Compositional dependence of phase formation and mechanical properties in three CoCrFeNi-(Mn/Al/Cu) high entropy alloys

Starting from three typical equiatomic CoCrFeNiMn, CoCrFeNiAl and CoCrFeNiCu high entropy alloys (HEAs), we systematically investigated the compositional dependence of phase formation and mechanical properties of 78 alloys by varying the atomic ratio of the constituent elements. It was found that the simple phase structures, including a single face-centered cubic (FCC) or body-centered cubic (BCC) phase, duplex FCC phases, duplex BCC phases, instead of intermetallics, could form within a broad compositional landscape in 68 out of the 78 alloys not limited to the equiatomic composition where the configurational mixing entropy is maximum. This fact indicates that it may be the nature of the constituent elements that leads to simple phase structure formation. With compositional variation, the microstructure and mechanical properties including hardness and tensile properties show corresponding changes. It was found that the hardness variation of samples within the same structure is smaller for the FCC than that of the BCC. Tensile results indicated that the tensile elongation of (CoCrFeMn)_(100−x)Ni_x (x = 0, 10 and 20) alloys increases with Ni addition due to the decreasing volume fraction of sigma phase. For the (CoCrFeAl)_(100−x)Ni_x (x = 27.3, 33.3, 38.5, 42.9 and 50) alloys, the yield strength decreases and tensile elongation increases with Ni addition due to decreasing volume fraction of BCC phase which is hard yet brittle. The present results are important to understand the phase formation and relationship between microstructure and mechanical properties in HEAs.     

Effect of nitrogen on microstructure and mechanical properties of CrMnFeVTi6 high entropy alloy

In order to evaluate interstitial strengthening effect on the properties of high entropy alloy (HEA), a nitrogen-doped CrMnFeVTi6 HEA was fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). XRD, SEM, TEM and FIB were used to characterize the phase composition and microstructure of this material. The sintered bulk HEA exhibits a microstructure comprising TiN_x, BCC, Laves and B2 phases. The HEA exhibits high yield strength (>2729 MPa) and hardness in lower temperature range of <380 °C. Quantitative calculations of the contributions from each strengthening mechanism in the BCC phase indicate that the interstitial strengthening by nitrogen is the dominant mechanism. Nitrogen additions in the BCC phase can produce a yield strength increase of ∼634 MPa/at.%, which is much higher than the strengthening effects of carbon or boron additions in other alloys. This demonstrates that adding nitrogen is a viable approach for enhancing the strength of HEAs.     

Effects of annealing at 800 and 1000 °C on phase precipitates and hardness of Al7Cr20FexNi73−x alloys

The effects of Fe content on the microstructure, phase constituents and microhardness of the as-cast, 800 °C- or 1000 °C-annealed Al₇Cr₂₀Fe_xNi_73?x (x=13?66) alloys were investigated. Not all these alloys are composed of the single FCC phase. The BCC and B2 phases are found. It is confirmed that the BCC phase in the Al₇Cr₂₀Fe₆₆Ni₇ alloy is transformed from the FCC phase at about 900 °C during cooling. While in the 800 °C-annealed Al₇Cr₂₀Fe₆₀Ni₁₃ alloy, the FCC phase is stable and the hardness decreases. After annealing at 1000 °C, for the precipitation of the B2 particles, the Al content in the FCC phase decreases, which results in decreasing of the alloy hardness. Moreover, after annealing at 800 °C, a small amount of Al-rich B2 particles precipitate at the phase boundary and some nanocrystal BCC phase precipitates in the FCC matrix, which increases the hardness of the Al₇Cr₂₀Fe_xNi_73?x (x=41?49) alloys. These results will help to the composition design and processing design of the Al?Cr?Fe?Ni based high-entropy alloys.     

