Microstructural evolution after thermomechanical processing in an equiatomic,single-phase CoCrFeMnNi high-entropy alloy with special focus on twin boundaries

The FCC-structured equiatomic CoCrFeMnNi high-entropy alloy was produced by arc melting and drop casting. After homogenization, the drop-cast ingots were cold rolled to sheets with six different final thicknesses (thickness reductions of 21, 41, 61, 84, 92 and 96%). Samples were cut from the rolled sheets and annealed for 1 h at temperatures between 400 and 1000 °C. The recrystallization temperature was then determined as a function of cold work by means of scanning electron microscopy and electron backscatter diffraction measurements. Additionally, Vickers indentation was performed on these samples. It was found that the microhardness first tends to increase slightly upon annealing below the recrystallization temperature but then drops steeply for higher annealing temperatures due to the onset of recrystallization. To study grain growth kinetics, samples that underwent 96% cold rolling were first recrystallized for 1 h at 800 °C, which is the lowest temperature at which complete recrystallization occurs, and then annealed at temperatures between 800 and 1150 °C for various times. The grain growth exponent was determined to be approximately n = 3, and the activation energy Q = 325 kJ/mol, both of which agree well with published values for this alloy. EBSD measurements were made in the as-recrystallized and grain growth samples to analyze the annealing twins. The density of annealing twins in the grain growth samples was found to depend only on grain size, i.e., it was independent of annealing temperature and time. No such correlation could be found for the as-recrystallized samples. These observations are discussed in the framework of existing theories for the formation of annealing twins.     

Grain refinement of a Fe3Al-based alloy using κ-Fe3AlC precipitate particles stimulating nucleation of recrystallization

Effects of particle distribution level on recrystallization were investigated in a Fe₃Al-based alloy containing coarse κ-Fe₃AlC precipitate particles. Volume fraction of 10–12% of rod-like κ particles with different size and interparticle spacing was introduced within the Fe₃Al matrix by changing the cooling rate from 1200 °C, which is above the precipitation temperature of the κ phase. These samples were warm rolled at 700 °C to a total reduction of 75%. Annealing of the warm rolled samples produced complete recrystallized structures. The average recrystallized grain size against interparticle spacing showed a valley-shaped curve with a minimum size of 20 μm. Orientation analyses of the warm rolled samples with high resolution EBSD method revealed that the valley shape of the curve may be explained by the particle stimulated nucleation density of recrystallization around κ particles, dependent on the particle distribution.     

Effects of electro-negativity on the stability of topologically close-packed phase in high entropy alloys

Topologically close-packed (TCP) phases with complex structures are often observed in high entropy alloys (HEAs). Currently, these TCP phases are garnering significant interest from both theoretical and experimental perspectives due to the ductility deterioration of the high strength HEAs. Alternatively, there are instances when TCP phases can actually benefit the mechanical performances of alloys, such as the wear resistance. Therefore, the stability of TCP phases should be taken into consideration in the alloy design. In this paper, the relationship between the TCP phase stability and the physicochemical/thermodynamic properties of alloying components in HEAs was systematically studied. The stability of TCP phases was found to correlate well with the electro-negativity difference (ΔX) for most of the reported HEAs. The stability of TCP phases is well delineated by the electro-negativity difference (ΔX): i.e., TCP phases are stable at ΔX > 0.133 except for some HEAs that contain a significant amount of aluminum.     

Mechanical properties at 2223 K and oxidation behavior of Ir alloys

Compressive strengths of twelve Ir-1at%X binary alloys were evaluated at 2223 K to understand the strengthening effects of alloying elements. The alloying elements were selected from group 4 to group 10 in the periodic table. The maximum strength of about 65 MPa was obtained in Ir–Hf and Ir–Zr alloys. The effect of adding multiple alloying elements on strength was also investigated. Isothermal oxidation behavior was investigated at 1373 K for Ir alloys including Nb and/or Hf. The weight of all the tested alloys decreased linearly during the oxidation test, indicating that Ir oxide evaporated during oxidation.     

