LAUSR

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