13th International Conference on Fracture June 16–21, 2013, Beijing, China 3 3.1. Microstructure Analysis on microstructures of as-cast and post-HIPped Ti-6Al-4V showed that both samples have the lamellar colony structure and alpha film was found along beta grain boundaries. The microstructure of as-cast samples is characterized by grain size of about 2-3 mm, alpha platelet thickness of about 1.5μm while post-HIPped samples by grain size of about 2-3 mm, alpha platelet thickness of about 4.5μm. The grain size and the alpha platelet thickness increased after post-HIPping due to heating for a long time during HIPping. 3.2. Fracture toughness The fracture toughness sample conditions do not meet all the validity requirements (B≥2.5(KIC/σy) 2), thus the testing results are considered to be an estimate of the fracture toughness. The measured fracture toughness values are 86, 83, 85 MPa m1/2 for as-cast samples which are lower than that of post-HIPped samples (95, 97, 93 MPa m1/2)which means the toughness was increased with increasing alpha platelet thickness. N.L. Richards also reported that the toughness obey straight-line relationships against the distance between the platelets i.e., the thickness of the alpha platelets[6]. SEM fractographs for the fracture surfaces of the as-cast and post-HIPped samples are shown in Figure 2. Optical images of microstructures in thin sections: (a) as-cast, (b) post-HIPped. (a) (b) Figure 3. 3D fracture surface of Ti64 fracture toughness samples: (a) as-cast, (b) post-HIPped. (a) (b)
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