Instytut Podstawowych Problemów Techniki

Streszczenie:

In this work, thermomechanical behavior of the superelastic Ti–26Nb, Ti–25Nb–0.3O and Ti–25Nb–0.3N (at. pct) shape memory alloys (SMAs) under load-unload tension was investigated using coupled techniques of infrared thermography and digital image correlation. Local and average characteristics were analyzed in the context of particular deformation stages. In the case of all the SMAs, during loading, first the temperature decreases due to the thermoelastic effect, which can serve to estimate true elastic strain. During further loading, the temperature significantly increases due to the forward stress-induced phase transformation, whereas during unloading, the temperature significantly decreases due to the reverse phase transformation. The average values of the temperature change generated due to the elastocaloric heating and cooling were 15.95 K, 14.94 K, 16.05 K and 17.79 K, 14.44 K, 19.83 K in the case of the Ti–26Nb, Ti–25Nb–0.3O and Ti–25Nb–0.3N SMAs, respectively. The kinematic fields demonstrated that the deformation during loading and unloading is inhomogeneous. It starts with the appearance of thin parallel bands perpendicular to the loading axis. These bands create larger areas with higher strain upon further loading and gradually reappear upon unloading. The results advance the comprehension of the thermal and kinematic aspects of tensile deformation of superelastic Ti-Nb-based SMAs

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Streszczenie:

A Ti–25Nb shape memory alloy (SMA) exhibits shape memory effect associated with stress-induced martensitic transformation from β to α″ phase. Addition of oxygen stabilizes the β phase and changes stress–strain response. Oxygen-added Ti–25Nb SMAs show a more distinct superelastic behavior. In this work, digital image correlation (DIC) was applied to investigate for the first time full-field deformation of Ti–25Nb, Ti–25Nb–0.3O and Ti–25Nb–0.7O (at. pct) SMAs. The specimens were subjected to loading–unloading tensile tests to study local and global mechanical characteristics related to activity of particular deformation mechanisms of the SMAs. Strain and strain rate fields were quantitatively compared at selected stages of each SMA’s deformation. It was found that the Ti–25Nb SMA exhibits a macroscopically localized Lüders-type deformation associated with the stress-induced phase transformation, whereas Ti–25Nb–0.3O and Ti–25Nb–0.7O SMAs show more discrete types of deformation related to activity of interstitial oxygen atoms. As a consequence, at particular stages of deformation, local values of strain rate of Ti–25Nb SMA were significantly higher than those of average strain rate. The results obtained in this paper provide a better understanding of the deformation mechanism in the oxygen-added Ti–25Nb based SMAs.

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Słowa kluczowe:

Ni-free shape memory alloys, interstitials atoms, stress-induced phase transformation, acoustic emission

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