On the continuous and discontinuous precipitation of the L12 phase in Cu-Ni-Al alloys

L12 precipitate hardened Cu-Ni-Al alloys offer high strength, corrosion resistance and anti-biofouling properties, making them useful in marine engineering applications. Optimisation of their mechanical properties requires a full understanding of their complex precipitate nucleation and coarsening mechanisms. In this work, the microstructural characteristics and hardness of three Cu-Ni-Al alloys with compositions of Cu(95-x)NixAl5 (x = 5, 15, 25 at%) were investigated in the homogenised state and following heat treatments at 700˚C for 1, 10, 100 and 1000 hours. L12 precipitates were observed in the alloys containing ≥ 15 at% Ni. In these alloys, the L12 phase was found to precipitate via both continuous and discontinuous routes following all exposures at 700˚C. The coarsening behaviours of the continuous and discontinuous L12 distributions were characterised and correlated to measurements of hardness and lattice misfit. The alloys containing 15 and 25 at% Ni exhibited peak hardness after 1 h at 700˚C, which corresponded to average particle diameters of 30 nm, respectively. These results were rationalised through calculations of the change in the critical resolved shear stress associated with the transition from weakly to strongly coupled superpartial dislocations. The discontinuous reaction was observed to be led by L12 phase formation, which extended into the neighbouring grain, ahead of the reorientation front of the matrix.