Fatigue Testing of Ni-based Superalloys for Gas Turbine Blades

Abstract

Single crystal Ni-base superalloys are used for turbine blades in aero engines and stationary gas turbines, where they operate under complex loading conditions, i.e. high temperature and complex stress state. The microstructure that provides high strength and high resistance against creep and fatigue is the well-known γ / γ’ microstructure with cubic γ’-precipitates separated by γ-channels, which is achieved by adding alloying elements and a stepwise solid solution annealing treatment. While in the γ’-phase Al, Ti, and Ta segregate, forming Ni3Al (Ni3Ti), solid solution strengthening of the γ-channels is provided by enrichment of Co, Cr, Re, and W [1,2]. During longer exposure at high temperatures, these strengthening elements also have the tendency to form undesired brittle phases, such as the so called TCP (topologically closed packed) phases namely the σ, μ, or Laves phase. TCP phases can have a deleterious effect on the creep properties of superalloys [3] and influence the fatigue properties (in thermal mechanical fatigue) due to the depletion of the γ matrix of strengthening elements in the vicinity of the TCP-phases [4]. However, observations made in high temperature fatigue experiments indicate that formation of TCP-phases may have beneficial effects on the evolution of fatigue damages. In this contribution, the influence of TCP phases on the crack propagation during high temperature fatigue tests has been investigated and will be discussed.