Abstract
Coarse inclusion materials have shown evidence of inclusion cracking ahead of a crack-tip by in-situ SEM observations, which degrades the fracture toughness. In-situ strengths of various inclusions have also been estimated in the preliminary analysis. The present work is aimed at numerical analyses of toughness degradation due to the existence of damaged inclusions with an A2091 alloy as its model material. Especially, microstructural control for toughness enhancement is discussed in the lights of the results.The investigation employs a combination of HRR singularity and an Eshelby model, which considers both elastic mismatches between a matrix and inclusions and back stresses due to rigidity of the inclusions, to conduct an estimation of internal stresses in the inclusions. The essential feature of the model is to predict crack initiation toughness and crack path morphologies using a mixed-mode fracture criterion. An appropriate criterion for the damage initiation, effects of a deflected crack-tip and shielding/antishielding effects due to the damaged inclusions are taken into consideration.The toughness is found to be degraded in the case of large matrix grains, high volume fraction of inclusions, and low fracture strength of the inclusions. When the spatial distribution of the inclusions is homogeneous, these effects are predicted less than 10%. However, the effect is remarkably pronounced when the inclusions are agglomerated. Some of these results are consistent with those by in-situ SEM observations reported elsewhere. The estimated lower boundary values of the fracture strengths are 2520 and 2440 MPa respectively for Al3Zr and Al3Ti particles. These values are of particular interests, because they are several times larger than the measured in-situ strengths of CuAl2 and Al2CuMg particles. Aligned weak inclusions on grain boundaries act to deflect the crack along the grain boundaries even when inferior crack propagation resistance within grain boundary PFZs is not considered.
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