查看更多>>摘要：Eutectic high-entropy alloys,composed of FCC/B2 phases with a narrow solidification interval and excel-lent fluidity,have become a new hotspot in additive manufacturing.Nevertheless,their microstructures exhibit significant sensitivity to processing parameters,feedstocks,and composition,ultimately limiting the alloys'engineering applications.Here,a hypereutectic Ala.7CoCrFeNi2.4 alloy with a low cracking sus-ceptibility index was designed by Thermo-Calc calculation and fabricated by laser powder bed fusion.Results show that the as-printed Al0.7CoCrFeNi2.4 alloy manifests a stable cellular structure,coupled with appreciable ultimate tensile strength(≥1200 MPa)and ductility(≥20％)over a wide range of process-ing parameters.After aging at 800 ℃ for 30 min,outstanding strength(1500 MPa)and elongation(15％)were obtained.Considerable mechanical properties after aging stem from a triple strengthening mecha-nism,i.e.,L12 nanoprecipitates and rod-shaped B2 particles within the FCC matrix,along with Cr-enriched spherical nanoparticles in the B2 phase.Meanwhile,hierarchical structure,i.e.,FCC dominated matrix,a discontinuous B2 phase,a precipitation-free zone in the B2 phase,and a K-S orientation relationship be-tween FCC and B2,facilitate to maintain excellent plasticity.These results guide designing HEAs by AM with controllable microstructures and outstanding mechanical properties for industrial applications.
查看更多>>摘要：A high-performance Ti-Ni-B alloy with good tensile properties and reduced mechanical anisotropy was developed by promoting the columnar to equiaxed transition(CET)of prior-β grains and modifying α-laths to equiaxed grains.Both Ni and B contributed to the refinement of columnar prior-β grains during the L→β phase transformation by generating constitutional undercooling.Compared with Ni,B had a su-perior capability of generating constitutional undercooling,which not only replaced a significant amount of Ni with a minor addition to reduce the formation of brittle eutectoid,but also reacted with Ti to form TiB to promote heterogeneous nucleation of α-Ti grains during the β→α phase transformation.Together with the restricted growth of α-laths induced by the refinement of prior-β grains,a fully equiaxed α-Ti structure was obtained.The competition between the negative effect of brittle eutectoid and the positive role of α-lath to equiaxed grain transition on the ductility of as-printed Ti-Ni-B alloys was fundamen-tally governed by the morphology of eutectoid and technically dependent on the Ni-B content.When the addition was 1.2Ni-0.06B(wt.％)or less,the positive effect of α-lath on equiaxed grain transition can effectively mitigate the issue of reduced ductility caused by brittle eutectoid.In contrast,at 1.8Ni-0.09B or greater,the negative effect of eutectoid dominated.New insights into microstructural design obtained through the aforementioned approach were presented and discussed.
查看更多>>摘要：Two-dimensional(2D)material MXene is a research hotspot in lubricating materials because of its unique layered structure,which provides weak interlayer interaction and easy shear ability.Herein,the liquid metal nanodroplets intercalated MXene were successfully prepared via pulsed laser treatment and self-assembly method.First,zero-dimensional(0D)nano gallium-based liquid metal(GLM)was synthesized by pulsed laser irradiation of bulk Ga80In20 in acetone.Then,the GLM nanodroplets were loaded onto 2D MXene nanosheets via the effect of electrostatic adsorption to prepare MXene@GLM composite material.The as-obtained GLM not only widens the interlayer distance between MXene nanosheets,making it more susceptible to interlayer shear,but also enhances its lipophilicity.The friction test results showed that the MXene@GLM has the best lubrication performance with 1.0 wt％additive.The coefficient of friction(COF)of base oil PAO-6 can decrease from 0.79 to 0.097,the wear volume is reduced by 90.3％,and the maximum load sustained reaches 950 N.The good tribological properties are mainly owing to the synergistic lubrication of GLM and MXene,which can form a continuous and firm tribofilm on the friction contact surface and avoid the direct contact of the friction pair.
