Fe50Mn30Co10Cr10 High-entropy Alloy Research Articles

Effects of microstructure on the evolution of dynamic damage of Fe50Mn30Co10Cr10 high entropy alloy

The Fe50Mn30Co10Cr10 high-entropy alloy (HEA) was subjected to plate impact loading with a single-stage light gas gun and the Doppler pin system (DPS) was used to measure the velocity of free surface particles of the sample during loading. The effects of microstructure on the evolution of spallation of the Fe50Mn30Co10Cr10 HEA were investigated by means of electron back scattered diffraction (EBSD), optical microscope (OM) and X-ray diffraction (XRD) techniques. The results showed that the voids are neither nucleated at the martensite (M)/matrix interface nor nucleated in the M aggregated area (M area) with high impact resistance as predicted by impact dynamics, instead, voids nucleated inside the matrix phase with low impact resistance. The reason is that the lattice expansion during M transformation leads to the formation of compressive residual stress at the M/matrix interface as well as M area and tensile residual stress in the matrix, which promoted the preferential nucleation of voids in the matrix. Void nucleation in the matrix was not random: the tri-junction of general high angle grain boundaries (HAGBs) and the grain boundaries of adjacent grains with a large difference in Taylor Factor (TF) value were prone to stress concentration under tensile loading, which were the priority position for void nucleation. The void nucleation was inhibited by twin boundary (TB) due to its stable structure and low energy. However, the intersections of HAGBs and termination TB are the possible position for void nucleation due to its high interface energy. The compressive residual stress at the M/matrix interface and inside of M area has a closure effect on the microcrack, which caused the microcrack avoided the M area and propagated in the matrix and finally stopped at the M/matrix interface.

Simultaneously enhanced strength and strain hardening capacity in FeMnCoCr high-entropy alloy via harmonic structure design

Harmonic structure (HS), consisting of coarse-grained (CG) areas uniformly embedded in three dimensional continuously connected ultrafine-grained (UFG) areas, is considered as an effective microstructural design strategy to achieve enhanced strength and ductility in metallic materials. In the present study, HS designed non-equiatomic FeMnCoCr high-entropy alloy samples with tunable shell fractions (ranging from ~16% to ~70%) were successfully prepared via controlled mechanical milling and subsequent sintering. Microstructure observations suggested that the shell region was composed of fully recrystallized UFGs with a mean grain size below 1 µm. Tensile test revealed that the HS designed samples exhibited simultaneously enhanced strength and strain hardening capability than those of the homogeneous structured counterpart. Particularly, the ultimate tensile strength and uniform elongation of the sample with a shell fraction of ~70% were 1228MPa and 12.4%, respectively, demonstrating superior strength-ductility synergy. The underlying mechanisms responsible for the enhanced mechanical properties were discussed.

Awakening the metastability of an interstitial high entropy alloy via severe deformation

The metastability of a typical non-equiatomic FeMnCoCr high-entropy alloy (HEA) system was demonstrated to be dormant upon co-doping with C and N interstitials, i.e., displacive γ-ε phase transformation disappears during tensile deformation. Interestingly, we found in the present study that the displacive phase transformation prevails again in the C-N co-doped HEA upon severe deformation via cold-rolling. The generation of ε-phase lamellae is found to be motivated by strain localization which facilitates the formation of shear bands. Such highly concentrated strains significantly intensify the local elastic fields to neighboring γ matrix and finally eke out the driving force to overcome the interstitials-enhanced energy barrier of ε-martensite transformation. The results suggest that the suppressed phase metastability is awakened, which inspires efforts to tune the phase metastability via modifying the strain state during alloy processing and gives new insights into the development of HEAs with desirable phase stability and mechanical properties.

Effects of the phase content on dynamic damage evolution in Fe50Mn30Co10Cr10 high entropy alloy

