Volume 2, Issue 4 (April 2005)
Temperature Dependent Fracture Toughness of a Single Crystal Nickel Superalloy
An important application of nickel base superalloys is in the industrial gas turbine (IGT) industry, where the high temperature capabilities of these alloys are exploited. Extensive testing has focused on the tensile and fatigue properties of single crystal versions of these alloys. The single crystals are utilized in the hottest parts of the turbine, where a detailed knowledge of material behavior is critical to design and life assessment. However, studies on the effects of temperature on fracture toughness have largely been absent in both past and present literature. Therefore, the goal of this research is to characterize the fracture toughness properties of a second-generation single crystal nickel superalloy at elevated temperatures. The experimental methods follow ASTM guidelines, as does the analysis for the elastic (K) and plastic (J integral) deformation. The toughness values were invalid for plain strain fracture toughness, but this should be expected from a structural material. In spite of this fact, both the K- and J-based analyses reveal consistent trends. Increased plasticity at elevated temperatures results in an overall increase in toughness as temperature increases. Unusual cases with inclined pre-cracks and microstructural defects did not technically yield valid results, although they were consistent with the nominal cases. Overall, the results indicate that the toughness of single crystal nickel alloys is actually quite high. Thus, the lack of work in this subject is understandable. However, it would seem that any design and lifting work would benefit from an understanding of such a fundamental property.