Temperature Dependent Stress–Strain Behavior and Martensite Stabilization in Magnetic Shape Memory Ni51.1Fe16.4Ga26.3Co6.2 Single Crystal

Abstract

The superelastic properties and stress-induced martensite (SIM) stabilization have been studied in a shape memory Ni51.1Fe16.4Ga26.3Co6.2 single crystal. The single crystal, characterized by a thermally induced forward martensitic transformation temperature around 56 °C in the initial state, has been submitted to compression mechanical testing at different temperatures well above, near and below the martensitic transformation (MT). After each mechanical test, the characteristic MT temperatures and the transformation enthalpy have been monitored by means of differential scanning calorimetry. At temperatures below MT, the stress–strain (σ–ε) curves show a large strain, around 6.0%, resulting from the detwinning process in the martensitic microstructure, which remains accumulated after unloading in the detwinned state of the sample as a typical behavior of the shape memory alloys (SMAs). After just two “σ–ε + heating” cycles the accumulation of strain was not observed any more indicating the formation of a two-way shape memory effect which consists in a spontaneous recovery of the aforementioned detwinned state of the sample during its cooling across the forward MT. Whereas the thermally induced shape recovery in conventional SMAs occurs at the fixed value of the reverse MT temperature, the heating DSC curves of the mechanically deformed martensite in the present work show a burst-like calorimetric peak at the reverse MT arising at temperatures essentially higher than the thermally activated one. This behavior is the result of the SIM stabilization effect. After a short thermal aging in the stress-free state, this effect almost disappears, showing a slight impact on the MT characteristic temperatures and the enthalpy. At temperatures higher than the transformation one, the SIM is not stabilized, as the mechanically induced martensite fully retransforms into austenite after the unloading. From the σ–ε curves, the critical stress, σc, as well as the values of Young’s moduli of martensite and austenite are determined showing linear dependences on the temperature with a slope of 3.6 MPa/°C.

Thermodynamic Analysis of Anomalous Shape of Stress–Strain Curves for Shape Memory Alloys

“…In fact, the above martensite stabilization is similar to the effect of the so-called SIM aging, where the stabilization is achieved by aging under uniaxial stress after the formation of stress induced martensite [11][12][13][14][15][16][17][18]. While the SIM aging effect is well interpreted by the symmetry confirming short range ordering [14,19,20], the interpretation of the anomalous shape of the stress-strain curves and the burst-like strain recovery still is in infancy [1,3,4,9] and mostly qualitative arguments were offered. There are only few articles in which more quantitative treatment can be found from two groups [1,3,[21][22][23].…”

“…Furthermore, the strain recovery of the stress induced martensite to austenite during heating can be very fast (burst-like recovery). For instance, it was observed that the DSC peak of this recovery was only about 10 −3 -10 −5 degree wide (instead of the usual 1-50 K wide transitions at typical 1-10 K/min rates) and was accompanied with jumping of the sample as well as with audible click [1][2][3][4][5][6][7][8][9][10]. In addition, the DSC peak appeared at higher temperature (by 10-60 K higher) than the corresponding peak of the reverse transformation measured after thermally induced cycling.…”

“…It is known that in some shape memory single crystals the stress-strain (σ~ε) curves, belonging to stress induced martensitic transformations at fixed temperature, can show anomalous shape: e.g., the uploading branch of the stress-strain curve, instead being the usual smooth, slightly increasing function, can have an overall negative slope with sudden stress drops on it [1][2][3][4][5][6][7][8][9][10] (Figure 1). This behaviour shows quite a wide variety.…”

“…It can be strongly anisotropic and can differ considerably along different crystallographic orientations (see e.g., Figure 1 in [2], where the upper branch of the σ~ε curves, along [100] A as well as [110] A directions in a Ni 49 Fe 18 Ga 27 Co 6 single crystal, was normal as well as anomalous, respectively). It is often observed that the transformation is not complete [1,[7][8][9], but there are also examples when the whole sample has been transformed [4,10]. Furthermore, the strain recovery of the stress induced martensite to austenite during heating can be very fast (burst-like recovery).…”

“…This indicates that the martensite structure formed during stress induced changes is more stable than the thermally induced one. Both the martensite stabilization, manifested in the shift of the DSC peak to higher temperatures during heating, and the anomalous stress-strain curves, are interpreted by the presence/competition of two different martensitic structural modifications [2,3,9]. These can be denoted as M 1 and M 2 structures, respectively.…”