Effect of additional elements on fracture toughness of (Mo0.85Nb0.15)Si2 C40/C11b lamellar-structured crystals

• Control of the toughness of lamellar-structured C40/C11b silicides was attempted.
• Toughness variation showed strong orientation and microstructure dependence.
• Toughness was governed by occurrence frequency of delamination and crack deflection.
• The refinement of microstructure by Ir-addition led to an increase in toughness.
• First-principles calculations proved the validity of the experimental results.

Variations in the fracture toughness of lamellar-structured C40/C11b silicides as a function of the microstructure were examined by using Cr-, Zr-, and Ir-added (Mo0.85Nb0.15)Si2 crystals. Fracture toughness variation showed strong orientation dependence, governed by the occurrence frequency of delamination and crack deflection. The refinement of the lamellar microstructure by Ir-addition led to an increase in fracture toughness when the lamella boundary was laid perpendicular to the loading axis, although it decreased at other orientations. The variation in fracture toughness when the alloyed crystal was loaded parallel to the lamellar boundary qualitatively showed good agreement with the predictions by first-principles calculations on the cohesive energy of the C40/C11b interface.

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