Comparison of microstructure and solidification behavior of rapid quenching with bulk undercooled superalloy

Microstructure and solidification behavior in rapid quenching (i.e., gas-atomized powders and melt-spun ribbons) of superalloy have been compared with bulk undercooled superalloy. The application of a molten salt denucleating technique combined with thermal cycle enables such investigation over a wide range of undercooling up to 210 K. The microstructure formation has been discussed for both methods of solidification with respect to undercooling, nucleation, and recalescence as well as recrystallization during post-recalescence. Comparison of the observed microstructure and morphologies indicates that the melt-spun ribbons and the gas-atomized powders with cooling rate above 10⁴ K sec^-1 crystallize only after achieving a large degree of undercooling, which becomes higher and higher with the increase of cooling rate. Furthermore, it should be noted that, grain refinements, which play a decisive role in the undercooled as-solidified structure, however, result from different sources in the rapid quenching process.