查看更多>>摘要：Heterogeneous structures(HS)materials have the potential to exhibit simultaneous improvement in strength and plasticity due to hetero-deformation-induced hardening(HDI)between multiple grain struc-tures.However,achieving HS in aluminum alloy can be quite challenging.In this study,7000 series alu-minum alloys are investigated by incorporating rare earth element Y to develop a dual-phase structure containing Al8Cu4Y and Al3(Y,Zr)phases.And a heterogeneous lamella structure(HLS)is formed through the synergistic effect of Al8Cu4Y which includes dynamic recrystallization nucleation during deformation,and Al3(Y,Zr)which hinders the growth of recrystallized grain by means of pinning dislocations and sub-grain boundaries.Substructures such as precipitates and nanocrystals are incorporated during the fabri-cation of HS,allowing for precise control over the volume fraction of fine grains by adjusting the ratio of two-scale second phases.When Y content reaches 0.3％,the 7Y55-2/heterogeneous lamella structure(HLS)samples exhibit a fine grain volume fraction of 76.5 vol.％,a tensile strength of 695 MPa,and an elon-gation of 16.6％.The alloy contains a large number of dislocations that preferentially induce the growth of η'phases along specific directions,thereby promoting their development.The multi-coupling effect and composite strengthening mechanisms in the"heterostructure-dislocation-precipitate"microstructure contribute to the intrinsic excellent strength and plasticity of the alloy.This study tackles the challenge of inverted strength and plasticity observed in Al-Zn-Mg-Cu-Zr alloys,offering novel insights that pave the way for further applications of heterogeneous materials.
查看更多>>摘要：Columnar grain structure caused anisotropy in mechanical properties,especially in elongation,is an im-portant concern for Ti-6Al-4 V alloy fabricated by directed energy deposition(DED).Several strategies have been proposed to reduce anisotropy by globularizing the grains,but these conventional approaches are costly and inefficient due to challenges faced during producing the columnar β-grain structures.How-ever,understanding the impact of columnar grain-related microstructures on the anisotropic deformation behavior is still necessary.Despite the recognition of the importance of grain boundary Widmannstättenα colony(αWGB)as a grain-related microstructure,it has received limited attention in available literature on anisotropy in mechanical properties.This study employed in-situ induction heating during DED to control αWGB formation,yielding three Ti-6Al-4 V samples with varying αWGB sizes.Anisotropic deforma-tion of αWGB and its impact on elongation in build and transverse directions were analyzed.αWGB width grew from 0.5 μm to 32.4 μm via diffusion-controlled growth due to reduced cooling rate.Transverse deformation led to dislocation movement and accumulation,causing early failure and worsened ductile anisotropy within αWGB.Notably,larger αWGB size significantly exacerbated anisotropy in ductility.This work underscores αWGB's role in anisotropic deformation and offers insights for optimizing mechanical properties in DED-fabricated titanium alloys.
查看更多>>摘要：Unique rapid solidified structure and nanocrystallization mechanism enable the Fe-based nanocrystalline alloys with high Cu content excellent soft magnetic properties and good manufacturability,and also re-sults in unusual phenomena in terms of alloying effects.In the present work,we systematically studied the influence rules of early transition elements on the rapid solidified structure and nanocrystallization behaviors of Fe-Si-B-Cu soft magnetic alloys with high Cu content and explored the related mechanisms.In terms of rapid solidified structure,the additions of early transition elements always inhibit the for-mation of pre-existing α-Fe crystals even eliminate them,and the additions that could produce larger atomic mismatch parameter(δ)and negative mixing enthalpy(ΔHmix)show stronger effects.In terms of nanocrystallization behaviors,the increases in δ and negative ΔHmix weaken the competitive growth between the pre-existing nanocrystals during annealing and then coarsen the nanostructure of the an-nealed alloys and deteriorate their magnetic softness,while the excessive increases in δ and negativeΔHmix could significantly suppress the growth of α-Fe crystals by diffusion inhibition during annealing and thus remarkable refine the nanostructure of the annealed alloys and improve their magnetic softness.