The use of diffusion multiples to examine the compositional dependence of phase stability and hardness of the Co-Cr-Fe-Mn-Ni high entropy alloy system

High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. Co-Cr-Fe-Mn-Ni quinary diffusion multiples were successfully created. Using these multiples, a quinary region of disordered of FCC was formed and examined using EDS and nanoindentation methods. From these techniques, maps of common HEA parameters (Ω, δ, ΔS_mix, ΔH_mix and Δχ) proposed in literature could be calculated and directly compared to observed phase stability. Similarly, hardness was examined as function of compositional complexity and atomic mismatch in the quinary disordered region in order to directly test the severe lattice distortion hypothesis. It was found that proposed HEA parameters were ineffective at single phase stability limits in the Co-Cr-Fe-Mn-Ni system. It was also observed that hardness did not correlate well to the maximum compositional complexity or to the maximum in atomic mismatch. This indicates the severe lattice distortion hypothesis is not the primary contributor to strengthening in the Co-Cr-Fe-Mn-Ni HEA system.     

A new strategy to design eutectic high-entropy alloys using mixing enthalpy

Although eutectic high entropy alloys (EHEAs) display homogeneously fine microstructure, excellent castability and promising industrial application potential, how to design eutectic compositions in high entropy alloys (HEAs) remains to be challenging. Here, a novel strategy, specifically, through calculation of mixing enthalpy, was used to locate eutectic compositions in HEAs. As a proof of this concept, a series of EHEAs were located and prepared following the mixing enthalpy method. Using this new strategy, eutectic compositions can be designed conveniently, once one can classify elements into two different groups. This new alloy design strategy can be readily adapted to locate new EHEAs.     

Effects of ternary additions on the microstructure and thermal stability of directionally-solidified MoSi2/Mo5Si3 eutectic composites

Effects of ternary additions on the microstructure and thermal stability of directionally-solidified MoSi₂/Mo₅Si₃ eutectic composites have been studied for twelve different elements (Ti, V, Cr, Fe, Co, Ni, Nb, Ta, W, Ir, B and C) paying special attention to the variation of lattice misfits and interface segregation behavior with ternary additions. Among six elements (type-1: Ti, V, Cr, Nb, Ta and W) with a relatively high solubility in MoSi₂ and Mo₅Si₃, Ta and W are found to be beneficial to microstructure refinement. All other six ternary elements (type-2: Fe, Co, Ni, Ir, B and C) with a negligibly low solubility in MoSi₂ and Mo₅Si₃ exhibit a strong tendency to segregate on MoSi₂/Mo₅Si₃ interfaces, resulting in both microstructure refinement and the modification of the interface morphology.     

Annealing effects on the microstructure and properties of bulk high-entropy CoCrFeNiTi0.5 alloy casting ingot

Most previous researches focused on small casting ingots prepared by arc melting, when studying high-entropy alloys. Large sized ingots were also necessary in exploring the existence of volume effects in the multi-principal element alloys. During the experiments, a large sized CoCrFeNiTi_0.5 alloy casting ingot was prepared by a medium frequency induction melting furnace. A slight volume effect occurred, reflecting mainly in the growth of crystalline grains and the increase of alloy hardness in the ingot. To investigate the effect of annealing temperature on microstructure and properties of CoCrFeNiTi_0.5 alloy, several samples taken from the ingot were annealed at 600 °C, 700 °C, 800 °C and 1000 °C respectively for 6 h. Almost no effects were found to the crystalline structure and elemental distribution when the samples were annealed below 1000 °C. The crystalline structure of CoCrFeNiTi_0.5 alloy was composed of one principal face-centered cubic (FCC) solid-solution matrix and a few intermetallic phases in the form of interdentrite. Dendrite contained approximately equivalent amount of Co, Cr, Fe, Ni and a smaller amount of Ti. When annealed below 1000 °C, the interdendrite stayed in (Ni, Ti)-rich phase, (Fe, Cr)-rich phase and (Co, Ti)-rich phase. After 1000 °C annealing, (Co, Ti)-rich phase disappeared, while (Ni, Ti)-rich phase and (Fe, Cr)-rich phase grew. The microhardness of the as-cast CoCrFeNiTi_0.5 alloy was 616.80 HV and the macrohardness was 52 HRC. The hardness of the samples stayed generally unchanged after annealing. This indicated a high microstructure stability and excellent resistance to temper softening that the CoCrFeNiTi_0.5 alloy exhibited.     

