Development of a Processing-Structure-Fatigue Property Model for Single Crystal Superalloys.

Abstract

Single crystal nickel-base superalloys have emerged as the materials of choice for high-temperature applications when significant resistance to fatigue loading is required. Although the fatigue life improvements due to a direct material substitution may be possible, additional gains are possible through advanced casting techniques in comparison to typical production processes. However, before integration in the manufacturing sector, optimization of a higher thermal gradient process for producing refined, homogeneous, single crystal components with improved fatigue properties is imperative.

In this dissertation, a higher cooling efficiency process, specifically, liquid-metal cooling (LMC) using Sn as the cooling medium, has been evaluated for potential fatigue property benefits of single crystal superalloy airfoil-sized components. A series of casting experiments were conducted using conventional radiation cooled Bridgman casting and the LMC process to compare the refinement in dendritic spacings and defect size distribution for 1.6 cm diameter rods. Casting conditions were selected to observe the trend in refinement with increasing cooling rate, as well as to identify the limit to structure refinement with the LMC process. Single-crystal René N5 and a modified version of the René N5 alloy were grown using both processes in order to evaluate the influence of segregation of alloying elements and defect occurrence with the addition of the refractory element, Ta. Newly developed statistical modeling techniques were employed to characterize the homogeneity in microstructure. Fatigue experiments were performed at 538oC (R = 0, f = 0.5 Hz), along with unique in-situ crack growth studies (R = 0.1, f = 20 kHz) to examine the influence of refinement on the fatigue life and crack propagation behavior.

The LMC process is capable of refining the dendritic spacings and maximum pore size by 60 and 65 pct, respectively. The primary initiation sites for fracture in the single crystals during low cycle fatigue (LCF) were casting pores that were located internally and near to the surface. These pores were strongly influenced the crack initiation life. An empirical-based processing-structure-fatigue property model that relates the critical aspects of processing conditions to fatigue life has been developed.