Effect of grain-boundary characteristics on castability of nickel-base superalloys
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Effect of Grain-Boundary Characteristics on Castability of Nickel-Base Superalloys JIAN ZHANG and ROBERT F. SINGER Hot tearing susceptibility of two Ni-base superalloys, IN792 and CM247, during directional solidification was studied. Effects of grain-boundary (GB) misorientation on castability were examined by casting single-crystal (SC) and bicrystal (BC) specimens. Crack-free specimens were obtained in castings with GB misorientation angles of less than 12 deg. Severe cracking occurred if the GB angle was greater than 25 deg. Secondary dendrite arm length right at the GB was found to be larger than in the GB vicinity and to increase with GB misorientation. The amount of eutectic melt and foreign element segregation is also larger at the GB. The greater susceptibility of the GB to hot tearing is almost certainly caused by reduced strength compared to the grain interior. The reduced strength is either due to reduced secondary arm bridging because of geometrical constraints or due to the existence of a thin liquid film as a result of stronger segregation.
I. INTRODUCTION
CRACK formation during directional solidification (DS) of Ni-base superalloys in investment casting is a well-known problem in the casting industry. Cracking occurs at the columnar grain boundaries (GBs) in the last stages of the DS process, and is also known as hot tearing, hot cracking, or solidification cracking. It has been well established that the DS versions of particular high chromium superalloys such as IN792* suffer from this problem prominently, while *IN792 is a trademark of Special Metals Corporation, Huntington, WV.
other alloys such as CM247* are much less prone to it.[1,2] *CM247 is a trademark of Cannon-Muskegon Corporation, MI.
Hot tearing phenomena as a source for bad castability have been studied extensively during the past decades.[3,4] The experience is largely built on binary alloy systems such as Al alloys[5,6,7] and organic alloys.[8,9] It is generally believed that (1) hot tearing occurs in a so-called “film stage” at the final stages of solidification, where the residual liquid is still more or less continuous; and (2) hot tearing is a GB phenomenon: the initiation or propagation of hot cracks is usually intergranular.[5,10] In the case of a DS process, where cracks develop along the broad faces of columnar grains, this means that only small cohesion forces are developed, essentially by bridging of secondary dendrite arms. If this little resistance is overcome by tensile forces and the grains are pulled apart, hot tearing can only be prevented by feeding of melt over long distances through narrow channels, i.e., through the thin film of remaining liquid on the GBs. It is well known that an alloy usually shows some mechanical strength even above the solidus temperature. For example, the so-called nil-strength temperatures determined in Gleeble JIAN ZHANG, formerly Research Associate, Department of Materials Science, University of Erlangen, 91058 Erlangen, Germany, is now Professor and Group L
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