Abstract
This review is focused on the influence of interstitial hydrogen and alloy compositional changes on the internal friction (IF) spectrum and elastic Young's modulus (E) of NiTi based shape memory alloys. In the martensitically transforming binary alloys Ni50+xTi50-x (x≤1.3) vacuum annealed and furnace cooled (H-free), besides the well known IF peak associated with the martensitic transition two additional non-thermally activated peaks (P150K and P200K′) are present due to some sort of second-order phase transitions. In martensitically transforming Ni50+xTi50-x and Ti50Ni50-yCuy alloys doped with hydrogen two thermally activated peaks, PTWH and PH, appear which originate from stress-assisted motions of H-twin boundary complexes and isolated H-elastic dipoles (Snoek effect), respectively. In a H-free martensitically non-trasforming alloy (x=2), besides the non-thermally activated peak P150K, a frequency dependent dip is observed in the E(T) curves at a temperature Tg. This dip is similar to that reported in the literature for two other non-transforming alloys (x=1.5 and x=2.5), which, however, were also found to exhibit a thermally activated IF peak just below Tg. Most likely, these two alloys were contaminated with hydrogen during the preliminary solubilization in argon atmosphere and subsequent water quenching treatments given to them. The Young's modulus dip and the lower temperature IF peak have been both attributed to a novel type of phase transition reported in the literature as “strain glass transition”. The introduction of hydrogen into the non-transforming alloy with x=2 enhances the Young's modulus dip and gives rise to the H-Snoek peak PH just below Tg, which clearly appears to be the counterpart of the peak observed in the alloys (x=1.5 and x=2.5) solubilized in argon atmosphere and water quenched. The conclusion was reached in the present work that this last peak is not related to the strain glass transition but is rather an H-Snoek relaxation.
Highlights
Shape memory alloys (SMA) are a class of functional materials that have been extensively investigated over the past decades.[1,2] Several experimental techniques have been used in these studies, among them a relevant role has been played by mechanical spectroscopy, which makes use of stress and strain as the stimulating and response field, respectively
A remarkable strain-amplitude dependence is displayed by P150K and P200K as shown by figure 3, where low-frequency dynamic mechanical analyzer (DMA) data taken at 10-3, 10-4 and 10-5 strain amplitudes have been reported as a function of temperature
H-free materials i) Binary alloys Ni50+xTi50-x, besides the transformation/transient peaks PAM and PMA, exhibit two additional damping features, P150K and P200K, whose temperatures are frequency-independent and whose height decreases with increasing frequency
Summary
Shape memory alloys (SMA) are a class of functional materials that have been extensively investigated over the past decades.[1,2] Several experimental techniques have been used in these studies, among them a relevant role has been played by mechanical spectroscopy, which makes use of stress and strain as the stimulating and response field, respectively. In the present review we focus our attention to some recently discovered features of the damping spectrum of NiTi based SMA7–31 regarding the influence of hydrogen and compositional structural defects on the response strain to an applied stress field. The damping peaks occurring at the martensitic transitions will be to some extent neglected here since they are the subject of most of the previous reviews
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
