Damage Evolution and Crack Growth in Nickel-Based Alloys during Ultrasonic Fatigue

Abstract

Experimental results on the fatigue damage of quasi defect-free materials in the VHCF range are presented. For nickel-based superalloys and pure nickel the likelihood of crack initiation at favorable grain morphologies is studied. Slip band and microcrack formation at the surface was observed even in run-out samples. Hence, microcracks were evaluated regarding their propagation capabilities according to grain orientation and barrier function of grain boundaries. In the VHCF regime crack initiation can shift from surface to subsurface, consequently early crack growth has to be studied by means of optical methods and indirect detection techniques or tomographic methods. In the study presented crack initiation and crack growth was monitored through optical observation and quasi 3-D observation by means of synchrotron radiation. For an as-received and a coarse-grained condition of pure nickel Ni201 fatigue crack growth in the VHCF regime occurs at deltaK as low as 3.54 MPam1/2 for a crack growth rate da/dN = 10E-12 m/cycle. The grain size had no effect on the threshold limit but crack growth retardation at grain boundaries and crack path deflection lead to lower crack growth rates for the coarse-grained condition In the nickel-based alloy Nimonic 80A the influence of microstructure on the intercrystalline crack initiation and propagation was confirmed. Here, the combination of the misorientation angle between two adjacent grains and the orientation of their boundary with respect to the external load defines the magnitude of stress concentration at grain boundaries.