Differential thermal analysis of nickel-base superalloys

Differential thermal analysis was performed on over 200 nickel-base superalloys that included model, experimental, developmental, and commercial alloys. About one-half of these alloys were studied during melting and solidification, and about three-fourths were studied using solid specimens. Large samples were employed to permit the use of internal thermocouples and to minimize both the effects of segregation and the changes in chemical composition resulting from the loss of strong oxide-forming components. The features of thermograms for solid specimens were identified by two calibration techniques: (1) Inserting thin bar specimens of four cast alloys into a gradient furnace so that all parts of the test specimen were heated at the same rate as in a DTA test to the temperature range of interest; water quenching preserved the entire range of microstructures for metallographic evaluation. (2) Heating (and cooling, when applicable) 35 disc specimens of various alloys to points of interest on the heating thermogram and especially on the cooling thermogram, at which point each specimen was water quenched and evaluated metallographically. The effects of alloying elements on liquidus temperature, carbide-formation temperature, and the extent of carbide dispersion in the cast structure, gamma' solvus temperatures (for DTA heating/cooling rates and for practical heat treatments and soaking treatments), incipient-melting temperature, and the width of the temperature "window" for solution-heat-treating of single-crystal alloys are presented. Some examples of problem solving by DTA related to cracking during casting caused by very-low-melting liquid and to microporosity related to the gamma + gamma' eutectic constituent are presented.