Yoshitaka Matsukawa

Abstract:

The useful lifetime of nuclear power plants is practically limited by embrittlement of non-replaceable reactor pressure vessels, induced by precipitation of minor alloying elements rather than irradiation damages. Establishing a mechanism-based predictive model of material embritlement (loss of ductility) is a goal of nuclear materials research community; however, this is also a long-standing challenge in fundamental physical metallurgy. Although the theory of dislocation is well established for quantitatively describing the strength of materials, the dislocation theory is incapable of directly describing the ductility thus far. Hence, the loss of ductility has often been indirectly scaled by the degree of hardening, based on a generally-accepted empirical rule that stronger materials exhibit less ductility. In the spirit that improving the quantitative precision of the modeling of hardening is a contribution to the precision improvement of the lifetime prediction, we have tried to further develop the theory of precipitation hardening by using advanced material characterization techniques. Precipitation occurs as a result of local enrichment of solute elements originally dissolved in the matrix. In a very early stage of solute agglomeration, clusters of solute elements have the same crystal structure as that of the matrix rather than the final product of precipitation. The crystal structure of precipitate particles was found to be a factor dominating their obstacle strength against gliding dislocations associated with deformation. Even in the case where the obstacle is softer than the matrix in terms of shear modulus, gliding dislocations are unable to cut through it when the slip plane inside the obstacle is not parallel with that in the matrix, because dislocations on atomic planes different from slip planes are practically sessile. Soft precipitates can be Orowan-type strong obstacles. The obstacle strength of precipitates changes during precipitation.