Microstructure and microhardness of high entropy alloys with Zn addition: AlCoFeNiZn and AlCoFeNiMoTiZn

High entropy alloys were designed from equiatomic multicomponent systems using powder metallurgy including mechanical alloying and sintering. The structure and morphology of the resulting alloys were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy techniques and their hardness values were also determined in the Vickers scale. The results indicate under the milling conditions used, the AlCoFeNiZn, AlCoFeNiMoTi and AlCoFeNiMoTiZn alloys crystallized forming BCC structures whereas the AlCoFeNi alloy presented two different phases, one with FCC structure and the other one with BCC. The synthesis method resulted in alloys with grain sizes in the nano scale having values between 4.1 and 9.4 nm on the powder form up to 40.1 nm after sintering phenomenon which lead to phase transformations which were more evident in the Mo-containing alloys. In addition, the AlCoFeNiZn and AlCoFeNiMoTiZn alloys did not show Zn traces after sintering as it was suggested by chemical analyses using energy dispersive spectroscopy, suggesting it is lost by evaporation during sintering process. Mo-containing systems exhibited the highest microhardness in both milled and sintered conditions.     

Alloying behaviour,thermal stability and phase evolution in quinary AlCoCrFeNi high entropy alloy

An equiatomic quinary AlCoCrFeNi high entropy alloy (HEA) has been synthesized by mechanical alloying. Milled powder after 30?h shows good chemical homogeneity and refined morphology with a mean particle size of ～4?μm. Solid solution phase with BCC crystal structure (a?=?2.89?±?0.02?Å) has been confirmed from XRD and transmission electron microscopy in the as-synthesized high entropy alloy. The milled alloy powder is not thermally stable. Differential scanning calorimetric (DSC) thermogram of 30?h milled powder exhibits the presence of a small peak at ～600?°C (873?K) with a thermal shift near the peak. This thermal shift indicates the diffusive type of phase transformation in this alloy while heating. The analysis of the in-situ heating X-ray diffraction patterns at various temperatures extends support to the diffusive nature of the phase transformation. Upon heat treatment, the disordered BCC solid solution phase partially transforms to Ni₃Al prototype L1₂ phase which precipitates at a lower temperature (350?°C (623?K)) as observed by in-situ XRD experiments. However, at high temperature annealing (575–800?°C (848–1073?K)) the evolution of a partially ordered BCC phase (B2) with lattice parameter (a?=?2.87?±?0.02?Å), and L1₂ phase (a?=?3.58?±?0.05?Å), along with tetragonal σ phase (a?=?8.8?Å and c?=?4.53?Å) are observed. Similar types of phases have also been identified after annealing and microwave sintering at 800?°C (1073?K) & 900?°C (1173?K) respectively. The transformation of ordered BCC phases along with two intermetallics such as L1₂ phase and σ phase suggests that the evolution of the high entropy phase in the milled condition leads to a combination of high entropy and medium entropy phases in the annealed condition.     

A novel refractory WMoVCrTa high-entropy alloy possessing fine combination of compressive stress-strain and high hardness properties

Refractory high-entropy alloys (HEAs) possess outstanding mechanical strength at room and high temperature but lack the room temperature ductility. A novel refractory equiatomic powder combination of WMoVCrTa was selected and verified for the feasibility of formation of solid solutions or else bulk-metallic glasses (BMGs) in the alloy based on the Guo et al.’s criteria and mismatch entropy criterion. The powder combination satisfies the above two criteria to crystallize in solid solution phases and inhibit the BMGs. Mechanical alloying characteristics of the powder mixture were determined. The particle size of the powder mixture decreased continuously during initial milling and later increased after 32 h of mechanical alloying. A homogeneous mixture was obtained after milling for 64 h. Crystallite sizes of the constituent elements in the powder mixture decreased continuously on progressive milling. A nanocrystalline powder was obtained by mechanical alloying. The powder milled for 64 h revealed a major BCC1, a minor BCC2 and small unknown phases. The lattice parameters of those BCC1 and BCC2 phases are 3.16 Å and 2.88 Å respectively. The alloy ingots were fabricated from the milled powder by vacuum arc melting technique followed by heat treatment. The ingot crystallizes in three phases such as a major BCC1, a minor BCC2 and a minor laves phase. The lattice parameters of these BCC1 and BCC2 phases are 3.05 Å and 2.85 Å respectively. Thereby, the BCC1 lattice of the milled powder contracts slightly after ingot fabrication. A fine combination of compressive stress and strain of 995 MPa and 6.2% respectively was achieved by the alloy at room temperature. Vickers hardness of the alloy was as high as 773 ± 20HV0.5. The density of the alloy was 11.52 g/cm³. The combination of excellent room temperature stress-strain and high hardness properties can enable the refractory HEA as a potential candidate for structural applications.     

