Formation and Morphology of Kurnakov Type D022Compound in Disordered fcc γ-(Ni, Fe) Matrix Alloys

Abstract

ABSTRACTFormation and morphology of D022compound Ni3V in γ-fcc alloys were investigated as a model case in order to understand fundamentals for microstructure control of new class of austenitic steels strengthened by Kurnakov-type GCP intermetallic compound. The formation process of the D022phase in γ matrix varies, depending on the composition and heat treatment temperature, and precipitation of D022takes place at temperature above T0in the γ+D022two phase region. The morphology of coherent precipitates of the D022compound is sensitive to misfit strains against matrix and can be controlled by alloying addition. The habit planes between the two phases become irrational, parallel to both directions of the invariant line and an a-axis of D022, since the lattice misfit becomes negative along a-axis (δa) and positive along c-axis (δc) of the D022. The calculation based on the lattice invariant theory as well as experimental results clearly demonstrate that the misfit strain ratio δc/δais a dominant factor to determine the habit plane. In addition, the shape of D022phase, either prism or plate, depends strongly on the magnitude of |δa|. These findings will extend to Ni-Fe-Nb-V quaternary system, by partial replacement of Ni with Fe and that of V with Nb.