查看更多>>摘要：High-strength Mg alloys have historically suffered from a challenge in achieving good ductility.Here,we report an asymmetric gradient nanostructure design prepared by ultrasonic severe surface rolling(USSR)at room temperature.Unlike conventional gradient-nanostructured materials that employ a hard-soft-hard sandwich structure,this new design incorporates a combined gradient distribution of grain microstructure and nanoprecipitates throughout the entire sample along the thickness direction.The nanoprecipitates are identified as the β-Mg17Al12 phase and are primarily generated through In-situ pre-cipitation promoted by the USSR-induced high-density dislocations and temperature increment.Benefit-ing from this unique microstructure,an outstanding strength-ductility synergy is achieved,with a yield strength of 372.8 MPa,an ultimate tensile strength of 453.3 MPa,and an elongation of 11.5％.The en-hanced strength can be attributed to several mechanisms,including grain boundary strengthening,dis-location strengthening,precipitation strengthening,twin strengthening,and hetero-deformation induced(HD1)strengthening.The HDI hardening and activation of multiple deformation modes also contribute to good ductility.This work provides a promising and effective method for overcoming the longstanding strength-ductility trade-off dilemma in Mg alloys.
查看更多>>摘要：Cellular microstructure is a unique feature in alloys fabricated by selective laser melting(SLM).Abundant efforts have been made to reveal the formation mechanism of cellular microstructures and its influences on mechanical performances,while its potential role in microstructure architecting during post-heat treatment is rarely explored.In this work,we investigated the features of cellular microstructures in an SLM-fabricated 18Ni(300)steel and revealed how this microstructure influences austenite reversion upon aging.Segregation of Ti and Mo is experimentally detected at cell boundaries.It is interestingly found that a distinctive reverted austenite network forms rapidly along cell boundaries during aging,whereas much less austenite is found in conventionally treated 18Ni(300)steels.The rapid austenite reversion in SLM-fabricated material proceeds mainly via the growth of retained austenite on cell boundaries while the nucleation and growth of new austenite grains is negligible.Phase-field simulations suggest austenite grows in a fast,partitionless manner along cell boundaries where the chemical driving force for austen-ite reversion is substantially enhanced by Ti and Mo segregations,but in a sluggish,partitioning manner towards cell interiors.Contrary to conventional views that austenite fraction should be confined to avoid strength reduction,current SLM-fabricated 18Ni(300)steel containing～13％cellular austenite is found to have higher tensile strength compared to its counterparts with negligible austenite.The design of austen-ite also shows its potential to enhance fracture toughness.The current study demonstrates that cellular structures could substantially alter austenite reversion behavior,providing a new route for microstructure architecting in additively manufactured steels.
查看更多>>摘要：The presence of excess Ta in high-temperature protective coatings can compromise the integrity of the Al2O3 scale on the surface,which has a negative impact on the oxidation behavior and reduces the service life.The effects of oxygen doping on the isothermal oxidation of three sputtered nanocrystalline coatings were investigated at 1100 ℃.The results indicated that oxygen doping inhibited the diffusion of Ta from the coating to the oxide scale,which was primarily attributed to the preferential oxidation of the Al in the coating.However,excess oxygen doping decreased the amount of Al available for the formation of the Al2O3 scale on the coating,thus reducing the inhibitory effect on Ta oxidation.Moreover,doping with excess O caused spalling of the oxide scale.Therefore,the right balance in O doping is crucial for suppressing Ta oxidation while maintaining the integrity of the oxide scale.
