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Open AccessFeature PaperArticle

Effect of Temperature and Texture on Hall–Petch Strengthening by Grain and Annealing Twin Boundaries in the MnFeNi Medium-Entropy Alloy

Institut für Werkstoffe, Ruhr-Universität Bochum, D-44780 Bochum, Germany
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Metals 2019, 9(1), 84; https://doi.org/10.3390/met9010084
Received: 21 December 2018 / Revised: 8 January 2019 / Accepted: 11 January 2019 / Published: 15 January 2019
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Abstract

Among equiatomic alloys of the Cr-Mn-Fe-Co-Ni system, MnFeNi was shown to exhibit a strong anti-invar behavior but little is known regarding its mechanical properties. The objective of the present study is to investigate Hall–Petch strengthening by grain and annealing twin boundaries in MnFeNi. For this purpose, seven different grain sizes between 17 and 216 µm were produced. Mean grain sizes (excluding annealing twin boundaries) and crystallite sizes (including them) were determined using the linear intercept method. Overall, 25% of the boundaries were found to be annealing twin boundaries regardless of the grain size. In some cases, two twin boundaries can be present in one grain forming an annealing twin, which thickness represents one quarter of the mean grain size. Based on a comparison of the mean twin thickness of different alloys with different stacking fault energy (SFE), we estimated an SFE of 80 ± 20 mJ/m2 for MnFeNi. Compression tests of MnFeNi with different grain sizes were performed between 77 and 873 K and revealed a parallel shift of the Hall–Petch lines with temperature. The interaction between dislocations and boundaries was investigated by scanning transmission electron microscopy (STEM) in a deformed specimen. It was found that a large number of dislocations are piling up against grain boundaries while the pile-ups at annealing twin boundaries contain much fewer dislocations. This indicates that annealing twin boundaries in this alloy are less effective obstacles to dislocation motion than grain boundaries. View Full-Text
Keywords: high-entropy alloys; CoCrFeMnNi; FeNiMn; hall–petch effect; dislocations/grain boundaries interactions; texture; mechanical properties high-entropy alloys; CoCrFeMnNi; FeNiMn; hall–petch effect; dislocations/grain boundaries interactions; texture; mechanical properties
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).
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Schneider, M.; Werner, F.; Langenkämper, D.; Reinhart, C.; Laplanche, G. Effect of Temperature and Texture on Hall–Petch Strengthening by Grain and Annealing Twin Boundaries in the MnFeNi Medium-Entropy Alloy. Metals 2019, 9, 84.

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