Acta Metallurgica Slovaca

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Year 2007

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Year 2007 No 1

Dobrovská J., Stránský K., Dobrovská V.

SEGREGATION BEHAVIOUR AND CHEMICAL HETEROGENEITY OF AS-CAST AND HEAT TREATED NICKEL-BASED SUPERALLOY IN 738LC

Abstract
The paper deals with study of segregation (microsegregation) behaviour of selected elements in the course of crystallization and solidification of Ni-based superalloy IN 738LC. The purpose of this paper is investigation of microsegregation and chemical heterogeneity of cast and heat treated IN 738LC Ni-based superalloy on the basis of experimental measurement of concentration distribution of the constituent elements in the dendrite core and interdendritic regions. The concentration data sets of aluminium, titanium, chromium, cobalt, nickel, niobium, molybdenum, tantalum and tungsten were measured in 101 points on the 500 ?m long line segment (the distance between individual points was 5 ?m). This line segment crossed several dendrites, i.e. several dendrite cores and interdendritic regions. The concentration data sets of these nine elements were determined using a microanalytical complex JEOL JXA-8600/KEVEX Delta V, Sesame and the method of energy dispersive X-ray spectroscopic microanalysis (EDS). Microsegregation parameters of all analysed elements have been calculated from concentration data sets with use of mathematical statistics and the original mathematical models. In this work an original mathematical model for calculation of effective partition coefficients is introduced. The model is based on processing of the measured concentration data sets. The effective partition coefficients calculated in this way inherently include in themselves both the effect of segregation in the course of alloy solidification and the effect of homogenisation, occurring during the solidification as well as during the cooling of alloy.
Segregation behaviour of elements was described with the help of distribution curves of dendritic segregation, indices of heterogeneity and effective partition coefficients. The above mentioned parameters were also determined for the same elements on samples after heat treatment at 850°C; in this case the parameters represented redistribution of elements after isothermal annealing. Diffusion processes at the crystallization of analysed alloy were characterised by dimensionless Fourier number. The Fourier number characterises an intensity of diffusion of elements in the solid phase in respect to the dendrite arms spacing and to local time of solidification. The model of Brody and Flemings was used to estimate the Fourier number. This model assumes a complete diffusion in liquid and incomplete solid-state back-diffusion.
There were found out the following main results: i) during solidification of this alloy Cr, Co, W and Mo segregated to the dendrite core; Ni, Ti, Al, Ta and Nb segregated in the inverse, i.e. they enriched the interdendritic regions; ii) Nb and Ta have the strongest segregation ability during solidification out of all the analysed elements; iii) dendritic heterogeneity of elements in the analysed alloy is at the average the lowest in as-cast state, i.e. without any applied heat treatment; and iv) the chemical microheterogeneity of analysed Ni-alloy increases with extended dwell duration at the same temperature (850°C). Obtained results are discussed and compared with the data from literature.

No 1 (2007), p. 7-16