Strengthening Mechanism of Cold-Rolled Co-Based Super Alloy for Diaphragm Valves under Room Temperature Tensile Loading

Cold-rolled Co-Cr-Ni alloy (Elgiloy) and Co-Ni-Cr alloy (SPRON 510) can be considered for diaphragm valve materials in gas valve systems. In this study, the effect of direct aging (DA) heat treatment on the tensile properties of Co-based alloys was investigated and the influence of microstructural changes on the tensile behavior was examined. The Co-Cr-Ni Elgiloy alloy exhibited a hexagonal close packed (HCP) phase formation through strain induced martensite transformation (SIMT) in both as-rolled and DA conditions, promoting finer grains due to increased sub-structured and recrystallized grains. In contrast, the Co-Ni-Cr SPRON 510 alloy showed a face centered cubic (FCC) single phase with a higher precipitates fraction and dislocation density. Both Co-based alloys showed improved hardness and strength after DA treatment, with the Co-Cr-Ni alloy demonstrating higher tensile strength and the Co-Ni-Cr alloy exhibiting higher yield strength and elongation. The Co-Cr-Ni alloy revealed fine shallow dimples and cleavage fracture mode, while Co-Ni-Cr showed larger dimples and river patterns on tensile fracture surfaces. The Co-Cr-Ni alloy’s deformation behavior was dominated by active SIMT due to low stacking fault energy (SFE), while the Co-Ni-Cr alloy primarily formed deformation twins due to higher SFE. The high tensile strength of the Co-Cr-Ni alloy was dominated by hardening effects due to a relatively fine grain size and SIMT behavior, while the high yield strength of the Co-Ni-Cr alloy was dominated by deformation twin, dislocation density, Taylor factor and high precipitate fraction.