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Fe49Mn30Cr10Co10C1 high entropy alloy (HEA) is produced by induction melting. The as-cast alloy is cold rolled and annealed at 900°C, to produce fine recrystallized structure before friction stir welding (FSW). The structure of the annealed alloy consists of a recrystallized face-centered cubic (fcc, γ) and hexagonal close-packed (hcp, ε) phases with volume fractions of 91% and 5%, respectively, as well as M23C6 carbides with the volume fraction of 4%. Sound weld without visible defects, such as porosity or cracks, are obtained. Friction stir welding results in a decrease in the average grain size from 7.0 to 1.9 μm in the stir zone. The volume fraction of the M23C6 carbides decreases to 1% after FSW. The alloy shows high yield strength and ultimate tensile strength of 475 MPa and 865 MPa, respectively, together with elongation of 70%.

Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500–900 °C for 1–50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600–900 °C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500–700 °C or a sigma phase particles at 600–800 °C. Moreover, an increase in the annealing time to 50 h at 600 °C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500–600 °C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600 °C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.

The effect of friction stir welding parameters on the structure and properties of Fe49Mn30Cr10Co10C1 high-entropy alloy welds was studied. Due to the development of the TRIP effect, the mechanical behaviour of this alloy was associated with the γ fcc - to - ε hcp martensitic transformation. In the initial condition, the microstructure of the program alloy comprised equiaxed fcc grains and small fractions of the hcp ε-martensite (~5%) and М23С6 carbides (~4%). Friction stir welding of the program alloy resulted in recrystallization of the stir zone and a decrease in the fraction of the carbides to 1–2%; however, the percentage of the hcp phase remained at nearly the same level as that in the initial condition. Post-welding tests showed a considerable increase in the strength and microhardness of the welds both due to the recrystallization-induced decrease in grain size and martensitic transformation.

Microstructure and mechanical properties of the Fe-Mn-Cr-Ni-Al system non-equiatomic high entropy alloys with a different Al content (x = 0–14 at.%) were studied in the present work. The Fe40Mn25Cr20Ni15 alloy was composed of the face-centered cubic (fcc) matrix phase with a small amount of coarse body-centered cubic (bcc) particles. Addition of a small amount of Al (x = 2–6) resulted in an increase in the fraction of the bcc phase to 26% and the formation of fine B2 precipitates within the bcc phase. At higher amounts of Al (x = 10 and x = 14) the microstructure consisted of coarse bcc matrix grains with the B2 precipitates inside. The alloys tend to become stronger with an increase in the Al content from 0 to 10 at.%; further increase in Al concentration did not influence strength considerably. The alloys exhibited pronounced softening with an increase in testing temperature from 25 to 400 °C–600 °C. Ductility of the alloys was high enough (>50%) at all temperatures. A quasi-binary Fe40Mn25Cr20Ni15-Al phase diagram was constructed using a ThermoCalc software and a TCHEA2 database; reasonable agreement between the experimental and predicted phase compositions of the alloys was obtained. It was suggested that an addition of the strong bcc-stabilizing and compound-forming Al to a bcc-prone Fe40Mn25Cr20Ni15 alloy is beneficial for the development of the alloys with the disordered bcc matrix and the embedded B2 precipitates having attractive mechanical properties.

Additive manufacturing (AM) allows printing parts of complex geometries that cannot be produced by standard technologies. Besides, AM provides the possibility to create gradient materials with different structural and physical properties. We, for the first time, printed gradient soft magnetic materials from paramagnetic powders (316L steel and Cu-12Al-2Fe (in wt.%) aluminium bronze)). The magnetic properties can be adjusted during the in-situ printing process. The saturated magnetization value of alloys reaches 49 emu g−1. The changes in the magnetic properties have been attributed to the formation of the BCC phase after mixing two FCC-dominated powders. Moreover, the phase composition of the obtained gradient materials can be predicted with reasonable accuracy by the CALPHAD approach, thus providing efficient optimization of the performance. The obtained results provide new prospects for printing gradient magnetic alloys.

Laser beam welding and friction stir welding of high entropy alloys (HEA) of the CoCrFeNiMn system were studied. The HEAs were produced by self-propagating high-temperature synthesis (SHS). Along with the principal elements, Al, C, S, and Si impurities were detected in the composition of the alloys. The as-cast alloys consisted of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6carbides. Laser beam welding resulted in the formation of a defect-free weld joint. Precipitation of nanoscale B2 phase particles in the weld zone leaded to a pronounced increase in microhardness from ~150 HV of the base material to ~220 HV in the fusion zone. Friction stir welding (FSW) of a recrystallized state of the HEA with the average grain size of 3-5 μm resulted in the formation of a fine microstructure with a grain size of ~1.5 μm in the most strained area. Noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the formation of nanosized M23C6carbides.

