Abstract
Iron-based Fe-Mn-Al-Ni shape-memory alloys are of rather low materials cost and show remarkable pseudoelastic properties. To further understand the martensitic transformation giving rise to the pseudoelastic properties, different Fe-Mn-Al-Ni alloys have been heat treated at 1473 K and quenched in ice water. The martensite, which is formed from a body-centred cubic austenite, is commonly described as face-centered cubic (f.c.c.), even though there are also more complex, polytypical descriptions of martensite. The presently studied backscatter Kikuchi diffraction (BKD) patterns have been evaluated, showing a structure more complex than simple f.c.c. This structure can be described by nanoscale twins, diffracting simultaneously in the exciting volume. The twinned structure shows a tetragonal distortion, not uncommon for martensite in spite of the lack of interstitial elements. These features are evaluated by comparing the measured BKD patterns with dynamically simulated ones.
Highlights
Iron-based Fe–Mn–Al–Ni shape-memory alloys (SMAs) show a remarkable pseudoelastic hysteresis (Vollmer et al, 2015, 2016, 2017, 2019; Omori et al, 2011, 2012, 2013) and are, of special interest
The backscatter Kikuchi diffraction (BKD) patterns taken from such regions can be tentatively indexed as either b.c.c. austenite or f.c.c. martensite [Fig. 3(c)]
The results presented here for the structure of the martensite, differ from the 53 polytype deduced by Omori et al (2012) based on selected-area electron diffraction of martensite generated in Fe43.5Mn34Al15Ni7.5 and from the apparently single-orientation f.c.c. structure as reported by Vallejos et al (2018)
Summary
Iron-based Fe–Mn–Al–Ni shape-memory alloys (SMAs) show a remarkable pseudoelastic hysteresis (Vollmer et al, 2015, 2016, 2017, 2019; Omori et al, 2011, 2012, 2013) and are, of special interest. The martensite is usually described with a face-centered cubic (f.c.c.) structure (Omori et al, 2011, 2012, 2013; Tseng et al, 2015a,b, 2016a,b; Vallejos et al, 2018; La Roca et al, 2015). A later backscatter Kikuchi diffraction study shows maps with large regions (of a size of several mm) of uniformly indexed martensite in Fe43.5Mn34Al15Ni7.5 alloys (Vallejos et al, 2018), where according to the authors an f.c.c. structure had been used for indexing. In the course of our own investigations on Fe–Mn–Al–Ni alloys (Walnsch et al, 2019a), even in the case of quite reasonable quality of the backscatter Kikuchi diffraction (BKD) patterns originating from martensite, we encountered difficulties during indexing when relying on an f.c.c. structure with Hough-space-based routines. The effect of the crystal structure and microstructure on the martensite in this alloying system will be described elsewhere
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