Atomic-scale structural characterization and twin formation mechanisms of μ phase within refractory Nb[sbnd]Ni alloy

The peritectic μ-Nb₇Ni₆ phase was prepared directly from liquid Nb₅₄Ni₄₆ alloy by melting spinning technique. The advanced precession electron diffraction analyses of nanograin orientation indicate that there are abundant {11¯02} twins in solidified microstructures. The atomic resolution image shows that there exist complex atomic arrangements of both {11¯02} stacking fault and twin together with a lattice offset between twin and matrix. The atomic structure mode and the interfacial atomic arrangements demonstrate that the {11¯02} twin of the μ-Nb₇Ni₆ compound is a kind of tension twin. The atomic site occupation of the μ-Nb₇Ni₆ phase was also derived from the atomic-scale elemental mapping. Furthermore, the density function theory calculation reveals that the {11¯02} twin originates from the slipping on the {11¯02} stacking fault crystallographic slipping plane along with the [101¯1] direction. Additionally, the crystallographic slipping plane is inconsistent with the crystallographic twinning plane, which further induces the matrix and twin are not completely mirror symmetric (with a lattice offset) about the {11¯02} twin plane. Finally, an extended formation criterion of {101¯2} twin in the crystals with hexagonal cell was proposed based on the experimental and calculation results of the μ-Nb₇Ni₆ crystal.