Precipitation of Gamma-Prime and Gamma-Double Prime Coherent Phases in Three-Phase Ni-V-Si Alloys

Abstract

Phase separation of γ (A1) supersaturated solid solution into A1, γ’ (L12) and γ” (D022) phases was investigated in two Ni-rich Ni-V-Si ternary alloys by means of transmission electron microscopy. When the alloys are annealed at 1073K, two different sequences of the phase separation are observed, depending on the chemical composition of the alloy: In Ni-17.0at%V-6.9at%Si alloy (A) at the D022 corner of three-phase field, first many D022 particles precipitate aligning along the <110> direction of the matrix and the so-called chessboard pattern is observed, followed by the formation of L12 phase at the interface between D022 and A1 phases. In Ni-12.1at%V-11.3at%Si alloy (B) at the L12 corner of the Gibbs triangle, cuboidal L12 particles precipitate arranging along the <100> direction, and then D022 phase is formed. As the phase separation proceeds, a selective growth/formation of the third phase (L12 in the alloy A, D022 in the alloy B) occurs: In the alloy A, L12 phase grows into D022 particle inside along the diagonal direction of D022 cube which is parallel to the a-axis of D022 tetragonal phase. In the alloy B, D022 forms on the {100} cube face of cuboidal L12 particle, arranging the c-axis of D022 perpendicular to the {100} cube face of L12 phase. As a result of such a selective growth/formation, the first phase D022/L12 is split off into two particles, which results in the formation of laminated structure consisting of D022 and L12 phases. The selective growth/formation is considered to occur so as to maintain the less elastic strain state.