In this paper, we report microstructure and mechanical properties evolution of the CoCrFeNiMn-type high entropy alloy, containing small amounts of Al and C, during cold rolling and subsequent annealing at 700-1100°C. In the initial as-cast condition the alloy has coarse-grained single face-centered cubic (fcc) phase structure. Cold rolling and annealing substantially refine fcc grains; in addition M23C6 type carbides appear. After annealing at relatively low temperatures (≤900°C), these particles are arranged in characteristic arrays aligned with rolling directions. The specific microstructure of the thermomechanically processed alloy is suggested to be the reason of the balanced combination of tensile strength and ductility.

Four non-equiatomic Fe-(Co, Mn)-Cr-Ni-Al-(Ti) high entropy alloys, namely Fe36Mn21Cr18Ni15Al10, Fe36Co21Cr18Ni15Al10, Fe35Mn20Cr17Ni12Al12Ti4, and Fe35Co20Cr17Ni12Al12Ti4 alloys, were produced by arc melting. Structures and compression mechanical properties of the as-cast alloys were examined. The Fe36Co21Cr18Ni15Al10 alloy had mostly a face-centered cubic (fcc) structure, while Fe36Mn21Cr18Ni15Al10 mainly consisted of a body-centered cubic (bcc) matrix with embedded B2 precipitates. An addition of Ti resulted in the formation of L21 precipitates of a cuboidal shape mostly in the Fe35Mn20Cr17Ni12Al12Ti4 and of a plate-like shape in the Fe35Co20Cr17Ni12Al12Ti4 alloys. In addition, a significant amount of the fcc phase (0.17) was found in the latter alloy. Good correlation between the average valence electron concentration (VEC) value and the amount of the fcc phase in the experimental alloys was found. The comparison of the experimental data with results obtained using a Thermo-Calc software and a TCHEA2 database demonstrated a lack of credibility in the L21 phase formation predicting. In terms of the mechanical properties, the Fe36Co21Cr18Ni15Al10 alloy was rather soft, while the Fe36Mn21Cr18Ni15Al10 and Fe35Mn20Cr17Ni12Al12Ti4 alloys had high strength at temperatures of ≤400 °C. The Fe35Co20Cr17Ni12Al12Ti4 alloy had the highest strength among the examined alloys and maintained the strength at temperatures up to 600 °C. The correlation between the mechanical properties and structure of the non-equiatomic Fe-(Co, Mn)-Cr-Ni-Al-(Ti) high entropy alloys and a potential for further improvements of the properties are discussed.

Butt-joint seam of a high entropy alloy (HEA) of the CoCrFeNiMn system was successfully obtained by friction stir welding (FSW). The HEA was produced by self-propagating high-temperature synthesis. Along with the principal elements, a small amount (0.9 at.%) of C was added to the alloy. The as-cast alloy was cold rolled and annealed at 900 °C to produce refined microstructure. The structure of the annealed alloy consisted of a recrystallized face-centered cubic matrix with a grain size of 9.2 μm and fine Cr-rich M23C6 carbides. FSW of the HEA resulted in microstructure refinement to d = 4.6 μm in the stir zone. A noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the microstructure refinement and some increase in the volume fraction of M23C6 carbides.

A new approach to increase the tensile performance of high entropy alloys (HEAs) by producing a duplex ultrafine-grained (UFG) structure was reported in this work. A novel HEA based on the CoCrFeNiMn system with substantial amounts of Al and C was used for the illustration of this approach. In the as-cast condition the alloy had almost entirely a single face-centered cubic (fcc) phase structure with an insignificant amount of M23C6 carbides. After cold rolling and annealing at 800–1000 °C an increased amount of fine second phases, namely M23C6 carbides and B2 phase, effectively pinned boundaries of recrystallized fcc grains. As a result, a duplex UFG structure composed of the recrystallized fcc grains and M23C6 and B2 particles was produced. The alloy with the UFG structure demonstrated attractive mechanical properties. For example, after annealing at 900 °C the alloy had the yield strength of 785 MPa, the ultimate tensile strength of 985 MPa, and elongation to fracture of 32%. The phase composition of the alloy in different conditions was compared with the equilibrium phase diagram obtained using a Thermo-Calc software. Strengthening mechanisms were qualitatively analyzed, and some possibilities for further improvement of strength of the alloy were discussed.

