FeCoCrNiMn High-entropy Alloy Research Articles

Fe-based metallic glass reinforced FeCoCrNiMn high entropy alloy through selective laser melting

Improving the comprehensive mechanical properties of metallic materials has attracted great attentions. Here, FeCoCrNiMn high entropy alloy composite with combined high strength and toughness was fabricated by selective laser melting. The intriguing finding is that two different high entropy phases in addition to amorphous phase were detected at the high-entropy alloy and metallic glass interfacial region. With increasing the fraction of Fe-based metallic glass, the composite exhibited enhanced strength above 1000 MPa and excellent fracture toughness over 100 MPa m1/2, demonstrating potential engineering applications. The above scenario is fundamentally understood according to the high mixing entropy and sluggish effects, as well as the little atomic size difference that facilitates the generation of new high entropy phase. The combined high strength and toughness of the composite is well understood in terms of the dispersion strengthening.

Corrosion resistance mechanism of the passive films formed on as-cast FeCoCrNiMn high-entropy alloy

The present work investigates the corrosion resistance of as-cast FeCoCrNiMn high-entropy alloy in borate buffer solution. Compared with 304 and 316L stainless steels steel, as-cast FeCoCrNiMn high-entropy alloy exhibited excellent passivation ability. This should be attributed to homogenization of alloying elements. Highly corrosion-resistant elements in as-cast FeCoCrNiMn alloys, for example, Cr, Co and Ni, could promote the formation of protective passive film in borate buffer solution. Moreover, combined effect of high mixing entropy and sluggish diffusion also could improve the corrosion resistance. The formation mechanism of the passive films on the surface of as-cast FeCoCrNiMn alloys satisfied the point defect model.

Short-range ordering induced serrated flow in a carbon contained FeCoCrNiMn high entropy alloy

Phase stability of high entropy alloys depends significantly on the compositional constituents. Here we report the joint effects of interstitial carbon and compositional inhomogeneity on the mechanical response, particularly the serrated flow, of a carbon doped FeCoCrMnNi high entropy alloy. Detailed microstructural characterizations show that compositional inhomogeneity, including heterogeneous short-range ordering structure, is formed in carbon doped FeCoCrNiMn HEA. The serrated flow on the stress-strain curves of carbon doped alloy at room temperature is attributed to the dynamic strain aging, which is presented based on the complex interactions between carbon, stacking faults and short-range ordering at the atomic scale.

Carbide precipitation strengthening in fine-grained carbon-doped FeCoCrNiMn high entropy alloy

Nano-sized carbides in carbon-doped high entropy alloys (HEAs) can induce remarkable precipitation strengthening, but the precipitation behavior and its effects on mechanical properties has not been clarified. In this study, a heavy-carbon doped equiatomic FeCoCrNiMn HEA with 1.3 at.% C addition (FeCoCrNiMn-1.3C) was prepared to provide an insight into the effect of carbides precipitation on the strength of the HEA. The results showed that FeCoCrNiMn-1.3C HEA exhibited a single FCC structure after homogenization treatment at 1220 °C for 6 h and the subsequent severe cold rolling (SCR) with 95% reduction. It was found that nano-sized M23C6 carbides can precipitate and exist stably in the SCRed FeCoCrNiMn-1.3C HEA during annealing at 700–1000 °C. These nano-sized M23C6 precipitations can substantially inhibit the recrystallized grain growth of FCC matrix in the SCRed FeCoCrNiMn HEA during annealing. The average grain size of the SCRed HEA can be maintained at 2.4 μm even after annealing at 1000 °C for 1 h due to the M23C6 precipitations inhibition on grain growth. The yield strength of ∼1040 MPa with total elongation of ∼12% has been achieved in FeCoCrNiMn-1.3C HEA after annealing at 700 °C for 1 h. Grain refinement and precipitation strengthening are the two major strengthening mechanisms of the present annealed FeCoCrNiMn-1.3C HEAs.

Effect of Annealing Temperature on Microstructure and Mechanical Properties of a Severe Cold-Rolled FeCoCrNiMn High-Entropy Alloy

