Abstract

The alloy system Al-(Co)-Cr-Fe-Ni contains compositional ranges where a solid state BCC-FCC phase transformation leads to dual-phase materials composed of face-centered cubic (FCC) and body-centered cubic (BCC) phases with nearly equal volume fraction. The microstructure arising from this transformation at slow cooling rates is the classical Widmanstatten structure, with FCC-laths and colonies growing from grain boundaries into the parent BCC-B2 grain. Very distinct microstructures are obtained, when Widmanstatten growth is kinetically suppressed e.g. during continuous cooling with high cooling rates. These novel microstructures are associated with the spinodal decomposition of the parent BCC-B2 such that FCC growth occurs during the spinodal decomposition or upon annealing from a metastable, fully spinodal state. We review the microstructures at case as function of the imposed cooling regimes for the Co-free medium entropy alloy AlCrFe2Ni2. One of them, termed ultrafine vermicular microstructure, involves a characteristic and novel crystal orientation relationship (OR) between FCC and BCC. We identify the common planes and directions of this OR using electron backscatter diffraction maps to be {111}^FCC∥{121 }^BCC and 〈1 01〉^FCC∥〈1 01 〉^BCC, respectively. Embedded is a second OR with {13 1 }^FCC∥{1 03}^BCC and 〈101〉^FCC∥〈010〉^BCC. We further show that the vermicular FCC phase contains a high amount of lattice strain and sub-grain boundaries with disorientation angles in the range from 2 to 12°, as a result of the solid-state phase transformation.

Highlights

  • High entropy and medium entropy alloys attracted wide interest, driven by the quest for multicomponent, yet single phase materials with face-centred-cubic (FCC), body-centred-cubic (BCC) and hexagonal-close-packed (HCP) structures while eliminating secondary phases (Steurer, 2020)

  • the quaternary alloy system Al-Cr-Fe-Ni proved equally prolific around compositions AlCrFe2Ni2

  • dual-phase materials composed of FCC

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Summary

Introduction

High entropy and medium entropy alloys attracted wide interest, driven by the quest for multicomponent, yet single phase materials with face-centred-cubic (FCC), body-centred-cubic (BCC) and hexagonal-close-packed (HCP) structures while eliminating secondary (and tertiary) phases (Steurer, 2020). Alloys like Al0.7CoCrFeNi, Al0.8CoCrFeNi and certainly many other alloys with reduced or fully eliminated Co content like AlCrFe2Ni2, are dual-phase materials composed of FCC and BCC phases with nearly equal volume fraction, typically with a Widmanstätten lath morphology of the FCC phase (De Jeer et al, 2017; Abuzaid and Sehitoglu, 2018). This microstructure type, known as “duplex microstructure”, is similar though not identical to the one well known from duplex steels (Ohmori et al, 1995; Knyazeva and Pohl, 2013). The domain ordering of the BCC-B2 in the spinodally decomposed state was investigated in detail by Linden et al (2017), while accounting for microsegregation inherited from solidification

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