Search

This study aimed at understanding the structure and properties of dual-phase eutectics in ternary, quaternary, and quinary alloys of the Al-Co-Cr-Fe-Ni system. The alloys at case were i) Ni48Fe34Al18, ii) Ni44Fe20Cr20Al16, and iii) Ni34.4Fe16.4Co16.4Cr16.4Al16.4. Samples in the form of cylindrical bars, diameter 10 mm × 150 mm, were produced by arc melting and suction casting from pure elements (>99.9 wt%). Bridgman solidification at low growth velocity was used to produce additional samples with large eutectic spacing and lamellae thickness of the two phases body-centered cubic (BCC)-B2 and face-centered cubic (FCC) in order to facilitate phase characterization by energy-dispersive X-ray analysis (scanning electron microscopy/energy-dispersive spectroscopy) and nano-indentation. In agreement with thermodynamic calculations, each of the phases was found to be multi-component and contain all alloying elements in distinct amounts. The mechanical properties of the individual phases were analyzed in relation to their composition using nano-indentation experiments. These measurements revealed some insights into “high-entropy effects” and their contribution to the elastoplastic response to indentation loading. Further analysis focused on as-cast as well as heat-treated samples comprising phase fraction measurements, micro-indentation, and miniature testing in three-point bending configuration. For optimum heat treatment conditions, a good balance of strength and ductility was obtained for each of the investigated alloys. Further work is necessary in order to assess their capability as structural materials.

High-entropy alloys, AlCrFe2Ni2Mox (x = 0.00, 0.05, 0.10, and 0.15), AlCoCrFeNi, and two quinary alloys with compositions close to its face-centered cubic and body-centered cubic component phases, are tested for corrosion resistance in 3.5 wt% NaCl. The materials with different microstructure produced by arc melting or ingot metallurgy are evaluated by several electrochemical techniques: measurements of open circuit voltage, cyclic potentiodynamic polarization, and electrochemical impedance spectroscopy. Microstructure, surface topography, and composition are systematically characterized by scanning electron microscopy and energy-dispersive x-ray spectroscopy. The results indicate that minor additions of Mo positively affect corrosion resistance of the AlCrFe2Ni2 alloy by hampering pit formation. The face-centered cubic phase in the equimolar alloy, AlCoCrFeNi, is proved to exhibit more noble corrosion potential and pitting potential, lower corrosion current density and corrosion rate than the body-centered cubic phase. Overall behavior of the investigated alloys is influenced by the manufacturing conditions, exact chemical composition, distribution of phases, and occurrence of physical defects on the surface.

The alloy system Al-Cr-Fe-Ni provides means for developing novel duplex materials composed of face-centered cubic (FCC) and body-centered cubic (BCC) phases with nearly equal volume fraction. We performed an alloy development study starting from the medium entropy alloy AlCrFe2Ni2 and adding small amounts of molybdenum in the following series (at.%): Al17Cr17Fe33Ni33, Al17Cr17Fe33Ni33Mo1, Al16Cr16Fe33Ni33Mo2, and Al16Cr16Fe33Ni33Mo3. We focused the research on samples with an ultrafine vermicular duplex microstructure, a unique structure requiring sufficiently high cooling rates to suppress the conventional Widmanstätten colony formation. The samples were produced by arc melting in buttons of 300 g each. We characterized the microstructure of the samples using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and electron backscatter diffraction (EBSD). The EBSD data revealed significant strain in the FCC phase resulting from the BCC→FCC phase transformation. We investigated mechanical properties of the samples by micro-indentation and 3-point bending in a miniature testing device. The test specimens were in the as-cast condition, as well as in distinct annealed conditions. Annealing treatments were carried out at 950 and 1100°C under argon. The annealing lasted from 10 min to 6 h, followed by water quenching. Prolonged annealing at 950°C of Mo-containing samples resulted in the formation of sigma-phase. Annealing at 1100°C safely avoided sigma-phase formation, while leading to a good balance between the flexural strength and ductility of these samples. Mechanical testing also included the well-established superduplex steel 1.4517 (DIN EN 10283/ASTM A890) as reference material.

In this paper we present and discuss phase equilibria in the quaternary Al–Cr–Fe–Ni and the quinary Al–Co–Cr–Fe–Ni alloy systems based on experimental data from DTA/DSC, SEM/EDS, and SEM/EBSD on as-cast and isothermally annealed samples. These data as well as literature data were used for developing a new Al–Co–Cr–Fe–Ni thermodynamic description by the CALPHAD approach. Considerable efforts in this direction have been made already and commercial databases for high entropy alloys are available, e.g., TCHEA4 and PanHEA. We focus on comparing our new thermodynamic database with computations using TCHEA4 for two section planes, i.e., quaternary AlxCrFe2Ni2 and quinary AlxCoCrFeNi, where x is the stochiometric coefficient. According to our new thermodynamic description a single-phase field BCC-B2 is stable over a wider temperature range in both section planes, giving rise to dual-phase microstructures along solid state phase transformation pathways. In the section plane AlxCoCrFeNi the two-phase field BCC-B2 + σ predicted by the new database is stable between 600 and 800°C, while in TCHEA4 this phase field extends to nearly 1000°C. Furthermore, our new database showed that the solidification interval with primary BCC-B2 phase in quinary as well as quaternary section planes is narrow, being in a good agreement with presented micro-segregation measurements. Additionally, computed phase fields and phase-field boundaries in the quinary section plane correspond well to the experimental results reported in the literature.

A duplex, nano-scale Co-free high-entropy alloy (HEA) based on AlCrFe2Ni2 was processed using laser material deposition (LMD). Process parameters in various beam diameter configurations, as well as deposition strategies, were used while the alloy microstructure was investigated in the as-built and heat-treated condition. Interlayer regions present a duplex microstructure composed of ultra-fine face-centered cubic (FCC)-platelets nucleated in a nano-scale structured body-centered cubic (BCC) phase. Rapid cooling during LMD induces the decomposition of the BCC phase into ordered and disordered nano-scaled structures. The hard and brittle BCC phase yields a high crack susceptibility during rapid cooling in the LMD process. A suitable processing strategy paired with a post-processing heat treatment was developed to solve this challenge. After heat treatment at 900°C and 6 h annealing time with subsequent furnace cooling, the material presented a homogeneous duplex ultra-fine FCC/BCC microstructure and high bending strength (2310 MPa) compared to a heat-treated cast duplex-steel (1720 MPa) while maintaining excellent ductility (no failure at 20% bending strain).

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 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 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}F⁢C⁢C∥{12⁢1¯}B⁢C⁢C and ⟨1¯01⟩F⁢C⁢C∥⟨1¯01¯⟩BCC, respectively. Embedded is a second OR with {1⁢3¯⁢1¯}F⁢C⁢C∥{1¯⁢03}B⁢C⁢C 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.