Study On Carbide Evolution And Related Properties Of Cobalt-base Superalloys

Cobalt-base superalloys have been widely used in industrial and aircraft turbine as turbine blade and inlet guide vane materials due to their intrinsic properties such as high melting temperature and superior thermal fatigue resistance. Thermal fatigue is one of the most important failure forms for guide vanes. Thermal fatigue crack mainly initiates at and then propagates along the carbides and grain boundaries. In order to improve the thermal fatigue properties of alloys, it is necessary to explore the ways to tune the morphology of carbides and grain boundaries. Heat treatment is an important means of affecting the type, morphology, size and distribution of carbides. Thus, the main purpose of this paper is to study the microstructure evolutions of two kinds of cobalt-base superalloys, DD640M and DD6509, under heat treatment and their effects on thermal fatigue behavior. Meanwhile, the stress-rupture properties of alloys under the corresponding conditions are also concerned. Additionally, in order to eliminate the effects associated with grain boundaries on carbide evolutions, the single crystal technology is applied. Not only that, the elimination of grain boundary is also beneficial to the improvement of thermal fatigue properties of alloys. This study provides a theoretical basis for the development and application of cobalt-base superalloys.The as-cast DD640M alloy contains two types of primary eutectic carbides:Cr-rich network M₇C₃ and ?Ta, Zr?-rich Chinese script MC carbides. During heat treatment at 1140?1260 0C for 4 h,the number and size of primary carbides decrease with temperature increasing. Furthermore, the carbide transformations of M₇C₃?M23C6 and the degeneration of MC carbide have been occurred during heat treatment.The in-situ transformation of M₇C₃ to M23C6 is first observed directly in cobalt-base superalloys. M23C6 carbide nucleates at the M₇C₃/matrix interface due to the coherent relationship between M23C6 carbide and the matrix,and grows towards M₇C₃ carbide.On the other hand, the primary MC carbide degenerates and releases a large number of Ti and W during heat treatment. The degeneration behavior of MC carbide is temperature dependent: MC decomposes into M6C carbide at lower temperature and dissolves partially into the matrix at higher temperature.The as-cast DD6509 alloy contains two types of primary eutectic carbides: ?Ta,Zr?-rich Chinese script MC and Cr-rich irregular block M23C6 carbides. During heat treatment at 1260?1330 ? for 4 h, primary carbides gradually dissolve into the matrix. The heat treatment at 1300 ? for 4 h causes the complete dissolution of primary M23C6 carbide. In addition, some Chinese script MC carbides are broken into fine particles.The solution treatment promotes the precipitation of finer and more homogeneously distributed secondary carbides during aging at 1000¹200 0C in DD640M and DD6509 alloys. Only secondary M23C6 carbides precipitate in the matrix for DD640M alloy. The precipitation of secondary MC and M23C6 carbides occurs in DD6509 alloy. Secondary MC carbides are distributed in the matrix while secondary M23C6 carbides mainly develop around the carbides.For DD640M alloy, the melting reaction of primary M₇C₃ eutectic carbide occurs at 1280 ?,and the re-solidified structure is M23C6 eutectic carbide. In addition,the melting process of primary M₇C₃ eutectic carbide is that M₇C₃ first transforms into M23C6 and then melts rather than directly melts. After heated at or above 1320 ?, the finer Chinese script MC eutectic carbide is produced after primary MC eutectic melting. For DD6509 alloy, the melting of primary M23C6 and MC eutectic carbides occurs at 1335 ? and 1340 ?, respectively. This study firstly focuses on the melting of primary eutectic carbides in cobalt-base superalloys, which provides a good reference for optimizing the chemical composition and carbide microstructure of cobalt-base superalloys.The thermal fatigue properties of DD640M and DD6509 alloys can be improved by using proper heat treatment process. For DD640M alloy, the 1260 ?/24 h heat-treated sample owes the best thermal fatigue property. The DD6509 alloy has the best thermal fatigue property after solution treatment at 1330 ?/24 h and followed by aging treatment at 1100 ?/100 h. The improvement of thermal fatigue resistance results from the optimization of the size and distribution of the carbides by heat treatment, which effectively hinders the initiation and propagation of thermal fatigue cracks.The rupture life of both DD640M and DD6509 alloys has been greatly improved after solution treatment, which is derived from the more stable microstructure, dispersed MC carbide morphology and the supersaturated matrix. The carbide transformations of M₇C₃ ? M23C6,MC ? M23C6, M23C6? M6C have occurred in the as-cast DD640M alloy during stress-rupture test. The thermal fatigue properties and stress-rupture properties of cobalt-base superalloys can be improved significantly by using heat treatment process, which changes the limited understanding of the effects of heat treatment on the mechanical properties, thus heat treatment should be given attention for cobalt-base superalloys.The type and stability of primary carbides are affected by different chemical composition of these two kinds of alloys. The thermal fatigue properties and rupture life of DD6509 alloy are better than that of DD640M alloy, which is attributed to the more stable carbide structure, higher carbide content and the precipitation of secondary MC carbide.