The Influence of Low and High-Cycle Fatigue on Dislocations Density and Residual Stresses in Inconel 718

The fatigue phenomena, as being one of the crucial importance for aircraft exploitation life, was investigated with different methods. The most important topic is to determine the state of the material and subsequently the moment of fatigue crack initiation. Many authors investigated the fatigue induced material damage and evolution of structure with different methods i.e.: scanning electron microscopy (Zhang et al. 2008), electron transmission microscopy (Hirsch and Whelan 1960), optical microscopy and two-beam interferometry (Kim and Laird 1978) or /and with modelling (Dingli et al. 2000). Nowadays, as the possibilities of diffraction methods were developed, so the conjoining of the diffraction image with the changes in the material structure can be applied to investigate the fatigue process in materials (Tahara et al. 2009; Huang et al. 2010).

The aim of this work is to find out the relationship between the residual stress and dislocation density evolution applying the X-ray diffraction methods. The dislocations multiply and reorganize during monotonic and cyclic deformation, so their evolution can be a valuable information for investigation of fatigue phenomenon (Huang et al. 2008).

The diffraction methods are non-destructive methods for quantitative analysis of grain level deformation (Korsunsky et al. 2004). In this study X-ray diffraction is employed to acquire the information about the evolution of elastic lattice strains and changes in dislocation density after fatigue cycling of Inconel 718 alloy.

X-ray diffraction has been employed to assess the damage level under high and low cycle fatigue conditions and under the tensile test. The objectives of the work were achieved by two X-ray diffraction techniques: the analysis of residual stresses changes and investigations of changes of full width at half maximum of diffraction peaks which can be a measure of dislocation density changes. The diffraction results were compared to the hardness measurements.