Study On The Stress Field Treatment And The Magnetic Field Induced Strain Of Ni-Mn-Ga Polycrystalline Alloys

In this work, two alloys with the nominal composition of Ni50Mn28.5Ga21.5and Ni5oMn3oGa2o were prepared by directional solidification. The composition, crystal structure, microstructure, and martensitic transformation temperatures of the two kinds of alloy were studied by Energy Dispersive Spectrometer (EDS), X-ray diffraction (XRD), Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM) and Electron Backscatter Diffraction (EBSD). Furthermore, the samples of two alloys were treated with the cycled compression by using Shimadzu AG-Xplus100KN material testing system. The magnetic field induced strain and the inter-variant relationship for the samples after compression treatment were illustrated.The results showed that there existed the significant Mn evaporation during the melting process. It should introduce additional Mn (2%) to compensate the loss of Mn. By this way, the as-solidified alloy can remain the designed the composition and the crystal structure of martensite.By comparison, it was found that the compression effect of the bulk samples with the equal length along three direction was better than the samples with other dimensions. The compression along three direction can result in higher residual strain than that of obtained by the compression along two directions. However, the twinning stress plateau of the compression along two direction was much lower. The more annealing time and the higher annealing temperature were favorable for the compression effect.The cycled compression can significantly reduce the twinning stress of Ni-Mn-Ga martensite. The twinning stress of5M martensite can be reduced to～5MPa and7M martensite to～2.5MPa. The cycled compression treatment can simply the twinning structure of the samples, resulting in the arrangement of the martensitic plates along the solidification direction or being a certain acute angle with the solidification direction. After compression, the martensitic plates thickness changed a lot, from the homogeneous thin plates to the inhomogeneous thick plates.The cycled compression treatment was confirmed to be favorable for the improvement of the magnetic field strain. In comparison with5M martensite, the improvement of magnetic field strain in7M martensite was much better.The compression treatment can induce the formation of new twinning system, where both the twinning plane of type Ⅰ twin and the direction of type Ⅱ twin have changed. Compared with the twinning shear (s=0.2299) of twinning system existing in the initial alloys, the twinning shear (s=0.1154) of type Ⅰ twin and type Ⅱ twin for the new twinning system was much smaller. Thus, the new twinning system should be activated much easier.