Composition dependence of structure,physical and mechanical properties of FeCoNi(MnAl)x high entropy alloys

This paper has provided a systematic study of the crystal structure, physical and mechanical properties of FeCoNi(MnAl)_x (0 ≤ x ≤ 2) high entropy alloys. As x increases, the crystal structure changes from FCC to FCC + BCC then to BCC solid solution. High saturated magnetization, low coercivity together with high electric resistivity are found in this system, indicating their potential applications as soft magnets. Meanwhile, the strength and hardness increase monotonously as x increases. Optimal balance of physical and mechanical properties are obtained in this system.     

粉末冶金FeCoNiCu0.4Al0.4高熵合金的热处理组织与性能

采用机械合金化(MA)与放电等离子烧结(SPS)相结合的方法制备出FeCoNiCu0.4Al0.4高熵合金,研究不同热处理温度对合金显微组织与力学性能的影响规律。结果表明:机械合金化后,FeCoNiCu0.4Al0.4高熵合金形成了单相的FCC固溶体,经1100℃SPS烧结后的块体组织仍为单相FCC结构,其压缩屈服强度、塑性应变和显微硬度分别为1165.1 MPa、45.2%和356.9 HV。经过热处理后,合金组织中生成了新的BCC相,且BCC相的含量随热处理温度的升高先增多后减少,500、600和700℃热处理后BCC相的含量分别为7%、30%和21%(体积分数)。退火态FeCoNiCu0.4Al0.4高熵合金的屈服强度随热处理温度的升高先升高后降低。当BCC相含量增多时,材料的屈服强度和硬度相应地提高,而塑性却显著降低。     

Microstructure and tensile properties of nanocrystalline (FeNiCoCu)1−xTixAlx high entropy alloys processed by high pressure torsion

In this study, an equal-atomic FeNiCoCu high entropy alloy (HEA) and a Ti and Al added (FeNiCoCu)₈₆Ti₇Al₇ HEA were subjected for high pressure torsion (HPT) up to 10 rotations. Microstructure observation and mechanical properties test revealed that significant grain refinement as well as enhanced strength could be obtained in both HPT processed alloys. The HPT processed FeNiCoCu HEA alloy shows nanocrystalline structure consisting of FCC matrix (grain size ∼100 nm) and FeCo-riched BCC phase. The HPT processed (FeNiCoCu)₈₆Ti₇Al₇ HEA alloy shows nanocrystalline structured FCC matrix (mean grain size ∼50 nm) and refined NiCoTiAl-riched particles (mean particle size ∼0.71 μm). The ultimate tensile strength of the HPT processed FeNiCoCu and (FeNiCoCu)₈₆Ti₇Al₇ alloys are 1402 MPa and 1849 MPa, respectively. The microstructure evolution during HPT and strengthening mechanisms of the HPT processed specimens were discussed.     

Phase stability,mechanical property,and electronic structure of Mg–Li system

First principle calculation reveals that the HCP, BCC, and FCC Mg_100?xLi_x phases are energetically favorable with negative heats of formation, and are predicted to be the most stable structures at 0 K when 0 ≤ x < 18, 18 ≤ x < 73, and 73 ≤ x ≤ 100, respectively. Calculation also shows that for Mg–Li phases there is an almost linear variation of bulk moduli with composition, and crystal structure has only a little effect on bulk moduli. In addition, it is found that Mg₃Li and MgLi have phase sequences of BCC → HCP → FCC and BCC → FCC under high pressure, respectively, and that the anomalous mechanical instability of the HCP MgLi phase would be attributed to its weak bonding and step-like electronic structure of valence bands.     

Strengthening CoCrFeNi high-entropy alloy by Laves and boride phases

To strengthen the face-centered-cubic (FCC) type CoCrFeNi high-entropy alloy (HEA) by in-situ reinforced phase, (CoCrFeNi)_100−x(NbB₂)_x (x=0, 2, 4, 6, 8, at.%) alloys were prepared. Phase constitution, microstructure, tensile mechanical properties of the alloys were studied, and the mechanisms were discussed. Results show that the microstructure of all the reinforced alloys consists of the matrix FCC phase, Laves phase, and (Cr₃Fe)B_x phase. The eutectic structure and (Cr₃Fe)B_x phases are formed in the interdendritic region, and the eutectic structure is composed of Laves and FCC phases. When x increases from 0 to 8, i.e., with increase of Nb and B elements, the volume fraction of Laves and (Cr₃Fe)B_x phases increases gradually from 0 to 5.84% and 8.3%, respectively. Tensile testing results show that the ultimate strength of the alloys increases gradually from 409 MPa to 658 MPa, while the fracture strain decreases from 75% to 1.6%. Fracture analysis shows that the crack originates from the (Cr₃Fe)B_x phase. The CoCrFeNi alloys are mainly strengthened by the second phase (Laves phase and boride phase).

