Small and long fatigue crack growth behaviour of a PM Ni-based superalloy, Udimet 720

Abstract

The fatigue crack growth of long and small surface cracks has been investigated in air under four-point bending conditions at a load ratio of 0.1 for a PM Ni-based superalloy, Udimet 720. Cracks were found to initiate from surface pores despite an increased level of inclusions introduced by doping. Attention has been focused on small fatigue crack growth at maximum stresses beyond the 0.2% proof stress of the material. Typical small fatigue crack growth behaviour was found, i.e. growth rates for small fatigue cracks were faster than for long fatigue cracks at all fatigue stress intensity factor ranges investigated. A faceted–striated transition on the fatigue fracture surfaces has been found for both long and small fatigue crack growth. For long fatigue cracks, this transition appears to occur when the reversed plastic zone size is approximately equal to the grain size. For small fatigue cracks, the transition crack depth, d 0, was found to be 10–20 times the grain size and independent of stress range employed.