Effect of grain boundary characteristics on hot tearing in directional solidification of superalloys
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he effect of grain boundary (GB) misorientation on hot tearing susceptibility of directionally solidified (DS) nickel-based superalloys was explored. We found that the castability of second generation nickel-based superalloy CMSX-4 is inferior to DS superalloy IN792, an alloy well known for bad castability. The castability of CMSX-4 is somewhat improved at a higher solidification rate. The hot tearing tendency increases with increasing GB misorientation angle. As feeding tendency becomes greater with increasing misorientation, this points to the importance of GB cohesion for solidification cracking in the alloy. Microstructure investigation reveals that hot tearing is associated with formation of continuous gamma and gamma prime eutectic films at the GB in CMSX-4. We assume that the gamma and gamma prime eutectic, which reflects the remaining liquid at the end of solidification, prevents the impinging dendrite arms from touching and in this way decreases cohesion.
I. INTRODUCTION
Single crystal (SC) turbine blades produced by directional solidification (DS) have led to dramatic improvements in gas turbine performance.1 Recent developments in SC superalloy compositions have aimed at the further increase of temperature capability. Progress was made by increasing refractory element additions such as tantalum (Ta), rhenium (Re), and tungsten (W) because of their important solid solution strengthening effect. However, the casting characteristics of these newer high-refractory alloys are significantly worse than those of the earlier alloys. Pollock et al.2–4 reported refractory alloying additions to promote the formation of freckle-type defects. McLean et al.5–7 found that the high-refractory superalloys yield a wider range of crystal orientations. The present article studies hot tearing susceptibility of these alloys. Crack formation along longitudinal grain boundaries (GBs) during the last stages of solidification is a wellknown problem in a DS nickel-based superalloy. The phenomenon is known as hot tearing, hot cracking, or solidification cracking in contrast to cold cracking or stress relief cracking, which occurs below the solidus temperature during further cooling. Hot tearing phenomena as a source for bad castability has been studied
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0284 J. Mater. Res., Vol. 21, No. 9, Sep 2006
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extensively over the past few decades.8,9 It is generally believed that hot tearing occurs in a so-called “film stage” at the last stages of solidification where the residual liquid is still more or less continuous, and that hot tearing is a GB phenomena: the initiation or propagation of hot cracks is usually intergranular.10,11 In the case of a DS process where hot tears develop along the broad faces of columnar grains, this means that only small GB cohesion forces are developed, essentially by the bridging of secondary dendrite arms. If this small amount of resistance is o
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