Synergy enhancement mechanism of strength and plasticity for superalloy foil by a strategy combining cryogenic and room-temperature rolling

In this study, a new method of foil preparation is proposed, adopting a strategy combining cryogenic rolling and room-temperature rolling, together with a short-time annealing treatment to achieve a superior synergy in strength and plasticity. Compared to the foils obtained by complete room-temperature rolling, the addition of the cryogenic rolling process increases the yield strength from 493 MPa to 563 MPa for samples annealing at 1000 °C, achieving excellent strength without sacrificing plasticity (12.33 %). The samples annealed at 950 °C exhibit superior yield strength (670 MPa), tensile strength (1023 MPa), and elongation (14.1 %). The microstructure characterization of each stage reveals that the cryogenic rolling process effectively suspends the dynamic recovery to limit the dislocation movement. The increased grain boundaries by severe plastic deformation reduce the average dislocation path and promote the interaction of dislocations. Meanwhile, the increased defects promote the precipitation of δ phase, which increases the hindering effect on dislocations and pinning the grain boundaries to obtain the fine-grained microstructure. Moreover, the synergistic effect of deformation twins, hierarchical SFs, and lomer-cottrell (L-C) locks, as well as extensive interactions between them contribute to the enhancement of the deformation resistance, which is rarely observed in room-temperature rolled foils. For the foils completely room-temperature rolled, the reduction of the deformation degree weakens the formation of defects and δ phase. The uneven deformation causes abnormal grain growth in local regions after annealing for a short time, resulting in poor mechanical properties. This study provides a new idea for foil preparation and microstructure regulation.