Fiber laser beam welding of a CoCrFeNiMn-type high entropy alloy (HEA) produced by self-propagating high-temperature synthesis (SHS) was reported in this work. The SHS-fabricated alloy was characterized by both ∼2 times reduced Mn content in comparison with that of the other principal components and the presence of impurities including Al, C, S, and Si. The as-fabricated alloy was composed of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M23C6 carbides. Successful defect-free butt joint of the alloy was obtained using a laser power of 2 kW and a welding speed of 5 m/min. Welding resulted in changes in texture and structure of the fcc matrix. In addition, precipitation of nanoscale B2 phase particles in the weld zone was observed. A pronounced increase in microhardness from (153 ± 3) HV 0.5 (base material) to (208 ± 6) HV 0.5 (fusion zone) was observed. The B2 phase precipitation after welding was found to be in a reasonable agreement with the ThermoCalc predictions. Quantitative analysis demonstrated that the increase in hardness can be associated with the B2 phase precipitation. Possibilities of the development of HEAs with intrinsic hardening ability after laser processing are discussed.

The effect of nitrogen on the structure and mechanical behavior of a CoCrFeMnNi high-entropy alloy after rolling and subsequent annealing was examined. The as-cast alloys doped with 0.5, 1.0, and 2.0 at% N were cold rolled to 80% thickness reduction. Further annealing in the temperature range of 700–1000 °C for 1 h has resulted in (i) development of recrystallization in the fcc matrix and (ii) precipitation of the M2N type nitrides in the alloys with 1.0 and 2.0 at% N. The recrystallization onset temperature lowered with an increase in the N content. Besides, the fcc grain size in the alloy with 2.0 at% N was considerably smaller than that in the alloys with a lower nitrogen content due to a strong pinning effect of the nitride particles. The cold-rolled alloys had high strength, but very limited ductility. The recrystallized microstructure has resulted in a much better strength-ductility synergy when tested at 293 K. For example, the alloy with 2.0 at% N after annealing at 900 °C had the yield strength of 673 MPa, ultimate tensile strength of 1021 MPa, and ductility of 47%. A decrease in the testing temperature to 77 K has resulted in a considerable increase in strength. For example, the same alloy had the yield strength and ultimate tensile strength of 1097 and 1548 MPa, respectively. The quantitative analysis of the strengthening mechanisms has revealed that (i) grain boundary strengthening provides the highest contribution at 293 K (ii) the increase in strength at 77 K is mostly associated with the lattice friction and interstitial solid solution hardening.

In this study, a new Nb30Mo30Hf20Co20 (at. %) refractory high entropy alloy (RHEA) comprising an intermetallic (Hf, Co)-rich B2 matrix and disordered (Nb, Mo)-rich bcc particles was introduced. The B2 matrix showed relatively high dislocation mobility, which, coupled with its continuity, allowed bridging and blunting of the cracks formed in the hard bcc particles during plastic deformation. The complex deformation behavior of the constitutive phases resulted in extraordinary high strain hardening capacity at 22–600 °C, exceeding single- and multi-phase RHEAs, together with the high strength. Our work endows new opportunities to develop high-performance alloys for high-temperature applications.

High-entropy alloys CoCrFeNiMnVKharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108 (Kharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108 = 0.25, 0.5, 0.75, 1) were prepared by vacuum arc melting. The structure and microhardness of the alloys have been studied in the cast state and after annealing at temperatures of 700–1100°C. It has been found that the alloys consist of the fcc (γ) solid solution and intermetallic sigma (σ) phase. The volume fraction of the σ phase increases with increasing vanadium content. As a result of annealing, phase transformations occur, including the precipitation of σ particles from the γ phase and, vice versa, the precipitation of γ particles from the σ phase. It has been shown that the change in the volume fraction of the σ phase upon annealing occurs due to the changes in the total content of σ-forming elements, chromium and vanadium, in accordance with the lever rule. With increasing temperature, the volume fraction of the σ phase varies nonmonotonically; first, it increases, then it decreases. The microhardness of the alloys correlates well with the change in the volume fraction of the σ phase. The mechanisms of the phase transformations and quantitative relationships between chemical and phase compositions of the alloys and their hardness are discussed.

A novel Fe 36 Mn 21 Cr 18 Ni 15 Al 10 high entropy alloy composed mostly of inexpensive components was produced by vacuum arc melting. In the as-cast condition the alloy had a dual-phase structure consisted of a bcc matrix and homogeneously distributed in the matrix cuboidal B2 ordered particles. The matrix phase was enriched with Fe and Cr, and B2 particles were composed mostly of Ni and Al. The alloy had high compression yield strength of 940–990 MPa at temperatures up to 400 °C. Further increase in temperature resulted in pronounced softening. The alloy could be compressed to 50% reduction without fracture at room temperature, but had a limited tensile ductility, especially at room temperature – elongation to fracture was only 2.5%. Annealing at 1200 °C did not change the structure of the alloy noticeably, but after annealing at 1000 °C an additional fcc phase appeared. The formation of the fcc phase caused pronounced softening. The comparison of the experimental results on the phase composition of the alloy with CALPHAD predictions showed reasonable agreement. Possibilities to increase stability of dual bcc-B2 structure were discussed.

