The BCC-FCC Phase Transformation Pathways and Crystal Orientation Relationships in Dual Phase Materials From Al-(Co)-Cr-Fe-Ni Alloys

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 facecentered 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 Widmanstätten structure, with FCC-laths and colonies growing from grain boundaries into the parent BCC-B2 grain. Very distinct microstructures are obtained, when Widmanstätten 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 AlCrFe 2 Ni 2. 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 {131} FCC {103} 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.

Microstructure and Thermal Analysis of Metastable Intermetallic Phases in High-Entropy Alloy CoCrFeMo0.85Ni

“…First of all, there is a good agreement between nominal and chemical composition of the alloy in as-received state confirming the consistency of semi-quantitative EDX measurements. In the as-received powder, there is minor elemental segregation across the powder particle in Figure 3 a, whilst the detailed SEM image in Figure 3 b shows fine alternating layers which have some similarity to the morphology formed via spinodal reaction in Al–(Co)–Cr–Fe–Ni HEA [ 18 ]. Further analysis on the chemical composition of the alternating layers is needed to clarify the reaction for the formation of the phases.…”

Hierarchical phase evolution in a lamellar Al0.7CoCrFeNi high entropy alloy involving competing metastable and stable phases

“…However, a recent study on this microstructure using detailed EBSD analysis revealed that this microstructure is not a result of eutectic reaction but a result of solid state decomposition of parent BCC grains into FCC and BCC [21]. This solid state decomposition leading to a Widmanstatten pattern of FCC lamellae in BCC matrix was accepted by other research groups [22]. Further, this alloy has stable intermetallic phases at lower temperatures which provides an opportunity to investigate the transformation pathways involved in forming the intermetallics in FCC and BCC based phases.…”

Phase-field approach to simulate BCC-B2 phase separation in the AlnCrFe2Ni2 medium-entropy alloy

“…Moreover, with the increase in aluminum content, the A1 field overlaps part of the miscibility gap, creating a region where the three phases can coexist at equilibrium. This combination of phases yields the possibility to obtain a large variety of microstructure morphologies, finely controlled by the alloy composition and the processing route [4].…”

“…As-cast Al n CrFe 2 Ni 2 alloys with composition n % 1 have shown exceptional tensile strength and ductility [2,5]. These properties have been attributed to the peculiar dual-phase microstructure consisting of A1 and B2 regions, formed simultaneously with the decomposition in the B2 phase [4]. Furthermore, the use of additive manufacturing techniques, such as laser powder bed fusion [6] and laser metal deposition [7], has revealed a strong dependency of the phase fractions and morphology on the cooling conditions.…”