Role of Cobalt in the Marage-Type Alloy Matrix

Abstract The new maraging alloys, which possess some of the highest strength and toughness combinations available in commercial alloys, are based on the iron-nickel martensites, further strengthened by precipitation reactions involving molybdenum, titanium, and aluminium. But enough (∼ 8%) cobalt is also needed to develop the desired properties, even though cobalt itself is absent in the precipitate phases. To determine the role of cobalt in these alloys, an iron-18 % Ni binary, a 4% Mo ternary, an 8% Co ternary, and a 4% Mo-8% Co quaternary alloy have been studied by transmission electron microscopy. The effect of cobalt on matrix strengthening was found to be based on cobalt lowering the stacking-fault energy (SFE) of the matrix. The lowered SFE discourages cross-slip and retards cell growth. The resulting increase in average dislocation density provides more nucleation sites for the precipitates, which stabilize the dislocation forest and increase interference for moving dislocations; thus strength is ...