Strain Aging of Fe-Co-Mo Alloys

Abstract

A strain aging stage and its reversion process by substitutional solute atoms corresponding to the low-temperature aging in maraging steels were found from electrical resistivity measurements. Ferritic iron alloys containing 8∼16%Co and 1.5∼4.5%Mo were continuously or isothermally annealed after cold-swaging up to 75%, and then the changes in electrical resistivity were measured in liquid nitrogen. During the continuous annealing at 1°C/min, the resistivity decreased in the temperature range of 150∼400°C and increased again in the range 450∼525°C. The amounts of the resistivity decrement showed good correlation with the cold reductions and the alloy contents. The stage of the resistance decrease has an apparent activation energy of about 34 kcal per mole. This stage can be considered as the strain aging due to the rich-zone or the atmosphere formation of substitutional solute atoms in the stress field of dislocations. On the other hand, the stage of the resistance increase at higher temperatures has an activation energy of about 60 kcal per mole. And this indicates that the reversion rate-controlled by the lattice diffusion of solute atoms takes place in the ferritic matrix.