查看更多>>摘要：(Ba(1-x)Bix)(Ti(1-x)Mg2x/3Tax/3)O3(BBTMT-x,x=0.075,0.1,0.125,and 0.15)ceramics were manufactured via a solid-phase reaction method.The pseudo-cubic BaTiO3(BT)as the primary phase and Ba4MgTi11O27 as the secondary phase were detected in BBTMT-x ceramics.The elongated rod-shaped grains therein be-came numerous as x increased.The introduction of Bi/Mg/Ta(BMT)elements transformed BT ceramics from ferroelectrics to relaxor ferroelectrics and induced the formation of short-range order polar nanore-gions(PNRs),which were beneficial for the preeminent energy storage properties(ESPs).The highest ESPs(a giant recoverable energy-storage density Wrec of 5.97 J cm-3 with a high-efficiency η of 87.4％)were achieved in BBTMT-0.1 ceramics at 710 kV cm-1.BBTMT-0.1 ceramics also possessed excellent fre-quency(1-500 Hz),temperature(30-150 ℃),and fatigue(cycle number of 1-100,000)stabilities.Finite element simulations(FES)demonstrated that elongated rod-shaped grains had stronger obstacles to the development of electrical branches,which was beneficial to improving the comprehensive ESPs.
查看更多>>摘要：Dynamic transformation(DT)of austenite(γ)to ferrite(α)in the hot deformation of various carbon steels was widely investigated.However,the nature of DT remains unclear due to the lack of quantitative analysis of stress partitioning between two phases and the uncertainty of local distribution of substitu-tional elements at the interface in multi-component carbon steels used in the previous studies.Therefore,in the present study,a binary Fe-Ni alloy with α+γ duplex microstructure in equilibrium was prepared and isothermally compressed in α+γ two-phase region to achieve a quantitative analysis of microstruc-ture evolution,stress partitioning,and thermodynamics during DT.γ to α DT during isothermal compres-sion and α to γ reverse transformation on isothermal annealing under unloaded condition after deforma-tion were accompanied by Ni partitioning.The lattice strains during thermomechanical processing were obtained via in-situ neutron diffraction measurement,based on which the stress partitioning behavior between γ and α was discussed by using the generalized Hooke's law.A thermodynamic framework for the isothermal deformation in solids was established based on the basic laws of thermodynamics,and it was shown that the total Helmholtz free energy change in the deformable material during the isothermal process should be smaller than the work done to the deformable material.Under the present thermody-namic framework,the microstructure evolution in the isothermal compression of Fe-14Ni alloy was well explained by considering the changes in chemical free energy,plastic and elastic energies,and the work done to the material.In addition,the stabilization of the soft α phase in Fe-14Ni alloy by deformation was rationalized since the γ to α transformation decreased the total Helmholtz free energy by decreasing the elastic and dislocation energies.
查看更多>>摘要：Slicing and post-treatment of SiC crystals have been a significant challenge in the integrated circuit and microelectronics industry.To compete with wire-sawing and mechanical grinding technology,a promis-ing approach combining laser slicing and laser polishing technologies has been innovatively applied to increase utilization and decrease damage defects for single crystal 4H-SiC.Significant material utiliza-tion has been achieved in the hybrid laser processes,where material loss is reduced by 75％compared to that of conventional machining technologies.Without any special process control or additional treat-ment,an internally modified layer formed by laser slicing can easily separate the 4H-SiC crystals using an external force of about～3.6 MPa.The modified layer has been characterized using a micro-Raman method to determine residual stress.The sliced surface exhibits a combination of smooth and coarse appearances around the fluvial morphology,with an average surface roughness of over Sa 0.89 μm.An amorphous phase surrounds the SiC substrate,with two dimensions of lattice spacing,d=0.261 nm and d=0.265 nm,confirmed by high-resolution transmission electron microscopy(HRTEM).The creation of laser-induced periodic surface nanostructures in the laser-polished surface results in a flatter surface with an average roughness of less than Sa 0.22 μm.Due to the extreme cooling rates and multiple thermal cy-cles,dissociation of Si-C bonding,and phase separation are identified on the laser-polished surface,which is much better than that of the machining surface.We anticipate that this approach will be applicable to other high-value crystals and will have promising viability in the aerospace and semiconductor industries.
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