Microstructural evolution during cold sheet rolling to 80% thickness strain and annealing at 600–1100 °C for 30 min of the CoCrFeNiMn high entropy alloy doped with 1 at.% of C and resulting mechanical properties of the alloy are reported. It is shown that in the initial homogenized (24 h at 1000 °C) condition the alloy has single fcc phase structure. Cold rolling is accompanied by dislocation slip, deformation twinning and formation of shear bands. Annealing at 600 °C after 80% cold rolling results only in partial recrystallization of cold-deformed structure, while an increase of the annealing temperature produces fully recrystallized microstructure. Comparison with the data on undoped CoCrFeNiMn alloy demonstrates that the addition of carbon pronouncedly increases dislocation activity simultaneously retarding deformation twinning during rolling and decreases the fraction of twin boundaries in the annealed condition. The effect of carbon can be attributed to an increase of stacking fault energy of the carbon-containing alloy. Cold rolling results in a substantial strengthening of the alloy; its ultimate tensile strength approaches 1500 MPa, but at the expense of low ductility. Good combination of strength and ductility can be obtained after annealing. For example, after annealing at 800 °C, the alloy has yield strength of 720 MPa, ultimate tensile strength of 980 MPa, uniform elongation of 21% and elongation to fracture of 37%. It is shown that the high strength of the annealed alloy can be attributed to (i) strong grain boundary strengthening; (ii) solid solution strengthening by carbon.

There is a strong belief that the CoCrFeNiMn alloy is an example of high entropy alloy with single disordered solid solution structure. Recently it was challenged by the demonstration of second phases in the alloy after low-temperature annealing; however, specific processing conditions (prolonged annealing or severe plastic deformation prior to annealing) were used to cause their formation. In this letter, we report the formation of Cr-rich second phases with tetragonal (sigma phase) and bcc structures in the CoCrFeNiMn alloy after room or cryogenic temperature rolling to 80% strain thickness and further annealing at 600–800 °C for 1 h.

The effect of plastic deformation under various conditions of the equiatomic CoCrFeNiMn alloy with single face-centered cubic phase structure was studied. The alloy was rolled at room and cryogenic temperatures, and uniaxially compressed at room temperature and temperatures of 600-1100°C with different height reductions. In addition, multiaxial forging at 900-1000°C was performed. Scanning and transmission electron microscopy, including EBSD analysis, was widely employed to characterize microstructure of the deformed alloy. At room and cryogenic temperatures, mechanical twinning and shear banding plays play dominant role in microstructure evolution. Extensive refinement of the microstructure occurs as the result of rolling with reduction of 80%. During deformation at 600-1100°C, discontinuous dynamic recrystallization takes place. The recrystallized grains size and their volume fraction increases with increase of deformation temperature. Multiaxial forging at 900-1000°C was used to produce fully recrystallized structure with average grain size of 6.7 μm. The alloy in the initial condition had low yield strength of 160 Mpa but remarkable tensile ductility of 68%. Rolling substantial increases yield strength to 1120-1290 MPa, but results in loss of ductility. After multiaxial forging the alloy has balanced combination of properties – yield strength of 280 MPa and elongation of 56%.

In present study, structure and mechanical properties of the AlxCrNbTiVZr (x = 0; 0.25; 0.5; 1) high-entropy alloys after arc melting and annealing at 1200 °C for 24 h are investigated. The CrNbTiVZr alloy is composed of body centered cubic (bcc) and C15 (face centered cubic) Laves phases while the AlxCrNbTiVZr (x = 0.25; 0.5; 1) alloys consist of bcc and two C14 (hexagonal close packed) Laves phases with different chemical compositions. Thermodynamic modeling predicts existence of two phases – bcc and C15 Laves phase and broadening of single bcc phase field due to Al addition. The density of the alloys decreases with the increase of Al content. The alloys are found to be extremely brittle at room temperature and 600 °C. The alloys have high strength at temperatures of 800–1000 °C. For example, yield strength at 800 °C increases from 440 MPa for the CrNbTiVZr alloy to 1250 MPa for the AlCrNbTiVZr alloy. The experimental phase composition of the AlxCrNbTiVZr alloys is compared with predicted equilibrium phases and the factors governing the transformation of C15 to C14 Laves phases due to Al addition to the CrNbTiVZr alloy analyzed. Specific properties of the alloys are compared with other high-entropy alloys and commercial Ni-based superalloys.

