| Summary: | Heat treatments of single crystals of Ni<sub>44</sub>Fe<sub>19</sub>Ga<sub>27</sub>Co<sub>10</sub> (at.%) shape memory alloys cause various microstructures of the high-temperature phase. The nanodomain structure, consisting of regions of the L2<sub>1</sub>- 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 M<sub>s</sub>, and the temperature intervals of the martensitic transformation. In the as-grown single crystals, the high-temperature phase has only the L2<sub>1</sub>-structure and the MT occurs at low (M<sub>s</sub> = 125 K) temperatures due to the motion of the practically single interphase boundary in narrow temperature ranges of 3–7 K. The reduction in the volume fraction of the L2<sub>1</sub>-phase to 40% and the formation of nanodomains (20–50 nm) of the L2<sub>1</sub>-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 40–45 K because of multiple nucleation of individual large (300–500 µm) martensite lamellae and their growth. After aging at 773 K for 1 h, the precipitation of nanosized particles of the ω-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 (20–50 μm) martensite lamellae.
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