QUANTITATIVE ANALYSIS OF CREEP STRENGTHENING FACTORS IN NI-BASE SINGLE CRYSTAL SUPER ALLOYS

Ni-base single-crystal (SC) superalloys with superior mechanical properties are being developed to improve the thermal efficiency of jet engines and land-based gas turbines. In this study, we investigate the relationships among alloy compositional factors, structural factors, and the creep-rupture life of Ni-base SC superalloys by multi-regression analysis with creep-tested data for 75 alloys. We also obtain three equations to predict creep-rupture lives at 900 degrees C/392 MPa, 1000 degrees C/245 MPa, and 1100 degrees C/137 MPa with excellent multi-correlation coefficients from 0.94 to 0.98. The gamma' phase composition, gamma' volume fraction, and lattice misfit were used as explanatory variables. The regression coefficients of the refractory elements including W, Ta, and Re were comparatively large at 900 degrees C/392 MPa and they decreased with an increase in temperature and decrease in stress. The coefficient for negative lattice misfit increased at higher temperatures and lower stresses, especially at 1100 degrees C/137 MPa. The coefficient of Ru showed a small value for all the conditions in this study. Consequently, it is clear that solution strengthening is effective in the lower-temperature and higher-stress regions, and that a larger negative misfit leads to a quicker formation of the rafted gamma/gamma' structure with a finer interfacial dislocation network. The finer dislocation network assures superior creep resistance by preventing glide dislocation from cutting the rafted gamma/gamma' structure. The addition of Ru in alloys is effective in suppressing topologically close packed (TCP) phases formation by expanding the solubility limit. The lattice misfit shifts toward a slightly negative value with the addition of Ru, but this effect was small with respect to the enhancement of creep strength.