The effect of the carbon content (0-2 at.%) on the structure and mechanical properties at room and cryogenic temperatures of CoCrFeNiMn-based high entropy alloys with reduced Cr concentration was studied. The as-cast alloys were cold rolled to a thickness reduction of 80% and annealed at 800°C for 1 hour. As a result, a fully recrystallized microstructure with a grain size of 6.4 μm was produced in the carbon-free alloy. The recrystallized grain size was much smaller (1.5 μm in the alloy with 2.0 at.% of C) due to the pinning effect of the precipitated M23C6 carbides. The yield strength of the alloys increased with an increase in the carbon concentration from 313 MPa to 636 MPa, while the elongation to fracture slightly decreased from 56% to 43%, respectively, in the alloys with 0 and 2 at.% of C. A decrease in the test temperature to 77K resulted in a significant increase in both the strength and ductility of the alloys. The alloys had high values of impact toughness of 140 J/cm2 and 85 J/cm2, respectively, in the alloys with 0 and 2 at.% of C. A decrease in the testing temperature did not have a noticeable effect on the impact toughness.

The non-equiatomic Al7.5Co21.75Cr5.5Fe21.75Mn21.75Ni21.75 high entropy alloy was studied. After cold rolling and annealing at 700-900 °C, a dual-phase ultrafine-grained structure composed of the partially recrystallized fcc matrix and B2 particles was produced; complete recrystallization of the fcc phase was observed after annealing at 1000 °C. The B2 precipitates formation provided a significant strengthening of the alloy in comparison with the single-phase state. The alloy demonstrated an attractive combination of mechanical properties, presenting yield and ultimate tensile strengths of 900 MPa and 1200 MPa, respectively, along with a ductility of 22%.

The effect of carbon content (x = 0, 0.5, and 2.0 at.%) on the phase composition, microstructure, and mechanical properties of thermomechanically processed CoCrFeMnNi system high-entropy alloys was studied. The molar fraction of Cr in the program Co1Cr0.25Fe1Mn1Ni1 alloy was reduced to 0.25 compared with the equiatomic alloy to increase the solubility of carbon in the face-centred cubic solid solution. The as-cast alloys were cold rolled to 80% thickness reduction and then annealed at 600–1000 °C for 1 h. The addition of carbon to the CoCrFeMnNi alloys resulted in an increase in the temperature at which recrystallisation starts and in the precipitation of the Cr-rich M23C6 carbide particles. The volume fraction of the second phase increased with an increase in the carbon content and decreased with increasing annealing temperature. The formation of precipitates in the carbon-doped alloys strongly limited the matrix grain growth owing to the pinning effect. The thermomechanical processing of the alloys resulted in a reasonable work-hardening capacity and, therefore, in rather high ultimate tensile strength and ductility. Increasing the carbon content led to a pronounced increase in strength and a weak decrease in the ductility of the CoCrFeMnNi alloys. After annealing at 800 °C, the yield strength of the alloys increased from 313 MPa to 636 MPa, whereas the total elongation decreased from 56% to 43% with an increase in the carbon percentage from 0 at.% to 2.0 at.%. Decreasing the testing temperature to 77 K led to a simultaneous increase in both strength and ductility of the alloys. The alloy with 2.0 at.% of carbon demonstrated an attractive combination of mechanical properties at cryogenic temperature, presenting a yield and ultimate tensile strength of 786 MPa and 1218 MPa, respectively, along with a ductility of 52%. The effect of the carbon content on the microstructure development and strengthening mechanisms was discussed.

Specimens of a CoCrFeMnNi high entropy alloy with different content of nitrogen (0.5–2.0 at.%) were prepared by induction melting. Microstructure and tensile mechanical behavior of the alloys in the as-cast condition were analyzed. The alloys with a low amount of N (0.5–1.0 at.%) had a single fcc phase coarse-grained microstructure. An increase in the content of N to 2.0 at.% resulted in the precipitation of a small amount (∼1%) of Cr-rich M2N nitride particles. The yield strength of the alloys increased in proportion to the percentage of nitrogen by 117 MPa/at% N at 293 K or by 316 MPa/at% N at 77 K. The observed increase in strength was attributed to solid solution hardening. Ductility of the alloy with 0.5 or 1.0 at.% of N increased with a decrease in the testing temperature while ductility of the alloy with 2 at.% dropped from 67% at 293 K to 8% at 77 K. The plastic deformation of the alloys at both 77 K or 293 K was mostly associated with planar dislocation slip. No signs of mechanical twinning were found even at 77 K. Thermo-Calc software was used to produce a quasi-binary CoCrFeMnNi–N phase diagram for comparing the experimental and calculated results. The effect of N on strengthening and deformation mechanisms at different temperatures was discussed.

