Quasi cleavage fracture of the bimodal size lamellar O phase microstructure of a Ti₂AlNb based alloy

The fracture mechanism of bimodal size lamellar O phase microstructure (BSLO microstructure) of Ti-22Al-25Nb alloy is studied. It is found that a BSLO microstructure fractures at room temperature via a quasi cleavage mechanism. The fracture is dominated by the cleavage of B2 phases. Micro cracks nucleate at O/O grain boundaries and O/B2 phase boundaries in the cleavage crack tip plastic zone. These micro cracks are blunted at B2 phase matrix. Fractured lamellar O phases form strip mini facets on cleavage planes and the blunted micro crack form tearing ridges. The mini facets and the tearing ridges comprise strip shallow dimples, which suggest the ductile fracture. It is determined that the crystal cleavage planes of B2 grains contain {100} and {110} via the SEM rotation operations and EBSD analyses. The cleavage cracks would be deflected at grain boundaries or at sub grain boundaries. The transformation of the crystal cleavage plane can lead to the deflection of a cleavage crack. The transformation of crystal planes can occur at either low angle boundaries or high angle boundaries. The newly defined cleavage factor f_C is effective to present the cleavage favorability of a crystal plane. It is found that B2 grains cleavage generally along a plane with high f_C but not always along the plane with the highest f_C since the cleavage of a grain is also affected by its adjacent ones.