Experimental determination of thermal and electrical properties of Ni-Ti shape memory wires

Abstract

The thermal and electrical properties of shape memory alloys (SMA) are known to be different in their austenitic and martensitic phases. This paper addresses the determination of the phase-dependent heat capacity, thermal conductivity and electrical resistivity of a SMA wire. While the heat capacity measurements are relatively straightforward using a differential scanning calorimeter, the determination of the thermal conductivity and the electrical resistivity are more difficult in view of the possible non-uniformity in the material state of a SMA wire during a phase transformation. Experimental procedures are developed and used to determine these properties in either phase and, along with a previously developed finite-element code accounting for the non-uniform material states, the austenite and martensite properties are determined from the experimental data. For the SMA wire tested, the thermal conductivities of the austenite and martensite phases are determined to be 2.8×10-2 J (mm S K)-1 and 1.4×10-2 J (mm S K)-1 respectively, a difference of 100%. The electrical resistivities of the austenite and martensite phases are determined to be 8.371×10-4 Ω mm and 9.603×10-4 Ω mm respectively, a difference of about 14.7%.

From the thermal and kinematical full-field measurements to the analysis of deformation mechanisms of NiTi SMAs

“…This simplified model was used in [4] to study superelastic NiTi tubes during homogeneous deformation stages. It is well known that material parameters such as k and C are strongly dependent on the phase type [10,11]. In this paper, a mean value was considered for the specific heat (C=590 Jkg -1 K -1…”

A homogenized free energy model for hysteresis in thin-film shape memory alloys

“…Both positive and negative values of ∆c have been measured [26,41,69] and some SMA models assume that ∆c = 0 with little or no experimental evidence [68,94]. The assumption that ∆c = 0 is equivalent to making a first order Taylor approximation to (18) …”

“…We describe temperature changes in the material via a simplified balance of internal energy, similar to the developments in [14,26,94]. In this case,…”

Phase Transformations in Electrically Conductive Ferromagnetic Shape-Memory Alloys, Their Thermodynamics and Analysis

“…As a result, phase transformation may be affected not only through magneto-mechanical coupling, but also through the produced Joule heat. This effect is also complicated because conductivity in shape-memory alloys depends not only on temperature, but mainly on the symmetry of the lattice [16,21,48,51]. For instance, variation of electric resistivity by tens of percents within the transformation of austenite to martensite or to R-phase has been experimentally documented [19,28].…”