Fabrication and properties of nanocrystalline Co0.5FeNiCrTi0.5 high entropy alloy by MA–SPS technique

Most of multi-component high entropy alloys were designed as equi-atomic or near equi-atomic and were mainly prepared by vacuum arc melting. The present paper reports synthesis of inequi-atomic Co_0.5FeNiCrTi_0.5 high entropy alloy by mechanical alloying and spark plasma sintering (MA–SPS). Alloying behavior, microstructure and properties of Co_0.5FeNiCrTi_0.5 alloy are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and instron testing system, respectively. Both BCC and FCC crystal structure phases are observed after MA, while a FCC phase and additional sigma phase are noticed after SPS. Moreover, numerous nanostructured phases are founded in the alloy after consolidated by SPS. The alloy with a density of 99.15% after SPS exhibits excellent comprehensive mechanical properties. The yield stress, compressive strength, compression ratio and Vickers hardness of the alloy are 2.65 GPa, 2.69 GPa, 10.0% and 846 HV, respectively. The fracture mechanism of this alloy is observed as cleavage fracture and intergranular fracture.     

Microstructure and mechanical properties of ultra-fine grained MoNbTaTiV refractory high-entropy alloy fabricated by spark plasma sintering

The MoNbTaTiV refractory high-entropy alloy(RHEA) with ultra-fine grains and homogeneous microstructure was successfully fabricated by mechanical alloying(MA) and spark plasma sintering(SPS).The microstructural evolutions,mechanical properties and strengthening mechanisms of the alloys were systematically investigated.The nanocrystalline mechanically alloyed powders with simple bodycentered cubic(BCC) phase were obtained after 40 h MA process.Afterward,the powders were sintered using SPS in the temperature range from 1500 ℃ to 1700 ℃.The bulk alloys were consisted of submicron scale BCC matrix and face-centered cubic(FCC) precipitation phases.The bulk alloy sintered at 1600℃ had an average grain size of 0.58 μm and an FCC precipitation phase of 0.18 μm,exhibiting outstanding micro-hardness of 542 HV,compressive yield strength of 2208 MPa,fracture strength of 3238 MPa and acceptable plastic strain of 24.9% at room temperature.The enhanced mechanical properties of the MoNbTaTiV RHEA fabricated by MA and SPS were mainly attributed to the grain boundary strengthening and the interstitial solid solution strengthening.It is expectable that the MA and SPS processes are the promising methods to synthesize ultra-fine grains and homogenous microstructural RHEA with excellent mechanical properties.     

Mechanical alloying and spark plasma sintering of CoCrFeNiMnAl high-entropy alloy

An equiatomic CoCrFeNiMnAl high-entropy alloy was synthesized by mechanical alloying, and alloying behaviors, microstructure and annealing behaviors were investigated. It was found that a solid solution with refined microstructure of 20 nm in grain size could be obtained after 30 h milling. As-milled powder transformed into a face-centered cubic phase above 500 °C. The as-milled powder was subsequently consolidated by spark plasma sintering at 800 °C, BCC phase and FCC phase coexisted in the consolidated HEA, which had excellent properties in Vickers hardness of 662 HV and compressive strength of 2142 MPa.     

Effect of aluminum on microstructural and magnetic properties of nanostructured (Fe85Ni15)97Al3 alloy produced via mechanical alloying

In the present study, mechanical alloying process was employed for preparation of the nanocrystalline (Fe₈₅Ni₁₅)₉₇Al₃ alloy through ball mill method. The structure, mechanical properties, and magnetic behavior of the alloy at various milling times of 0, 4, 16, 32 and 64 h were studied by X-ray analysis, vibrating sample magnetometer (VSM) and scanning electron microscopy (SEM) measurements. The bcc Fe(Ni) phase was successfully formed within 32 h ball-milling. It was found that an increment in the milling time leads to higher lattice parameter while it decreases the grain size from 172 to 16 nm. Also, the VSM test results indicated that by increasing the milling time to 32 h, the saturation magnetization (M_s) and coercivity (H_c) increased.     

