Soldification segregation in ruthenium-containing nickel-base superalloys

Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.