Abstract
Grain boundary cracking during directional solidification of columnar grained nickel base superalloys has been investigated. A quantitative test has been developed that allows a classification of the castability of directionally solidified nickel base superalloys: CM247LC showed excellent castability, while IN792 showed severe cracking. The cracking was found to originate at a temperature near the solidus temperature (hot tearing). A number of experimental alloys based on the composition of IN792 were tested, with the effects of variations in hafnium, aluminium/titanium, carbon, boron and zirconium contents being examined. A combination of 0%B and 0·01%Zr shows significant promise. A number of approaches were applied to explain the cracking susceptibility. A relationship was found between a small nil ductility region, obtained by Gleeble testing, and good alloy castability for IN792 and CM247LC. A correlation between the eutectic content of the alloy and its cracking tendency was also found. At low (<5%) and high (>22%) amounts of eutectic, a significant reduction in cracking was apparent, while with intermediate amounts (15%), severe cracking was observed.
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