Microstructure and mechanical properties change by rare earth Dy added in as-cast Co-Ni-Al ferromagnetic shape memory alloys

By means of X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectrometry and compression tests, the influences of the rare earth Dy added in as-cast Co-Ni-Al ferromagnetic shape memory alloys on microstructure and mechanical properties were investigated. The results indicated that the microstructure of Co-Ni-Al-Dy consists of the matrix and the Co-rich γ phase. Wherein, the rare earth Dy segregated in the γ phase and scarcely any Dy was observed in the matrix. With the increasing of Dy content, the discrete-type γ phase disappeared gradually and the rod-type one grew up to be a network of dendritic-type γ phase at the grain boundary during this time. When Dy rose to 0.5 at%, rare earth intermatallic compound Co5Dy precipitated in the Co-rich γ phase. Meanwhile, the increases of the volume fraction of γ phase and grain refinement significantly improved the mechanical properties. Nevertheless, a large number of Co5Dy intermatallic compounds precipitated in the γ phase would weaken the strength and ductility of the alloy when the Dy content rose from 0.5 at% to 1 at%.