查看更多>>摘要：Eutectic high-entropy alloys(EHEAs),as a classification of high-entropy alloys(HEAs),have received worldwide interest due to superior fluidity and attractive properties.However,other than the FCC+B2 EHEA system,most other reported EHEA systems show inherent brittleness during tensile loading at room temperature,which limits their advanced engineering application.In this work,a novel spheroidization+recrystallization(SR)strategy for synergistic strengthening and plasticizing of the brit-tle CoCrFeNi2(V6B3Si)o.149 was proposed.The superior combination of strength and ductility was achieved by tailoring spherical M3B2+recrystallized FCC duplex phases.Based on this strategy,the yield strength and elongation were improved from 565±15 MPa and 2.3％±03％to 841±24-1278±20 MPa and 14.7％±0.5％-22.5％±1.2％,with an increase of 48％-126％and 539％-878％,respectively.The synergistic increment in the strength and ductility of SR-FCC+M3B2 EHEAs exceeds all reported fur-ther strengthened FCC+B2 EHEAs.Meanwhile,such simple thermo-mechanical processing is suitable for large-scale industrial production.The high strength results from the back stress provided by the dual heterogeneity of FCC grain sizes and soft FCC/hard M3B2.The good ductility is attributed to the dislo-cation movement path released by spheroidized M3B2 and a more uniform stress distribution caused by the recrystallized FCC.This work provides a new strategy for synergistic strengthening and plasticizing of the brittle EHEAs to meet industrial reliability requirements.
查看更多>>摘要：Achieving a superior strength-ductility combination for fcc single-phase high entropy alloys(HEAs)is challenging.The present work investigates the in-situ synthesis of Fe49.5Mn30CoioCr10Co.5 interstitial solute-strengthened HEA containing 0.5 wt.％Nb(hereafter referred to as iHEA-Nb)using laser melt-ing deposition(LMD),aiming at simultaneously activating multiple strengthening mechanisms.The effect of Nb addition on the microstructure evolution,mechanical properties,strengthening and deformation mechanisms of the as-deposited iHEA-Nb samples was comprehensively evaluated.Multiple levels of het-erogeneity were observed in the LMD-deposited microstructure,including different grain sizes,cellular subgrain structures,various carbide precipitates,as well as elemental segregation.The incorporation of Nb atoms with a large radius leads to lattice distortion,reduces the average grain size,and increases the types and fractions of carbides,aiding in promoting solid solution strengthening,grain boundary strengthening,and precipitation strengthening.Tensile test results show that the Nb addition significantly increases the yield strength and ultimate tensile strength of the iHEA to 1140 and 1450 MPa,respectively,while maintaining the elongation over 30％.Deformation twins were generated in the tensile deformed samples,contributing to the occurrence of twinning-induced plasticity.This outstanding combination of strength and ductility exceeds that for most additively manufactured HEAs reported to date,demon-strating that the present in situ alloying strategy could provide significant advantages for developing and tailoring microstructures and balancing the mechanical properties of HEAs while avoiding conventional complex thermomechanical treatments.In addition,single-crystal micropillar compression tests revealed that although the twining activity is reduced by the Nb addition to the iHEA,the micromechanical prop-erties of grains with different orientations were significantly enhanced.
