| Research Abstract |
The intermetallic compound Ni_3Nb (δ) is of interest as a strengthener of Ni base alloys because of its phase stability up to the melting temperature. The crystal structure of the δ phase is ordered orthorhombic (DO_a structure), the lowest crystal symmetry among the A_3B compounds with close-packed structure. Thus, in order to use the δ phase as a strengthener, it is important to find the way to change its precipitation morphology more fine and uniform. The misfit accommodation between the δ and γ phases by alloying is one of the effective ways to change its morphology. In this study, phase equilibria among the phases including γ-nickel solid solution and Ni_3Nb-δ in Ni-rich corner of Ni-Nb-M (M : Fe, Co, Al, Ti, V) ternary systems at elevated temperatures have been examined. We have found that the solid solubility of δ phase field in these systems, and the crystal structure becomes more symmetrical by the addition these element but Al.
Based on the knowledge of phase diagrams and crystal symmetry, , the effect of Fe, for example, on precipitation of the δ phase in the γ Ni solid solution was studied. The addition of Fe drastically refines the morphology of the δ precipitates. TEM weak beam analysis revealed the a-axis misfit dislocation density increases with Fe content. This is because of the increase in the lattice misfit along the a-axis. The introduction of the misfit dislocations makes the growth kinetics sluggish, which is responsible for the refinement of the δ precipitates in Fe containing alloys. The effects of other elements on the precipitation kinetics and morphology of the δ phase have been demonstrated, and the alloy design using the δ phase by controlling the morphology has been demonstrated.
|
