Low cycle fatigue and dwell-fatigue of diffusion coated superalloy Inconel 713LC at 800 °C

Abstract

The charm of diffusion coatings is mainly in their protective function of the surface of gas turbine components, which are often subjected to fatigue-creep loading in extremely hostile environments. Their impact gains in importance with the increasing time that the components are exposed to high temperatures and aggressive environments. This motivates the authors to study the low cycle fatigue behaviour of Al-Cr coated nickel-based superalloy Inconel 713LC. Polycrystalline diffusion Al-Cr coating was produced by out-of-pack Cr-modified aluminising chemical vapour deposition technique on the surface of cylindrical fatigue specimens manufactured by investment casting. Continuous low cycle fatigue (CLCF) tests and dwell-fatigue (DF) tests where a 10-minute tensile dwell was included in each cycle were conducted in strain control mode with constant total strain amplitude at 800 °C. The fatigue behaviour was characterised by cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves. The Al-Cr coating provided excellent protection against surface oxidation of Inconel 713LC, especially during dwell-fatigue tests. The substrate as well as the Al-Cr coating were examined in as-coated conditions and after fatigue loading through scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Dislocation arrangement in the substrate material was investigated in a transmission electron microscope (TEM). The microstructural observations and the discussion on the degradation mechanisms of surface-treated superalloy have been provided for both regimes of cycling.