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Keynote Lecture

Effect of grain size, temperature and texture on mechanical properties of the CrCoNi medium-entropy alloy

Wednesday (26.09.2018)
14:30 - 15:00 S1/01 - A5
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In a group of equiatomic solid solution alloys with a fcc crystal structure, Wu et al. showed that the medium-entropy alloy CrCoNi exhibits the best combination of strength and ductility in the CrMnFeCoNi system [1]. The reason for these superior mechanical properties was shown to be due to the fact that the CrCoNi alloy has a lower stacking fault energy and deformation twinning occurs at lower strains than in the CrMnFeCoNi [2]. Recently, attempts were made to increase the strength of CrCoNi by precipitation hardening similarly to polycrystalline superalloys. Zhao et al. alloyed CrCoNi with 3 at.% Ti and 3 at.% Al to form γʼ-precipitates in a γ-matrix [3]. Therefore, basic data regarding the CrCoNi ternary system are of prime importance for a better understanding and the development of new superalloys. The first study of grain size strengthening at room temperature was carried out by Yoshida et al. [4]. However, the Hall-Petch relationship, especially at different temperatures, for this alloy has not received much attention so far. In this study, mechanical tests between 77 and 873 K were carried out for grain sizes between 3 µm and 175 µm. Grain size and texture analyses are performed using backscattered electron imaging and backscatter electron diffraction, respectively. The effect of temperature and texture on grain size strengthening (Hall-Petch relationship) is investigated. Interestingly, the Hall-Petch slope is found to be independent of temperature for temperatures between 77 and 673 K. However, two domains with different slopes are observed at 873 K, i.e. for large grain sizes the slope of the Hall-Petch relationship is similar to that found at lower temperatures while for smaller grains the slope is significantly lower suggesting the occurrence of a grain boundary mediated deformation mechanism at high temperature and small grain sizes. The authors acknowledge funding by the German Research Foundation (DFG) through projects B5/B7 of SFB/TR 103.

[1] Wu et al., ACTA MATER 81, 2014, pp. 428-441

[2] Laplanche et al., ACTA MATER 128, 2017, pp. 292-303

[3] Zhao et al., ACTA MATER 138, 2017, pp. 72-82

[4] Yoshida et al., SCRIPTA MATER 134, 2017, 33-36

Mike Schneider
Ruhr-Universität Bochum
Additional Authors:
  • Tanguy Manesau
    Université de Toulouse Ensiacet
  • Prof. Dr. Guillaume Laplanche
    Ruhr-Universität Bochum
  • Prof. Dr. Gunther Eggeler
    Ruhr-Universität Bochum
  • Prof. Dr. Easo George
    Oak Ridge National Laboratory