Effects Of Co And Cu Doping On Phase Transformation And Magnetic Properties Of NiMnGa Alloys

The martensitic transformation and the magnetic properties of Ni-Mn-Ga ferromagnetic shape memory alloys have a strong composition dependence, which can be modified through composition tuning or alloying. In the present work, the master Ni5oMn28Ga22alloy was alloyed through Cu substitution for Mn, Co substitution for Ni, and Cu and Co simultaneous substitution for Mn and Ni, respectively. Three group polycrystalline alloys with the nominal composition of Ni5oMn28-xCuxGa22(x=0,1,2,3,4,5,6,7), Ni50-xCoxMn28Ga22(x=l,2,3,4,5,6,7) and Ni50-xCoxMn28-xCuxGa22(x=1,2,3,4,5,6,7) were prepared. Based on differential scanning calorimetry (DSC), magnetic property measurement system(MPMS), vibrating sample magnetometer (VSM), X-ray diffraction (XRD), scanning electron microscope (SEM) and material testing system, the influence of Co and Cu substitution on the martensitic transformation behaviors, magnetic properties, crystal structures, microstructures and mechanical properties was studied systemically.According to DSC and MPMS test, it was found that replacing Mn with Cu into Ni5oMn28Ga22can increase martensitic transformation temperatures and decreasae the Curie temperature. On the contrary, replacing Ni with Co will decrease the martensitic transformation temperature and increase the Curie temperature. Moreover, replacing Mn with Cu and replacing Ni with Co at the same time, with the increase of the adding element content, the martensitic transformation temperatures tent toincrease. The Curie temperature showed the trend of slow increase up to Co and Cu with3%, then it decreased quickly with the increase of the Co and Cu content.XRD measurement results showed that the room temperature phase of Ni5oMn28Ga22alloy is5M martensite. When Cu is added to1%and2%, the room temperature phase remained5M martensite. With Cu addition of3-7%, the room temperature phase is the mixture of7M and NM martensite. The room temperature phase of all the Co added alloys were austenite. For the simultaneous addition of Co and Cu, the alloy remained5M martensite at room temperature at the content of addition elements with1%. When Co and Cu contents were2-7%, the room temperature phase transformed to the mixture of7M and NM martensite, similar to the situation with Cu substitution for Mn.Based on the SEM observations, it was shown that the microstructure of5M martensite was characterized with the broad plates. Moreover, much thinner plates with a certain bending were found inside the broad plates. For the co-existing microstructure of7M and NM martensite, the plate thickness of NM martensite was obviously higher than that of7M martensite. Further observation revealed that the NM martensite plates consisted of paralleled thin plates.According to VSM measurements, all three groups of alloy were in ferromagnetic state at room temperature. In the Cu doping alloys, with the increase of the content of Cu, the saturation magnetization first decreased and then increased to the maximum with Cu content of3%. After that, saturation magnetization decreased again. The Co addition can result in the fluctuation of the saturation magnetization and in general the increase of the saturation magnetization.The Cu addition can result in the increase of fracture strength of the alloy when Cu content was1%. Further addition (≥2%) weakened the mechanic properties. There was no general tread for the influence of Co addition on the fracture strength. However, the fracture strength of all the Co added alloys was higher than that of master Ni50Mn28Ga22alloy, suggesting the Co addition exerted a certain strength for mechanic properties. For the simultaneous substitution with Co and Cu, the fracture strength of the alloys increased gradually when Co and Cu content was1-4%. After that the fracture strength first increased and then decreased.