Complementary In Situ Characterization of Superelasticity in Fe–Mn–Al–Ni–Ti Shape Memory Alloys by Acoustic Emission, Digital Image Correlation and Infrared Thermography

Abstract

Coupled in situ investigations were conducted on a Fe–Mn–Al–Ni–Ti single crystal deformed in compression and two Fe–Mn–Al–Ni–Ti oligo-crystals deformed in tension. Acoustic emission measurements were employed to monitor the degradation of superelasticity and the stabilization of martensite due to dislocation processes. These observations were corroborated by the application of digital image correlation and infrared thermography measurements. A poor strain reversibility and a premature plastification of the parent phase were observed in case of the single crystal due to an unfavourable crystal orientation. A contradictory transformation behaviour of the two oligo-crystals was observed, with one specimen showing a promising strain reversibility and characterisitic signs of degradation, and the other specimen exhibiting a limited strain reversibility due to an unusual confinement of the martensitic phase transformation to an unfavourably oriented grain. In the former case, an increase in the dislocation density within five cycles was detected through a shift of the acoustic signals’ median frequencies. In the latter case, a strong coupling between martensite nucleation and dislocation generation led to a pronounced martensite stabilization after one loading cycle. For all specimens, temporal sequence effects related to the coupling of martensite nucleation and dislocation generation were detected by means of acoustic emission.