Austenite-martensite Transition Research Articles

An integrated model to predict the microstructure evolution and the mechanical behaviour of a two-phases pseudo-elastic SMA

Shape memory alloys (SMAs) are more and more used in many fields of mechanics and medicine. The reason for the success is the ability to recover the initial shape also after high values of deformation. During the deformation, the SMAs are able to change the structure from an initial one, often named as austenite, to a final one, named martensite. This transformation takes place at low temperature, and no recrystallization occurs, but the bulk of grains is interested by a structure changing. For that, this phenomenon cannot be qualified as a traditional structure transformation, but it is much more correct to talk about the transition of phase. The austenite-martensite transition is reversible, allowing a perfect recovery of the shape shown in the austenite phase.In this work, a model able to describe the microstructure evolution related to the mechanical behaviour of a SMA has been proposed. Results of FEM simulations have been compared with experimental ones both in terms of microstructure modifications and mechanical behaviour.

Element-specific electronic structure and magnetic properties of an epitaxialNi51.6Mn32.9Sn15.5thin film at the austenite-martensite transition

An austenite-martensite transition was observed in a 100-nm-thick ${\mathrm{Ni}}_{51.6}{\mathrm{Mn}}_{32.9}{\mathrm{Sn}}_{15.5}$ film by temperature-dependent resistivity and magnetization measurements, revealing a martensite starting temperature of ${M}_{S}\ensuremath{\approx}260$ K. The influence of the structural phase transition on the electronic structure and the magnetic properties was studied element specifically employing temperature-dependent x-ray-absorption spectroscopy and x-ray magnetic circular dichroism. In addition, density functional theory calculations have been performed to study the electronic and magnetic properties of both phases. It is shown that off-stoichiometric Ni-Mn-Sn alloys can exhibit a substantial magnetocrystalline anisotropy energy in the martensite phase. For Mn a change of the electronic structure and a strong increase of the ratio of orbital to spin magnetic moment ${m}_{l}/{m}_{S}$ can be observed, whereas for Ni nearly no changes occur. Applying an external magnetic field of $B=3$ T reverses the change of the electronic structure of Mn and reduces the ratio of ${m}_{l}/{m}_{S}$ from 13.5 to $\ensuremath{\approx}1$ % indicating a field-induced reverse martensitic transition.

Hysteresis and phase transitions for one-dimensional and three-dimensional models in shape memory alloys

By means of the Ginzburg–Landau theory of phase transitions, we study a nonisothermal model to characterize the austenite-martensite transition in shape memory alloys. In the first part of this paper, the one-dimensional model proposed by Berti et al. [“Phase transitions in shape memory alloys: A non-isothermal Ginzburg-Landau model,” Physica D 239, 95 (2010)] is modified by varying the expression of the free energy. In this way, the description of the phenomenon of hysteresis, typical of these materials, is improved and the related stress-strain curves are recovered. Then, a generalization of this model to the three-dimensional case is proposed and its consistency with the principles of thermodynamics is proven. Unlike other three-dimensional models, the transition is characterized by a scalar valued order parameter φ and the Ginzburg–Landau equation, ruling the evolution of φ, allows us to prove a maximum principle, ensuring the boundedness of φ itself.

Growth and magnetic control of twinning structure in thin films of Heusler shape memory compound Ni2MnGa

Twin structure engineering in sputtered films close to the Heusler stoichiometry Ni2MnGa (001) is used to demonstrate temperature and magnetic control of the phase transformation behavior. A custom heating apparatus integrated with a commercial microscope allows the observation of the austenite-martensite transition in epitaxially clamped films. Intermartensitic twin boundaries on cantilevers released from the epitaxial strain by focused ion beam etching are shown to move in response to an applied magnetic field with a strength of 0.6 T. We also report the observation of two coexisting twin morphologies.

Martensitic transformation temperature hysteresis narrowing and magnetocaloric effect in ferromagnetic shape memory alloys Ni–Mn–Ga

The hysteresis narrowing and magnetocaloric effect (MCE) were studied in the Ni–Mn–Ga alloys. Ni 54Mn 24Ga 22 and Ni 51.9Mn 27Ga 21.1 were selected in order to obtain specific characteristic temperatures of alloys and to study an influence of several factors on the hysteresis width ( δ) of basic austenite–martensite (AM) transformation. The effect of martensite crystal lattice softening due to intermartensitic transition (IT) taking place immediately after AM transformation on δ was examined in alloy 1. MCE was measured in alloy 2, in which Curie temperature ( T C ) and austenitic start temperature ( A S) coincides. It was demonstrated that the hysteresis width δ reaches about 2 K, if IT is enough close to AM transition. MCE was found to have a maximum at T C≈ A S.

Microstructure, Phase Sequence and Superelasticity in Highly Oriented MBE-Grown NiTiCu Shape Memory Thin Films

Using MBE deposition highly oriented NiTiCu shape memory thin films were accomplished. The austenite-martensite transition in these films, single or two-stage transformation depending on the quantity of the Cu additions, occurs with comparable transition temperatures as known for sputter-deposited films. Mechanical stress measurements reveal a deformation recovery in the order of 400 MPa. Superelasticity spans to 1-1.5% strain as referred to spherical indentation data. The microstructure is remarkable in that the crystallites are oriented within {+-}5 along the film plane normal as observed by X-ray diffraction. The occurrence of this preferential order is deduced to the formation of a Ti-rich (Ti{sub 2}Ni) phase at the film-substrate interface providing a highly oriented initial crystallization template. The results described in this study outline the perspectives for tailoring the microstructure and transformation performance of shape memory thin films, especially in regard to technological applications. (orig.)

Compatibility conditions for microstructures and the austenite–martensite transition

Abstract We explore theoretically the possibility of austenite–martensite transformations more complicated than those envisaged by the crystallographic theory, and make corresponding experimental predictions. The use of the Hadamard jump condition in this context is justified, and leads also to results on non-attainment of minimum energy.

Thermal Cycling Effect on the Premartensitic and Martensitic Transition in a Ti Rich NiTi Alloy

The study was carried on a Ti-rich NiTi shape memory alloy. The electrical resistance and the elastic modulus were determined versus temperature. The aim of this work is to study the thermal cycling effect on the transformation features as the temperatures or the phase succession as well as the structural modifications. The major results of this work concern the R-phase occurrence and its evolution. We have shown that the premartensitic phase could exist in a narrow range of temperature on cooling and is revealed only after a few cycles. In fact, its presence and stability is directly linked to the diminishing of Ms. The main condition for the austenite to be transform in a rhombohedral structure is that Ms becomes low enough to be below Tr. Interrupted cooling alms also show that contrarily to the austenite-martensite transition, the austenite-R-phase transition doesn't exhibit any hysteretic behaviour.

The 'turn over' transition of suberic acid.

A LATTICE transition which is not induced by nuclei or boundaries but takes place simultaneously in the whole of a crystal by the jump of a whole lattice plane is termed ‘turn over’ (Umklappen). Such transitions are observed with certain metals (cobalt, thallium) and especially in the austenite–martensite transition. When, in 1941 in this laboratory1, the phenomenon was discovered for the first time with an inorganic salt (potassium dichromate), we were informed by Prof. L. Kofler (Innsbruck) that a similar ‘flashlike’ transition can be observed with suberic acid crystals. The features of this ‘turn over’, as we have observed it, are described here.