Study On Crystal Structure And Microstructure Of Novel Ni-Mn-Ga Magnetic Shape Memory Alloys

Ni-Mn-Ga magnetic shape memory alloys (MSMAs) with chemical composition close to Ni2MnGa have received great attention during recent years due to their giant magnetic shape memory effect and fast dynamic response. However, there still remain many key fundamental issues unresolved, which greatly hinders further improvement of the functional performances of these MSMAs. In this work, the crystal structure, martensitic transformation crystallography and texture evolution of Ni-Mn-Ga MSMAs are systematically studied. The influence of alloying with Co element on structural and magnetic transformation characteristics of Ni-Mn-Ga MSMAs is also investigated.The crystal structure and phase transformation of Ni-Mn-Ga alloys are investigated by in situ neutron diffraction technique, taking advantage of the acute discernment of nearby elements in the periodic table of neutron diffraction. It is shown that Ni48Mn3oGa22 has a cubic, L21 Heusler structure from 373K to 293 K. Its crystal structure changes into a seven-layered orthorhombic martensitic structure when cooled to 243K. No substantial change of the neutron diffraction pattern is observed upon further cooling to 19K, indicating that there is no intermartensitic transformation in this alloy. Ni53Mn25Ga22 has a tetragonal I4/mmm structure from 20K to 403K. An abrupt jump in unit-cell volume around room temperature, corresponding to an endothermic peak in the heating differential scanning calorimetry (DSC) curve, is observed, which indicates a pretransformation in the martensitic phase of Ni53Mn25Ga22.The martensitic transformation crystallography of Ni53Mn25Ga22 is experimentally studied by electron backscatter diffraction (EBSD) and theoretically predicted by the crystallographic phenomenological theory. Self-accommodated martensitic microstructure is observed at room temperature in the Ni53Mn25Ga22 alloy annealed at 1073K. There are only two martensitic variants distributed alternately in each initial austenite grain. The two variants have a compound twinning relationship with the twinning elements K1=(112), K2=(112),η1=[111],η2=[111], P=(110) and s=0.379. The misorientation between them is-82°around<110> axis. The interface plane between the neighboring martensitic twins is found to be (112), which coincides with the twinning plane. The ratio of the relative amounts of twins within the same initial austenite grain is-1.70. The main orientation relationship between austenite (A) and martensite (M) is Kurdjumov-Sachs (K-S) relationship with (111)A//(101)M, [110]A//[111]M. Based on the crystallographic phenomenological theory, the calculated habit plane is (0.690-0.102 0.716)A, and the magnitude, direction and shear angle of the macroscopic transformation shear are 0.121, [-0.709 0.105 0.698]A and 6.88°, respectively. Nanoscale twins inside the micrometer scale martensitic lamellae are observed in the Ni53Mn25Ga22 alloy annealed at 1173K. The internal nanotwins within one martensitic lamella have a compound twinning relationship. Two kinds of lamellar interfaces, i.e. interpenetrated inter-lamellar interface and stepped intra-lamellar interface, are observed. The orientation relationships between the nanotwins connected by the two kinds of interfaces are determined.Ni-Mn-Ga(-Co) MSMAs are successfully hot forged together with the stainless steel jackets. Neutron diffraction measurement shows that strong texture exists in the hot-forged ingots. The texture in the hot-forged Ni48Mn25Ga22Co5 alloy changes significantly after room temperature deformation, and after subsequent quenching it recovers to its initial state before room temperature deformation. The texture evolution in the Ni48Mn25Ga22Co5 alloy during room temperature deformation and subsequent heat treatment is closely related to its thermally controlled macroscopic shape memory effect.The substitution of Co for Ni in Ni53-xMn25Ga22Cox(x=0-14) alloys proves very efficient in increasing the Curie temperature. It only slightly decreases the martensitic transformation temperature when the Co content is less than 6%. In contrast, an abrupt decrease of martensitic transformation temperature is observed when the Co content exceeds 6%, probably due to the atomic disorder as a result of the addition of a large amount of Co. It is suggested that the substitution of a small amount of Co for Ni is helpful to the development of MSMAs with high martensitic transformation temperature and high Curie temperature.Insights into the fundamental aspects such as microstructure, crystallography, phase transformation and alloying in Ni-Mn-Ga MSMAs are of great significance to the improvement of the functional performances of the present Ni-Mn-Ga alloys and to the design of new promising MSMAs.