Bulk equiatomic AlCrFeTi medium entropy alloy was produced by mechanical alloying (MA) and spark plasma sintering (SPS). Powders with a homogeneous chemical composition composed of amorphous and bcc phases were obtained due to MA. Bulk, almost porosity free specimen of the alloy with the measured density of 5.53 ± 0.07 g × cm−3 was obtained after SPS. A mixture of a (Fe, Ti)-rich C14 Laves phase, a Cr-rich bcc phase, and an Al-rich L12 phase was found in the as-sintered specimens. The average size of grains/particles of the different phases was 115 ± 100 nm. Annealing at 1000°С resulted in some coarsening of the microstructure to the grain/particle size of 140 ± 120 nm and in an increase in the fraction of the bcc and L12 phases, yet the microstructure was found to be relatively stable. After SPS the AlCrFeTi alloy had a high microhardness of 1090 ± 120 HV, which decreased after annealing to 870 ± 70 HV. Compression tests have meanwhile revealed low ductility of the alloy after SPS – the alloy exhibited brittle fracture at room temperature and became ductile at only 800 °C. After annealing the alloy demonstrated reasonable ductility already at 700 °C. Also, the annealed alloy had a remarkable specific yield strength of 258 kPa × m3/kg at 700 °C. The experimental data on the phase composition of the alloy was compared with the Thermo-Calc predictions, and the relationships between the structure and properties of the alloy were discussed.

CoCrFeNiMn-0.9%C high entropy alloy (HEA) was produced by self-propagating high-temperature synthesis. The as-sintered alloy was cold rolled and annealed at 900°C to produce refined microstructure before friction stir welding (FSW). The structure of the annealed alloy consisted of a recrystallized face-centered cubic (fcc) matrix with a grain size of 9.2 µm and fine M23C6 carbides with 2% volume fraction. Good quality of butt joints after FSW was observed. It was established that an increase of the tool rotation speed from 600 to 1000 rpm increased the average grain size from 3.2±0.7 µm to 4.6±2.4 µm, respectively. The structure of the stir zone contained M23C6 carbides in all conditions. Hardness and strength of the joints decreased with the increase of tool rotation speed due to higher grain size, while ductility increased.

Characteristics of mechanical behavior during superplastic flow and associated microstructural evolution in the wrought AlCoCrCuFeNi high-entropy alloy were studied. The alloy had complex microstructure with fine grain/particle size of ≈2.1 μm. 4 different phases with volume fractions from 7% to 46% and different deformation characteristics were found in the alloy. Very high tensile elongations of up to 1240% were observed during deformation at temperatures of 800°C–1000°C and at strain rates of 10-4 s-1–10-1 s-1 despite presence of pronounced softening stage followed by steady state flow stage. Microstructure of the alloy after tensile testing was studied in detail. Phase transformations were analyzed employing thermodynamic modeling and their role in strain accommodation is discussed.

A series of Fe40Mn28Ni32 − xCrx (x = 4, 12, 18, 24 (at.%)) multicomponent alloys was prepared by vacuum arc melting. The Fe40Mn28Ni28Cr4, Fe40Mn28Ni20Cr12 and Fe40Mn28Ni14Cr18 alloys were ductile single phase fcc solid solutions. The Fe40Mn28Ni8Cr24 alloy had intermetallic sigma phase matrix and was extremely brittle after homogenization. The tensile properties of the Fe40Mn28Ni28Cr4, Fe40Mn28Ni20Cr12 and Fe40Mn28Ni14Cr18 solid solution alloys were examined in recrystallized condition with average grain size of ~10 μm. The yield strength increased from 210 MPa of the Fe40Mn28Ni28Cr4 alloy to 310 MPa of the Fe40Mn28Ni14Cr18 alloy. The elongation to fracture of the alloys decreased from 71% to 54%, respectively. Solid solution strengthening by the constitutive elements of the alloys was calculated using Labush approach. Strong solid solution strengthening by Cr was predicted. Gypen and Deruyttere approach was used to estimate solid solution strengthening of the Fe40Mn28Ni32 − xCrx alloys. Good correlation between predicted solid solution strengthening and the experimental yield strength values was found. Keywords: High entropy alloy, Alloy design, Mechanical properties, Solid solution strengthening

Microstructure evolution in high-entropy alloy CoCrFeNiMn during uniaxial compression to a height reduction of true strain of ≈1.4 in the temperature interval 600–1100 °C was studied. Although some differences was observed in the mechanical behavior of the alloy and the activation energy of deformation in warm (below 800 °C) and hot (above 800 °C) temperature intervals, microstructure evolution at all studied temperatures was found to be accompanied by discontinuous dynamic recrystallization (dDRX). During hot deformation recrystallization was primary associated with nucleation of new grains on the initial grain boundaries, while in the warm interval dDRX was mainly observed in shear bands. The volume fraction of the recrystallized structure was respectively 0.085 and 0.95 at 600 and 1000 °C and the recrystallized grain size was found to be 0.2 and 40.4 µm for 600 and 1100 °C, respectively.

