Abstract
In situ tensile tests employing digital image correlation were conducted to study the martensitic transformation of oligocrystalline Fe–Mn–Al–Ni shape memory alloys in depth. The influence of different grain orientations, i.e., near-〈001〉 and near-〈101〉, as well as the influence of different grain boundary misorientations are in focus of the present work. The results reveal that the reversibility of the martensite strongly depends on the type of martensitic evolving, i.e., twinned or detwinned. Furthermore, it is shown that grain boundaries lead to stress concentrations and, thus, to formation of unfavored martensite variants. Moreover, some martensite plates seem to penetrate the grain boundaries resulting in a high degree of irreversibility in this area. However, after a stable microstructural configuration is established in direct vicinity of the grain boundary, the transformation begins inside the neighboring grains eventually leading to a sequential transformation of all grains involved.
Highlights
Shape memory alloys (SMAs) are able to show large recoverable strains due to a fully reversible thermo-elastic phase transformation [1]
In situ tensile tests employing digital image correlation were conducted to study the martensitic transformation of oligocrystalline Fe–Mn–Al–Ni shape memory alloys in depth
Fe–Mn–Al–Ni is characterized by a low temperature dependence of the critical stress for martensitic transformation (0.53 MPa K-1) [11] making it attractive for large scale damping applications in numerous fields
Summary
Shape memory alloys (SMAs) are able to show large recoverable strains due to a fully reversible thermo-elastic phase transformation [1]. Up to now, the majority of studies focused on single crystalline [12, 13, 16, 19, 22,23,24, 26, 27] and oligocrystalline states [11, 28,29,30], since the superelastic performance of Fe–Mn–Al–Ni SMAs is significantly influenced by the relative grain size with respect to the cross section of the samples [31]. High resolution in situ tensile tests supported by digital image correlation (DIC) were conducted to investigate the influence of individual grain orientations and of grain boundaries on the martensitic transformation in oligocrystalline structures. The correlation between the theoretical transformation strains and the experimentally determined local strains is exploited to provide for novel insights into the evolution of martensite during loading and further shed light on the reverse transformation during unloading
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