Effect of Crystal Orientation on Fatigue Failure of Single Crystal Nickel Base Turbine Blade Superalloys

Abstract

High cycle fatigue (HCF) induced failures in aircraft gas turbine and rocket engine turbopump blades is a pervasive problem. Single crystal nickel turbine blades are being utilized in rocket engine turbopumps and jet engines throughout industry because of their superior creep, stress rupture, melt resistance, and thermomechanical fatigue capabilities over polycrystalline alloys. Currently the most widely used single crystal turbine blade superalloys are PWA 1480/1493, PWA 1484, RENE' N-5 and CMSX-4. These alloys play an important role in commercial, military and space propulsion systems. Single crystal materials have highly orthotropic properties making the position of the crystal lattice relative to the part geometry a significant factor in the overall analysis. The failure modes of single crystal turbine blades are complicated to predict due to the material orthotropy and variations in crystal orientations. Fatigue life estimation of single crystal turbine blades represents an important aspect of durability assessment. It is therefore of practical interest to develop effective fatigue failure criteria for single crystal nickel alloys and to investigate the effects of variation of primary and secondary crystal orientation on fatigue life. A fatigue failure criterion based on the maximum shear stress amplitude /Delta(sub tau)(sub max))] on the 24 octahedral and 6 cube slip systems, is presented for single crystal nickel superalloys (FCC crystal). This criterion reduces the scatter in uniaxial LCF test data considerably for PWA 1493 at 1200 F in air. Additionally, single crystal turbine blades used in the alternate advanced high-pressure fuel turbopump (AHPFTP/AT) are modeled using a large-scale three-dimensional finite element model. This finite element model is capable of accounting for material orthotrophy and variation in primary and secondary crystal orientation. Effects of variation in crystal orientation on blade stress response are studied based on 297 finite element model runs. Fatigue lives at critical points in the blade are computed using finite element stress results and the failure criterion developed. Stress analysis results in the blade attachment region are also presented. Results presented demonstrates that control of secondary and primary crystallographic orientation has the potential to significantly increase a component S resistance to fatigue crack growth with- out adding additional weight or cost. [DOI: 10.1115/1.1413767

Effects of secondary orientations on long fatigue crack growth in a single crystal superalloy

“…The transformations from original material compliance to the new specimen coordinate can be determined as the following [2] …”

“…In this case, fatigue cracks may propagate in different orientations if the secondary orientation is not controlled during manufacturing. Therefore, there is a great need to understand the fatigue initiation, threshold and FCG behaviors of turbine blade alloys [2]. However, due to the material anisotropy, fatigue responses and failure modes of single crystal blades are too complicated to predict.…”

“…The [0 0 1] low modulus orientation of single crystal superalloy has the best low cycle fatigue (LCF) resistance [1,2], so the primary orientation of single crystal turbine blades is controlled in [0 0 1] orientation, in order to obtain the longest LCF life for the turbine's start-up and shut-down cycle. However, the secondary orientations, which are normal to the solidification direction, are neither specified nor controlled in the manufacturing process, and thus can vary from 0°to 90° [3].…”

“…However, due to the material anisotropy, fatigue responses and failure modes of single crystal blades are too complicated to predict. Up to date, limited work has been done on the modeling of fatigue crack propagation in association with the secondary orientation effects [2,3,[5][6][7]. Ever since the pioneer work of Paris and Erdogan [8], the famous Paris equation has been successfully employed in characterizing FCG behaviors of most polycrystalline materials.…”

“…However, the secondary orientations, which are normal to the solidification direction, are neither specified nor controlled in the manufacturing process, and thus can vary from 0°to 90° [3]. Early investigations on single crystal blades assumed that it was unnecessary to control the secondary orientation [4], yet more recent studies have shown that the high cycle fatigue performance [2], the FCG response [5], and the notch-tip plastic behaviors [3,6] are appreciably affected by the secondary orientations of single crystal blades.…”