Microstructure, martensitic transformation and anomalies in c′-softening in Co–Ni–Al ferromagnetic shape memory alloys
Introduction
Ferromagnetic shape memory alloys (FSMAs) have been intensively studied since the discovery of the magnetically induced reorientation (MIR) effect by Ullakko et al. [1], [2]. The ability of these alloys to deform reversibly under the action of the external magnetic field with strain amplitudes up to 10% [3], [4] makes them highly interesting from the perspective of possible smart applications, including magnetically driven actuators, micromanipulators, pumps, etc. [5], [6]. These materials are important not only from a practical point of view but also from the theoretical perspective as the coupling between two ferroic orderings (ferroelasticity and ferromagnetism) in FSMAs opens new challenges for mathematical modeling at different spatial scales [7], [8], [9], [10]. As well as the Ni–Mn–Ga [1], [3], [11] and Fe–Pd [12], [13] systems, which exhibit MIR, attention should be paid to the other ferromagnetic Heusler alloys undergoing martensitic transformation [14] as this transformation is a basic precondition for MIR.
One important class of FSMAs is Co–Ni–Al, since the thermomechanical properties of this material can be sensitively tuned by heat treatment [15], [16], [17] and moreover the alloying elements for this system are cheap. These alloys undergo martensitic transformation from the cubic (B2) to tetragonal (L10) structure [18], [19], which is similar to the high-temperature non-modulated martensite in Ni–Mn–Ga [20]. In the tetragonal structure there are only three different variants of martensite, which limits the type of possible martensite microstructure in comparison with the rich structures of modulated martensites in Ni–Mn–Ga [21], [22]. However, contrary to previous belief, the tetragonal structure does not preclude MIR as recently shown by Sozinov [23]. Despite the simple martensite microstructure in Co–Ni–Al alloys the processes accompanying both the forward and reverse martensitic transformation can be rather complex due to the presence of fine γ-phase (A1) precipitates in transforming B2 matrix (e.g. [18]).
Murakami et al. [17] observed that the martensitic transformation in Co–Ni–Al alloy was preceded by a tweed-like modulation of austenite lattice. This precursor phenomenon, called the premartensitic transition, is a well-known phenomenon in Ni–Mn–Ga alloys [24]. In Ni–Mn–Ga the premartensitic modulation is magnetoelastic in origin [25] and its formation results in pronounced, anomalous changes of elastic constants of the austenite phase [26], [27]. On the other hand, Brown et al. [28] used neutron powder diffraction to follow the structure evolution of Co38Ni33Al29 during the transformation. They concluded that the presence of premartensitic phase should be indicated by a weak additional diffraction peak appearing in the vicinity of the transition temperature, but in contrast to Murakami et al. they found that the martensitic transition in this material proceeded without any significant intermediate step.
Since the transition in Co–Ni–Al belongs to the cubic-to-tetragonal class, the changes of elasticity preceding the transition are related to the transverse accoustic TA2 phonon branch, i.e. to shearing along the {1 1 0} planes with polarization [33]. The resistance of the lattice to such shearing is described by the coefficient c′, defined as using three independent elastic constants of the cubic material (c11, c12 and c44). Similarly to other shape memory alloys, this shear elastic coefficient is significantly lower than the coefficient c44 representing shears along the principal {1 0 0} planes. The anisotropy factor A = c44/c′ is usually larger than 10. The coefficient c′ softens further with decreasing temperature towards the transformation. For this reason, we refer to so-called anomalous c′-softening. As discussed in Ref. [27], [29], the most appropriate method for accurate determination of the c′ coefficient of such highly anisotropic cubic materials is resonant ultrasound spectroscopy (RUS) [30], [31]. RUS is an experimental technique based on measurements of resonant frequencies of free elastic vibrations of a small sample. The lowest resonant frequencies correspond to the shearing vibration related to the softest shear coefficient and thus detecting a limited number of the first few resonances is sufficient for accurate determination of c′. Moreover, when RUS is applied to a fully non-contact regime using lasers for both generation and detection of the vibration modes, it enables very sensitive tracing of the c′ coefficient with temperature. As shown in this paper, even very weak changes in the dc′/dT slope or other anomalies can be detected. On the other hand, the experimental requirements of RUS, in particular the need for the sample to have a perfect parallelepiped shape, limits to some extent the applicability of this method to material in the early stages of martensitic transition, as also shown in the experimental part of this paper.
The main aim of this paper is to probe elastic softening and expected premartensitic phenomena by measurements of the (magneto) elastic properties of Co–Ni–Al single crystals in the vicinity of the martensite starting temperature Ms and to relate these to the microstructure and structural changes during transformation. A comparison with the prototypical magnetic shape memory compound Ni–Mn–Ga is discussed. The results of contactless RUS measurement indicate that additional anomalous softening is very weak and there is no sign of premartenstic phenomena. The observed small anomalies can be ascribed to the presence of martensite nuclei stress-induced in the vicinity of γ-phase precipitates.
Section snippets
Materials and methods
Five different samples were used in the experiments, referred to as “single-phase”, “as-cast” and “annealed at X temperature”, where X = 1523, 1548 and 1573 K. The samples were cut from single-crystal ingots grown by the Bridgman method in argon atmosphere using two different growth rates, 17 and 104 mm h−1. The nominal composition of the initial alloy was Co38Ni33Al29. All samples were cuboids with approximate dimensions 3.2 mm × 2.8 mm × 2.3 mm and the orientation of sides was (0 1 −1) × (1 0 0) × (0 1 1).
The
Martensitic transformation and microstructure
Fig. 1 shows the DC magnetization as a function of temperature for samples annealed at 1573 K and as-cast. The drop in magnetization indicates the onset of martensitic transformation due to higher magnetic anisotropy of the martensite. While for the as-cast and the one-phase material no apparent transition occurred until 10 K, all annealed materials exhibited a reversible martensite transition with a martensite start temperature between 190 and 195 K which slightly increased with increasing
Summary
RUS measurements of the elastic coefficients (constants) indicate that there are pronounced differences between the elastic properties of Co–Ni–Al samples with the same stoichiometry but with different heat treatments and/or phase composition, resulting in different transformation behaviour. While the c11 coefficient is approximately constant in all investigated samples, the shear elastic coefficients differ significantly at room temperature. The difference can be related to the presence of the
Acknowledgments
This work has been financially supported by the Czech Science Foundation Grant Nos. P107/11/0391 and 101/09/0702, and by the Academy of Sciences of the Czech Republic, bilateral international collaboration project No. M100761203.
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