Development of high nickel austenitic steels for the application to fast-reactor cores, (I) - Alloy design with the aid of the d-electrons concept

The design of high nickel austenitic steels for the core materials of the f ast reactors was performed following the d-electrons concept devised on the basis of molecular orbital calculations of transition metal based alloys. In this design two calculated parameters are mainly utilized. The one is th e d-orbital energy level (Md) of alloying transition elements, and the othe r is the bond order (Bo) that is a measure of the covalent bond strength be tween atoms. Using the <(Md)over bar>-<(Bo)over bar> phase stability diagra m accurate prediction become possible for the phase stability of the austen ite phase and 5% swelling at 140dpa for nickel ions. Here <(Md)over bar> an d <(Bo)over bar> are the compositional average of Md and Bo parameters, res pectively. On the basis of the phase stability diagram and preliminary expe riments, guidelines for the alloy design of carbo-nitraides precipitated hi gh nickel austenitic steels were constructed. Following the guidelines seve ral new austenitic steels were designed for the fast reactors core material .