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One of the most effective ways to increase the processing plasticity of advanced superalloys (heat-resistant nickel-based alloys) is the formation of an ultrafine-grained (UFG) microstructure in bulk semi-finished products. Such a microstructure is a necessary condition for the manifestation of the structural superplasticity effect in the technological processes of manufacturing products from such superalloys. One of the most promising methods for producing UFG microstructures is thermomechanical treatment (TMT) according to the multiple isothermal forging scheme. It has been shown that the EK79 superalloy after TMT, with a gradual decrease in the processing temperature from 0.88 to 0.62 Ts (where Ts is the strengthening phase dissolution temperature) leads to the transformation of the initial microduplex fine-grained microstructure into a mixed UFG microstructure. Such a mixed UFG microstructure consists of: 1) relatively coarse (inherited from the fine-grain microstructure) particles – γ'-phase with a size of 3.0±0.8 μm; 2) γ-grains, and incoherent γ'-phase particles with a size of 0.3–0.5 μm; 3) strengthening coherent intragranular γ'-phase particles with a size of 0.05–0.1 μm, released upon cooling from the TMT temperature to room temperature. During uniaxial compression tests, the EK79 superalloy with such microstructure, demonstrates low-temperature superplasticity in the temperature range of 800–1000 °C. It has been found that an increase in the deformation temperature up to 1000 °C, leads to the increase of γ-phase grains to micron size. The maintenance of superplastic properties in the presence of relatively coarse incoherent particles in the microstructure of the second phase (γ'-phase) is apparently related to the fact that the deformation is localised in the UFG component.

Duplex stainless steels (DSSs) are widely used in the oil and gas industry. Unfortunately, fusion welds of these alloys might exhibit inappropriate microstructures, face cracks, and even failures during service. In this context, friction hydro-pillar processing (FHPP) is a suitable friction-based welding technique for repairing cracks in offshore engineering with promising outcomes. Accordingly, the present work evaluates the effects of FHPP on one of the most widely used DSS known as SAF 2205. The welded joint features were evaluated by optical microscopy and X-ray diffraction (XRD). Furthermore, mechanical properties were evaluated by microflat tensile tests and microhardness, and pitting corrosion resistance was analyzed by the ASTM G48 Method A. Intense grain refinement in the dual-phase microstructure induced by the FHPP led to the formation of a microduplex microstructure, which resulted in enhanced microhardness and mechanical properties. The pitting corrosion mainly occurred near the weld interface, for which the formation of Cr2N nitrides and the consequent decrease in pitting resistance equivalent number (PREN) was found to be a significant factor. [ABSTRACT FROM AUTHOR]