| The microstructures, martensitic transformations, mechanical properties and magnetic-field-induced strain (MFIS) of Ni54.75Mn20.25-xFexGa25 and Ni54.75Mn13.25Fe7Ga25-xCox magnetic shape memory alloys have been systematically investigated by means of optical microscopy, TEM, DSC, XRD, compressive tests, ac-susceptibility and MFIS measurements. The effect of Fe and Co addition on martensitic transformation temperature and the improvement of mechanical properties have been discussed. Ni-Mn-Fe-Ga magnetic shape memory alloy thin films have been fabricated by magnetron sputtering technique, and the surface morphology, microstructure and transformation behavior have been investigated.It is shown that the martensitic transformation temperatures and the type of martensite in Ni54.75Mn20.25-xFexGa25 (x=0, 1, 3, 5, 7) alloys are significantly influenced by Fe content. The martensitic transformation temperatures decrease with the Fe content increasing. The Curie temperature firstly decreases and then increases with the increase of Fe content. The type of martensite is non-modulated tetragonal when the Fe content is less than 5 at.%, however, the type of martensite is 7M (orthorhombic) when the Fe content is up to 7 at.%.Compressed fracture strength increases from 467MPa to 580MPa, and fracture strain increases from 5.2% to 8.7% with increasing the Fe content from 0 to 7 at. %, respectively. The fracture type changes from intergranular crack to the mixture of intergranular and transgranular cleavage crack.The experimental results show that the microstructure and the constitution phase change with the Co content increasing. The alloys are of single-phase martensite when the content of Co is less than 2 at.%, and the face-centered cubic structureγphase appears when Co content is more than 2 at.%. The amounts ofγphase increase with the increase of Co content. The Co content has significant influence on martensitic transformation temperatures and the type of martensite. The martensitic transformation temperatures firstly increase and then keep unchanged with increasing Co content. Curie temperature firstly increases and then decreases with increasing the Co content. It achieves the maximum when the content of Co is 1.5 at. %. The type of martensite is 7M orthorhombic without Co addition, and it changes to be non-modulated tetragonal after doping Co. Compressed fracture strength and fracture strain increase significantly with the Co content increasing. The fracture strength and strain are 2098 MPa and 38.3%, respectively, when the Co content is up to 6 at. %. The fracture exhibits transgranular cleavage crack. The increase ofγphase and the change of fracture type are the main reason for the improvement of ductility.It is also found that the magnetron sputtering process has remarkable influence on the surface characteristics and chemical compositions of Ni-Mn-Fe-Ga alloy thin films. As the sputtering power ranging between 245W and 405W, Ni content of the thin films decreases with the sputtering power increasing, whereas Mn and Fe contents increase with increasing the sputtering power and Ga content almost keep a constant. The surface roughness and the average particle size of thin films increase with the increase of Ar working pressure and sputtering power. The as-deposited Ni54.75Mn13.25Fe7Ga25 thin film is partially crystallized. After annealing at 973K for 10 minutes, the film is fully crystallized and associated with the appearence ofγphase. The grain size is about 500nm, which are two orders of magnitude smaller than that of bulk material with same composition. The film undergoes L21→7M martensitic transformation during cooling.
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