查看更多>>摘要：Although the rate-dependence of metals has been widely researched,the deformation mechanism under small-scale impact conditions lacked exploration and in-depth understanding.Using quasi-static nanoin-dentation(strain rate,SR,＜1 s-1)and high strain-rate nano-impact(SR＞103 s-1)with a pyramidal Berkovich tip,this study investigates the influence of SR on the deformation response of an aluminium single crystal(110).The underlying microstructural variance was analyzed using on-axis TKD and TEM.The results show that the impact deformation involves great elastic recovery and different substructural characteristics.In contrast to the uniform sub-grain substructure with medium and high-angle grain boundaries formed during quasi-static indentation,the substructure formed under impact has a more heterogeneous nature including microbands near the surface and sub-grains underneath with dominant low-angle grain boundaries.The significant change in substructure for the impact deformation comes from suppressed thermally activated dislocation motion,leading to the conversion of dislocation glide from wave-like(quasi-static)to planar regime(impacting),and the insufficient rearrangement of geomet-rically necessary dislocations(GNDs).The heterogeneous microstructure develops due to the competition between high strain-rate-induced planar-slip and strain gradient-induced GND rearrangement,as well as the uneven distribution of SR and strain gradient.Moreover,the underlying incipient mechanism of the microband is proposed,in which successive primary dislocations are nucleated at the surface and glide perpendicular to flanks to pile up.Finally,the influence of SR on indentation size effects is discussed.
查看更多>>摘要：Refractory high-entropy alloys(RHEAs)are considered to be a promising candidate for elevated temper-ature applications.Nanocrystalline(NC)RHEAs are supposed to exhibit many different high-temperature mechanical behaviors in comparison with their coarse-grained(CG)and ultrafine-grained(UFG)counter-parts.However,the creep behaviors of NC RHEAs,which must be well evaluated for high-temperature applications,are largely unknown because it is difficult to produce bulk quantities of NC RHEAs for creep tests.In the present work,an equiatomic bulk NC VNbMoTaW RHEA with an average grain size of 67±17 nm was synthesized by mechanical alloying(MA)and the subsequent high-pressure/high-temperature sintering.The creep tests were performed on bulk specimens by compression at high tem-peratures(973 and 1073 K)under different stresses(70-1100 MPa).The creep resistance of the bulk NC VNbMoTaW is slightly lower than that of the bulk CG VNbMoTaW,but much higher than that of previ-ously reported CG and UFG HEAs.The derived activation volume,stress exponent,and activation energy of bulk NC VNbMoTaW indicate that the creep deformation is dominated by grain boundary diffusion.The creep deformation is controlled by the diffusion of Mo and Nb elements,which have the two slowest grain boundary diffusivities among the five alloying elements.The present work provides a fundamental understanding of the creep behavior and deformation mechanism of NC RHEAs,which should help design advanced creep-resistant RHEAs.
查看更多>>摘要：The mechanical properties of as-cast metallic materials depend strongly on the size and shape of grains,which are critical microstructural parameters dictated by the interplay of nucleation and growth of crys-talline solids during solidification.In our experiments,the microstructure transition from coarse colum-nar crystals into fine equiaxed crystals for dilute Al-Mn-Si alloys was achieved by using sub-rapid so-lidification with the addition of Al-5Ti-1B grain refiner.The average grain size of Al alloy was reduced from a millimeter size to 73 pm.Through temperature gradient calculation,we found that the acqui-sition of fine equiaxed crystals could be attributed to the existence of a high number density of TiB2,acting as effective nucleation sites with an increase in total undercooling.Furthermore,the curvature su-percooling,constitutional undercooling,thermal undercooling,and kinetic undercooling during sub-rapid solidification were quantitatively determined for given solidification rates.Our results showed that con-stitutional undercooling,rather than thermal undercooling,was primarily responsible for the formation of fine equiaxed grains,with the assistance of Al-5Ti-1B grain refiner.This work provides a new insight into the grain refining mechanism under sub-rapid solidification.
查看更多>>摘要：Deformation-induced twinning or martensitic transformation can improve the work-hardening capabil-ity of alloys with face-centered cubic(FCC)structures and suppress strain localization.The stacking fault energy(SFE)of alloys plays a key role in determining deformation mechanisms and mechanical prop-erties.This study developed V-bearing high-Mn steel with a tensile strength of 1288 MPa and uniform elongation of 36％by tactfully designing the composition.Precipitation of V-carbides was selected to strengthen the steel and tune the global SFE of the matrix by settling carbon.Stronger work-hardening capability due to lower SFE and finer twin/matrix lamellae provided the steel with good ductility,while precipitation strengthened it.
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