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    Effect of extrusion on Ni55.5Mn25Ga18.5Mo2 shape memory Heusler alloy
    (Elsevier Ltd, 2023) Dal, Serkan; Eskil, Murat; Demirel, Bilal
    In this study, the effects of the extrusion process on the phase structure and shape memory properties of NiMn type Heusler alloy were investigated. For this, Ni55.5Mn25Ga18.5Mo2 alloy obtained by melting method in vacuum induction furnace was extruded at 950 °C, 1000 °C and 1050 °C, respectively. XRD analyzes of the obtained specimens were performed to compare crystallographic differences. Afterwards, metallurgical microscope and SEM photographs were obtained to see the phase structures of the specimens. Phase transformation temperatures were also determined by DSC analysis. In addition, EDS and MAPPING observations of elemental analysis were made. Then, Vickers micro-hardness analyses were performed to examine the effects of extrusion process on microhardness. As a result, the shape memory properties were negatively affected by the increase in extrusion temperature. Generally, with the increase in this temperature, the amount of high temperature phases increased significantly, as micro-hardness values slightly increased. In addition to this, martensitic phase structures decreased with increasing extrusion temperature and martensite and austenite transition temperatures could not be observed in DSC curves.
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    The effects of homogenization time on the crystal structure and hardness of NiMnGaMo alloy
    (Elsevier, 2021) Dal, Serkan; Demirel, Bilal; Eskil, Murat
    This study is based on X-ray diffraction (XRD) and DSC analysis, Scanning Electron Microscopy (SEM) and Optical Microscope observations and Vickers Micro Hardness Test results with structural effect and phase transformations on NiMnGa shape memory alloys with different composition. The addition of different elements to the shape-memory NiMnGa alloys and the heat treatment of the alloys are extremely useful for improving the shape memory, magnetic, mechanical, chemical and other physical properties. The NiMnGaMo alloys were successfully obtained using a vacuum induction melting furnace and then homogenized by heat treatment at 950 degrees C for 2, 24 and 96 h, respectively. After homogenization, all samples were cooled in ice water. Crystallographic results calculated from XRD data were evaluated by comparison with the data obtained by DSC analysis, Scanning Electron Microscope (SEM), Optical Microscope and Vickers Micro Hardness Test Device. The results show that the alloy with a higher Mo amount has a smaller crystallite size and the L2(1) lattice volume increases with heat treatment time. The change on microhardness values were discussed with the change on the obtained lattice constants.