniaxial compression at 1000 °C using strain rates of 0.1 and 10 s−1. The texture and microstructure evolution within austenite was similar in character for both the strain rate used. The observed large-scale subdivision of austenite grains/islands into complex-shaped deformation bands, typically separated by relatively wide transition regions, has been attributed to the complex strain fields within this phase. Organised, self-screening microband arrays were locally present within austenite and displayed a crystallographic character for a wide range of austenite orientations. The microband boundaries were aligned with the traces of {1 1 1} slip planes containing slip systems having high, although not necessarily the highest possible, Schmid factors. The slightly lower mean intercept length and higher mean misorientation obtained for the sub-boundaries at the higher strain rate can be ascribed to the expected more restricted dynamic recovery processes compared to the low strain rate case. Dynamic recrystallisation within austenite was extremely limited and mainly occurred via the strain-induced migration of the distorted original twin boundaries, followed by the formation of multiple twinning chains. [ABSTRACT FROM AUTHOR]

The microstructure evolution during hot deformation of a 23Cr–5Ni–3Mo duplex stainless steel was investigated in torsion. The presence of a soft δ ferrite phase in the vicinity of austenite caused strain partitioning, with accommodation of more strain in the δ ferrite. Furthermore, owing to the limited number of austenite/austenite grain boundaries, the kinetics of dynamic recrystallisation (DRX) in austenite was very slow. The first DRX grains in the austenite phase formed at a strain beyond the peak and proceeded to <15% of the microstructure at the rupture strain of the sample. On the other hand, the microstructure evolution in δ ferrite started by formation of low angle grain boundaries at low strains and the density of these boundaries increased with increasing strain. There was clear evidence of continuous dynamic recrystallisation in this phase at strains beyond the peak. However, in the δ ferrite phase at high strains, most grains consisted of δ/δ and δ/γ boundaries. [ABSTRACT FROM AUTHOR]

In the current study, the propensity of austenite-ferrite interface to sigma phase precipitation was investigated in a duplex stainless steel. The crystallographic characteristics of the interphase interfaces significantly affected the tendency to sigma phase precipitation. The lattice misorientation was found to be an insufficient criterion to fully define the interface character and ultimately the propensity of austenite-ferrite interface to sigma phase precipitation. The interphase interfaces with a rational orientation relationship, considering all five crystallographic parameters, generally exhibited the lowest sigma precipitation propensity. This could be attributed to the high sigma precipitation nucleation barrier energy and low diffusion rate at (semi-)coherent interphases. [ABSTRACT FROM AUTHOR]