Martensitic Transformation And Microstructure Of Ni-Mn-Ga Magnetically Controlled Shape Memory Alloys

The characteristics of martensite transformation and microstructure of Ni-Mn-Ga alloys were investigated by means of resistivity measurement, optical microscopy, transmission electron microscopy and X-ray diffraction analysis.An intersecting or overlapping type of martensite morphology was observed. Most martensite grow along the solidification direction and only a few martensite are perpendicular to the boundaries of the austenite during cooling.Two kinds of martensitic transformations were examined inNi43.85Mn20.09Ga25.05,Ni54.48Mn2l.69Ga23.83, Ni45.03Mn32.gGa22.17,nI54.9mN19.6-Ga25C0.5 and Ni55.4Mn2o.3Ga22.gC 1.4 alloys. Only the first martensite transformation occurred in NiMn(GaSi), NiMn(GaGe) and NiMn(GaCo) alloys. The Si and Ge additions obviously decrease the first martensitic transformation temperature of the alloys and depress the second martensitic transformation completely. The substitution of Co for Mn has hardly any influence on martensite transformation temperature; the substitution of Co for Ga increases the first phase transformation temperature and the thermal hysteresis of martensitic transformation.The X-ray diffraction spectrum of L21 structure accords with that of face-centered cubic structure. There are three kinds of martensite structure in the alloys: non-modulated martensite in Ni56.3Mni9.4Ga22.?Ci.6, Ni48.19Mn27.1Ga21.52Co3.19 and Ni47.99Mn27.43Ga20.32Co4.26 alloys, five-layered martensite in Ni54.4sMn21.69Ga23.83 alloy and seven-layered martensite of the first phase transformation and five-layered martensite of the second phase transformation in Ni54.85Mn20.09Ga25.05 alloy.The results of transmission electron microscopy (TEM) show that the martensite in Ni54.48Mn21.69Ga23.83 alloy is five-layered martensite, and thatof Ni563Mn19.4Ga22.7C1.6 alloy is non-modulated one, which are consistentwith the results of X-ray diffraction. The treatments of face-centered cubic structure for the L21 structure and base-centered monoclinic structure for five-layered modulated martensite were applied when analyzing the results of TEM. The orientation relationship between austenite and martensite of Ni54.48Mn21.69Ga23.83 alloy martensite transformation was studied.The effect on Ni-Mn-Ga martensite transformation temperature of composition was analyzed in detail. The martensite transformation temperature of Ni-Mn-Ga alloys can be evaluated as Ms = 1858-17.23X Mn -49.02XCa -61.92XSi -65.43XGe+1.24Co -51.6XC -58.92XIn by means of multiple linear regression, where XMn, XCa, XSi, XGe, XCo, Xc and XIn are mole percent numbers for elements Mn, Ga, Si, Ge, Co, C and In. Both electron concentration and austenite lattice parameter influence martensite transformation temperature, however, electron density (valence electrons per volume) may be a more appropriate or accurate factor regarding the martensitic transformation temperature, based on the electron theory of metal and our results. With increasing electron density, the phase transformation temperature increases.The crystallography data was acquired by applying phenomenological theory of martensite crystallography to Ni-Mn-Ga alloys. Only when assuming that the plane and direction of the lattice-invariant shear are - and respectively is the calculation ofphenomenological theory of martensite crystallography self-congruent. The martensite transformation crystallography such as theoretical habit plane was dependent on the alloy composition.