INFLUENCE OF SEGREGATION ON LIQUID DENSITY IN THE MUSHY ZONE OF DZ483 Ni--BASED SUPERALLOY

Abstract

Ni-based superalloys have been widely applied in advanced aeroengine as gas turbine blades and vanes. The freckles in superalloys formed during directional solidification have deleterious influence on the properties of the alloys. The generation of freckles is associated with the local liquid density gradient in the mushy zone, which is obviously influenced by microsegregation of alloy elements. However, the individual contributions of the various elements to the total density variation are still not well known. Therefore, the effect of microsegregation on the liquid density variation in DZ483 Ni-based superalloy was investigated by isothermal solidification together with liquid quench method. Solidification microstructures were observed by optical microscope and SEM, and the compositions of the solids and the residual liquid were determined by EDS. Based on the compositions of residual liquids, the densities of liquids at different temperatures were calculated. The results show that the onset solidification temperature of DZ483 alloy is a little bit below 1335 ,a ndM C, which is enriched with Ta and Ti, formed at about 1325 . The segregation coefficients of different elements show that W and Co are negative segregation elements, Ta and Ti positive segregation elements, while Al and Cr show little segregation. The density of the residual liquid generally decreases as the decrease of temperature, with the exception that it increases somewhat from 1325 to 1315 . Calculation results show that temperature has insignificant influence on liquid density, and variation of density is mainly due to microsegregation. Segregations of Mo and Ta lead to the increase of density, but segregations of Ti and W present opposite effect. Contribution of each element to the variation of the liquid density