AlxCoCrFeNi is a prominent high entropy alloy system with varying crystal structure from FCC to BCC depending on aluminum content. The mechanical behavior of Al0.7CoCrFeNi with dual phase FCC+BCC ...microstructure has been compared with that of single phase FCC Al0.3CoCrFeNi. Both quasi-static and dynamic strain rate regimes were investigated. Hypo-eutectic Al0.7CoCrFeNi showed much higher strength due to fine lamellar microstructure with a large number of FCC-BCC interphase boundaries. But this also leads to lower strain rate sensitivity due to the long-range nature of these interfaces, overcoming them is indifferent with temperature elevation to assist slip, thus making them athermal barriers. Both these precipitation hardenable alloys were aged to induce precipitation of ordered L12 in the FCC phase. This coherent nano-scale L12 precipitate caused a significant increase in the yield strength of both single-phase and dual phase structures while reducing the strain rate sensitivity (SRS) only slightly. L12 precipitation in FCC matrix greatly enhanced twinning during dynamic deformation. Large-scale deformation twins were observed in coarse Al0.3CoCrFeNi FCC and FCC + L12 microstructures. The scale of deformation twins was much smaller in the dual phase Al0.7CoCrFeNi whose refined lamellae width retarded twinning. The lamellar structures, nevertheless, had higher work hardening due to their higher dislocation density storage capability.
High-entropy alloys (HEAs) can offer exceptional strain-rate sensitivity (SRS) due to high lattice-friction and strong solid-solution strengthening, and in some cases, low stacking fault energy. In ...this study, SRS of a single phase FCC HEA- Al0.3CoCrFeNi was investigated at two grain sizes. Due to its outstanding Hall-Petch coefficient, grain size becomes a crucial microstructural feature in determination of strength and SRS. Change in SRS due to grain refinement was derived to be inversely proportional to associated strength gain, in coarse microstructures where grain size >> dislocation forest cell size. This correlation was proven using two microstructures with grain sizes of 12 µm and 150 µm, with yield strengths of 313 MPa and 145 MPa, and demonstrated SRS of m = 0.029 and 0.064, respectively. SRS was also derived to increase linearly with strength contribution from thermal short-range obstacles. The slope reflects the maximum upper limit on SRS possible upon elimination of all other obstacles of long-range nature. This limiting value of SRS was derived to be 0.118 for this HEA.
Often the experimentally-observed, single-phase high entropy alloy (HEA) is the result of second-phase precipitation constrained by thermodynamic and kinetic factors. Using Al0.3CoCrFeNi as a ...candidate HEA, this paper demonstrates the strong influence of thermo-mechanical processing on the transformation pathway adopted for isothermal second-phase precipitation. A traditional thermo-mechanical processing route comprised of homogenization cold-rolling solution treatment in the single fcc phase region, followed by a precipitation anneal at a lower temperature, results in a homogeneous distribution of nanometer scale-ordered L12 (gamma prime-like) precipitates within the fcc matrix. In contrast, if cold-rolling is followed directly by annealing at the precipitation temperature, then the resulting microstructural evolution pathway changes completely, with concurrent recrystallization of the matrix fcc grains and precipitation of B2 and sigma phases, largely at the grain boundaries. These experimentally observed variations in transformation pathway have been rationalized via the competition between the thermodynamic driving force and activation barrier for second-phase nucleation in this alloy, coupled with the kinetics of the process. The microstructural variations that result from these dramatically different phase transformation pathways can lead to some rather exceptional mechanical properties that can be varied over a large range even for a single Al0.3CoCrFeNi HEA composition.
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Refractory high-entropy alloys (RHEAs) have recently attracted much attention, primarily due to their mechanical properties at elevated temperatures. However, the equilibrium phase-stability of these ...alloy systems is not well established. The present investigation focuses on the phase stability of Al0.5NbTa0.8Ti1.5V0.2Zr RHEA at temperatures ranging from 600 to 1200 °C. The detailed phase characterization involves coupling of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The stable phases present at these temperatures are (i) 1200 °C—body-centered cubic (BCC) matrix with nano-B2 precipitates; (ii) 1000 °C and 800 °C—a BCC matrix phase with Al–Zr rich hexagonal closed packed intermetallic precipitates and, (iii) 600 °C—a BCC + B2 microstructure, comprising a continuous BCC matrix with discrete B2 precipitates. These results highlight the substantial changes in phase stability as a function of temperature in RHEAs, and high-entropy alloys in general, and also the importance of accounting for these changes especially while designing alloys for high temperature applications.
The microstructure and magnetic properties of three face-centered cubic (FCC) FeCoNiCrCu(x) high entropy alloys (HEAs) (x = 0, 0.5, 1) are investigated. Interestingly, addition of the nonmagnetic ...element Cu to FeCoNiCr HEA is found to enhance exchange interactions and low temperature saturation magnetization. The paramagnetic to ferromagnetic Curie transition temperature increases from 85 K for FeCoNiCr to 118 K for FeCoNiCrCu. This is counterintuitive since Cu is nonmagnetic; however, atom probe tomography revealed Cu rich clusters containing 5 at% Ni and 1 at% each of Fe, Co, Cr, within FCC matrix, these clusters altered the matrix composition and consequently its magnetic properties.
