Formation and development of C40/C11b lamellar structure in NbSi2/MoSi2 crystals

An aligned C40/C11b lamellar structure is formed in as-grown (Mo0.85Nb0.15)Si2 crystals after they are annealed at temperatures of 1673–1873 K in air. Here, a complete C40/C11b lamellar structure was formed at 1873 K/20 h, and lamellar boundaries were formed by the motion of ledges containing dislocations on the (0001)C40 plane. Further, curved dislocations were observed in some coarse C11b lamellae but never in the C40 lamellae. This is attributed to the lattice strain accumulated during coarsening of the C11b lamellae and subsequently relaxed by dislocation glide in the C11b phase which bears a relatively high crystallographical symmetry compared with the C40 phase. The critical lamellar spacing of C11b lamellae for introducing curved dislocations is approximately 0.80 μm.