On the formation mechanism of hierarchical inter- and intragranular precipitations in a cast Cu-17Ni-3Al-based alloy

Cast Al-containing cupronickel alloys are the promising materials for marine applications. However, due to addition of multiple alloying elements, the microstructural development and transformation products become rather complicated that must be fully understood to optimize the mechanical properties. In this contribution, the microstructure characteristics of a cast Cu-17Ni-3Al-based alloy are systematically investigated using a combination of experiments and theoretical calculations. Two novel hierarchical precipitations at both the grain boundaries and grain interiors are revealed. The intergranular precipitates (in micron and submicron scales) consist of G phase, Cr-rich phase and B2 phase, where the Cr-rich phase and G phase are mainly eutectic products that can serve as a nucleation site for the B2 phase at low temperature. By contrast, the intragranular precipitates (in nano-scale) are mainly comprised of L1 2 -ordered phase with types of primary and secondary ones. For the first time the splitting of L1 2 phase is observed in the Cu alloys, where the splitting product is FeCrNi-rich phase with a disordered FCC structure and fully coherent interface with the L1 2 phase. This may support the splitting mechanism of thermodynamically driven rather than the pure elastic driven as remaining disputable in Ni-based alloys. First-principles calculations are performed to address the alloying substitution effect on the nucleation of L1 2 phase in terms of the interfacial energy. The preferred occupation site by Cu at the Cu/Ni 3 Al interface is revealed. The present study may shed new light on the alloy design of high-performance cupronickel alloys by utilizing hierarchical precipitation strategy. • The intergranular precipitates of G phase and Cr-rich phase severe as nucleation sites for B2 phase based on their mutual coherent orientation relationship. • The intragranular precipitates are mainly comprised of L1 2 -ordered phase with primary and secondary types differing in size and morphology. • Splitting of the L1 2 phase into FeCrNi-rich phase with disordered FCC structure is captured for the first time in Cu alloys. • The preferred incorporation of Cu into L1 2 phase can be attributed to the significant reduction of interfacial energy via Cu occupying the Al lattice site.