From Brittleness to Superplasticity: Tailoring the Properties of Fe–Cr–Co Alloys

Fe–Cr–Co alloys are deformable hard-magnetic materials that combine high coercivity with the possibility of conventional thermomechanical processing. In this work, the transition from limited ductility to superplastic deformation was investigated in industrial 23Kh15KT (Fe–23Cr–15Co) and 30Kh23K (Fe–30Cr–23Co) alloys by varying phase composition and phase morphology. Tensile tests were conducted at 800–1050 °C and strain rates of 1.6 × 10−4–1.6 × 10−1 s−1. The microstructures were characterized using optical microscopy, scanning electron microscopy, transmission electron microscopy, and X-ray diffraction to evaluate phase constitution, grain size, and the evolution from lamellar to globular microduplex morphologies. The ultrafine-grained globular structures exhibited features characteristic of superplastic deformation during high-temperature tensile testing. The most favorable tensile response was observed at 900 °C for 23Kh15KT and at 950 °C for 30Kh23K. Under these conditions, the globular microduplex states showed high elongation to failure and strain-rate sensitivity coefficients of approximately m = 0.43 and m = 0.47, respectively. In contrast, the initially lamellar 23Kh15KT structure showed pronounced strain hardening and only partial fragmentation and spheroidization of the lamellae. In 30Kh23K, the lamellar morphology transformed more completely into a fine globular microduplex structure, which was accompanied by stable high-temperature deformation behavior. The results demonstrate that grain refinement alone is insufficient to promote superplasticity in Fe–Cr–Co alloys; the formation of a fine globular microduplex morphology is also required to provide favorable conditions for grain-boundary-sliding-mediated deformation. Complementary compression experiments revealed progressive fragmentation and spheroidization of lamellar structures at elevated temperatures. These findings provide a structural basis for the thermomechanical processing and potential superplastic forming of complex-shaped Fe–Cr–Co alloy components.