An ultrafine-structured FeCoCrNiMn high-entropy alloy (HEA) was produced by severe cold rolling (SCR) with a 95 pct reduction, and the effect of annealing temperature over a wide range from 573 K to 1373 K (300 °C to 1100 °C) on the microstructure and mechanical properties of this SCRed HEA was systematically investigated. The results show that the microstructure of the single-phase FCC HEA after SCR is greatly refined and mainly comprises ultrafine crystalline, high-density dislocation cell and networks, which substantially improve the hardness and strength. The SCRed HEA starts to recrystallize at an annealing temperature of 773 K (500 °C) and is fully recrystallized at 973 K (700 °C) after 1 hours. The σ phases with a tetragonal structure form upon annealing at 773 K (500 °C), leading to a further enhancement in hardness and strength for the annealed HEA compared to the SCRed HEA. The average grain size remains below 500 nm when the SCRed HEA is annealed at 923 K (650 °C) for 1 hours, indicating that the ultrafine-structured HEA after SCR and subsequent annealing treatment possesses excellent microstructural thermal stability. The SCRed HEAs annealed at 923 K and 973 K (650 °C and 700 °C) exhibit excellent high strength–ductility combinations due to the formation of the fully recrystallized ultrafine-grained microstructure. The strain-rate sensitivity parameter (m value) with different grain sizes, from ultrafine to coarse, was evaluated by strain-rate jump tests. The m value of the HEA increases monotonically with increasing grain size and annealing temperature. Moreover, high-temperature tensile tests demonstrated that the annealed HEAs exhibit superplastic deformation behavior at 973 K and 1073 K (700 °C and 800 °C), and the formation of a Cr-rich phase is found during high-temperature deformation. It is believed that the plastic deformation facilitates the phase decomposition of fine-grained HEAs and grain boundary sliding is the key mechanism of high-temperature deformation.

Effects of carbon on the microstructures and mechanical properties of FeCoCrNiMn high entropy alloys

FeCoCrNiMn based high entropy alloys (HEAs) doped with different carbon contents were prepared by vacuum arc melting. The effects of carbon on phase composition and mechanical properties of the HEAs were investigated. After cold rolling and annealing processes, fine M23C6 carbides with an average size of ~230 nm were observed in the HEAs doped with carbon. The volume fraction of M23C6 reaches 8.9% in the alloy doped with 1.0 at% carbon. The precipitation of M23C6 particles exerts a strong pinning force for the migration of grain boundary, which results in the formation of refined grains. Compared to those of the HEA without carbon, the yield strength and ultimate tensile strength of the HEA doped with 1.0 at% carbon increase from 378.7 MPa to 643.8 MPa, and from 700.1 MPa to 887.6 MPa, respectively, while the uniform elongation decreases from 43.5% to 24.9%. The strengthening mechanisms for the increase in the mechanical properties by carbon doping are attributed to the refinement of the grains, lattice friction stress and the precipitation of the M23C6 particles.

Microstructure and mechanical properties of FeCoCrNiMnAlx high-entropy alloys prepared by mechanical alloying and hot-pressed sintering

To improve the strength and hardness, Al-containing FeCoCrNiMn high-entropy alloys (HEAs) were fabricated by mechanical alloying (MA) and hot-pressed sintering. The effects of Al concentration on the microstructure and mechanical properties of the alloys were examined. It was found that the Al-containing alloys consisted of the matrix fcc or fcc + bcc duplex solid solution phases and a small quantity of M7C3 + M23C6 (where M = Cr, Mn, Fe) carbides and Al2O3 phases. A high Al concentration induced bcc precipitates in the alloys, and with the increase of Al concentration, the crystalline structure of the matrix solid solution phase changed from fcc to bcc. The addition of Al obviously strengthened the alloys, especially the alloys that contained fcc + bcc duplex solid solution phases. For example, the yield strength, compressive strength and hardness of alloy FeCoCrNiMnAl0.7 reached as high as 2230 MPa, 2552 MPa and 622 HV, respectively. The high strength/hardness was attributed mainly to the finer grains and the bcc precipitation.

Microstructure and mechanical properties of FeCoCrNiMn high-entropy alloy produced by mechanical alloying and vacuum hot pressing sintering

Abstract FeCoCrNiMn high-entropy alloys were produced by mechanical alloying (MA) and vacuum hot pressing sintering (VHPS). Results showed that the nano-crystalline alloy powders were obtained by MA and the corresponding phase structures were composed of FCC matrices and low amounts of BCC and amorphous phases. After VHPS, the BCC phases almost disappeared, simultaneously with the precipitation of σ phases and M 23 C 6 carbides. An increase of sintering temperature resulted in grain growth of the precipitated phases. As the sintering temperature was increased from 700 to 1000 °C, the strain-to-failure of the alloys rose from 4.4% to 38.2%, whereas the yield strength decreased from 1682 to 774 MPa. The bulk FeCoCrNiMn HEAs, consolidated by VHPS at 800 °C and 900 °C for 1 h, showed relatively good combination of strength and ductility.

