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

The peritectic mu-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 {1 (1) over bar 02} twins in solidified microstructures. The atomic resolution image shows that there exist complex atomic arrangements of both {1 (1) over bar 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 {1 (1) over bar 02} twin of the mu-Nb7Ni6 compound is a kind of tension twin. The atomic site occupation of the mu-Nb7Ni6 phase was also derived from the atomic-scale elemental mapping. Furthermore, the density function theory calculation reveals that the {1 (1) over bar 02} twin originates from the slipping on the {1 (1) over bar 02} stacking fault crystallographic slipping plane along with the [10 (1) over bar1] 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 {1 (1) over bar 02} twin plane. Finally, an extended formation criterion of {10 (1) over bar2} twin in the crystals with hexagonal cell was proposed based on the experimental and calculation results of the mu-Nb7Ni6 crystal.