Large improvement in ductility of ultra-high strength CoCrNi-base alloy by one-stage grain refinement

Pursuing a superior combination of strength and ductility in the ultra-high strength range is always a target, as it brings the benefit of reducing the weight and increasing the safety of the alloy during usage. Composition and process design are important ways to achieve the goal. The CoCrNi-base FCC alloy has low yield strength but very high ductility, so it becomes a high-potential alloy system for this goal. In this work, we use a small addition of Ti to achieve precipitation hardening and a one-stage grain refining process to provide a fine microstructure in the CoCrNi-base alloy. This research reveals that the one-stage grain refining process by rolling and annealing will yield finer initial grains, providing higher strain energy and more nucleation sites for more uniform precipitates and higher recrystallization ratios through cold-rolling and ensuing 600 °C heat treatment. The behavior of annealed texture and grain growth ability is different as well. Under identical conditions, yield strength (YS) and ultimate tensile strength (UTS) are in a comparable range of 1300–1400 and 1800–2000 MPa, while total elongation improves by a factor of 1.5–2 when grain sizes are preliminarily reduced by one-stage grain refinement. The reasons for this improvement are discussed.