Tham khảo tài liệu 'titanium part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 232 5 Alpha Beta Alloys corresponding scale for the crack depth a at the top of the diagram . It can be seen that the propagation curves for the short corner cracks lie between the microcrack curves self-initiated surface cracks on round smooth specimens and the macrocrack curves measured on CT specimens. The ranking and position of the corner crack curves for the two microstructures indicate that the range of crack front lengths covered by this data is slightly beyond the crossover of the two microcrack curves. In this case the retarding effect of the developing rough crack front profile for the lamellar microstructure is about equal to or slightly larger than the intrinsically slower crack propagation behavior of the bi-modal microstructure. As pointed out above macrocracks have different crack front profiles in fully lamellar and bi-modal microstructures. Figure illustrates this point by looking at the crack front in a plane perpendicular to the crack propagation direction. The large difference in crack front profile between the two microstructures is striking. The crack front profile for the coarse lamellar microstructure is very rough and the steps with the height Z discussed earlier in Sect. see also Fig. are clearly visible and lie in a very unfavorable orientation for crack propagation. In contrast the crack front profile of the bi-modal microstructure is relatively smooth. In agreement with this relatively smooth crack front profile of the bi-modal microstructure the fracture toughness of bi-modal microstructures of the Ti-6Al-4V alloy is around 55 MPa m1 2 being only slightly higher than the fracture toughness of the fine lamellar microstructure 50 MPa m1 2 but much lower than the toughness of the coarse lamellar structure 75 MPa m1 2 described already in Sect. . Fig. . LCF curves R of the a p alloy IMI 834 see Fig. for microstructures bi- modal 1 and bi-modal 2 Microstructure and Mechanical Properties 233 Table .
