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Journal Articles Applied Physics Letters Year : 2020

Paramagnetism and martensite stabilization of tensile strained NiTi shape memory alloy

E. Gaudry
M. Ponçot
Pascal Boulet
B. Kierren
Thomas Hauet


We present an experimental and theoretical study of Pauli paramagnetism and martensite stabilization in a near equiatomic NiTi shape memory alloy. We demonstrate a direct correlation between strain-induced shear of the B19' NiTi lattice and its electronic and thermodynamical features. An increase of the monoclinic angle β from 97.4 to 98 degrees induces a 7 % decrease of the magnetic susceptibility because of a shift and deepening of a dip in B19' density of state at the Fermi level. It also produces a decrease of the B19' enthalpy, which translates into an increase of the martensite-to-austenite transition temperature by 60 K. Near equiatomic NiTi alloys carry remarkable properties such as shape memory effect and pseudoelasticity. Both effects are born of first-order reversible phase transformation between low-symmetry monoclinic B19' martensite phase and high-symmetry cubic B2 austenite phase. Tuning the transition temperatures and martensite transformation hysteresis has been a constant objective. It has yet been achieved by adjusting NixTi1-x concentration [1] or by substituting element in ternary NiTiX alloys (with X=Hf, Pd, Cu,…)[2,3]. Geometrical compatibility of B19' and B2 phases [4], change of valence electron number [5] and thermodynamics [1,6] have been investigated to explain the influence of theses chemical changes on NiTi-based alloys characteristics. Interestingly, no direct correlation between electronic features, transition temperatures and NiTi lattice distortion for a fixed concentration has yet been demonstrated, whereas the complex microstructure of NiTi under strain has been heavily studied [7-9]. Moreover, magnetism, which is known as a good probe of the electronic properties, has been rarely studied in NiTi alloys [10-13], although its impact on the magnetic resonance imaging of NiTi-based stents and biomedical implants has often been highlighted [14,15]. Here, we perform systematic X-ray diffraction (XRD) and magnetization measurements in order to characterize the influence of the tensile strain on both the martensite-to-austenite transition temperature and magnetic susceptibility of a NiTi sheet. Ab-initio calculations correlate the measured strain-induced shear of the martensite lattice to particular features of the B19' energy and density of states at the Fermi level. Figure 1. Magnetic susceptibility χ of NiTi versus temperature for as grown sample (black curve), 5% strained sample (red curve), 16.5% strained sample (blue curve). We use NiTi (50.6 at% Ti; 49.4 at% Ni ± 0.5%) free-standing polycrystalline 20µm thick sheets, grown by DC magnetron sputtering as described in details in Ref. [16]. Fig.1 shows the DC Magnetic susceptibility χ measured by SQUID magnetometer as a function of temperature under a constant magnetic field of 2T. Magnetic field is applied in the plane of the NiTi sheet. Note that χ does not depend on the measurement direction in the film plane because of the polycrystalline nature of the NiTi sheet. As-grown NiTi sample overcomes a hysteretic transition between a value M= 23 mJ.T-2 .kg-1 in the martensite phase and A=32.5 mJ.T-2 .kg-1 in the austenite phase, in agreement with [10]. A magnetic transition temperature of around 360 K (330K) matches the martensite-to-austenite (respectively austenite-to-martensite) transition in XRD curves, in agreement with [8,17] for similar concentrations. Tensile tests are performed using a Deformation Device System
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hal-02948734 , version 1 (25-09-2020)



A. Kyianytsia, E. Gaudry, M. Ponçot, Pascal Boulet, B. Kierren, et al.. Paramagnetism and martensite stabilization of tensile strained NiTi shape memory alloy. Applied Physics Letters, 2020, 117 (12), pp.122411. ⟨10.1063/5.0020145⟩. ⟨hal-02948734⟩
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