13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- crack initiation, short crack propagation and long crack propagation, which might be important in some cases. Another option, which is linked to physical properties of the material, is a microstructural-based short fatigue crack propagation model for lifetime prediction [6]. Since the loads arising in components associated with the internal structure are usually in the area of high cycle fatigue, up to 99% of the total lifetime can be spent with crack initiation and the propagation of short fatigue cracks. Thus, modeling short crack propagation in virtual microstructures promises a flexible and reliable approach for lifetime calculations. This paper presents firstly some selected experimental results from a study of the crack initiation and microstructurally short fatigue crack propagation mechanisms in two forged Ti6Al4V alloys. Then a mechanism-based model will briefly be introduced which tries to describe quantitatively the phenomena observed. This model will be verified by comparing experimental and predicted results. Finally it will be shown that by means of the generation of virtual microstructures the model is capable to identify microstructural parameters which are significant for a purposeful microstructure optimization with respect to fatigue resistance. 2. Material and experimental details The Ti6Al4V alloy under investigation was delivered by Böhler Schmiedetechnik, where round bar stocks were forged into “V-shaped” pieces from which all specimens were machined. Two different heat treatments were applied after the forging process, (i) mill-annealing (ma) and (ii) solution heat treatment (sht). The most important chemical elements of the composition are given in Tab. 1. The analysis was done using spark emission spectroscopy. The figures represent mean values from three measurements and are within normal scatter. Some Fe content was found probably resulting from impurities of the alloying elements or from the process routine. Table 1. Chemical composition in wt.% Element Al V Fe Ti Concentration 6.5 3.52 0.133 bal. Micrographs using a scanning electron microscope with backscattered electron detector giving a channeling grain contrast of the resulting microstructures are shown in Fig. 1. A typical bi-modal microstructure was obtained in both conditions, thus no distinctive differences are present. The microstructures consist of primary alpha grains (αp-grains) and colonies of secondary alpha lamellae (αs-lamellae). A little content of remaining β-phase (app. 5-8%) can be found between the lamellae or at triple points. The primary alpha grains of the sht microstructure are fully recrystallized due to the applied dwell time at a high temperature near the forging temperature, while the ma condition was cooled directly after forging, so some grains are only partially recrystallized. This can be deduced from orientation measurement data exhibiting large misorientation variations in single primary alpha grains along with subgrain boundaries.
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