Microstructural Evolution with a Wide Range of Solidification Cooling Rates in a Ni-Based Superalloy
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ased superalloys, especially directionally solidified (DS) and single-crystal (SC) alloys, have been used extensively in the aerospace industry due to their exceptional properties at elevated temperatures.[1,2] The primary fabrication process for DS and SC blades with high performance is DS casting. The key parameters in the DS technique are the thermal gradient (G) ahead of the solid/liquid (S/L) interface and the withdrawal rate (V). Therefore, a large percentage of the literature is concentrated on studying the effect of directional solidification parameters (mainly G and V) on microstructural evolution and on establishing a relationship between them. Liu et al.[3] reported that the evolution of the S/L interface morphology was changed in the sequence of planar, cellular, cellular-dendritic, dendritic, and superfine dendrite-cellular interface with increasing V. It is noted that the superfine dendrite-cellular interface with fewer side branches is obtained under a high thermal gradient condition and at a fast withdrawal rate. It is well known that primary dendrite arm spacing (k1) is proportional to G–0.5V–0.25 and secondary dendrite arm spacing (k2) can be given as proportional to
YONGJUN ZHANG, Doctoral Student, BIN HUANG, Vice Professor, and JIANGUO LI, Professor, are with the School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted July 11, 2012. Article published online February 8, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
(GV)–1/3.[4] Whitesell et al.[5] systematically studied and summarized dendrite arm spacing as a function of directional solidification parameters (G and V) in a wide range of Ni-based superalloys. In addition, G changes with increasing V during directional solidification. Actually, cellular/dendrite arm spacing is a function of GV, which is defined as the solidification cooling rate in the directional solidification process. In order to describe exactly the cooling parameters in a different process, the symbol T is used hereafter to represent the cooling rate. However, the single solidification process generally possesses a limited cooling effect, based on which the established model has its limitation in the application of a wide range of cooling effects. Therefore, it is necessary to investigate the evolution of microstructure in a wider range of solidification cooling rate. The purpose of this study is to show the microstructural evolution characteristics (S/L interface and dendrite arm spacing) with increasing T for a Ni-based superalloy prepared by various casting techniques, including conventional casting (CC), zone melting liquid metal cooling (ZMLMC), suction casting, spray casting, and laser surface remelting. Also, the statistical data from the literature were analyzed for a wide variety of nickelbased superalloys and a model was established, which can be used to predict dendritic growth at given cooling rates in the solidification process of Ni-based superalloys. The n
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