     

Effects of V Addition on Microstructural Evolution and Mechanical Properties of AlCrFe2Ni2 High-Entropy Alloys

The effects of vanadium addition on the microstructural evolution and mechanical properties of AlCrFe₂Ni₂ high-entropy alloy (HEA) were investigated. The results showed that the AlCrFe₂Ni₂V_0.2 HEA was composed of FCC phase, disordered BCC phase, and ordered BCC (B2) phase. With the increase in vanadium content, the formation of FCC phase was inhibited, and a transition from FCC phase to BCC phase occurred. The FCC phase disappeared completely when the value of x exceeds 0.4 in AlCrFe₂Ni₂V_x HEAs. Besides, the amplitude-modulated microstructure morphology transformed from a B2 phase matrix with dispersed BCC nano-phase into an alternating interconnected B2 and BCC phases. Vanadium element has the function of stabilizing BCC phase and B2 phase in AlCrFe₂Ni₂V_x alloys. The hardness of AlCrFe₂Ni₂V_x alloys increased from HV 332.4 to HV 590.7, while the yield strength increased from 765 to 1744.6 MPa with increasing vanadium content, which was mainly due to the decreasing content of FCC phase and the solid solution strengthening of vanadium element. At the same time, the compression ratio of the alloys decreased with the disappearance of the FCC phase. Among the alloys, the AlCrFe₂Ni₂V_0.2 alloy possessed the most excellent comprehensive mechanical properties with yield strength, fracture strength, and compressive ratio 1231.1, 2861.9 MPa, and 44.5%, respectively.     

Microstructure and mechanical properties of the TiZrNbMoTa refractory high-entropy alloy produced by mechanical alloying and spark plasma sintering

The equiatomic refractory high entropy alloy (HEA) TiZrNbMoTa was investigated. The alloyed powders with face-centered cubic (FCC) structured solid solution phase were prepared by mechanical alloying (MA) and then sintered by spark plasma sintering (SPS) at 1300, 1400, 1500, and 1600 °C. The microstructure and mechanical properties of the bulk alloy were investigated. The body-centered cubic (BCC) structured solid solution phase and the ZrO₂ phase precipitated from the FCC structured solid solution phase during cool-down from sintering. The highest compression fracture strength (3759 MPa) and fracture strain (12.1%) were obtained in the refractory HEA sintered at 1400 °C. The grain boundary strengthening, precipitation strengthening, solid solution strengthening, transformation-induced plastic (TRIP) effect, and toughening effect of the ZrO₂ phase are the important factors for the high strength and ducitily of the refractory HEA prepared in this study.     

Microstructure and mechanical properties of Laves phase-reinforced Fe–Zr–Cr alloys

Multi-phase Fe_90?xZr₁₀Cr_x alloys with 0 ≤ x ≤ 10 containing cubic C15 and hexagonal C14/C36 Laves phases have been prepared by copper mold casting. The microstructure of the samples consists of micrometer-sized Laves phase particles embedded in an ultrafine eutectic matrix of alternating lamellae of α-Fe and Laves phases. Room temperature compression tests of the binary alloy reveal a high strength of 1900 MPa combined with a plastic strain of about 9%. The addition of Cr improves the plastic strain up to 17% while reducing the strength only by about 70 MPa. The increased plastic deformation is linked to the specific structural features of the Laves phases. For the binary alloy, shearing and crack formation within the C15 phase limits plastic deformation. In contrast, in the samples containing Cr no shearing occurs within the C14/C36 phases and crack formation, which is observed at the particle/ferrite interface, is retarded.     

