Effect of Cu addition on nanocrystallisation of Al–Ni–Sm amorphous alloy

The microstructure and the partitioning of solute atoms during the course of primary crystallisation in Al88Ni8Sm4 (numbers indicate at.%) and Al88Ni7Sm4Cu1 melt spun amorphous alloys have been investigated using the techniques of transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP). TEM observations on the specimens annealed at 124 °C for different times suggest that the primary crystallisation in both alloys is through nucleation and growth. A large number of nanometer scale α-Al particles are formed after annealing at 220 °C for 10 min. The substitution of Cu for Ni results in refinement of the nanocrystalline microstructure. The 3DAP results on the annealed Al88Ni7Sm4Cu1 alloy indicate that solute atoms of Ni, Sm and Cu are rejected from the α-Al particles and partition into the remaining amorphous phase. No nanometer scale Cu clusters were observed. Therefore, in the present alloy the refinement of nanocrystalline microstructure by the substitution of Cu for Ni is not through heterogeneous nucleation on nanometer scale Cu clusters, which has been shown to occur in FINEMET and Fe–Zr–B nanocrystalline soft magnetic alloys. It is suggested that the reduction of grain size of α-Al particles by the substitution of Cu for Ni arises from a reduction of the nucleation barrier.