The Fe50Mn30Co10Cr10 high entropy alloy with different martensite phase content were dynamic loaded at 190 m/s and 420 m/s impact velocity respectively by one-stage light gas gun. The effect of phase content on dynamic damage evolution in this alloy was firstly studied. Results showed that after hot-rolling and quenching (HRQ), the mass percentage of martensite in the HRQ sample was 28.3 wt%, and the size of martensite aggregated area (M area) was large (∼49 μm). The sample cold-rolled and quenched (CRQ) after HRQ treatment had more martensite (46 wt%), and M areas in the CRQ sample were uniformly distributed and relatively smaller (∼10 μm) than that of the HRQ sample. Voids nucleated in the matrix where the impact impedance was lower than the martensite. The compressive residual stress in the M area and tensile residual stress in the matrix induced by the martensitic transformation inhibited and promoted nucleation of voids. The uniform distributed tensile residual stress areas with smaller size in the CRQ sample was more, which led to the more nucleation sites, higher nucleation density, and higher initial damage rate. Meanwhile, voids were difficult to coalesce due to larger space between voids and limit of growth by the small-sized M areas, thus the damage evolution rate of the CRQ sample was relatively lower. Therefore, the spall strength of the CRQ sample (1.89 GPa) was smaller than that of the HRQ sample (2.21 GPa) under the impact velocity of 190 m/s. Microcrack propagation “avoided” the small-sized and uniformly distributed M area, so the turning path and direction deviation of microcrack were shorter and smaller. Then the microcracks in the CRQ sample tended to coalesced into a large-sized crack, which lead to a higher crack propagation rate and lower spall strength of the CRQ sample (1.98 GPa) under higher impact velocity of 420 m/s. Furthermore, the microcrack in the CRQ sample could propagate along a “coalescence path” that formed at the interface of the adjacent M areas, which may have further increased the rate of crack propagation, and resulted in the lower spall strength of the CRQ samples.

Thermodynamics-kinetics of twinning/martensitic transformation in Fe50Mn30Co10Cr10 high-entropy alloy during adiabatic shearing

Thermodynamics-kinetics of twinning/martensitic transformation during adiabatic shearing in the Fe50Mn30Co10Cr10 high-entropy alloy was firstly quantitatively analyzed. The results indicated that twins and no martensite ε phase were observed within the adiabatic shear band. The critical stress for martensitic transformation was greater than the applied shear stress after 35.2 µs (72.4 µs whole deformation time), and the driving force for ε→γ phase transformation was increasing with increased deformation time, so the original ε phase decreased and finally disappeared. Before 38.3 µs, the critical stress for twinning was lower than the applied shear stress, and twinning occurred. Therefore, twins were observed.

Adiabatic Shear Susceptibility of Fe50Mn30Co10Cr10 High-Entropy Alloy

Dynamic loading of the Fe50Mn30Co10Cr10 high-entropy alloy (HEA) was carried out using a split Hopkinson pressure bar at room temperature. The effects of microstructures and loading strain rates on the adiabatic shear susceptibility of an Fe50Mn30Co10Cr10 HEA were evaluated for the first time by comparing metallographic observation, the stress collapse time, the critical values of strain corresponding to the adiabatic shearing initiation, and the adiabatic shearing formation energy per unit volume. The results show that the adiabatic shear susceptibility increases with the increase of loading strain rate. The smaller the grain size and the more hexagonal close-packed (hcp) phase of the Fe50Mn30Co10Cr10 HEA, the greater the strain hardening effect and the stronger the localized deformation resistance under the same dynamic loading conditions, and the lower the adiabatic shear susceptibility.

The effect of strain rate on mechanical properties and microstructure of a metastable FeMnCoCr high entropy alloy

The relationship between mechanical properties and microstructures of a metastable dual-phase high entropy alloy Fe50Mn30Co10Cr10 under uniaxial tensile testing at different strain rates (10-3 s-1~103 s-1) has been studied systematically. As the strain rate increases, yield strength, ultimate tensile strength and uniform elongation decrease first and then increase. Namely, when the strain rate is 10-3 s-1, yield strength and ultimate tensile strength are 280 MPa and 720 MPa, respectively, with the uniform elongation of 64.2%. When the strain rate is increased to 1 s-1, yield strength and ultimate tensile strength are 300 MPa and 672 MPa, respectively, and uniform elongation decreases to 49.2%. When the strain rate reaches 103 s-1, yield strength and ultimate tensile strength increase to 380 MPa and 810 MPa, respectively, while uniform elongation is elevated to 67%. As dynamic deformation is affected by the adiabatic heating, the stacking fault energy of the alloy is increased by ~13 mJ m-2 at a strain rate of 103 s-1 compared with that in the quasi-static condition. Under quasi-static loading, martensitic transformation is the dominant deformation mechanism. Under dynamic loading, when the strain is low the deformation induced phase transformation dominates, whereas as the loading proceeds mechanical twinning becomes the dominant deformation mode. At the same time, the adiabatic temperature rise under dynamic tests also causes a reverse transformation from ε-martensite to austenite. Accordingly, the release of internal stress and the formation of soft and ductile austenite jointly contribute to the elevated uniform elongation of the material. Both mechanical twinning and martensitic reverse transformation promote the microstructure to be dynamically refined, so that the alloy shows the good plasticity while maintaining the high ultimate tensile strength at dynamic strain rates.

