Large Anhysteretic Deformation of Shape Memory Alloys at Postcritical Temperatures and Stresses. Issue 2 (18th August 2017)

• Record Type: Journal Article
• Authors: Chernenko, Volodymyr A.; L'vov, Victor A.; Kabra, Saurabh; Aseguinolaza, Ivan R.; Kohl, Manfred; Hosoda, Hideki; Barandiaran, Jose M.
• Other Names: Entel Peter guestEditor.; Fähler Sebastian guestEditor.
• Is Part Of: Physica status solidi. Volume 255:Issue 2(2018)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2017-08-18
• Subjects: critical point -- martensitic transformation -- neutron diffraction -- postcritical anhysteretic deformation -- thin films; Solid state physics -- Periodicals; Solids -- Periodicals; Atomic structure -- Periodicals; 530.41
• DOI: 10.1002/pssb.201700273
• Languages: English
• ISSNs: 0370-1972

Abstract

Abstract : Magnetic and nonmagnetic shape memory alloys (SMAs) exhibit thermoelastic martensitic transformations (MTs) which are hysteretic due to their first‐order nature. According to the thermodynamic Landau theory of phase transitions, which assumes ideal thermoelastic equilibrium at each point of the MT interval, the hysteresis is explained by the different limits of stability for austenite and martensite in the phase diagram. No interactions on the phase boundaries are taken into account. In the real alloys, the hysteresis of MT is related not only to the stability intervals of two phases but also to the processes of nucleation and growth of the resultant phase inside the parent phase. In turn, the features of these processes are related to the heights of energy barriers caused by the incompatibility of austenitic and martensitic lattices, crystal defects and some other physical factors. However, the defects, normally, play a minor role in the width of MT hysteresis if compared to the thermodynamic and crystallographic factors. A reduction of hysteresis of MT in SMAs, being crucial for technology, presents a challenging problem for science. A decrease of hysteresis width of MT was observed recently for the single crystals of ferromagnetic SMAs such Ni–Fe(Co)–Ga and Fe–Pd on approaching of their transformation paths to the critical point in stress–temperature phase diagram. Moreover, the superelastic and shape memory properties characterized by the nearly‐zero hysteresis width were observed in the postcritical transformational regime. Here we show that both the Landau‐type theory of ferroelastic phase transitions and neutron diffraction experiments carried out under axial compression describe the essential features of these properties. We also interpret the experimentally observed anhysteretic phenomena in Ni–Mn–Ga thin films and nanobeam actuators in terms of their postcritical state. Abstract : Minimization of hysteresis accompanying the first order martensitic transformation is of paramount importance in the majority of applications of shape memory alloys (SMAs). Chernenko et al. found experimentally and theoretically a disappearance of hysteresis in SMAs exhibiting anomalously low shear modulus (of about several GPa). The anhysteretic shape memory and superelasticity effects (see schematic graph) have been observed at the critical point ( σ *, T *) in the stress–temperature ( σ–T ) phase diagram and in the postcritical state induced by stressing of alloy at T > T *.