Effect of grain size on the fracture toughness of nanocrystalline FeMoSiB

In a former paper, by employing a new technique - laser drilling - to produce artificial cracks, fracture toughness K sub Ic of FeMoSiB nanocrystalline ribbon samples have been successfully measured, and typical features of crack surface have been investigated. However, the measurements were performed for samples with a specific grain size; it did not tell how K sub Ic varies with the microstructure. Furthermore, microhardness measurements show an abnormal Hall-Petch relationship below a critical grain size. It is thus interesting to understand: 1. Whether or not the values of K sub Ic vary with grain size? 2. How K sub Ic correlates with the microstructure? 3. Relationships between plastic deformation and fracture resistance. With these problems in mind, fracture toughness tests are performed on FeMoSiB nanocrystalline ribbon samples with varied grain size, and efforts are made to try to get some clues of the fracturing mechanism of nanocrystalline materials.