Consideration of particle-strengthening mechanism of copper-precipitation-strengthened steels by atom probe tomography analysis

Atom probe tomography (APT) analysis of copper-strengthened steels was performed to elucidate the strengthening mechanism due to precipitated copper particles. The dependence of the particle diameter on the interaction (resistance) force per copper particle was examined by performing tensile tests and estimating the mean particle spacing in copper-added steels with various aging times. With increasing particle diameter, the interaction force dramatically increased below 3 nm and then gradually increased above 4 nm. On the other hand, the transformation of coherent bcc to semi-coherent 9R copper particles was observed at a diameter of approximately 4–6 nm. The dependence of the diameter on the interaction force indicates that the main mechanism of particle strengthening is due to the difference in elastic modulus between the copper particles and the iron matrix, and not the misfit strains of coherent bcc copper particles. The results also show that the balance between the diameter, number density, volume fraction and interaction force produced the region of maximum particle strengthening.