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A B S T R A C T Fatigue crack growth at room temperature and its relation to the local microstructure is studied for four different γ-TiAl-alloys with microstructures ranging from coarse and fully lamellar to fine and partly lamellar. It is shown that the number of cycles to failure depends strongly on the efficiency of the first barrier to crack extension, as crack growth rates may increase rapidly once this barrier has been breached by a specific crack. The crack extension behaviour for two typical barriers (colony boundary and twin boundary) is studied using high-resolution optical and scanning electron microscopy.

A B S T R A C T The relation is studied between crack initiation from micro-notches in a fully lamellar intermetallic -TiAl alloy and the local strain field. These micro-notches were introduced using femtosecond-laser ablation and had dimensions below the average colony size. The specimen under investigation was then subjected to fatigue loading. Continuous monitoring using a travelling optical microscope allowed detecting microcracks at an early stage. Prior to fatigue loading, a sustained load was applied and the local strain field was determined using digital image correlation. This was supplemented by a Finite Element analysis of the notches and their neighbourhood. It was found that a crack was initiated from a notch causing high normal strains in lamella direction, whereas no crack was initiated from notches with high shear strains. a = Notch depth [μm] b = Notch width [μm] c = Notch length [μm] da/dN = Crack growth rate [mm/cycle] DIC = Digital image correlation E i = values of elastic modulus [MPa] FE = Finite element method FSD = Forward scatter diode G ij = Values of shear modulus [MPa] R p0.2 = Yield strength [MPa] R p0.01 = Elastic limit [MPa] ΔK = Range of stress intensity factor [MPa√m] ε, ε ij = Strain tensor V ij = Values of Poisson's ratio σ = Stress [MPa]