| Summary: | 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.
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