Research On Crystal Structure And Performances Of NiMn-based Alloys

In recent years, people have been researching on magnetic shape memory alloys, which shape memory effect were caused by changes of external magnetic field. NiMn-based alloy--especially Ni-Mn-Ga and Ni-Co-Mn-In alloys have been widely investigated. This paper researches this two family alloys.In this paper, the crystal structure and formation mechanism of precipitates in Ni45Co5Mn36.7ln13.3alloy are studied. TEM analysis indicates that the crystal structure of precipitates is fcc structure with lattice constant of0.365nm. It is speculated that the precipitates is y phase which presence helps reduce the brittleness Ni-Co-Mn-In alloys and make the alloy more practical in use. The formation mechanism of precipitates is studied from the aspect of NiMn deformation alloys. After thermal deformation from β+γ phase region, the martensite structure was found in Ni52Mn48alloys, which indicated that the composition offset of β phase.For Ni45Co5Mn37In13alloy, XRD analysis finds martensite structure at room temperature. The lattice parameters of martensite structure simulated by Powdercell is a=0.4617nm b=2.7821nm c=0.6115nm.The detail modulated structures of martensite phase for Ni45Co5Mn37In13alloy is investigated by TEM. Alloys quenched from850℃and furnace cooled from the same temperature is found to transform into different modulated structure. For alloy quenched form850℃, the martensite structure shows four-layer and seven-layer modulated structure. Dark field images indicate antiphase structure. The quenched alloy is not perfectly modulated in HRTEM image. Selected area electron diffraction patterns of four-layer modulated structure are indexed according to orthographic structure with lattice constant of a=1.026nm, b=0.534nm, c=0.452nm. Furnace cooled samples mainly show seven-layer modulated structure. HRTEM images indicate that furnace cooled samples modulated structure is better than quenched samples, and has micro-twin relationship.In addition, the effect of4%Cu atom addition on the microstructure and performance of polycrystalline Mn rich Ni-Mn-Ga-Cu magnetic shape memory alloys are investigated by OM (optical microscopy), XRD, DSC, and VSM. The OM shows that Ni-Mn-Ga-Cu alloys exhibit dendrite segregation for cast samples. At room temperature, alloys are single martensite phase after quenching form900℃. XRD indicates that the martensite structures of these alloys are seven-layered martensite at room temperature. DSC curves reveal that martensitic transformation temperatures and enthalpy changes are in accordance with e/a. VSM measurements find that the magnetization of bulk samples and powder samples was different, which demonstrate that in the Ni-Mn-Ga-Cu alloy the phase transformation between ferromagnetic martensite and paramagnetic martensite, are very sensitive to the changes of strength and composition.