Castability of directionally solidified nickel base superalloys

Grain boundary cracking during directional solidification of columnar grain ed nickel base superalloys has been investigated. A quantitative test has b een developed that allows a classification of the castability of directiona lly solidified nickel base superalloys: CM247LC showed excellent castabilit y, while IN792 showed severe cracking. The cracking was found to originate at a temperature near the solidus temperature (hot tearing). A number of ex perimental alloys based on the composition of IN792 were tested, with the e ffects of variations in hafnium, aluminium/titanium, car bon, boron and zir conium contents being examined. A combination of 0%B and 0.01%Zr shows sign ificant promise. A number of approaches were applied to explain the crackin g susceptibility. A relationship was found between a small nil ductility re gion, obtained by Gleeble testing, and good alloy castability for IN792 and CM247LC. A correlation between the eutectic content of the alloy and its c racking tendency was also found. At low (<5%) and high (>22%) amounts of eu tectic, a significant reduction in cracking was apparent, while with interm ediate amounts (15%), severe cracking was observed. MST/3958.