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We report on interfacial characteristics and chemistry of bonded Mg-Fe interfaces welded using friction stir assisted scribe technique (FaST). Two pairs of dissimilar joints: (AZ31-DP590) and (Pure ...Mg-DP590) were studied to shed light on joining mechanisms responsible for bonding of "immiscible" pairs of Mg and Fe. We present first direct experimental evidence of presence of oxide layer, Al segregation by atom probe tomography and nano steel grains close to interface by transmission electron microscopy study.
Tuning deformation mechanisms is imperative to overcome the well-known strength-ductility paradigm. Twinning-induced plasticity (TWIP), transformation-induced plasticity (TRIP) and precipitate ...hardening have been investigated separately and have been altered to achieve exceptional strength or ductility in several alloy systems. In this study, we use a novel solid-state alloying method-friction stir alloying (FSA)-to tune the microstructure, and a composition of a TWIP high-entropy alloy by adding Ti, and thus activating site-specific deformation mechanisms that occur concomitantly in a single alloy. During the FSA process, grains of the as-cast face-centered cubic matrix were refined by high-temperature severe plastic deformation and, subsequently, a new alloy composition was obtained by dissolving Ti into the matrix. After annealing the FSA specimen at 900 °C, hard Ni-Ti rich precipitates formed to strengthen the alloy. An additional result was a Ni-depleted region in the vicinity of newly-formed precipitates. The reduction in Ni locally reduced the stacking fault energy, thus inducing TRIP-based deformation while the remaining matrix still deformed as a result of TWIP. Our current approach presents a novel microstructural architecture to design alloys, an approach that combines and optimizes local compositions such that multiple deformation mechanisms can be activated to enhance engineering properties.
The electrochemical instability of ether-based electrolyte solutions hinders their practical applications in high-voltage Li metal batteries. To circumvent this issue, here, we propose a dilution ...strategy to lose the Li
/solvent interaction and use the dilute non-aqueous electrolyte solution in high-voltage lithium metal batteries. We demonstrate that in a non-polar dipropyl ether (DPE)-based electrolyte solution with lithium bis(fluorosulfonyl) imide salt, the decomposition order of solvated species can be adjusted to promote the Li
/salt-derived anion clusters decomposition over free ether solvent molecules. This selective mechanism favors the formation of a robust cathode electrolyte interphase (CEI) and a solvent-deficient electric double-layer structure at the positive electrode interface. When the DPE-based electrolyte is tested in combination with a Li metal negative electrode (50 μm thick) and a LiNi
Co
Mn
O
-based positive electrode (3.3 mAh/cm
) in pouch cell configuration at 25 °C, a specific discharge capacity retention of about 74% after 150 cycles (0.33 and 1 mA/cm
charge and discharge, respectively) is obtained.
The reciprocating sliding wear behavior of two high entropy alloys, CoCrFeMnNi and Al0.1CoCrFeNi, was evaluated in dry and marine environments. Both the alloys showed better wear performance in ...marine environment as compared to dry condition, indicating negative synergy of wear and corrosion. Al0.1CoCrFeNi was more wear resistant compared to CoCrFeMnNi in both environments. Accelerated electrochemical corrosion tests were carried out to quantify the effect of passive layer on marine wear behavior. Al0.1CoCrFeNi showed lower corrosion rate, higher pitting resistance and greater degree of passivation. A strong correlation was found between the electrochemical polarization resistance and wear resistance.
•Reciprocating sliding wear behavior of high entropy alloys in dry and marine environments was evaluated.•Al0.1CoCrFeNi alloy showed significantly higher resistance to wear in both environments compared to equimolar CoCrFeMnNi.•Dry and marine wear resistance was attributed to high surface hardness and pitting corrosion resistance respectively.
•Laser additive processing for high-throughput analysis of high entropy alloys.•A combinatorial approach for assessing composition-microstructure-microhardness.•Compositionally graded AlCrFeMoVx ...(0 < x < 1) high entropy alloys (HEAs) assessed.•A new single solid solution bcc based equi-atomic HEA is reported.•High solubility of V offers a broad range of solid solution strengthening.
The current study discusses a novel combinatorial high-throughput approach for assessing the composition-microstructure-hardness relationship, using laser deposited compositionally graded AlCrFeMoVx (0 < x < 1) high entropy alloy (HEA) as a candidate system. The composition gradient was achieved from AlCrFeMo (with 0.3 at. % V) to AlCrFeMoV (with 18.5 at. % V) over a length of ∼20 mm, deposited using the laser engineered net shaping process from a blend of elemental powders. Scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were used to characterize the alloys. A single solid solution (SS) body-centered cubic (bcc) structure is observed throughout the compositional range. The high solubility of V in this novel alloy system offers a broad range of solid solution strengthening of a compositionally complex but structurally simple bcc matrix. The hardness of the alloy increases from 485 Hv to 581 Hv on increasing V from 0.3 to 18.5 at. %. The solid solution hardening model for dilute solutions underestimates the strengthening of the alloy. The current study presents a novel and efficient method for microstructural screening of the bulk alloys for optimization of the microstructure and properties.
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