Evolution behavior of γ-D03 phase in Cu-15Ni-8Sn alloy and the corresponding effects on alloy property

Abstract

Cu-15Ni-8Sn alloy ingots were prepared by vacuum melting, and the evolution behavior of γ-D03 phase in casting and subsequent heat-treatment processes were systematically investigated, especially the key influencing factors of its distribution, morphology and quantity. Because of the Sn enrichment towards the solidification front, the particulate γ-D03 phase would directly form from the molten alloy in the solidification process, while the lamellar and needlelike ones are then precipitated out from the supersaturated matrix in the post-solidification cooling process. After complete dissolution in the solid-solution treatment, the γ-D03 phase would re-nucleate again at the grain boundary and grow into the adjacent grains in the aging process, forming discontinuous precipitation (DP) colony. The reduction of the number of nucleation sites and the decrease of the free energy difference could both inhibit the formation of γ-D03 phase. As a result, an appropriate addition of B offers an ideal suppression effect, and leads to a decrease of DP content from 40 to 4 vol% after 16 h aging at 673 K. In terms of performance, for every 1 vol% decrease in DP content in the as-aged microstructure, the macro-hardness of alloy increases by 0.75 HB and the electrical conductivity decreases by 0.035% IACS. Furthermore, the in-coherent interface between α-FCC matrix and γ-D03 phase is the weakest link in alloy, and it decreases the tensile strength from 890 to 760 MPa by increasing the DP content from 4 to 40 vol%.