Thermal activation and lattice misfit induced material degradation in a low alloy steel

Abstract

Carbides, one of the principal phases of material strengthening, are often associated with material properties. For low alloy steels in nuclear pressure vessels, carbide coarsening rate is often deemed as low or fixed and thermal aging happens only after a considerable period of time. However, the coarsening mechanism in this kind of material has yet been fully understood. Here we investigated the evolution of microstructure and cementite during thermal aging of a low alloy steel. The preferential growth direction of cementite was found to be determined by its innate lattice arrangement and misfit interfaces with the matrix, which was verified by first-principles calculations. A first-of-its-kind four-stage mechanism of cementite coarsening and transformation was proposed. We discovered carbide coarsening associated with material degradation is well represented by more difficult grain rotation and slip blocking, and average carbide length and upper shelf energy are linearly related. This work not only reveals the ever-believed aging-free low alloy steels are ageable, but empowers future design and handling of long-life materials.