Abstract:The effect of post annealing on the phase transformation of Ni₅₂Mn₂₄Ga₂₄ ferromagnetic shape memory alloy particles prepared by ball milling was studied. Ni₅₂Mn₂₄Ga₂₄ alloy particles at micron scale were prepared successfully by ball milling the crushed bulk alloy. SEM observation reveals that the shape of the as-milled particle is regular polygon and a lot of cracks can be seen at the surface of the particles. For as-milled particles, the widening of characteristic peak can be found in the XRD pattern, and no transformation characterization can be detected by DSC. Post annealing at the elevated temperature will recover the transformation behavior of milled particles to the same level as that of bulk sample. It is shown that with increasing annealing temperature above 400 ℃, M_s decreases and A_s increases, while the magnetic transition temperature keeps constant. XRD results indicate that the change of grain size of the particles results in such an effect of post annealing.

Key words: NiMnGa particle; phase transformation; ball milling; ferromagnetic shape memory alloy