Effects of Nb and Zr addition on introduction of misfit dislocations onto α₂/γ lamellar boundaries and change of lattice misfit in fully lamellar TiAl alloy

Changes in microstructures of α₂/γ lamellar boundaries was studied on fully lamellar Ti-38Al-xM (at%, x = 0, 3, M = Zr, Nb) alloys over a wide range of average lamellar thickness from 290 to 20 nm. The lamellar structure was formed by isothermal aging and lamellar thickness was controlled by changing aging temperature. The addition of Nb or Zr alters the lattice mismatch between α₂ and γ phases. At lamellar boundaries the mismatch is accommodated by elastic deformation and/or by introducing misfit dislocations. The elastic deformation totally accommodates the lattice mismatch (coherent boundary) when γ lamellae are thin enough. Misfit dislocations are introduced to accommodate partly the mismatch when γ lamellar thickness exceeds a critical value λ_c. Density of the misfit dislocations saturates to an upper limit d_s in thick lamellar structures. The critical value of λ_c decreases and the saturation level d_s increases with increasing lattice mismatch. Zr increases and Nb decreases the lattice mismatch.