Al-Permalloy (Ni71.25Fe23.75Al5) obtained by mechanical alloying. The influence of the processing parameters on structural,microstructural, thermal,and magnetic characteristics

A new alloy, permalloy alloyed with aluminum, has been obtained by mechanical alloying using elemental powders as raw materials. The new alloy was obtained in powder form by adding an amount of 5 wt% of Al to permalloy (75% Ni and 25% Fe, wt.%) resulting Al-Permalloy with Ni71.25Fe23.75Al5 composition. The alloy was obtained using different ball to powder ratio (BPR): 4:1, 8:1 and 17:1 and keeping the rest of the mechanical alloying/milling parameter constant. The BPR influences the time required for the alloy synthesis and alloy characteristics. The time required for alloy synthesis as FCC single phase varies from 4 h when using a BPR of 17:1 to 8 h when using a BPR of 4:1. A more compact cubic structure is obtained when using a BPR of 8:1. The particles are flattened for all BPRs used, but upon changing the BPRs particles shape is changing and become more or less flattened. Large particles have been obtained when using BPRs of 4:1 and 17:1 and finer particles when using a BPR of 8:1. Curie temperature of the Al-Permalloy is depending on synthesis conditions varying from 478 °C to 501 °C. The higher saturation magnetization has been found when using a BPR of 8:1. The powder characteristics evolution upon increasing the milling time for all three BPRs is discussed in the light of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, differential scanning calorimetry, particles size analyses and magnetic investigation.     

Influence of mechanical alloying on structural,thermal, and magnetic properties of Fe50Ni10Co10Ti10B20 high entropy soft magnetic alloy

A multicomponent with functional Fe₅₀Ni₁₀Co₁₀Ti₁₀B₂₀ (at.%) high entropy soft magnetic alloy powders were produced from the elemental powders by mechanical alloying (MA). The MA processes were carried out under argon gas atmosphere at a speed of 250 rpm, carrying milling and rest in every 20-min period to prevent the mixture from overheating. Scanning electron microscopy and energy-dispersive X-ray spectroscopy, X-ray diffraction, differential thermal analysis, and vibrating sample magnetometer analysis were utilized to characterize various powdered samples with respect to MA time (0–50 h). The results show that in the first 2.5 h of MA, the mixture of crystalline phases transformed into a nanocrystalline supersaturated α-Fe solid solution phase. With prolonging milling time, the amorphous phase appeared after 20 h of MA. In the final stage of MA (50 h), the saturation magnetization (Ms) and the coercivity (Hc) were 89.7 emu/g and 32.5 Oe, respectively, proposing the alloy as a very good high entropy soft magnet in nature.

     

The effect of alloying with hafnium on the thermal stability of chromium bronze after severe plastic deformation

The effect of alloying with 0.9 wt% hafnium on the thermal stability of the Cu-0.7%Cr alloy after severe plastic deformation by high pressure torsion has been studied by microhardness and resistivity measurements, optical microscopy, and transmission electron microscopy. The data obtained were used to construct the curves of the temperature dependences of microhardness and resistivity for the alloy samples in a temperature range of 50–550 °C and the curves of the microhardness distribution over the sample diameter. The effect of preliminary heat treatment on the stability of the structure formed upon deformation is considered. It is shown that alloying with hafnium decreases the grain size from 250 to 120 nm, increases the strength, and elevates the temperature of the beginning of alloy softening upon heating. In addition, the introduction of hafnium to the alloy results in the occurrence of aging upon heating, and the strength reaches maximum (3177 MPa) at a temperature of 450 °C.     

Synthesis of a lightweight Ti–10Al–5Mg (wt.%) alloy by mechanical alloying followed by compaction and sintering

A lightweight Ti–10Al–5Mg (wt.%) alloy was synthesized by mechanical alloying followed by cold compaction and sintering. A metastable hcp α-Ti(Al,Mg) solid solution was obtained after 5 h of milling. The c/a ratio of the hcp structure was found to increase with milling time. The particle size distribution of the mixture becomes broader and the dp50 value decreases as the milling time increases. The structure of the 900 and 1,100 °C sintered samples consist of an α-Ti(Al) solid solution matrix with α₂-Ti₃Al and MgO precipitates. Values of 11 and 188 GPa were obtained for the Hardness and Young’s modulus of 1,100 °C sintered sample, respectively, confirming the strength improvement of the Ti-based alloy.     