High temperature deformation behavior of carbon-containing FeCoCrNiMn high entropy alloy

High temperature compressive deformation behaviors of a carbon-containing FeCoCrNiMn high entropy alloy (HEA) was investigated at temperatures ranging from 700 °C to 1000 °C, and strain rates from 0.001s−1 to 1s−1. The microstructure mainly consists of fcc solid solution. The data obtained from the flow curves was employed to develop the constitutive equation, and the apparent activation energy (Q) was determined to be 385.43 kJ/mol. The size of the dynamically recrystallized grains decreased with the increasing value of Zener-Hollomon (Z) parameter. At high Z condition, dislocation cross slip can act as the main deformation mechanism. At intermediate Z condition, dislocation cross slip accompanied by discontinuous dynamic recrystallization (dDRX) becomes the main deformation mechanisms. At low Z condition, continuous dynamic recrystallization (cDRX) becomes the dominant softening mechanism. Nano carbides can be precipitated at intermediate Z and low Z condition, which prevent grain from growing up and promote the nucleation of DRX.

Effect of Ti and C additions on the microstructure and mechanical properties of the FeCoCrNiMn high-entropy alloy

To improve the yield strength of an FeCoCrNiMn high-entropy alloy (HEA), elemental Ti and C were doped into the alloy. Subsequently, an in situ synthesized carbides particle-strengthened HEA matrix composite was prepared by mechanical alloying (MA), followed by a vacuum hot-pressing sintering (VHPS) method. The TiC nanoparticles were distributed along the grain boundaries. The microstructure of the alloy contained a face-centered cubic (FCC) solid solution as the matrix phase and small amounts of TiC, M23C6 and M7C3 (where M = Cr, Mn, Fe) carbides. The addition of elemental Ti and C significantly improved the room-temperature compressive yield strength of the FeCoCrNiMn HEA from 774 MPa to 1445 MPa (an 86.7% increase), accompanied by a decrease in the compressive strength and plasticity. Grain boundary strengthening and precipitation strengthening are the main strengthening mechanisms of the alloy doping with elemental Ti and C.

Controllable fabrication of a carbide-containing FeCoCrNiMn high-entropy alloy: microstructure and mechanical properties

ABSTRACTPrevious studies have reported that high carbon contents in FeCoCrNiMn high-entropy alloys lead to carbides precipitating from the alloys. Typically, carbides are used to improve the strength of alloys but also lead to decreased ductility. However, the strength and ductility of alloys can be improved when carbides shape, size and distribution are carefully controlled. Therefore, a carbide-containing FeCoCrNiMn alloy with 2 at.-% carbon was prepared by arc melting, and its microstructure and mechanical properties were further tuned by cold rolling with subsequent annealing treatment. The yield strength and uniform elongation of the resultant alloy were excellent, reaching 581 MPa and 25%, respectively, due to the additive combination of various strengthening mechanisms, such as solid-solution hardening, grain-boundary hardening and precipitation hardening.

Effect of Carbon Addition on the Cast and Rolled Microstructures of FeCoCrNiMn High Entropy Alloys

In this study, the effect of carbon addition the cast and rolled microstructures of Cantor alloy type FeCoCrNiMn high entropy alloys. Both as-cast FeCoCrNiMn and FeCoCrNiMnC0.1 alloys have dendritic microstructure. Small particles, which may be associated carbon addition exist in the dendrite arms in FeCoCrNiMnC0.1 alloy. After homogenization treatment at 1327K for 24 hrs., dendritic structure was completely eliminated after annealing. Dendritic structure was converted to the structure with elongated grains, especially for carbon added FeCoCrNiMnC0.1. The development of elongated grains is associated with the direction of the primary arms in the dendritic structure. Carbides are segregated at the grain boundaries in FeCoCrNiMnC0.1 alloy. It also appears that growth of grains is impeded by the segregation of carbides. It is apparent that the grain boundary precipitates are Cr-rich. Both the strength and ductility of FeCoCrNiMnC0.1 increased over FeCoCrNiMn with the addition of 0.1 wt. % carbon. The increase of ductility in FeCoCrNiMnC0.1 may be caused by the rapid hardening in FeCoCrNiMnC0.1 due to dislocation-solute interaction.

Incipient plasticity and dislocation nucleation of FeCoCrNiMn high-entropy alloy

Instrumented nanoindentation was conducted on a FeCoCrMnNi high-entropy alloy with a single face-centered cubic structure to characterize the nature of incipient plasticity. Experiments were carried out over loading rates of 25–2500μNs−1 and at temperatures ranging from 22 to 150°C. The maximum shear stress required to initiate plasticity was found to be within 1/15 to 1/10 of the shear modulus and relatively insensitive to grain orientation. However, it was strongly dependent upon the temperature, indicating a thermally activated process. Using a statistical model developed previously, both the activation volume and activation energy were evaluated and further compared with existing dislocation nucleation models. A mechanism consisting of a heterogeneous dislocation nucleation process with vacancy-like defects (∼3 atoms) as the rate-limiting nuclei appeared to be dominant.