Structure and microhardness of the three-component Ni-Mn-In alloy after different modes of thermal cycling treatment

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The influence of various modes of thermal cycling treatment on the microstructure and properties of ferromagnetic alloy Ni47Mn42In11 was studied. The modes of thermal cycling treatment differed in the heating temperature (363 – 573 K) and holding time, the cooling temperature being 77 K for all modes. The number of heating-cooling cycles was varied from 1 to 30. The microstructure of Ni47Mn42In11 alloy in the initial state and after thermal cycling treatment was studied by optical metallography and scanning electron microscopy. A magnetic structural transition is observed in the alloy during cooling from the high-temperature region at a temperature about Т ≈ 300 – 310 K. In the initial state, the structure of the alloy has two phases and consists of the L21 phase and martensite. The martensite crystals are grouped in packets of 80 – 150 μm width, which are misoriented relative to each other by angles of 60 or 120 degrees. The boundaries of the coarse grains are predominantly smooth in the initial state. Structural studies showed that after thermal cycling treatment the martensitic crystals reached the grain boundaries and deformed them, and boundaries of a serrate form appeared. Micro-X-ray spectral analysis did not reveal precipitates of the second phases along the grain boundaries after thermal cycling treatment. The stress level was estimated after annealing and subsequent thermal cycling treatment by electron back scattered diffraction method. It was shown that with an increase in the number of heating-cooling cycles the stress level in the material increased. After the thermal cycling treatment the microhardness of the investigated alloy increased. It was shown that there was no unequivocal dependence of the microhardness value on the temperature of heating upon the thermocycling processes. The maximum increment in microhardness was detected after 20 cycles of treatment by in the 473 K ↔ 77 K mode.