Experimental Research On The Microscopic Fatigue Properties Of Nickel Based Super Alloy

The fatigue properties at the micro scale in the areas of stress concentration of Ni based superalloy GH4169 have been studied in the thesis, by using microscopic optical measuring techniques. Concretely speaking, the research includes the following aspects:(1) A experimental approach for investigation of in-plane displacement at the grain scale in polycrystalline materials has been proposed, by coupling digital image correlation method and a new type of microscope—digital microscope. The measuring errors of this approach have been analyzed. It is shown that the accuracy of this approach can fulfill the requirement of measuring deformation at the grain scale. As an example of application, a uniform tension test and a fatigue experiment have been presented for a Ni based alloy specimen GH4169, and inhomogeneous strain distribution maps at the grain scale have been obtained.(2) By coupling scanning electron microscope and digital speckle correlation method, the fatigue residual strain distributions at the grain scale in the areas of stress concentration of GH4169 specimens have been obtained. The results showed that there is a trend of accumulation for the residual strains, the strain distributions at the micro scale are inhomogeneous, and the residual strain gradients are large. The residual strains have relations to the grain boundaries.(3) By coupling scanning electron microscope and digital speckle correlation method, the relations between residual strains at the grain scale and micro-crack initiation and propagation have been studied. It is shown that the micro-cracks are more likely to initiate in or near the areas with large residual strain at the micro scale, and propagate along the large-residual-strain paths.(4) By using the digital microscope and scanning electron microscope, the mechanisms of fatigue micro-crack initiation and propagation of GH4169 have been studied at room temperature and high temperature(450℃), respectively. The propagation rates of micro-cracks have been measured, too. The result showed that micro-crack initiates and propagates independently as a single crack. The main propagation way of cracks is transgranular propagation, and there is still small part of intergranular propagation. Pause and transient acceleration also occur during crack propagation. It is thought that the reason for the pause and transient acceleration is the different hindering effects of the grain boundaries in different directions at the micro scale, and the hindering effects of machining defects (for example, nicks).