Study of the intermetallic growth in copper-clad aluminum wires after thermal aging

Study of the solid-state diffusion between copper and aluminum was carried out in the temperature range [573–673] K in order to better understand the aging mechanisms which occur in copper-clad aluminum thin wires. A complete microscopic analysis was performed to evaluate the interface composition and corresponding microstructure. The intermetallic phases developed during annealing identified by TEM and X-Ray diffraction analysis are respectively Al₂Cu, AlCu, and Al₄Cu₉. A fine layer containing nanometric copper grains was also depicted and identified as a diffusion-induced recrystallization region. These results agree with EDXS analysis and nanoindentation measurements. The effective heat of formation model was used to evaluate the first phase(s) which happens in the interface and the sequence formation of intermetallic compounds during annealing. This model finely describes the metallurgical aging of copper-clad aluminum wires and explains the presence of only three intermetallic compounds in the interface between copper and aluminum.     

Optimisation of precipitation for controlling recrystallisation of wrought Fe3Al based alloys

A Fe–26Al–5Cr (at.%) single-phase (:A2/B2/D0₃) alloy and two-phase (+TiC) alloys with different amounts of TiC particles have been hot rolled at 800 °C and the kinetics of static recrystallisation have been studied. In the alloys with a high amount of TiC, needle-like TiC of more than 1 μm in length formed during cooling after homogenisation in the single-phase region and coarsened during hot rolling. The large particles cause particle stimulated nucleation (PSN) and hence accelerate recrystallisation. In order to accomplish both strengthening by precipitates and inhibition of recrystallisation that deteriorates room-temperature ductility, a thermo-mechanical treatment consisting of hot deformation with a low amount of precipitates and a subsequent heat treatment for further precipitation is proposed. This process is difficult to carry out in the (Fe–26Al–5Cr)–TiC system due to the high precipitation temperature of TiC. The precipitation temperature is significantly decreased by replacing TiC by VC or MoC.     

Microstructural changes and estimated strengthening contributions in a gamma alloy Ti–45Al–5Nb pack-rolled sheet

Thin sheets made of a gamma-titanium aluminide alloy, Ti–45Al–5Nb, produced by a pack-rolling process, were evaluated for microstructure variation and evolution taking place during aging and annealing treatments. The as-received sheet material was characterized by remarkably high yield strength, 810 MPa, and a complex bimodal microstructure. The microstructure consisted of a matrix of twinned gamma-phase grains and fine-lath lamellar grain microconstituent, together with a dispersed ultra-fine-grained gamma + alpha-2 mixture microconstituent. High-temperature isothermal aging treatments changed the microstructure to a stable mixture of gamma-phase grains (matrix) and coarse alpha-2-phase particles, having size distributions and volume fractions that were specific to the aging temperature. A concurrent strength loss reflects this trend and results in a stable strength level of 550 MPa upon aging at 1000 °C for 144 h. Using composition estimates from the phase-boundary shifts that occur from the Nb addition to a Ti–45Al base alloy and, the rule of mixtures, an analysis was made to show that the gamma-phase matrix has an intrinsic strength of 178 MPa. This is a significant intrinsic strength level, well over that of 70 MPa for the Ti–45Al binary alloy. This is rationalized as the solid-solution strengthening effect from shifts of the Ti and Nb levels in the gamma phase and, by an added effect due to increased oxygen solubility in the gamma phase. The overall strength of Ti–45Al–5Nb, however, is roughly the same as that of Ti–45Al, and this is explained by a drastic reduction in the volume fraction of alpha-2-phase in Ti–45Al–5Nb alloy, which is a result of the Nb-induced phase-boundary shifts.     

