AN ELECTRONIC APPROACH TO ALLOY DESIGN AND ITS APPLICATION TO NI-BASED SINGLE-CRYSTAL SUPERALLOYS

The d-electron concept was applied to the design of nickel-based singl e-crystal superalloys. This concept was devised on the basis of molecu lar orbital calculations for transition-metal-based alloys. Two calcul ated parameters were mainly utilized in this alloy design. One is the d-orbital energy level (Md) of alloying transition elements and the ot her is the bond order (Bo), which is a measure of the covalent bond st rength between atoms. Using these parameters, accurate prediction is p ossible for the appearance of the gamma + gamma' eutectic phase, the t opologically close-packed phases and the alpha-W phase in the alloys. Also, the strengthening of both the gamma and gamma' phases in the all oys can be treated with these parameters. On the basis of this concept , single-crystal superalloys were designed successfully. Six kinds of designed alloys lay in the compositional range .%Ta-(1.8-2.6)mol.%W-(0 .6-0.9)mol.%Mo-0.25mol.%Re. It was confirmed experimentally that these designed alloys had excellent creep rupture properties, high hot corr osion resistance, low density and low material cost. In this respect t he designed alloys are much superior to any single-crystal superalloys published in the literature to date.