Microstructure and Magnetic Property Variation with Addition of Rare Earth Element Dy in Co-Ni-Al Ferromagnetic Shape Memory Alloy

The influence of rare earth element Dy on martensitic transformation and magnetic properties of Co-Ni-Al alloy was studied. The results showed that the microstructure of the sample has a dual-phase structure (γ-phase and martensite). The rare earth element Dy was segregated in Co-rich γ-phase and took the place of Co after its addition into the Co-Ni-Al alloy. As Dy content increased to over 0.5 at. %, the grain was refined and the rare earth intermetallic compounds Co5 Dy were precipitated in y-phase. Meanwhile, one-step thermo-elastic martensitic transformation occurred in the sample, wherein the phase transformation temperature significantly increased with rising Dy content. The martensite had a tetragonal L10 structure with a (111) twinning plane. Furthermore, the sample exhibited obvious hysteresis behaviors in the magnetic hysteresis loops. In addition, the saturation magnetization, coercivity, retentivity and magnetocrystalline anisotropy were significantly enhanced owing to the bigger radius of Dy which took the place of Co in the alloy.