Phase formation in mechanically alloyed AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys

Alloying behavior and phase transformations in Al_xCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) multi-component high entropy alloys that are synthesized by mechanical alloying were studied. Two FCC phases along with a BCC phase were formed in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi, while a single B2 phase was observed in higher Al containing alloys Al_2.5CoCrCuFeNi and Al₅CoCrCuFeNi. DSC analysis indicates that BCC phase present in the alloys could be Fe–Cr type solid solution. A detailed analysis suggests that two melting peaks observed during DSC in lower Al containing alloys can be attributed to that of Cu–Ni and Fe–Ni FCC solid solutions. The BCC phase disappears in Al_0.45CoCrCuFeNi and AlCoCrCuFeNi at high temperatures during DSC. However, Al₅CoCrCuFeNi retains its B2 structure despite of heating in DSC. Further, phases present in these alloys retain nanocrystallinity even after exposure to high temperatures. A critical analysis is presented to illustrate that solid solution formation criteria proposed for high entropy alloys in the literature are unable to explain the phase formation in the present study of alloys. Besides, these criteria seem to be applicable to high entropy alloys only under very specific conditions.     

Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process.     

Novel microstructure and properties of multicomponent CoCrCuFeNiTix alloys

CoCrCuFeNiTi_x (x values in molar ratio, x = 0, 0.5, 0.8 and 1.0) were prepared by arc melting of the pure elements and suction casting under an argon atmosphere. Both CoCrCuFeNi and CoCrCuFeNiTi_0.5 alloys form a single FCC solid solution. While the alloys of CoCrCuFeNiTi_0.8 and CoCrCuFeNiTi are basically composed of primary FCC solid solution and eutectic mixture of FCC phase and Laves phase of Fe₂Ti type. The yield strength of the alloys increases from 230 MPa to 1272 MPa with the increase of Ti addition, among which the CoCrCuFeNiTi_0.5 alloy, especially, exhibits compressive strength of up to 1650 MPa together with extensive work hardening and large plastic strain limit of 22%. An interesting transition from paramagnetism to superparamagnetism has been discovered due to the appearance of nanoparticles embedded in the amorphous phase.     

FeCoNiCrCu0.5Alx高熵合金的结构和性能（英文）

研究Al含量和热处理对FeCoNiCrCu0.5Alx多主元高熵合金的相结构、硬度和电化学性能的影响规律。随着Al含量的增加,铸态合金的相结构由FCC相向BCC相转变。当x从0.5增加到1.5时,FeCoNiCrCu0.5Alx高熵合金的稳定结构由FCC结构向FCC+BCC双相结构转变。BCC相的硬度高于FCC相的,在氯离子及酸性介质中BCC相的耐腐蚀性均优于FCC相的。FeCoNiCrCu0.5Al1.0铸态合金具有高硬度和良好的抗腐蚀性能。     

High strength and good ductility of casting Al-Cu alloy modified by PrxOy and LaxOy

A modified Al-Cu alloy with high tensile strength and ductility of about 574.0 MPa and 10.4%, respectively, was obtained by adding multiple rare earth oxides (Pr_xO_y and La_xO_y) as modifier. Compared with the unmodified Al-Cu alloy, the tensile strength and ductility of the modified sample were increased by 24.3% and 42.5%, respectively. The improvement both in the strength and ductility may attribute to the finer crystal grains and dendrites, more homogeneously distributed θ′ phase precipitates and the intermetallic compounds formed at the crystal grain boundaries as well as in the space of the dendrites.     

