偏析对DZ483镍基高温合金糊状区内液相密度的影响

为研究DZ483合金在凝固过程中的元素偏析行为对糊状区内液相密度的影响,采用等温凝固结合水淬法对合金进行不同温度下的凝固实验.利用OM和SEM观察试样的凝固组织,利用EDS测量固相和剩余液相的元素含量,并根据剩余液相的成分计算其密度.结果表明,DZ483合金的开始凝固温度略低于1335℃,在1325℃液相中开始析出富含Ta和Ti的MC;W和Co为负偏析元素,Mo,Ta和Ti为正偏析元素,而Al和Cr几乎不发生偏析;剩余液相的密度随温度降低基本呈下降趋势,但在1325至1315℃之间有所回升.计算结果表明,温度对液相密度的影响很小,合金元素的偏析对液相密度的影响占主要地位.其中,Mo和Ta的偏析导致液相密度增加,而Ti和W的偏析导致液相密度明显降低.备合金元素的偏析对密度变化的贡献由大到小顺序为:Ti>Ta>W>Cr>Mo>Al>Co.MC的形成消耗了大量的Ti和Ta,对合金元素的偏析造成一定的影响,从而导致液相密度在1325至1315℃之间有所增加.

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

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℃,and MC,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 is analyzed.The sequence of contributions of alloy elements to the variation of total liquid density is Ti>Ta>W>Cr>Mo>Al>Co.The formation of MC consumes an abundant of Ti and Ta,resulting in the increase of liquid density from 1325 to 1315℃.