13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- a b Figure 8. SEM fractographs of grain facets in Allvac718plus (700°C dwell) a) oxidized surface with voids b) surface at lower magnification with one open and one closed secondary crack with voids. Figure 9. SEM fractograph showing the voids in the grain boundary shown in fig. 8b at higher magnification. 4. Discussion The results from studies of the crack growth process of IN718 and Allvac718Plus under fatigue and dwell time loading conditions at high temperature will be discussed. The focus will be put on the effects of events taking place during the hold time periods. The crack growth under the influence of temperature and cyclic loading is transgranular at 450°C with clear evidence of planar slip in the grains surrounding the crack going over to intense slip close to the fracture surface with phenomena like nano twins and local recrystallisation appearing. The crack path is sometimes branched and not as transgranular as during room temperature cycling indicating that the grain boundaries play a role even during base line cracking. With increasing temperature the crack path becomes more branched and the plastic deformation is less pronounced. This indicates that the environmental effect is gradually increasing with temperature. The hold time experiments consist of applied constant load cycles between 90 to 21660 s where each cycle is separated by unloading to zero load level. The observed macroscopic change in growth rate caused by those events can unfortunately not by traced on a micro level. The reason for that is probably that the crack advance is a non continuous process. Gas phase embrittlement, GPE by oxygen is by far the most proposed damage mechanism in this case [12]. Still very little is 250nm 2.5μm 1μm
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