Microstructure and spin polarization of quaternary Co2Cr1−xVxAl,Co2V1−xFexAl and Co2Cr1−xFexAl Heusler alloys

The microstructures, magnetic properties and spin polarization of quaternary Co₂Cr_1−xV_xAl, Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys were investigated. Phase separation into A2 and B2/L2₁ structure occurs in Co₂CrAl and Co₂Cr_0.6Fe_0.4Al, whereas Co₂FeAl exhibits a single-phase B2 structure. The ordered L2₁ structure becomes more stable with increasing vanadium concentration (x ⩾ 0.35). The saturation magnetization measured at 5 K for Co₂Cr_1−xV_xAl alloy changes from 1.4 to 2.0 μ_B when x increases from 0.0 to 0.5 and then becomes 1.4 μ_B for x = 1.0. This behavior can be attributed to the variation in the local magnetic moment of Co atoms. The saturation magnetization of Co₂V_1−xFe_xAl and Co₂Cr_1−xFe_xAl alloys increases with increasing Fe concentration. The spin polarization decreases from 0.62 to 0.56 with increasing x for Co₂Cr_1−xFe_xAl alloy. Also, the spin polarization decreases with increasing x for Co₂Fe_1−xV_xAl and Co₂Cr_1−xV_xAl alloys. Possible reasons for the reduced spin polarization in these alloys are discussed.     

Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys

We investigated the microstructure and mechanical properties of Ti₂₀Zr₂₀Hf₂₀Nb₂₀X₂₀ (X = V or Cr) high-entropy alloys (HEA), produced by induction melting and casting in inert atmosphere. The structures of these alloys were studied via X-ray diffractometry and scanning electron microscopy. Results show that Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ has mainly the body centered cubic (BCC) structure, whereas the BCC matrix of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ contains small amount of Cr₂Nb and Cr₂Hf intermetallic compounds. Ti₂₀Zr₂₀Hf₂₀Nb₂₀V₂₀ alloy shows the high strength and the homogeneous deformation under compression at room temperature. The strength and hardness of Ti₂₀Zr₂₀Hf₂₀Nb₂₀Cr₂₀ alloy are further enhanced by the Cr-containing Laves phases segregated during casting. The structural and mechanical properties remained almost unchanged after a short time (10 min) heat treatment at 573, 773, 973 and 1173 K indicating the resistance to working temperature peaks for these two alloys. Ab initio calculations predict ductile behavior for these and similar refractory HEAs. The theoretically calculated Young's modulus E is in good agreement with the experimental ones.     

添加Si/Ti对粉末冶金FiNiCrCo高熵合金物理力学和化学性能的影响（英文）

用粉末冶金法制备FiNiCrCoSi_x和FiNiCrCoTi_x高熵合金(HEAs)。其中x为0、0.3%、0.6%和0.9%(质量分数)。所有合金中均形成均匀分布的固溶体。与FiNiCrCo高熵合金相比,加入Si和Ti后,合金的密度和硬度提高,研究合金在不同载荷下的磨损率。结果表明,含0.3%Si和0.9%Ti的高熵合金磨损率最低。用XRD、SEM和EDX研究HEAs的相组或、晶粒尺寸和显微组织。通过H₂SO₄、HNO₃和HCl溶液浸泡实验,研究Si和Ti含量对烧结态FiNiCrCoSi_x和FiNiCrCoTi_x HEAs腐蚀行为和表面形貌的影响。在样品表面观察到不同尺寸的均匀腐蚀和局部点烛,且FiNiCrCoSi_0.3合金具有更小,更少的蚀坑,和优异的显微组织     

Microstructure evolution of Al0.6CoCrFeNi high entropy alloy powder prepared by high pressure gas atomization

The influence of cooling rate on the microstructure of Al_0.6CoCrFeNi high entropy alloy (HEA) powders was investigated. The spherical HEA powders (D₅₀≈78.65 μm) were prepared by high pressure gas atomization. The different cooling rates were achieved by adjusting the powder diameter. Based on the solidification model, the relationship between the cooling rate and the powder diameter was developed. The FCC phase gradually disappears as particle size decreases. Further analysis reveals that the phase structure gradually changes from FCC+BCC dual-phase to a single BCC phase with the increase of the cooling rate. The microstructure evolves from planar crystal to equiaxed grain with the cooling rate increasing from 3.19×10⁴ to 1.11×10⁶ K/s.