Role of TCP phases on crack propagation in high temperature fatigue of single crystal Ni-based superalloys

Abstract

Single crystal Ni-base superalloys used for gas turbine blades operate at high temperature and under complex stress state. High strength and high resistance against creep and fatigue is provided by the γ-/γ’- microstructure with cubic ’-precipitates separated by channels. Solid solution strengthening of the channels is provided by enrichment of Co, Cr, Re, and W [1, 2]. However, these strengthening elements have the tendency to form brittle phases, such as the so called TCP (topologically closed packed) phases. TCP phases can have a deleterious effect on the creep properties of superalloys [3] and influence the fatigue properties due to the depletion of the γ-matrix of strengthening elements in the vicinity of the TCP-phases [4]. Own observations made in high temperature low cycle fatigue (LCF) experiments indicate that formation of TCP-phases may have beneficial effects on the evolution of fatigue damages. LCF tests have been performed on a second generation single crystal superalloy with the nominal chemical composition of CMSX-4® at a test temperature of 950°C. Microstructural analyses of tested specimens show that cracks initiate at pores and propagate along two possible paths, i.e. (i) perpendicular to the loading direction through the γ-channels or (ii) along {111}-slip planes. The formation of TCP-phases seems to be linked with the presence of stress-/strain-concentrations since they only appear along crack paths and slip bands and at pores. This assumption is backed by preliminary EBSD-investigations, which reveal crystallographic misorientation at these locations. Slight crack path deviations of mode I cracks at TCP-phases indicate that TCP-phase formation can act as obstacles for cracks propagating along γ-channels. TCP-phases evolving along slip planes may also act as obstacles against further propagation of shear bands and may even enable subsequent healing of sheared microstructure, as suggested by TCP-phases aligned along slip planes in apparently undisturbed microstructure.