Synthesis,microstructural investigation and compaction behavior of Al0.3CrFeNiCo0.3Si0.4 nanocrystalline high entropy alloy

This research article focused on developing Al_0.3CrFeNiCo_0.3Si_0.4 nanocrystalline high-entropy alloy (HEA) by mechanical alloying. The initial powders mixture was ball milled for 1 hr (HEA-1 h), 5 hr (HEA-5 h), 15 hr (HEA-15 h) and 25 hr (HEA-25 h) at ball to powder mass ratio (BPR) of 15:1 and a speed of 300 rpm. The mechanical alloying time was varied from 1 to 25 hr to ensure the nanocrystalline nature and attainment of steady state in HEA powders. The structure of the developed HEAs was characterized by means of X-ray diffraction (XRD), Laser particle size analyzer (LPSA), and various electron microscopes (TEM and FEGSEM with EDS). HEA-25hr sample exhibited the crystallite size of 13.8 nm with lattice strain of 0.67% obtained from XRD which matched the result by TEM. The formation of a solid solution (SS) with a uniform elemental dispersion was observed with a major BCC stable structure and a minor FCC structure in HEA-25 h sample. The HEA-25 h sample revealed an average particle size of 386.2 nm (89.8% peak intensity) with Polydispersity Index (PDI) value of 0.364 which confirmed the uniform distribution of particles over a narrow range of particle size. The synthesized powders were consolidated to green compacts with a loading rate of 1 mm/min at different compaction pressures (25, 50, 75, 100, 150, 200, 400, 600, 800, 1000, and 1100 MPa) for examining the powder particles packing. Several compaction models (both linear and non-linear) were discussed to establish the density-pressure relationship of developed HEAs. The results revealed that the milling time has influenced the relative density. HEA-1 h sample was exhibited the relative density of 0.76 whereas HEA-25 h sample was produced the relative density of 0.6 indicating more strength and more amount of strain hardening occurs in MAed HEA-25 sample in addition to the entropy effect for the same composition.     

Structural,thermal and vibrational characterization of mechanical alloyed In50Te50

Structural, chemical, thermal and vibrational studies of InTe produced by mechanical alloying were carried out using X-ray diffraction, energy dispersive spectroscopy, transmission electron microscopy, differential scanning calorimetry and Raman spectroscopy. The main crystalline phases formed after 2 h of milling were the tetragonal (TlSe-type) and high-pressure cubic (NaCl-type) InTe phases. Minority cubic In₂O₃ phase was also nucleated. Mean crystallite size and phase fraction variations with the increase of the milling time were obtained from Rietveld analyses. The distribution of the particle size (centered at about 39 nm) was obtained by images of transmission electron microscopy. Differential scanning calorimetry measurements showed no evidence of nonreacted materials and Raman measurements showed peaks that can be associated with the InTe (TlSe-type) phase and/or the existence of molecular structures of Te (chains/rings). The structural stability of the nanocrystaline phases of the In₅₀Te₅₀ sample milled for 15 h was attested by systematical X-ray diffraction measurements performed up to one year after its production.     

Structural and microstructural characterizations of nanocrystalline hydroxyapatite synthesized by mechanical alloying

Single phase nanocrystalline hydroxyapatite (HAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO₃ and CaHPO₄ powders in open air at room temperature, for the first time, within 2 h of milling. Nanocrystalline hexagonal single crystals are obtained by sintering of 2 h milled sample at 500 °C. Structural and microstructural properties of as-milled and sintered powders are revealed from both the X-ray line profile analysis and transmission electron microscopy. Shape and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Particle size of HAp powder remains almost invariant up to 10 h of milling and there is no significant growth of nanocrystalline HAp particles after sintering at 500 °C for 3 h. Changes in lattice volume and some primary bond lengths of as-milled and sintered are critically measured, which indicate that lattice imperfections introduced into the HAp lattice during ball milling have been reduced partially after sintering the powder at elevated temperatures. We could achieve ~ 96.7% of theoretical density of HAp within 3 h by sintering the pellet of nanocrystalline powder at a lower temperature of 1000 °C. Vickers microhardness (VHN) of the uni-axially pressed (6.86 MPa) pellet of nanocrystalline HAp is 4.5 GPa at 100 gm load which is close to the VHN of bulk HAp sintered at higher temperature. The strain-hardening index (n) of the sintered pellet is found to be > 2, indicating a further increase in microhardness value at higher load.     

