Effect Of Heat Treatments On The Phase Transformation And Magnetic Properties Of Ni-Co-Mn-Sn Alloy Ribbon

Ni-Co-Mn-Sn ferromagnetic shape memory alloy is an important promising magnetic refrigerant material due to its giant magnetocaloric effect.A ferromagnetic shape memory alloy ribbon with a nominal composition of Ni₃₈Co₁₂Mn₄₁Sn₉ was prepared by the vacuum melt-spinning technique.The microstructure,martensitic transformation and magnetic properties of differently treated ribbons were investigated systematically by SEM,DSC,XRD and VSM.The results show that in the as-spun ribbon,single martensite microstructure is found.Annealing at 700°C for all times and 800°C/0.5h do not lead to the formation of second phase.Further increasing the annealing temperature to 800°C and the holding time to 5 and10 h,a small amount of second phases occur.900°C annealing precipitates the second phase irrespective of the holding time.As compared with the matrix,the content of Co in the second phase is much greater and Sn content is much lower.EDS and XRD results indicate that the second phase is a Co-rich and Sn-poor phase with a BCC structure,i.e.,?phase.DSC analysis shows that all the ribbons undergo a typical thermoelastic martensitic transformation.It should be noted that,upon cooling and heating,only one-step forward and reverse transformation occurs in the ribbons after annealing at 700 and 900°C,while two-stage forward and reverse transformation occurs in the ribbons annealed at 800°C.In comparision with the as-spun ribbon,the transformation temperatures are slightly increased by annealing at 700 and 800°C but greatly lowered by annealing at 900°C,which can be ascribed to the composition change of matrix resulting from the precipitation of second phases.Martensitic transformation temperatures are gradually shifted to higher values during the initial thermal cyclings and then become stable for all the ribbons.Martensitic reverse transformation temperatures are decreased by several centigrade after the first thermal cycling and kept constant after that for the as-spun,700 and 800°C-annealed ribbons.For the ribbons annealed at 900°C for 5 and 10 h,martensitic reverse transformation temperatures are not affected by thermal cycling.It is shown that the all the as-annealed ribbons exhibit one-way shape memory behavior and the ribbon annealed for 30 min at 900°C has the largest shape recovery ratio.Isothermal magnetization curves show that for the as-spun ribbon and the ribbons after annealing at 700 and 800oC,martensitic reverse transformation can be induced at a low magnetic field,which is usually 7 kOe.For the ribbon annealed at 700oC for 10 h,a magnetic field of 3 kOe is large enough to realize the martensitic reverse transformation.In contrast,for the ribbons after annealing at 900oC,a much higher magnetic field up to 12kOe is required to induce the reverse transformation.The magnetic field-induced reverse transformation is characterized by the step-like jump of magnetization occurring within the reverse transformation range.The as-spun ribbon and the ribbons after annealing at 700 and 800oC for 0.5 h exhibit giant magnetocaloric effects far above room temperature because the temperature regime showing large magnetic entropy changes?S_m?is above 400 K.The peak value of S_m for the as-spun ribbon is 15.3 J/kg K,which is increased to 16.0 J/kg K after annealing 700°C for0.5 h.Further increasing annealing temperature and holding time causes a decrease of S_m.All the ribbons containing the second phase have smaller S_m than the as-spun ribbon except that annealed for 5 h at 900°C whose peak S_m is 14.5 J/kg K.