FCC High Entropy Alloys Research Articles

Design of new face-centered cubic high entropy alloys by thermodynamic calculation

A new face-centered cubic (fcc) high entropy alloy system with non-equiatomic compositions has been designed by utilizing a CALculation of PHAse Diagram (CALPHAD) - type thermodynamic calculation technique. The new alloy system is based on the representative fcc high entropy alloy, the Cantor alloy which is an equiatomic Co- Cr-Fe-Mn-Ni five-component alloy, but fully or partly replace the cobalt by vanadium and is of non-equiatomic compositions. Alloy compositions expected to have an fcc single-phase structure between 700 °C and melting temperatures are proposed. All the proposed alloys are experimentally confirmed to have the fcc single-phase during materials processes (> 800 °C), through an X-ray diffraction analysis. It is shown that there are more chances to find fcc single-phase high entropy alloys if paying attention to non-equiatomic composition regions and that the CALPHAD thermodynamic calculation can be an efficient tool for it. An alloy design technique based on thermodynamic calculation is demonstrated and the applicability and limitation of the approach as a design tool for high entropy alloys is discussed.

Predicting yield strengths of noble metal high entropy alloys

Recent data on the Noble metal (Pd-Pt-Rh-Ir-Au-Ag-Cu-Ni) high entropy alloys (HEAs) shows some of these materials to have impressive mechanical properties. Here, a mechanistic theory for the temperature-, composition-, and strain-rate-dependence of the initial yield strength of fcc HEAs is applied to this alloy class, with inputs obtained through “rule-of-mixtures” models for both alloy lattice and elastic constants. Predictions for PdPtRhIrCuNi are in good agreement with available experiment and the model provides useful insights into this system. The model is then used to explore other alloy compositions within this broad class to guide design of new stronger Noble metal HEAs.

Understanding the mechanical behaviour and the large strength/ductility differences between FCC and BCC AlxCoCrFeNi high entropy alloys

The deformation behaviour of three direct laser fabricated AlxCoCrFeNi high entropy alloys were examined. One alloy had a face-centered cubic (x = 0.3) structure, one had a two-phase structure comprising an ordered (B2) and disordered BCC phase (x = 0.85), and the third alloy contained a mixture of all three phases (x = 0.6). The deformation behaviour of all three alloys was examined by mechanical testing, scanning electron microscopy and transmission electron microscopy. The dislocation density of specimens was measured using x-ray diffraction. For the FCC alloy, it was found that the correlation between dislocation density, applied stress and compressive strain all benchmark closely with the behaviour of austenitic stainless steel. It is therefore found that this alloy does not show any unique deformation behaviours of note, and like austenitic steels, show a low yield point, moderately high work hardening rates and excellent plasticity. For the case of the alloy with a high aluminium concentration, which had a two-phase B2+BCC microstructure, an exceptionally high yield strength of 1400 MPa was observed. Nano-hardness examination of this alloy showed that the B2 phase had an exceptionally high hardness value. Since the alloy contained a large volume fraction of this hard intermetallic B2 phase, is was concluded that the B2 phase is responsible for the high strength of this alloy. It was shown that this alloy behaves like a composite, and its strength can be predicted closely using a simple rule of mixtures approach. The poor tensile ductility of BCC high entropy alloys has been suggested to be the result of the inability of the B2 phase to accommodate shape change, leading to interphase cracking that results in brittle fracture characteristics.

Serration behavior and negative strain rate sensitivity of Al0.1CoCrFeNi high entropy alloy

Temperature dependent deformation mechanism of Al0.1CoCrFeNi high entropy alloy (HEA) was studied using monotonic and strain rate jump tests at various test temperatures in a coarse-grained single phase FCC HEA. The tensile properties of Al0.1CoCrFeNi HEA exhibited a modest temperature dependence in the tested range of 300–673 K. At an initial strain rate of 10−5 s−1, the serration type was a function of the test temperature. Furthermore, the strain rate sensitivity of the flow stress changed from positive to zero to negative once the unstable plastic deformation region due to dynamic strain aging was attained.

Theory of strengthening in fcc high entropy alloys

High Entropy Alloys (HEAs) are a new class of random alloys having impressive strength and toughness. Here, a mechanistic, parameter-free, and predictive theory for the temperature-, composition-, and strain-rate-dependence of the plastic yield strength of fcc HEAs is presented, validated, and applied to understand recent experiments. To first order, each elemental component in the HEA is considered as a solute embedded in the effective matrix of the surrounding alloy. Strengthening is then mainly achieved due to dislocation interactions with the random local concentration fluctuations around the average composition. The theory is validated against molecular simulations on model Fe-Ni-Cr alloys. Hall-Petch-corrected yield strengths in Ni-Co-Fe-Cr-Mn fcc HEAs are then predicted using only available experimental information, and good quantitative agreement is achieved. The theory demonstrates the origins of the high strength and detailed trends with composition, materials parameters, temperature, thus identifying the key measurable/calculable material properties needed for design and optimization of fcc HEAs, and is a general model for fcc random alloys.