Nanocrystalline Al/Al12(Fe,V)3Si alloy prepared by mechanical alloying: Synthesis and thermodynamic analysis

Al–Al₁₂(Fe,V)₃Si nanocrystalline alloy was fabricated by mechanical alloying (MA) of Al–11.6Fe–1.3V–2.3Si (wt.%) powder mixture followed by a suitable subsequent annealing process. Structural changes of powder particles during the MA were investigated by X-ray diffraction (XRD). Microstructure of powder particles were characterized using scanning electron microscopy (SEM). Differential scanning calorimeter (DSC) was used to study thermal behavior of the as-milled product. A thermodynamic analysis of the process was performed using the extended Miedema model. This analysis showed that in the Al–11.6Fe–1.3V–2.3Si powder mixture, the thermodynamic driving force for solid solution formation is greater than that for amorphous phase formation. XRD results showed that no intermetallic phase is formed by MA alone. Microstructure of the powder after 60 h of MA consisted of a nanostructured Al-based solid solution, with a crystallite size of 19 nm. After annealing of the as-milled powder at 550 °C for 30 min, the Al₁₂(Fe,V)₃Si intermetallic phase precipitated in the Al matrix. The final alloy obtained by MA and subsequent annealing had a crystallite size of 49 nm and showed a high microhardness value of 249 HV which is higher than that reported for similar alloy obtained by melt spinning and subsequent milling.     

Fabrication and characterization of Ni–W solid solution alloys via mechanical alloying and pressureless sintering

Ni–W solid solution alloy powders and sintered compacts were fabricated via mechanically alloying and pressureless sintering of powder batches with the compositions of Ni–xW (x = 20, 30, 40 wt.%). The crystallite size of the powders were between 11 nm and 17 nm, which decreased with increasing W contents, where a microhardness value of 6.88 GPa for the Ni powders MM’d for 48 h increased to 9.37 GPa for the Ni40W powders MA’d for 48 h. The MM’d/MA’d powders were sintered at 1300 °C for 1 h under Ar and H₂ gas flowing conditions. X-ray diffraction (XRD) patterns of the sintered Ni, Ni20W and Ni30W samples revealed the presence of only the solid solution phase, whereas the presence of elemental W and Ni₄W intermetallic phase were observed in the XRD patterns of the sintered Ni40W sample. Among all sintered samples, the sintered Ni sample had the highest relative density value of 96.36% and the lowest microhardness value of 1.59 GPa. The relative densities of the sintered samples decreased with increasing W amounts, contrary to microhardness values which increased with W contents. Moreover, microstructural characterizations via scanning electron microscope and electron backscatter diffraction, room temperature compression tests and sliding wear experiments were conducted in order to reveal the effects of W contents on the properties of the sintered Ni–W alloys.     

Mechanical alloying of biocompatible Co–28Cr–6Mo alloy

We report on an alternative route for the synthesis of crystalline Co–28Cr–6Mo alloy, which could be used for surgical implants. Co, Cr and Mo elemental powders, mixed in an adequate weight relation according to ISO Standard 58342-4 (ISO, 1996), were used for the mechanical alloying (MA) of nano-structured Co-alloy. The process was carried out at room temperature in a shaker mixer mill using hardened steel balls and vials as milling media, with a 1:8 ball:powder weight ratio. Crystalline structure characterization of milled powders was carried out by X-ray diffraction in order to analyze the phase transformations as a function of milling time. The aim of this work was to evaluate the alloying mechanism involved in the mechanical alloying of Co–28Cr–6Mo alloy. The evolution of the phase transformations with milling time is reported for each mixture. Results showed that the resultant alloy is a Co-alpha solid solution, successfully obtained by mechanical alloying after a total of 10 h of milling time: first Cr and Mo are mechanically prealloyed for 7 h, and then Co is mixed in for 3 h. In addition, different methods of premixing were studied. The particle size of the powders is reduced with increasing milling time, reaching about 5 μm at 10 h; a longer time promotes the formation of aggregates. The morphology and crystal structure of milled powders as a function of milling time were analyzed by scanning electron microscopy and XR diffraction.     

Influence of solution and aging on the microstructures and mechanical properties of complex deformed WE93 alloy

Optical microscopy, scanning electron microscopy, transmission electron microscopy, hardness testing, and mechanical property testing were performed to study the influence of solution (T4) and aging (T6) on the microstructures and mechanical properties of the WE93 magnesium alloy. A reasonable solution treatment and an aging regime were developed, and the fracture features of the alloy in different states were analyzed. Results show that complex deformation produces microstructures that are largely characterized by deformation-precipitated Mg–Y phase (Mg₂₄Y₅), in addition to those with cubic Mg–Y and Mg–MM phases, which are both undissolved after homogenization. The optimum solution treatment condition for the alloy is a holding temperature of 490 °C for 2 h. After solution treatment, the precipitated Mg–Y phase re-dissolves and grain size grows to a limited extent, which may be attributed primarily to the pinning effect of the Mg–MM phase on the grain boundary. The reasonable aging regime was maintained at 225 °C for 40 h. After the solution and aging treatments, the ultimate tensile strength of the alloy at room temperature reaches 375 MPa but the elongation is only 3%. As indicted by the fracture behavior of the alloy, the secondary cracks of the extruded alloy and the solid-solution alloy occur mainly in the Mg–MM phase with few transcrystalline fractures. After peak aging, however, transcrystalline cracks appear on the grains at room and high temperatures. Under a multi-strengthening mechanism, the mutual coordinating effect may depend primarily on service temperature.     

