The mechanism of θ- to α-Al2O3 phase transformation.

• θ- to α-Al 2 O 3 phase transformation followed a "synchro-shear" model. • The details of transformation were mapped out through first-principles calculations. • There was a critical size of ~22.88 nm for θ-Al 2 O 3 before it transformed to the α-Al 2 O 3. ga1 The mechanism of θ- to α-Al 2 O 3 phase transformation is proved by the "synchro-shear" model through the first-principles calculations. The proposed model is based on a reorganization of oxygen anions sublattice from f.c.c. to h.c.p. which is achieved by series of shears of the oxygen closed-stacking plane (20 1 ̅) along the direction of [102] and a migration of Al-cations to the near octahedral interstices to form a "honey comb" lattice. On this basis, it is concluded that there is a critical size of ~22.88 nm at which θ-Al 2 O 3 is thermodynamically able to transform to an α form. The obtained data is in a good agreement with the experimental observations. [ABSTRACT FROM AUTHOR]