Extent of back diffusion during solidification of experimental nickel based single crystal superalloy

Abstract

The microsegregation inherited from the directional solidification and quenching (DSQ) of an experimental third generation single crystal superalloy is characterised using electron probe microanalysis (EPMA). A statistical treatment of the data is used to estimate the composition of the solid in the mushy zone. Comparison of the experimental data with the predictions from a numerical model indicates that the results cannot be rationalised without acknowledging that back diffusion is occurring; it is demonstrated that the magnitudes of the diffusion coefficients which need to be invoked are consistent with recent experimental measurements of the coefficients deduced from diffusion couples.

Solute Redistribution during Planar and Dendritic Growth of Directionally Solidified Ni-Base Superalloy CMSX-10

“…Tewari et al [17] have reported a similar result, that the contribution of back-diffusion to microsegregation is negligible for the elements Ta, Al, Ti, Co, and W, when the cooling rate is about 7.3 K/min in the Ni-base superalloy PWA-1480. In contrast to these studies, Thirumalai et al [14] have reported that their microsegregation results cannot be rationalized without acknowledging the occurrence of back-diffusion during directional solidification of the experimental Ni-base superalloy RR2100 at a cooling rate of 10 K/min ( G = 10 K/mm and R = 1.0 mm/min). Nevertheless, they could not explain why the solute content of the dendrite core in mushy zone from the dendrite tip to the dendrite bottom remains unchanged.…”

“…Of the techniques available, random sampling approach, which is commonly known as point matrix scan technique, is widely used for characterizing microsegregation in Ni-base superalloys [3,4,7,[11][12][13][14]. This technique has become prevalent because it allows characterizing microsegregation with no dependence on the plane of examination and simply provides a continuous profile with respect to the fraction of solid ( s ).…”

A Comparative Study of the γ/γ′ Eutectic Evolution During the Solidification of Ni-Base Superalloys

“…the microsegregation would be negligible depending on the diffusivity of each solute element when the cooling rate is increased up to a certain limit. [16,30,33,34] The back-diffusion effect with respect to the cooling rate in CMSX-10 and CMSX-4 can be qualitatively evaluated on the basis of c/c¢ eutectic volume fraction, because all the c¢ forming elements in both alloys were found to be segregated to liquid during the solidification of primary c (Table II). Figure 10 shows a schematic illustration of the back-diffusion effect on the formation of c/c¢ eutectic, which is based on the quasi-binary Scheil model.…”

Experimental and Numerical Modeling of Segregation in Metallic Alloys

“…[18] One can cite characterization by electron probe microanalyses on samples solidified in wellcontrolled conditions, e.g., using directional solidification and quenching. [19][20][21][22] Because the fraction of the phases can also be measured as a function of the temperature or local composition, it is then possible to conduct a comparison with predictions. Not only could the combined interpretation of such experimental and modeling analyses explain the effect of solid diffusion, but it was also used to identify the effect of the nucleation undercooling of a second phase.…”