The image shown here is of ceramics, where they varied the grain size and measured the strength. For example, let's take Al2O3. There we observed that strength first increased from 3 μm to 95 μm. But after 95 μm, strength was decreasing (dislocation slip can explain this behaviour).
Q 1. Why does strength decreases is we reduce the size from 95 μm to 3 μm, (many people have suggested looking for inverse hall-Petch relation but that is valid for nanocrystalline material)
Q 2. Why do TiO2 and Nb2O5 are behaving very different?