DAMAGE EVOLUTION DURING COMPRESSIVE HOLD SUSTAINED PEAK LOW CYCLE FATIGUE OF A NL-BASED SINGLE-CRYSTAL SUPERALLOY Swapnil Patil 1 , Shenyan Huang 2 , Mallikarjun Karadge 2 , Doug Konitzer 3 , Akane Suzuki 2 1 General Electric Global Research, Bangalore, Karnataka 560066, India 2 General Electric Global Research, Niskayuna, NY 12309, USA 3 General Electric Aviation, Cincinnati, OH 45215, USA Keywords: hold-time fatigue, crack growth, hysteresis loops, creep Abstract The damage evolution in a Ni-base single-crystal superalloy during compressive hold sustained peak low cycle fatigue (SPLCF) was investigated by conducting series of interrupted SPLCF tests using René N5 at 982 and 1093 °C. Evolution of hysteresis loops and microstructure in the bulk away from surface cracks were analyzed to understand bulk damage accumulation. Tensile stress developed during SPLCF was found to be a key factor for crack propagation and life. Bulk deformation away from cracks showed a signature of dominant creep deformation, including γ' rafting and formation of interfacial dislocation networks. Crack depth measurements along with detailed microstructural investigations of cracks were performed to understand crack growth behavior. Based on the experimental results, a comprehensive SPLCF crack growth model was developed by combining a finite element method (FEM) simulation of oxide induced crack growth and linear elastic fracture mechanics (LEFM) based approach. Introduction Sustained-peak low cycle fatigue (SPLCF) resistance is one of the important properties required for Ni-based single-crystal superalloys. SPLCF deformation involves fatigue, creep and oxidation, and it is important to understand how these degradation processes contribute to the SPLCF damage evolution. SPLCF damage can be divided into two parts: bulk damage accumulation and crack nucleation followed by propagation. Literature involving SPLCF behavior of single crystal superalloys is quite limited [1-7]. Typical bulk microstructure after compressive SPLCF failure in CMSX-4 ® (CMSX-4 ® is a registered trademark of Cannon Muskegon Corporation) and René N5 were reported as the formation of discontinuous p-type rafts during compressive dwell, caused by dislocation activities in both vertical and horizontal γ channels during creep-fatigue interaction [1,5,7]. These rafts seem to have less resistance to dislocation motions than the long continuous rafts developed during creep. However, the SPLCF bulk damage mechanisms and damage evolution during cyclic deformation are not well understood from these earlier studies. Many cracks were found to initiate from surface oxide spikes at high temperatures [1-7]. To explain the surface nucleation and propagation of SPLCF crack Evans and colleagues [10-12] proposed a mechanism of oxide assisted crack growth. However, their oxide assisted crack growth mechanism and/or fatigue crack growth mechanism alone cannot explain observed failure life in SPLCF experiments. In this study, several interrupted SPLCF tests were conducted to understand the SPLCF crack initiation and growth mechanisms along with the bulk damage evolution in a Ni-base single-crystal superalloy René N5. Focus was restricted to compressive SPLCF deformation. Analyses of hysteresis loops and microstructure evolution in the bulk away from surface cracks were performed to identify bulk damage and its correlation with SPLCF life. Based on detailed microstructural investigations of cracks, a comprehensive SPLCF crack growth model was developed in this study. A finite element method (FEM) simulation of oxide induced crack growth and linear elastic fracture mechanics (LEFM) based approach was employed to model SPLCF life. Experimental Procedures SPLCF tests were conducted on a bare Ni-base single-crystal superalloy René N5 with loading direction parallel to [001] crystallographic orientation within 8 degrees of misorientation. Tests were conducted isothermally at 1093 ˚C and 982 ˚C in air, under strain-controlled condition with A = –1 and total strain ranges 0.4, 0.7 and 0.9%. The specimen was held in compression at a peak compressive strain for 2 minutes, and strain was then returned to zero. Tests were terminated at about 10, 25, 50, 75 and 100% of the expected SPLCF life. Detailed microstructural characterization using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) was conducted on longitudinal sections of the specimens. TEM specimens for bulk deformation studies were prepared by conventional slicing and electropolishing technique while specimens from crack tip locations were prepared by focused ion beam milling (FIB). Fractography was also performed on specimens tested to failure. Hysteresis Loop Analysis Typical SPLCF stress-strain hysteresis loops are illustrated in Figure 1(a). Reasonable repeatability in hysteresis loops was confirmed by comparing data of interrupted tests and the test to failure. With increasing cycle, loop shifts to the tensile stress direction mainly due to the stress relaxation during the 2 minute hold, which leads to tensile mean stress. Stress and strain parameters from loop analysis are also defined schematically in Figure 1(a). As an example, evolution of maximum, minimum, mean stresses, total stress range, cyclic stress range, stress relaxation at 982 ˚C and 0.7% total strain range is plotted in Figure 1(b), which can be divided into three stages throughout the life. At early stage of SPLCF within the first 20~50 cycles, stress relaxation during 2 minute hold continuously decreases and cyclic stress range defined as total stress range extracted by stress relaxation increases, which indicate cyclic hardening. Plastic strain range per cycle decreases accordingly. The intermediate 959 Superalloys 2016: Proceedings of the 13th International Symposium on Superalloys Edited by: Mark Hardy, Eric Huron, Uwe Glatzel, Brian Griffin, Beth Lewis, Cathie Rae, Venkat Seetharaman, and Sammy Tin TMS (The Minerals, Metals & Materials Society), 2016