Influence of phase constituent and proportion in initial Al–Cu alloys on formation of monolithic nanoporous copper through chemical dealloying in an alkaline solution

An alkaline medium has been used to fabricate monolithic nanoporous copper (NPC) ribbons through chemical dealloying of melt-spun Al–Cu alloys with 33–50 at.% Cu. The results show that phase constituent and proportion in the initial alloys have a key influence on the formation of NPC. The alloy ribbons comprising one or a combination of Al₂Cu and AlCu can be fully dealloyed in alkali solution only when there is no or a minor AlCu in the initial alloys. Additionally, the length scales of ligaments/pores in NPC can be broadly modulated by simply changing the amount of AlCu in the initial alloys.     

Oxidation behavior at 300–1000 °C of a (Mo,W)Si2-based composite containing boride

The oxidation behavior of a (Mo,W)Si₂ composite with boride addition was examined at 300–1000 °C for 24 h in dry O₂. The oxidation kinetics was studied using a thermobalance, and the oxide scales were analyzed using a combination of electron microscopy (SEM/EDX, FIB, BIB) and XRD. Accelerated oxidation was found to occur between 500 °C and 675 °C, with a peak mass gain at 625 °C. The rapid oxidation is attributed to the vaporization of molybdenum oxide that leaves a porous and poorly protective silica layer behind. At higher temperature (700–1000 °C) a protective scale forms, consisting of a dense SiO₂/B₂O₃ glass.     

Site-occupation behavior and solid-solution hardening effect of rhenium in Mo5SiB2

The site-occupation behavior of Re in Mo₅SiB₂ (T₂) was studied both theoretically and experimentally, and the effect of Re on the solid-solution hardening of T₂ was investigated by taking into account the off-stoichiometry of the T₂ phase. Mo–Si–B quaternary alloys containing 1.4 at.% Re were produced using a conventional Ar arc-melting technique, and the cast samples were homogenized at 1800 °C for 24 h in an Ar atmosphere. High-resolution high-angle annular dark-field scanning transmission electron microscopy observations strongly suggest that Re preferentially occupies the Mo sites in the Mo–B layers of the T₂ unit cell, which was confirmed by the site-occupation behavior predicted by first-principles calculations. Nanoindentation measurements indicate that the hardness of the T₂ phase is affected by both the off-stoichiometry and Re addition.     

Crystal structure, phase stability and elastic properties of the Laves phase ZrTiCu2

The crystal structure of the ternary Laves phase ZrTiCu₂ with unusual stoichiometry has been determined from combined refinement of X-ray powder, X-ray single crystal and neutron powder intensity data. The derived structure is of type MgZn₂ (space group P6₃/mmc) with lattice parameters a = 0.51491(3) nm, c = 0.82421(8) nm. Crystal symmetry and composition reveal a high degree of atomic disorder, because Ti and Zr atoms share the 4f sites, whereas Ti and Cu atoms are found at the 6h sites. The 2a sites, however, are exclusively occupied by Cu. Lattice parameters for alloys Zr_1−xTi_1−xCu_2+2x (annealed at 800 °C) as a function of the concentration of Cu for a constant ratio of Zr/Ti = 1 vary in a nonlinear way, which is consistent with the described complex atomic substitution mechanism. At a load of 2 N the micro-hardness was measured to be 7.5 ± 0.3 GPa, which is significantly larger than for most of the binary Ti–Cu or Zr–Cu phases. By a density functional theory ab initio approach the site preferences of Zr, Ti and Cu were calculated indicating that a random mixture of Ti and Cu atoms at the 6h lattice sites is a key factor to stabilize the proposed structure, which is unique for a Laves phase. Lattice parameters, elastic constants and shear moduli for polycrystalline ZrTiCu₂ were also derived. The Vickers hardness of 6.2 GPa was estimated by applying a correlation between shear modulus and hardness. Data as calculated by the ab initio approach are in good agreement with the experimental findings.     

