In-situ SEM study on effect of inclusion particles on fracture behavior in Al-Li system alloy

In-situ SEM observations of specimen surfaces during a 3-point bend test have been made to characterize the microscopic damage initiation and the fracture behavior of an Al-Li system alloy. Damage initiation from inclusion particles larger than several micrometers is always attributed to their breakage. Neither interfacial debonding nor interfacial cavitation is observed for such coarse particles. Some kinds of such coarse inclusion particles are fractured several hundred micrometers ahead of a main crack tip. Simultaneous extension of microcracks and the main crack, and subsequent incorporation of the microcracks into the main crack are observed. In such a way, it is clarified that the fracture of the Al-Li alloy is significantly affected by the coarse inclusions. A combination of HRR singularity and Eshelby type internal stress analysis has been used to calculate the actual fracture strengths of various inclusion particles. Fracture strength of both CuAl₂ and Al₂CuMg particles is about 710 MPa for the diameter ranging 5 and 8 μm, and the fracture strength decreases with increasing particle size. Damage initiation in Al₃Zr and Al₃Ti particles has not been observed in this study. The lower bound values of the fracture strength of these two kinds of particles are estimated as 900 to approximately 1000 MPa by the same analysis.