Формирование микроструктуры типа «ожерелье» методом ковки в сплавах Гейслера системы Ni-Mn-Ga-Si как способ повышения механических свойств

Abstract

The standard technique for melting Heusler alloys involves melting in an argon-arc furnace on a water-cooled copper crucible. As a result of intensive crystallization of the ingot, large elongated crystals are formed in the structure. The thermo-mechanical treatment of such a structure is ineffective, since microcracks are formed along the boundaries of large crystals as a result of internal stresses and martensitic transformation, along which then, during processing, cracks develop and the workpiece is destroyed. It is shown that additional heat treatment by the vacuum induction remelting makes it possible to eliminate this effect. As a result of the treatment, an equiaxed granular microstructure with a grain size of about 200 μm is formed. However, as a result of remelting in a quartz crucible, silicon atoms diffuse into the bulk of the ingot. Its content varies in the range of 1-2% depending on the duration of exposure in the molten state. A uniform distribution of silicon, without the formation of additional phases is shown by energy-dispersive analysis. Thus, the Ni54.1Mn19.6Ga24.6Si1.7, Ni56.2Mn18.8Ga23.2Si1.8, Ni57.4Mn18.2Ga22.7Si1.7 and Ni56.5Mn20.1Ga22.3Si1.2 alloys were obtained. A partially recrystallized structure of the "necklace" type was formed in the alloys by the multiaxial isothermal forging at 950-973 K and the true degree of deformation e=1.9...3.9. The initial large grains with a size of 100-200 microns are surrounded by a layer of fine-grained structure. The thickness of the interlayer at the periphery of the workpiece is about 5 grains. In the center of the workpiece, the proportion of the fine-grained structure may prevail over the coarse-grained structure. The study of cyclic and fatigue strength was carried out by the method of three-point bending during cyclic tests in the temperature range of martensitic transformation using the example of the Ni54.1Mn19.6Ga24.6Si1.7 alloy in the initial state and the Ni57.4Mn18.2Ga22.7Si1.7 alloy in the forged state. It is shown that the “necklace” type microstructure, compared to the equiaxed microstructure, demonstrates a twofold advantage in cyclic strength and a fivefold advantage in fatigue strength.