The effect of heat treatment on mechanisms of martensitic transformations in ferromagnetic Ni49Fe18Ga27Co6 single crystals

Abstract

Ferromagnetic alloys based on Ni-Fe-Ga-(Сo) undergo thermoelastic В2(L2 1 )-10М-14М-L1 0 martensitic transformations (MTs) and are capable of reversible changes in size and shape when subjected to thermal action, mechanical stresses and magnetic fields. The Ni-Fe-Ga-(Сo) alloys possess reserve plasticity unlike the brittle Ni 2 MnGa alloys, and can be used in a variety of current technological applications [1]. The authors of [2,3] investigated the magnetic properties and characteristic temperatures of thermoelastic MTs in Ni-Fe-Ga-(Сo) polycrystals as a function of their chemical composition and heat treatment. It was shown that due to a changing content of Co and precipitation of particles of the γ(А1)-and γ′(L1 2 )-phases during annealing at the temperatures within 673-973 K, one can control the temperature of the onset M s and end M f of the direct and A s , A f reverse MTs, as well as the Curie temperature and other characteristics of the material. On the other hand, no systematic investigations of the effect of ageing on the mechanisms of MTs and functional properties of the Ni-Fe-Ga-Co crystals have been conducted so far.The present work deals with the effect of heat treatment on the mechanisms of development of thermoelastic MTs using single crystals of Ni 49 Fe 18 Ga 27 Со 6 (at.%). The experimental procedure is detailed in [4]. For the experiments we selected single crystals in the following structural states: initial as-grown crystals (I), quenched after annealing at 1373 K, 25 min (II), (quenched + aged at 673 K, 1 h (III), quenched + aged at 673 K, 4 h (IV), quenched + aged at 823 K, 0.5 h (V)) It has been established that the crystals after being grown (I) at room temperature are in a single-phase state. The high-temperature phase is ordered as L2 1 . This is evidenced by the X-ray diffraction peaks corresponding to the superstructure reflections 113 and 331 for the L2 1 -structure in the SADPs. The quenched single crystals (II) are in a two-phase state, with large, 5-10 μm particles of γ-phase precipitated in austenite. Subsequent annealing runs (III-V) result in the formation of bimodal heterophase structure in the austenite phase: large γ-phase particles 5-10 μm long (Fig. 1, particles А) and small γ′-phase particles [3] 5-30 nm long after ageing (III and IV) and up to 150-300 nm after ageing (V) (Fig. 1, particles В).Shown in Fig. 2 are the dependences of electrical resistance on temperature for the initial and heat-treated single crystals. Precipitation of these particles results in changes in the mechanisms of MT development. For one thing, the temperature intervals of the direct Δ 1 = M s -M f and reverse Δ 2 = A f -A s MTs in single crystal specimens (II-V) are by more than a factor of 7 larger compared to the initial state (I) (Fig. 2). Secondly, the crystals after quenching (II) and ageing at 673 K (III and IV) exhibit an MT of the second type according to the classification of Tong-Wayman, and the reverse transformation starts at A s < M s . The single crystal specimens after growing (I) ...

One-way and two-way shape memory effect in ferromagnetic NiFeGaCo single crystals

“…As shown previously, these particles allow for modifying the properties of the alloy, i.e. not only the strength but also the phase transformation temperatures and the transformation hysteresis properties are affected [9].…”

“…2). Related transmission electron microscopy (TEM) studies have shown [9] that in these single crystals a bimodal microstructure is formed. In addition to the large γ-phase particles, small γ′-phase particles with a size of 10-30 nm are precipitated (Fig.…”

“…As shown for TiNi alloys [6] this method is an effective way to obtain the TWSME. In NiFeGa(Co) single crystals nonequiaxed dispersed particles form, which have a fcc structure (γ-phase) or L1 2 structure (γ′-phase) [8,9]. As shown in Refs.…”

Shape Memory Effect and Superelasticity in Single Crystals of High-Strength Ferromagnetic Alloys

“…Stepped heat treatments of as-grown Ni 49 Fe 18 Ga 27 Co 6 single crystals were conducted to achieve a high level of austenite strength properties and maintaining plasticity: 1) high-temperature annealing at 1373 K for 25 min, followed by quenching in water to increase the crystal plasticity through the precipitation of large γ-phase particles (disordered fcc structure) [15]; 2) lowtemperature ageing at 673 K for 1 h (type-II) and at 823 K for 0.5 h, (type-III) followed by cooling in air to result in precipitation of γ'-phase particles, allowing control of the characteristic temperatures of MT and functional properties of crystals [15].…”

Physics of Thermoelastic Martensitic Transformation in High-Strength Single Crystals

“…The increase in the stress hysteresis with increasing temperature at T > 473 K can be connected with the generation of defects and increasing diffusive mobility of defects at the stress-induced MT. The reduction of stress hysteresis at T > 530 K in NiFeGa and at T > 630 K in NiFeGaCo can be connected with ageing processes, which take place in these crystals [88,97]. However additional investigations are necessary to understand the physical reasons for the non-monotonic temperature dependence of the stress hysteresis at temperatures over 473 K.…”