The Effect of Thermal Treatment on Microstructure and Thermal-Induced Martensitic Transformations in Ni44Fe19Ga27Co10 Single Crystals

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 3–7 K. The reduction in the volume fraction of the L21-phase to 40% and the formation of nanodomains (20–50 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 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.

The effect of heat treatment on microstructure and martensitic transformation temperatures in Ni44Fe19Ga27Co10 single crystals

“…Alloying with Co of 10 at.% or more leads to the another lattice structure of second phase (ω-like phase) that was shown in the works [9,10]. In our previous work [12], we confirmed the precipitation of the ω-like phase in Ni 44 Fe 19 Ga 27 Co 10 (at.%) single crystals aged at 773 K for 1 h. However, at the same time, we observed the large γ′-phase particles in as-grown Ni 44 Fe 19 Ga 27 Co 10 single crystals [11,12]. Unfortunately, there are no other works on heterophase NiFeGaCo alloys with a cobalt content of C Co 10 at.%.…”

“…Single crystals were preliminary annealed at 1448 K for 1h, followed by water quenching ('initial quenched crystals'). The annealing temperature was chosen based on our previous work [12] to form a homogeneous structure without particles. The DSC 404F1 calorimeter was used to obtain the DSC exothermic curve for initial quenched crystals by continuous heating from room temperature to 1100 K with speed of 20 K min −1 .…”

“…During each aging the samples were put into a furnace with fixed temperature providing fast heating. The aging temperature and time were chosen from the previous studies on NiFeGa alloys [1,8,12] to investigate the temperature ranges and crystal structures of second phase precipitation and clarify their effect on martensitic transformation. The number of samples after different treatments was obtained: (1) annealing at 1448 K for 1 h; (2) (1) + aging at 623 K for 16 h;…”

“…The microstructure of initial quenched crystals was researched in detail in [11,12]. The high-temperature phase did not contain the second phases and consisted of nanodomains L2 1 and B2-structures.…”

“…The volume fraction of B2-structure was about 60%. The reasons for the presence of B2-domains in the initial quenched single crystals were discussed in [12] and associated with the small time of annealing (1 h) for disordering of L2 1 -phase of as-grown single crystals.…”