Toward a better understanding of strain incompatibilities at grain boundaries in the analysis of fatigue crack initiation at low temperature in the Udimet^TM 720 Li superalloy

¹ Institut Pprime, CNRS – ENSMA - Université de Poitiers, UPR CNRS 3346, Department of Physics and Mechanics of Materials, ENSMA – Téléport 2, 1 avenue Clément Ader, BP. 40109, 86961 Futuroscope Chasseneuil Cedex, France
² Turbomeca – SAFRAN group, Materials Processes & Investigations Department, 64511 Bordes Cedex, France

^a Corresponding author: baptiste.larrouy@ensma.fr

Abstract

Low cycle fatigue properties of polycrystalline γ-γ′ Ni-based superalloys are dependent on many factors such as temperature, environment, grain size and distribution of the strengthening phases. Under LCF conditions at intermediate temperatures, an intergranular crack initiation could be observed. In this paper we propose to analyze the local conditions favouring such an intergranular cracking mode considering the high strength C&W Udimet^TM720 Li alloy, widely used for manufacturing high pressure turbine disk for aeroengine applications. Tensile and fatigue tests were performed in air in the 20–465 ^∘C range of temperature on micro-samples in order to focus on plasticity and damage processes developed near grain boundaries. A special attention was paid on the slip transfer between neighbouring grains taking into account their local crystallographic orientations. In some specific crystallographic configurations, small zones were detected at the tip of slip bands presenting an intense elastic/plastic activity. Although they are limited in size, they are associated to local crystalline rotations. High levels of local strain/stress were also evaluated in these volumes using an EBSD pattern cross correlation technique. The development of such specific zones was investigated at different stages of the tensile and LCF behaviour and was identified as leading to micro-cracks initiation for both solicitation modes.