Crystal structure, microstructure, microhardness and compression properties of CoCrFeMnNiVx (x = 0, 0.25, 0.5, 0.75, 1) high entropy alloys were examined. The alloys were produced by vacuum arc melting and studied in as-solidified and homogenized (annealing at 1000 °C for 24 h) conditions. The CoCrFeMnNi alloy was a single-phase fcc solid solution in both conditions. The CoCrFeMnNiV0.25 alloy had a single-phase fcc structure in as-solidified condition, but ∼2 vol.% fine particles of a sigma phase precipitated after annealing. The alloys with x = 0.5, 0.75 and 1.0 contained the sigma phase already in as-solidified condition. The sigma-phase volume fraction increased with an increase in the V content, and in CoCrFeMnNiV the sigma phase became the matrix phase. After homogenization treatment, the volume fraction of the sigma phase increased in all three alloys by ∼8% due to additional precipitation of fine particles inside the fcc phase. Phase composition and microstructure of the alloys was analyzed employing criteria for solid solution/intermetallic phase formation. The effect of alloys’ chemical composition on the volume fraction of constitutive phases was discussed. A modified valence electron concentration (VEC) criterion, which takes into account localized lattice distortions around V atoms, was suggested to correctly predict sigma phase formation in the CoCrFeNiMnVx alloys. It was demonstrated that the volume fraction of sigma phase was proportional to the cumulative Cr and V concentration. Mechanical properties of the alloys were greatly affected by the sigma phase. The CoCrFeMnNi and CoCrFeMnNiV0.25 alloys were soft and ductile, but an increase in the sigma-phase volume fraction resulted in continuous strengthening and loss of ductility.

The crystal structure, microstructure, microhardness and compression mechanical properties of AlxNbTiVZr (x = 0, 0.5, 1, 1.5) high entropy alloy were examined. In the as solidified conditions, the alloys consisted from bcc matrix and C14 Laves phase. After homogenisation, the NbTiVZr alloy was bcc solid solution, whereas in Al containing alloys, C14 Laves phase and Zr2Al particles were found in the bcc matrix. Volume fraction of second phase increased with Al concentration. Increase in Al content results in gradual decrease in density of the alloys from 6.49 g cm− 3 of the NbTiVZr to 5.55 g cm− 3 of the Al1.5NbTiVZr alloy. The microhardness of the alloys was higher in the alloys with higher Al content and was generally proportional to the volume fraction of second phase particles. The compression yield strength of the alloys was of 960–1320 MPa, and NbTiVZr alloy was stronger than Al containing alloys. The ductility of the alloys gradually decreased with increase in Al content. The factors determi...

High-pressure torsion (HPT) is applied to a face-centered cubic CoCrFeMnNi high-entropy alloy at 293 and 77 K. Processing by HPT at 293 K produced a nanostructure consisted of (sub)grains of ~50 nm after a rotation for 180°. The microstructure evolution is associated with intensive deformation-induced twinning, and substructure development resulted in a gradual microstructure refinement. Deformation at 77 K produces non-uniform structure composed of twinned and fragmented areas with higher dislocation density then after deformation at room temperature. The yield strength of the alloy increases with the angle of rotation at HPT at room temperature at the cost of reduced ductility. Cryogenic deformation results in higher strength in comparison with the room temperature HPT. The contribution of Hall–Petch hardening and substructure hardening in the strength of the alloy in different conditions is discussed.

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20–23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (

Microstructure and mechanical properties of equimolar composition alloys FeCrCoNi, FeCrCoNiV, FeCrCoNiMn and FeCrCoNiMnV were studied in as-solidified and annealed conditions. The FeCrCoNi and FeCrCoNiMn alloys were single-phase FCC solid-solutions in both conditions. However, the FeCrCoNiV and FeCrCoNiMnV alloys consisted of the intermetallic σ-phase matrix with a tetragonal crystal lattice and precipitates of a disordered FCC phase. The crystal structures of these alloys were found to be not affected by annealing. A number of criteria were considered to explain phase composition of the studied alloys. It was shown that poor compatibility of V with other alloying elements caused significant distortions of FCC solid solution and thus promoted formation of the σ phase. Tensile and compressive properties of these alloys together with their microhardness were determined. Significant strengthening accompanied by the loss of ductility due to formation of the σ phase was demonstrated in the V containing alloys. The characteristics of the microstructure formation in the studied alloys were discussed.

