Use of Infrared Thermography for Inspection of Tensile Deformation of Ti-25Nb-0.5O and Ti-25Nb-0.5N Shape Memory Alloys ^†

The stress- or temperature-induced martensitic transformation from the cubic β phase to the orthorhombic α″ phase is responsible for superelasticity or a shape memory effect in Ni-free Ti-based shape memory alloys (SMAs) [1,2]. However, the addition of oxygen and nitrogen interstitials changes the tensile characteristics of these SMAs [3,4,5]. For instance, the Ti–25Nb (at.%) SMA exhibits a shape memory effect and a Lüders-type deformation, whereas oxygen-added Ti-25Nb-based SMAs present different but still inhomogeneous superelastic deformation [6]. The goal of this work is to compare the thermomechanical behavior of two SMAs, with compositions Ti–25Nb–0.5O and Ti–25Nb–0.5N, subjected to load–unload tension.

The alloys were prepared by an Ar arc melting method using pre-melted sponges of Ti (purity: >99.7%) and pure Nb (purity: 99.9%). The oxygen and nitrogen concentrations of the alloys were adjusted by amounts of TiO₂ and TiN powders (purity: 99.9%), respectively. Further processing included selected heat treatments and cold rolling, specified in [1]. The load–unload tension was performed using an MTS 858 testing machine (MTS SYSTEMS CORPORATION, Eden Prairie, MN, USA) at a strain rate of around 0.02 1/s. The gauge area of the specimen was 6 mm × 4 mm. During the experiments, a mid-wave infrared camera FLIR Phoenix (Teledyne FLIR LLC, Wilsonville, OR, USA), with high thermal sensitivity up to 0.02 K and a recording frequency of 200 Hz, was used to investigate thermal effects accompanying the deformation of the Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs. Simultaneously, a sCMOS PCO Edge 5.5 camera (Excelitas Technologies Corp., Pittsburgh, PA, USA ) was used to record a sequence in a visible spectrum for further processing using digital image correlation (DIC) to determine the kinematic fields of these alloys in tension. The camera settings were as follows: an image size of 931 pixels × 1280 pixels, with the pixel size equal to 7.7 μm, and the recording frequency of the camera was equal to 100 Hz. An open-source 2D digital image correlation program, Thermocorr, was also used to couple temperature fields. More details on the methodology are provided in [7]. Examples of the experimental results obtained for Gum Metal (Ti–23Nb–0.7Ta–2.0Zr–1.2O; at.%) using this approach can be found in [8,9].

The temperature ΔT and strain ε_yy fields of Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs under load–unload tension captured at specific stages of deformation identified in the stress and temperature change vs. strain curves are shown in Figure 1a and Figure 1b, respectively. The stress–strain plots of the Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs show hysteretic behaviors characterized by superelasticity with the same recovery strains of ε_r = 2%. However, the hysteresis loop is notably narrower in the case of the Ti–25Nb–0.5N SMA. The letters marked in the stress and temperature vs. strain curves correspond to the following instants of tension: A_O/N and A*_O/N—stress σ_tr at minimum temperature ΔT_min; B_O/N and B*_O/N—maximum stress σ_max and maximum temperature ΔT_max; C_O/N and C*_O/N—σ = 0 MPa and temperature after unloading ΔT_un. Subscripts O and N refer to those SMAs doped with oxygen and nitrogen, respectively. During loading, first, the average temperature decreases due to the thermoelastic effect 0—A*_O/N, and, with further loading, the temperature significantly increases due to the forward stress-induced phase transformation A*_O/N–B*_O/N. In the case of SMAs, ΔT_min can be an indicator of the transformation stress σ_tr. During unloading, first, the temperature significantly decreases due to the reverse stress-induced phase transformation, and, at the final stage, the temperature slightly increases due to the thermoelastic effect B*_O/N–C*_O/N.

Although the strain rate applied during the experiments was relatively high to imitate adiabatic conditions, the heat exchange with the surroundings could slightly affect the results. Moreover, the aforementioned deformation mechanisms are believed to be dominant during specific stages of tension. However, in this class of alloys, deformation twinning could also occur, especially in the final stage of loading, similarly to other metastable β-Ti alloys [10].

The critical parameters identified based on the stress and temperature change vs. strain curves of the Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs are listed in

Table 1. Critical parameters identified based on the stress and temperature change vs. strain curves of the Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs.

Composition	Minimum Temperature, ΔTmin	Transformation Stress, σtr	Temperature Increase, ΔTeH	Temperature Decrease, ΔTeC
Ti-25Nb-0.5O	−0.89 K	236 MPa	11.34 K	8.42 K
Ti-25Nb-0.5N	−0.66 K	204 MPa	14.63 K	16.32 K

. They include transformation stress σ_tr at minimum temperature ΔT_min, as well as temperature increase during loading ΔT_eH and temperature decrease ΔT_eC during unloading.

The temperature ΔT and strain ε_yy fields captured at selected instants of tension demonstrate that the deformation process was inhomogeneous. The maximum local values of ΔT_max and ε_{yy_max,} determined at B*_O/N and B_O/N, respectively, are provided in

Table 2. Maximum local values of ΔT_max and ε_{yy_max} determined at B*_O/N and B_O/N.

Composition	Maximum Local Temperature, ΔTmax	Maximum Local Strain, εyy_max
Ti-25Nb-0.5O	11.93 K	4.6
Ti-25Nb-0.5N	15.24 K	2.5

.

The values of ΔT_max were rather close to those of ΔT_eH. However, the values of ε_{yy_max} were quite higher than those of ε_r.

Our results show that infrared thermography is a useful technique to track the peculiar temperature changes of the Ti–25Nb–0.5O and Ti–25Nb–0.5N SMAs. The temperature fields captured at selected stages of loading revealed heat sources associated with the deformation process. The thermal effects were discussed in view of the kinematic characteristics obtained using DIC. It was shown that temperature changes can serve to identify particular deformation stages of the SMAs considered in this study.