A1→D022 transformation and multi-variant structure of Ni3V

The formation mechanism and morphology of the tetragonal compound Ni3V (D0(22)) quenched from the disordered fee (A1) state have been examined by TEM using a Ni-25at%V. The alloy is fully transformed to D022 in the quenched state. The resulting microstructure exhibits a fine brick-like "multi-variant structure (MVS)" consisting of one major and two minor variants with c axes perpendicular each other, having coherent interfaces of {102)(D022) and {100)(D022). The major variant particles with a quadrangular-prism shape are alternately arrayed with two minor variants, one parallel to {102)(D022) and the other {100)(D022), respectively. The thermal APDs are formed in the all variants, but the average size within the major is larger than that in the minors. The c/a ratio of the D0(22) in the MVS is 2.006, much smaller than the value (2.04) in equilibrium state. From all these facts, the transformation of Al --> D0(22) is assumed to proceed through a selective nucleation and growth of tetragonal A6 phase, followed by ordering to D0(22). The formation of the tetragonal phase produces an anisotropic strain field due to the contraction and expansion of the a and c axes against the cubic matrix. The strain field makes {102)(D022) and {100)(D022) with the invariant lines become habit, and also drives the selective nucleation and growth of minor variants to minimize the elastic strain energy, thereby leading to the multi-variant structure.