Microstructure and Mechanical Behavior of High-Entropy Alloys

High-entropy alloys (HEAs) have generated interest in recent years due to their unique positioning within the alloy world. By incorporating a number of elements in high proportion, usually of equal atomic percent, they have high configurational entropy, and thus, they hold the promise of interesting and useful properties such as enhanced strength and alloy stability. The present study investigates the mechanical behavior, fracture characteristics, and microstructure of two single-phase FCC HEAs CoCrFeNi and CoCrFeNiMn with some detailed attention given to melting, homogenization, and thermo-mechanical processing. Ingots approaching 8 kg in mass were made by vacuum induction melting to avoid the extrinsic factors inherent to small-scale laboratory button samples. A computationally based homogenization heat treatment was given to both alloys in order to eliminate any solidification segregation. The alloys were then fabricated in the usual way (forging, followed by hot rolling) with typical thermo-mechanical processing parameters employed. Transmission electron microscopy was subsequently used to assess the single-phase nature of the alloys prior to mechanical testing. Tensile specimens (ASTM E8) were prepared with tensile mechanical properties obtained from room temperature through 800 °C. Material from the gage section of selected tensile specimens was extracted to document room and elevated temperature deformation within the HEAs. Fracture surfaces were also examined to note fracture failure modes. The tensile behavior and selected tensile properties were compared with results in the literature for similar alloys.

Polycrystalline elastic moduli of a high-entropy alloy at cryogenic temperatures

CrMnCoFeNi is a FCC high-entropy alloy (HEA) that exhibits strong temperature dependence of strength at low homologous temperatures in sharp contrast to pure FCC metals like Ni that show weak temperature dependence. To understand this behavior, elastic constants were determined as a function of temperature. From 300 K down to 55 K, the shear modulus (G) of the HEA changes by only 8%, increasing from 80 to 86 GPa. This temperature dependence is weaker than that of FCC Ni, whose G increases by 12% (81–91 GPa). Therefore, the uncharacteristic temperature-dependence of the strength of the HEA is not due to the temperature dependence of its shear modulus.

Nanoscale phase separation in a fcc-based CoCrCuFeNiAl0.5 high-entropy alloy

Nano-scale phase separation is reported in a nominal single-phase, high-entropy alloy (HEA), which was characterized using scanning transmission electron microscopy (STEM) combined with atom probe tomography (APT). Despite the fact that X-ray diffraction exhibits a single face-centered-cubic (fcc) phase feature of the as-cast alloy prepared by melt spinning, selected area electron diffraction reveals weak L12 ordering in the as-spun alloy. High-resolution STEM shows the presence of two coherent nanophases with distinct L12 and fcc structures, coupling with compositional segregations. The ordering of the L12 domains is enhanced after annealing at 500°C. Electron energy loss spectroscopy and APT analyses reveal that the L12 nano-phase is enriched with Fe, Co, Cr and Ni, while the fcc domains are a Cu-rich phase. The nano-scale phase separation can effectively minimize the lattice distortions caused by the atomic size difference in the constituent elements, which may offer structural insights into the unusual mechanical behavior and phase stability of fcc HEA.

Explore the possibility of forming fcc high entropy alloys in equal-atomic systems CoFeMnNiM and CoFeMnNiSmM

The multi-principal high-entropy alloys (HEAs) are promising new alloys. However, it is a challenge to screen out the suitable composition from the diverse combinations. Referring to the prototype AuCu3 with AB3-L12 structure, where it becomes a face-centered cubic (fcc) structure if element A and B are the same element, the site occupying tendencies of the elements and thermodynamic functions are predicted by using the sublattice model supported with first-principles total energy calculations. By considering the Gibbs energy of formation and the configurational entropy, the fcc HEAs in available literatures are examined, and the results of the quinary system with equal-atomic composition CoFeMnNiM and the hexbasic system with equal-atomic composition CoFeMnNiSmM are reported, respectively, where the element M is selected from the rest of the periodical table. When M=Cr, Zn, Ru, Rh, Pd, Re, Os, Ir, or Pt in the quinary systems CoFeMnNiM and when M=Ru, Pd, or Pt in the hexbasic systems CoFeMnNiSmM, respectively, the alloys are recommended to be potential fcc HEAs. The new approach opens a new way to mine the rich ores of HEAs.

Formation of ordered/disordered nanoparticles in FCC high entropy alloys

Abstract This study comprises the microstructure characterization of four as-cast CoCrFeNiAl0.3, CoCrFeNiTi0.3, CoCrFeNiMo0.3 and CoCrFeNiAl0.3Mo0.1 FCC high entropy alloys. Massive nanoparticles are observed in the CoCrFeNiAl0.3, CoCrFeNiTi0.3 and CoCrFeNiAl0.3Mo0.1 alloys, whereas no nanoparticle is found in the CoCrFeNiMo0.3 alloy. These nanoparticles are multi-elemental solid solutions instead of inter-metallic compounds, and the crystal structure possesses both the ordered L12 and the disordered FCC structures. It is suggested that the formation of nanoparticles is dominated by the effects of the high mixing entropy, the sluggish cooperative diffusion of atoms, and the relatively negative mixing enthalpies of the atomic pairs between Al (or Ti) and other compositional elements. Furthermore, the formation of L12 ordered nanoparticles should fulfill the criteria of alloy composition having both more than 25 at.% of Ni (Ni could be partially substituted by Co) and at least 6.25 at.% of Al or Ti.

Synthesis of titanium aluminide-titanium boride composites by combustion reaction: X. Qu et al (Central South University of Technology, Changsha, Hunan, China)

We present a review of solid solution strengthening models for random concentrated solid solutions of which high entropy alloys are an interesting subset. High entropy alloys (HEAs) usually refer to a class of multicomponent alloys in equal or near equal concentrations. These complex compositions break the conventional notion of solutes and solvents. Few attempts have been made to extend the conventional solute strengthening models to HEAs. Among these, the model based on an average effective medium, does not include any adjustable parameter, allows all model inputs to be computed by atomistic simulations, and has predicted the strength of fcc HEAs in good agreement with experiments. The basic concepts of this theory is explained and its capabilities are compared with few other existing models for solute strengthening of HEAs.