Effect Of Copper Content On The Microstructure And Magneocaloric Properties Of Ni₅₀Mn₂₅Ga_25-xCu_x Alloy

In this paper, 3 kinds of Ni50Mn25Ga25-x Cux alloys(x=2.2, 3.0, 3.8) with different compositions were prepared by vacuum induction melting method. The microstructures of Ni-Mn-Ga-Cu alloy were observed by using scanning electron microscopy(SEM) and optical microscope(OM). X-ray diffraction(XRD) and differential scanning calorimetry(DSC) were utilized to analyze the phase transformation of Ni-Mn-Ga-Cu alloys, no second phase was found. Ni50Mn25Ga25-x Cux microwires(x=3.8) with diameter of 20 to 60μm were prepared by melt-extraction method. Chemical ordering annealing and internal stress release treatment were applied. Magnetocaloric effect of the microwires after annealing was analyzed on the physical property measurement system(PPMS). Finally, room temperature mechanical properties of the as-extracted and annealed microwires were studied by using a dynamic mechanical analyzer(Q800 DMA) under the tension mode, the results show that Cu doping Ni-Mn-Ga significantly reduced the brittleness of Ni-Mn-Ga alloy.Cu element was choose to take the place of Ga element and act as the fourth element in Ni50Mn25Ga25-x Cux with different composition of x=2.2, 3.0 and 3.8. No secondary phase was found in all alloys after polishing and etching using OM and SEM. XRD results showed that all three alloys are single martensite at room temperature. DSC analysis indicated that the phase transformation temperature increased with increasing Cu content. Actually, the valence electron concentration would be changed with the increase of Cu content, thus, the transformation temperature changed. The Curie temperature of the alloys maintained 70℃and less affected by the Cu element content.Higher Cu content of 3.8 was chosen to prepare the microwires due to the composition loss during melt-extraction and the later heat treatment process. Chemical ordering annealing and internal stress release treatment was applied. PPMS was used to measure the martensitic transformation of the annealed microwires and coupled martensitic transformation and Cuire temperature was found which was considered to have better magnetocaloric effect.At 356 K the microwires has a maximum magnetic entropy change, so designed the isothermal magnetization curves to calculate the microwires’ maximum magnetic entropy change is-5.3J/kg·K at 5T, 359 K.Different gauge lengths of 2mm, 5mm and 10 mm were chosen to test the mechanical properties of the microwires at room temperature. The 2mm gauge length sample exhibited the best fracture strength and elongation and was chosen to perform all the mechanical properties of the microwires in this work. Tensile tests were performed at microwires with different diameter of 30μm, 40μm, 50μm. The tensile strength decreased from 713 MPa to 447 MPa with increasing diameter while the elastic deformation remained unchanged of 1.52%、1.62%、1.68%, respectively. The maximum plastic strain of 3.89% was found in microwires with diameter of 30 μm, indicating excellent ductility. However, 0.57% and 0.62% plastic strain were found in microwires with diameter of 40 and 50 μm, indicating a decreasing ductility with increasing diameter. The microstructure and the defects formed during fabrication were considered to be the key factors affecting the mechanical properties of the microwires By comparing the mechanical properties of alloy with and without Cu, the mechanical properties of Ni-Mn-Ga alloy can be improved significantly. In addition, the fracture reliability evaluation of Ni-Mn-Ga-Cu alloy fiber was carried out. As-extracted Ni-Mn-Ga-Cu alloy microwires has higher Weibull modulus m and threshold stress value sigma u, indicating higher fracture reliability and safety in comparison to the ternary Ni-Mn-Ga microwires.