13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- cracks especially dwell time cracking at hold times with varying length. 2. Experiments Two alloys IN718 and Allvac718Plus have been subjected to fatigue testing with dwell time. The testing was performed in servohydraulic testing machines at constant stress amplitude using SEN specimens [8] with DCPD crack monitoring at high temperature using a radial furnace or induction heating [8, 15, 16]. Specimens with a crack were studied either as cross sections or as fracture surfaces using an Hitachi S70 analytical SEM with an annular Back scatter detector making it possible to produce images by Electron Channeling Contrast Image (ECCI). This method has been used with great success to study damage mechanisms in super alloy single crystals [17, 18]. The studied specimens have been chosen from crack growth experiments performed with Kb-type of specimens with cross sections of 4.3 X 10.2 mm with a 0.2 mm sparc cut starter notch. A baseline series of specimens were run at 0.5 Hz, R=0.005and the hold time testing (90s, 2160s and 21600s) was performed at 450, 550, 650 and 700°C in lab air environment except for Allvac718Plus that was also tested in pure oxygen atmosphere and the results presented as crack growth curves in ref. [5, 16, 19-21]. 3. Results In order to be able to distinguish between crack growth at different temperatures with and without dwell the results from crack growth without dwell will be presented first. In figure 1 the crack zone for baseline cracking at 450°C and 650°C is shown. a b Figure 1. SEM-ECCI micrograph showing cross section of the crack tip region of IN718 subjected to base line fatigue at a) 450°C showing high slip activity and 650°C with lower slip activity. The lower temperature shows clear channeling contrast from plastic deformation caused by homogeneous slip on primary slip systems and in a narrow zone close to the crack localized more concentrated multiple slip. The crack path is transcrystalline with slight crack branching. The crack growth mechanism seems to be based on a weakening of interface at the crack tip by plastic deformation. For the higher temperature the crack path is more intergranular with more crack branching and the slip intensity is less pronounced. The specimens subjected to dwell show more intergranular fracture and branching Figure 2 and the plastic deformation is more pronounced at 550°C but the fracture mode is varying along the crack 10μm 10μm
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