**Author details**

but in case of martensitic morphology, it is the width of individual α plates. Increased crack path tortuosity leads to enhanced crack deflection and a resulting increase in the FCP resistance

**Figure 19.** Comparison of crack front roughness profiles in the zone adjacent to the pore. (a) As welded condition and

The effect of grain size on the notch sensitivity is well known for steels. Murakami [37] investigated microcrack propagation starting from artificial holes with diameters of 35–500 µm in steels. They concluded that defects smaller than a critical size are non‐damaging (not detrimental) to fatigue strength, and the critical size is smaller for materials having higher static strength. Harder material is more sensitive to notches and defects. A larger decrease in fatigue strength for materials of higher static strength was found. This trend is generally in reasonable agreement with our findings, although we investigated different materials. Based on the results for steels, we can also assume the existence of non‐damaging permissible defects for titanium alloys because micromechanisms and models for the behaviour of cracks ema‐ nating from sharp notches do not depend on the material. However, further investigations must be carried out to prove this assumption. If the dependence of critical size on the micro‐ structure of the FZ is obtained, an obvious way to exclude the detrimental effect of pores on fatigue is to provide a welding technique leading to smaller pores than the critical size. However, it should be kept in mind that in the absence of pores or if the size of defects is lower than the critical size, PWHT will have an opposite effect on fatigue; that is, fatigue strength will be reduced after high‐temperature annealing. This was shown in the work of Babu et al. [7] for electron beam welded joints. The fatigue cracks in their work originated at the surface, implying that internal defects were insignificant. PWHT at temperatures of approximately

Fully penetrated Ti‐6Al‐4V butt joints were produced by LBW with filler wire. The effect of PWHT was analysed in terms of microstructural features, microhardness, residual stress

distribution and fatigue performance. The following conclusions can be drawn:

cycles compared to that of the as‐

900°C reduced the fatigue strength of the joints at 2·× 106

welded condition and those annealed at lower temperatures.

and the overall fatigue performance.

136 Study of Grain Boundary Character

(b) annealed at 850°C for 1 h.

**5. Conclusions**

Fedor Fomin\* , Volker Ventzke, Falk Dorn, Nikita Levichev and Nikolai Kashaev

\*Address all correspondence to: fedor.fomin@hzg.de

Institute of Materials Research, Materials Mechanics, Helmholtz‐Zentrum Geesthacht, Geesthacht, Germany