Deformation behavior of an ordered B2 phase in Ti–25Al–25Zr alloy

The mechanical behavior of the B2 phase in alloy Ti–25Al–25Zr has been studied under compression. True stress–strain curve exhibits similar behavior to those of typical B2 intermetallics such as NiAl and FeAl. The alloy exhibits highest yield strength in comparison to those reported in other titanium based B2 alloys with around 2% plastic strain. The microstructural characterization of specimen after compression reveals that the B2 phase transforms to an orthorhombic martensitic phase during compression.     

Hydrogen sorption properties of Ti–Cr alloys synthesized by ball milling and cold rolling

Three Ti–Cr alloys with nominal compositions of TiCr_x (x = 2, 1.8 and 1.5) were synthesized by cold rolling and ball milling of as-cast ingots, and their microstructures and hydrogenation properties were studied. X-ray diffraction showed that TiCr_x transformed from a mixture of C14 and C15 Laves phases to a metastable BCC phase after 5 h of ball milling under argon. Cold rolling did not lead to the formation of a metastable BCC phase but only to the reduction of TiCr_x size particles under 20 nm. Surprisingly, the hydrogen absorption/desorption curves of cold rolled and ball milled samples at 323 K were quite similar. This result proves that hydrogen storage properties do not depend only on microstructure and that cold rolling could be an interesting method to synthesize hydrogen storage materials.     

Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys

Although refractory high-entropy alloys have exceptional strength at high temperatures, they are often brittle at room temperature. One exception is the HfNbTaTiZr alloy, which has a plasticity of over 50% at room temperature. However, the strength of HfNbTaTiZr at high temperature is insufficient. In this study, the composition of HfNbTaTiZr is modified with an aim to improve its strength at high temperature, while retaining reasonable toughness at room temperature. Two new alloys with simple BCC structure, HfMoTaTiZr and HfMoNbTaTiZr, were designed and synthesized. The results show that the yield strengths of the new alloys are apparently higher than that of HfNbTaTiZr. Moreover, a fracture strain of 12% is successfully retained in the HfMoNbTaTiZr alloy at room temperature.     

Microstructure and mechanical properties of a newly developed high strength Mg54.7Cu11.5Ag3.3Gd5.5Sc25 alloy

Starting from the bulk glass-forming composition Mg_59.5Cu_22.9Ag_6.6Gd₁₁, the development of a bulk glassy matrix composite alloy with high strength and ductility was attempted by adding a large amount of Sc to induce the formation of the MgSc intermetallic compound. However, copper-mould casting of the modified composition, Mg_54.7Cu_11.5Ag_3.3Gd_5.5Sc₂₅, as rods with 3 and 5 mm diameter, yields besides the expected MgSc, a second (CuSc) and a third (Mg₃Gd-type) intermetallic. These compounds are surrounded by a Mg-rich matrix, possibly composed of glassy regions and a Mg-based terminal solid solution. Although the obtained microstructure deviates largely from the expected one, the new alloy exhibits very high strength, i.e. 790 MPa, and visible plastic deformation, i.e. 0.9%. Shear dimples were found in MgSc.     

Effect of ball milling on structure and thermal stability of Al84Gd6Ni7Co3 glassy powders

The influence of ball milling on microstructure and thermal stability of the gas-atomized Al₈₄Gd₆Ni₇Co₃ glassy powder has been investigated as a function of the milling time. The results show that the traces of crystalline phases present in the as-atomized powder decrease gradually with increasing the milling time. The thermal stability of the fcc-Al primary phase increases while the thermal stability of the intermetallic phases decreases with increasing milling. Moreover, significant improvement in hardness occurs after milling, which is attributed to the amorphization of the residual crystalline phases present in the as-atomized powder. These results demonstrate that milling is an effective way for amorphizing the residual crystalline present in the amorphous matrix and to control the thermal stability of the material.