Synthesis,structure, and mechanical response of Cr0.26Fe0.24Al0.5 and Cr0.15Fe0.14Al0.30Cu0.13Si0.28 nanocrystallite entropy alloys

The present research work has concentrated to synthesize nanocrystalline (NC) Cr_0.26Fe_0.24Al_0.5 (medium entropy alloy, 3E-MEA) and Cr_0.15Fe_0.14Al_0.30Cu_0.13Si_0.28 (high-entropy alloy, 5E-HEA) non-equiatomic (equal weight fraction) alloys through mechanical alloying (MA); which studied the influence of entropy effect on structural properties, microstructural characterization, and mechanical behaviour. Further, the same non-equiatomic ratio of two coarse grain alloys (CGAs) was manufactured by conventional powder metallurgy (PM) route (blending method, 3E-CGA, 5E-CGA) for comparison. All synthesized powders were hot-pressed (HPed) at 723 k for 30 min subsequently mechanical properties in terms of compressive stress-strain and hardness were examined. The samples of as-milled powders, HPed, and fractured were investigated using X-ray diffraction (XRD) and advanced electron microscopes. The HPed sample of 3E-MEA of Cr_0.26Fe_0.24Al_0.5 produced 94% BCC and 6% FCC crystal structures due to more dissolution of Al atoms in the stronger bonding atoms of Cr-Fe lattice. Whereas 5E-HEA of Cr_0.15Fe_0.14Al_0.30Cu_0.13Si_0.28 sample has exhibited 72.1% FCC phase and 27.9% BCC phase due to balance between the dissolution of FCC elements (Al, Cu, Si) and BCC elements (Cr, Fe). Further, 3E-MEA and 5E-HEA have exhibited the ultimate compressive strength (UCS) of 1278 ± 6.75 MPa and 2060 ± 2.8 MPa respectively whereas the corresponding conventionally blended alloys produced 268 ± 5 MPa and 615 ± 3 MPa for 3E-CGA and 6E-CGA respectively. Vicker’s hardness strength (VHS) of 5E-HEA of Cr_0.15Fe_0.14Al_0.30Cu_0.13Si_0.28 has exhibited 68% more when compared to 3E-MEA of Cr_0.26Fe_0.24Al_0.5, 3.26 times higher compared to blended alloys. Further, several strengthening mechanisms on the mechanical behaviour of MEA and HEA were investigated in which dislocation strengthening mechanisms followed by solid solution strengthening mechanisms have influenced more as compared to grain boundary strengthening mechanisms.     

Influence of Al content on thermal stability of nanocrystalline AlxCoCrFeNi high entropy alloys at low and intermediate temperatures

Thermal stability of mechanically alloyed nanocrystalline Al_xCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high entropy alloys (HEAs) has been investigated for the low and intermediate temperature range of 673–1073 K. Single phase FCC structure is observed in the as milled CoCrFeNi. A mixture of FCC and BCC phases is exhibited by × = 0.3, 0.6 and 1, alloys where the volume fraction of BCC increases with increasing Al content. Phase evolution in heat-treated Al_xCoCrFeNi HEAs proceeds via increasing BCC fraction at 673 K, followed by subsequent reduction at elevated temperatures. For each alloy, the major phase observed in as milled condition and it is retained even after prolonged exposure at the 1073 K. Al favors the formation of the BCC phase due to its high affinity to form ordered B2 structures with constituent elements Co, Fe and Ni. Thermal exposure of Al_xCoCrFeNi HEAs also leads to the formation of Cr₇C₃, owing to the higher negative free energy of carbide formation for Cr among other constituents. Transmission electron microscopy (TEM) investigations substantiated that nanostructure of milled powder is maintained even after the heat treatment. Grain growth factor for quinary HEAs is relatively lower than quaternary CoCrFeNi owing to their slower rates of diffusion.