High entropy alloys (HEAs) with a face-centered cubic (fcc) structure are considered as promising structural materials, in particular due to their impressive ductility and toughness at cryogenic temperature; at the same time strength of these HEAs is often quite low. An addition of interstitial elements like carbon substantially increases the strength of the fcc HEAs at room temperature, however the effect of C on cryogenic properties has not been properly studied. Therefore in this work we examined cryogenic tensile behavior of the fcc high entropy alloys with different carbon content (0–2 at.%). The alloys had non-equiatomic proportions of principal elements, i.e. Co1Cr0.25Fe1Mn1Ni1. The lower Cr concentration in comparison with the equiatomic alloy led to the higher solubility of carbon confirmed by both ThermoCalc calculations and experimental results; only in the alloy with 2 at.% C a small (

Laser beam welding was used to produce butt joints from the CoCrFeNiMn-type high entropy alloy. The alloy in the initial condition had an fcc single-phase coarse-grained structure. Laser welding resulted in the M7C3-type carbides precipitation in the fcc matrix. The carbide particles precipitation resulted in a considerable increase in microhardness from 150 HV 0.5 for the as-sintered condition to 205 HV 0.5 in the fusion zone. Laser beam welding had a negligible effect on both static mechanical properties and fatigue behaviour of the alloy. The endurance limit of either type of specimens (i.e. with and without welding seam) was 200 MPa. Fracture of all specimens with the laser beam welded seams occurred in the base material area during both tensile and fatigue testing. Weak effect of welding on static/fatigue behaviour of the alloy can be attributed to the higher hardness of the fusion zone, resulting in strain localization in the base material area. An increase in load resulted in activation of secondary slip systems and formation of deformation twins in fatigue specimens.

A novel high entropy alloy based on the CoCrFeMnNi system with substantial amounts of Al and C was studied. After cold rolling and annealing at 973-1273 K a duplex ultra-fine grained structure composed of the recrystallized fcc grains and M23C6 and B2 particles was produced. Analysis of the coarsening behavior of grains and particles growth suggested that kinetics of both was controlled by volume diffusion. The apparent activation energy of structure coarsening during recrystallization was evaluated.

A cast AlCoCrCuFeNi high-entropy alloy was multiaxially forged at 950°C to produce a fine homogeneous mixture of grains/particles of four different phases with the average size of ~2.1 μm. The forged alloy exhibited unusual superplastic behavior accompanied by a pronounced softening stage, followed by a steady-state flow stage, during tensile deformation at temperatures of 800°C–1000°C and at strain rates of 10−4–10−1 s−1. Despite the softening stage, no noticeable strain localization was observed and a total elongation of up to 1240% was obtained. A detailed analysis of the phase composition and microstructure of the alloy before and after superplastic deformation was conducted, the strain rate and temperature dependences of the flow stress were determined at different stages of the superplastic deformation, and the relationships between the microstructure and properties were identified and discussed.

Extensive multistep forging at 950 °C was applied to the cast AlCuCrFeNiCo high-entropy alloy to transform the cast coarse dendritic structure into a fine equiaxed duplex structure consisting of the mixture of BCC and FCC phases, with the average grain/particle size of ∼1.5 ± 0.9 μm. Tensile properties of the alloy in the as-cast and forged conditions were determined in the temperature range of 20–1000 °C. The hot forged alloy was stronger and more ductile during testing at room temperature, than the as-cast alloy. The yield stress (YS), ultimate tensile strength (UTS), and tensile ductility ( δ ) of the forged condition were 1040 MPa, 1170 MPa, and 1%, respectively, against 790 MPa, 790 MPa and 0.2% for the as-cast condition. In both conditions, the alloy showed brittle to ductile transition (BDT), with a noticeable increase in the tensile ductility within a narrow temperature range. In the as-cast condition, this transition occurred between 700 and 800 °C, while in the forged condition, it was observed between 600 and 700 °C. With an increase in the testing temperature above the BDT, a continuous decrease in tensile flow stress and an increase in tensile ductility were observed. In the temperature range of 800–1000 °C, the forged alloy showed superplastic behavior. The tensile elongation was above 400% and reached 860% at 1000 °C.

Recently developed high entropy alloys (HEAs) have complex chemical composition, however, the relationship between the choice of constitutive elements and their concentration and phase composition and microstructure of HEAs is not fully understood at the moment, as well as the potential effect of annealing on the phase composition of HEAs. In this report, we present data on microstructure of the CoCrFeNiMn-based alloys with different V content in as-solidified state and after annealing at temperatures of 700–1000°C. It is shown that alloying with V results in formation of intermetallic sigma phase in as-solidified state. Annealing has pronounced effect on morphology and volume fraction of sigma phase. The temperature corresponding to maximum volume fraction of sigma phase is shown to be significantly dependent on V concentration.