The Effect of Thermal Treatment on Microstructure and Thermal-Induced Martensitic Transformations in Ni44Fe19Ga27Co10 Single Crystals
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abstract
Heat treatments of single crystals of Ni44Fe19Ga27Co10 (at.%) shape memory alloys cause various microstructures of the high-temperature phase. The nanodomain structure, consisting of regions of the L21- and B2-phases, and nanosized particles are the main parameters that change during heat treatments and determine the mechanism of nucleation and growth of martensite crystals, the size of thermal-induced martensite lamellae, the temperature Ms, and the temperature intervals of the martensitic transformation. In the as-grown single crystals, the high-temperature phase has only the L21-structure and the MT occurs at low (Ms = 125 K) temperatures due to the motion of the practically single interphase boundary in narrow temperature ranges of 37 K. The reduction in the volume fraction of the L21-phase to 40% and the formation of nanodomains (2050 nm) of the L21-and B2-phases due to annealing at 1448 K for 1 h with quenching causes an increase in the MT temperatures by 80 K. The MT occurs in wide temperature ranges of 4045 K because of multiple nucleation of individual large (300500 m) martensite lamellae and their growth. After aging at 773 K for 1 h, the precipitation of nanosized particles of the -phase in such a structure additionally increases the MT temperatures by 45 K. The MT occurs due to the multiple nucleation of packets of small (2050 m) martensite lamellae.
