The Effect of Casting Conditions on the High-Cycle Fatigue Properties of the Single-Crystal Nickel-Base Superalloy PWA 1483

Abstract

Different solidification conditions are employed to produce sets of single-crystal samples with different primary dendrite arm spacings, i.e., 600 lm, 400 lm, and 250 lm. The material with the smaller dendrite arm spacing is shown to possess considerably increased high-cycle fatigue life. Fatigue cracks are found to originate from shrinkage porosity rather than from carbides, in which the size of the largest pores in the samples scales with the primary dendrite arm spacing. Fatigue life can be rationalized using a fracture mechanics approach based on a KitagawaTakahashi plot. The impact of the results with respect to the development of new commercial casting processes that produce higher temperature gradients and cooling rates will be discussed.

Very High Cycle Fatigue of Ni-Based Single-Crystal Superalloys at High Temperature

“…This is in agreement with VHCF results obtained at room temperature on SX [38] and DS alloys. [39] It appears that the smaller the pore size is, the higher the VHCF life [see Figure 3(b) focused on AM1 and MCNG alloys], which is in agreement with previous studies done in the LCF [18,20,21] and HCF [18,19] regimes when environment has (almost) no impact. Since the casting pore size in the specimen volume (and more specifically, the tail of the pore size distribution toward larger sizes) is directly connected to the solidification process, [22] then the LMC solidification process shows a high improvement in the VHCF life in comparison to the Bridgman one.…”

“…Crack initiation (and life) under these fatigue conditions are mainly dependent on the local stress concentration close to the pores and hence, mainly process dependent. [19,20] B. Impact of Defect Nature at R = À 1 Both eutectics and carbides are defects that do not present a prominent role on the VHCF life in comparison with pores.…”

“…Both alloys present crack initiation from pores full of small cracked carbides at 20 kHz, 1000°C, and R = À 1, similar to studies done on Rene´N5 in the LCF regime at 538°C [20] and PWA1484 in the HCF regime at 800°C. [19] However, MAR-M200+Hf is a SX alloy containing much more carbides than these two alloys. At 650°C in the LCF regime, crack initiation mainly originates from internal clusters of carbides that can reach a size of up to 200 lm.…”

“…[18][19][20][21] One aim of this study was to investigate the impact of the pore size on the VHCF properties using three AM1 and two MCNG materials processed differently. Table I presents the size range of the casting pores that acted as crack initiation sites for these alloys, and also for the other ones.…”

Development and Application of an Optimization Protocol for Directional Solidification: Integrating Fundamental Theory, Experimentation and Modeling Tools

“…Production implementation of the LMC process for blades provided a 2-10x increase in growth rate [3]. The process has demonstrated 3-9x increase in cooling rate, corresponding to a 50% reduction in microstructure scale with reduced defect occurrence [4][5][6][7][8][9][10][11][12][13][14][15][16][17]. The refined microstructure has resulted in a 3-7x improvement in fatigue life on simple geometries [17].…”