The enhanced static recrystallization kinetics of a non-equiatomic high entropy alloy through the reverse transformation of strain induced martensite

The present work deals with the static recrystallization behavior of a cold rolled non-equiatomic Fe50Mn30Co10Cr10 high entropy alloy. Towards this end, the experimented alloy was cold rolled to a total thickness reduction of 60% and was then isothermally annealed in the temperatures range of 750–850 °C for various periods of time followed by air coolling. It was found that the reverse transformation of the strain induced martensite phase during continues heating results in formation of lath-shaped reversed regions holding high density of dislocation tangles and stacking faults. During isothermal annealing, these highly stored energy regions serve as preferred nucleation sites of statically recrystallized grains. The recrystallization kinetics has been analyzed in terms of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. Interestingly, the Avrami exponent ranged from 1.62 to 1.84 and deviates from the ideal values that was discussed considering the various capability of thermal martensite, strain induced martensite and face center cubic phase for static recrystallization. The activation energy of recrystallization is found to be much lower than that of bulk diffusion of each constituent. This is attributed to the progressive substructure development which provides the high track diffusion through the low angle boundaries.

Joint contribution of transformation and twinning to the high strength-ductility combination of a FeMnCoCr high entropy alloy at cryogenic temperatures

The microstructure-mechanical property relationships of a non-equiatomic FeMnCoCr high entropy alloy (HEA), which shows a single face-centered cubic (fcc) structure in the undeformed state, have been systematically investigated at room and cryogenic temperatures. Both strength and ductility increase significantly when reducing the probing temperature from 293 K to 77 K. During tensile deformation at 293 K, dislocation slip and mechanical twinning prevail. At 173 K deformation-driven athermal transformation from the fcc phase to the hexagonal close-packed (hcp) martensite is the dominant mechanism while mechanical twinning occurs in grains with high Schmid factors. At 77 K athermal martensitic transformation continues to prevail in addition to dislocation slip and twinning. The reduction in the mean free path for dislocation slip through the fine martensite bundles and deformation twins leads to the further increased strength. The joint activation of transformation and twinning under cryogenic conditions is attributed to the decreased stacking fault energy and the enhanced flow stress of the fcc matrix with decreasing temperature. These mechanisms lead to an elevated strain hardening capacity and an enhanced strength-ductility combination. The temperature-dependent synergy effects of martensite formation, twinning and dislocation plasticity originate from the metastability alloy design concept. This is realized by relaxing the equiatomic HEA constraints towards reduced Ni and increased Mn contents, enabling a non-equiatomic material with low stacking fault energy. These insights are important for designing strong and ductile Ni-saving alloys for cryogenic applications.

Microstructure and tribological performance of Fe50Mn30Co10Cr10 high-entropy alloy based self-lubricating composites

The high-entropy alloy based self-lubricating composites were successfully synthesised by mechanical alloying (MA) and spark plasma sintering (SPS). In this paper, Fe50Mn30Co10Cr10 was chosen as matrix and graphene nanoplatelets were chosen as solid lubricant dispersed phase. The results showed that the matrix maintains a single FCC structure before or after the solid lubricants added. The tribological properties of the composites were better than the matrix. In room temperature, the coefficients of friction (COF) of composites were 62% lower than the Fe50Mn30Co10Cr10. In the meantime, the wear loss can hardly be detected by the device. The results are discussed in terms of mechanisms controlling the lubrication process in composites with matrix.

Microstructure and mechanical properties of graphene reinforced Fe50Mn30Co10Cr10 high-entropy alloy composites synthesized by MA and SPS

In present work, the Fe50Mn30Co10Cr10 metal-matrix composites using graphene as a reinforced phase were synthesized via mechanical alloying (MA) and spark plasma sintering. Alloying behavior, phase evolution and mechanical properties were investigated. The results show that during the MA, a single FCC structure was formed within 50 h ball milling. After sintered, a single face-centered cubic structure was obtained. The addition of graphene increased the room temperature yield strength to the maximum value of 903 MPa, which was 48.7% higher than the Fe50Mn30Co10Cr10 base, accompanied by a decrease in elongation. The results are discussed in terms of mechanisms controlling the strengthening process in composites with matrix.