Introduction of nanotwins into nanoprecipitations strengthened CoCrNiMo_(0.2) alloy to achieve strength and ductility trade-off:A comparative research

Normal strengthening methods through precipitations and deformation obviously enhance the strength of metallic materials while resulting in the sacrifice of ductility,and synergistic improvement of strength and ductility is currently an urgent requirement.Herein we developed a cryogenic deformation combined with an annealing method to fabricate CoCrNiMo_(0.2) medium entropy alloy,which achieved an ultrahigh strength of 1.8 GPa with synergistic improvement in strength and ductility.Microstructure,mechanical performance,and strengthening mechanisms of the developed alloys were investigated compared with that prepared by the regular room temperature deformation method.It was found that high-density nanotwins were produced in CoCrNiMo_(0.2) MEA via cryogenic deformation.Fine grains,hard precipitations,and high volume fraction of nanotwins greatly strengthened the alloy,obtaining a yield and ultimate tensile strength of 1400 MPa and 1800 MPa.Ductility improvement of the developed alloy was mainly attributed to the production of deformation nanotwins due to the lower stacking fault energy,which greatly increases the dislocation storage ability,and thus,the ductility of the alloy was enhanced.

High strength and ductility achieved in friction stir processed Ni-Co based superalloy with fine grains and nanotwins

The trade-off between strength and ductility has been an enormous difficulty in the field of materials for an extended time due to their inverse correlation. In this work, friction stir processing(FSP) was for the first time performed to high-strength and high-melting-point Ni-Co based superalloy(GH4068),and enhanced strength and ductility were achieved in FSP samples. At room temperature, the FSP sample demonstrated significantly higher yield strength and ultimate tensile strength(1290 and 1670 MPa)than that of the base material(BM, 758 and 904 MPa) and advanced wrought GH4068 alloy(982 and 1291 MPa), concurrent with high tensile ductility(~24%). Compared with the BM, 70% higher yield strength of the FSP sample results from the remarkable contribution of grain-boundary and nanotwin strengthening, which has been confirmed by the multimechanistic model studied in this work. More importantly, with increasing temperature, an excellent strength-ductility synergy was obtained at 400 ℃,i.e., the yield strength of the FSP sample was increased by more than 50% compared with the BM(from789 to 1219 MPa);more interestingly, the elongation was also significantly increased from 17.9% in the BM to 28.5% in the FSP sample. Meanwhile, the Portevin-Le Chatelier effect was observed in the engineering stress-strain curve. The occurrence of this effect may be attributed to the interaction between solutes and defects like twins and mobile dislocations. Moreover, the grain refinement mechanism of FSP samples was proved to be discontinuous dynamic recrystallization.

Theoretical study of the effects of alloying elements on Cu nanotwins

Nanotwins form in many metallic materials to improve their strength and toughness.In this study,we thoroughly studied the alloying effects of 10 common metal and nonmetal elements on Cu nanotwins by density functional theory(DFT).We calculated the segregation energies to determine if Cu nanotwins attract both the metal and nonmetal alloying elements;these segregation energies were then decomposed to mechanical and chemical components.The Cu-Sn bonds are different from other metal alloying elements,and the strong bond between Cu and the nonmetal element results in the negative values of the chemical contribution.Furthermore,the temperature and concentration have different effects on the nanotwin formation energy of the metal and nonmetal alloying elements.As determined by the Generalized Stacking Fault Energy,Al and nonmetals can inhibit the migration of Cu nanotwin boundary,and the effects of Li,Mg,and Sn are opposite.Our theoretical study serves as the foundation for the engineering nanotwin structures through alloying elements,the elements that may lead to new alloy compositions and thermomechanical processes,and are important complements to the experimental research.

Interfacial enhancement of Ag and Cu particles sintering using(111)-oriented nanotwinned Cu as substrate for die-attachment

The electroplated(111)-oriented nanotwinned-Cu(nt-Cu) film was utilized as substrate for Ag and Cu sinter joining to improve the weak interface connection between the metal paste and the substrate.It was found that both Cu and Ag sinter joints using(111)-oriented nt-Cu film exhibited a higher bonding strength than that using traditional random-oriented Cu film.Especially,the joints sintered with Cu paste on(111)-oriented nt-Cu film possessed a higher bonding strength of 53.7 MPa at the sintering condition of 300 °C,0.4 MPa in N2 atmosphere,compared to that on random-oriented Cu film with a value of 31.3 MPa.The results show that as metal substrate layer,the(111)-oriented nt-Cu film can improve the connection performance of Ag and Cu sinter joints,which could further promote their application in dieattachment technology for the next-generation power semiconductors.

Phase-field simulation of the coupled evolutions of grain and twin boundaries in nanotwinned polycrystals

Nanotwinned polycrystals exhibit an excellent strength-ductility combination due to nanoscale twins and grains. However, nanotwin-assisted grain coarsening under mechanical loading reported in recent experiments may result in strength drop based on the Hall-Petch law. In this paper, a phase-field model is developed to investigate the effect of coupled evolutions of twin and grain boundaries on nanotwin-assisted grain growth. The simulation result demonstrates that there are three pathways for coupled motions of twin and grain boundaries in a bicrystal under the applied loading, dependent on the amplitude of applied loading and misorientation of the bicrystal. It reveals that a large misorientation angle and a large applied stress promote the twinning-driven grain boundary migration. The resultant twin-assisted grain coarsening is confirmed in the simulations for the microstructural evolutions in twinned and un-twinned polycrystals under a high applied stress.

Strengthening and softening in gradient nanotwinned FCC metallic multilayers

Plastic-deformation behaviors of gradient nanotwinned(GNT)metallic multilayers are investigated in nanoscale via molecular dynamics simulation.The evolution law of deformation behaviors of GNT metallic multilayers with different stacking fault energies(SFEs)during nanoindentation is revealed.The deformation behavior transforms from the dislocation dynamics to the twinning/detwinning in the GNT Ag,Cu,to Al with SFE increasing.In addition,it is found that the GNT Ag and GNT Cu strengthen in the case of a larger twin gradient based on more significant twin boundary(TB)strengthening and dislocation strengthening,while the GNT Al softens due to more TB migration and dislocation nucleation from TB at a larger twin gradient.The softening mechanism is further analyzed theoretically.These results not only provide an atomic insight into the plastic-deformation behaviors of certain GNT metallic multilayers with different SFEs,but also give a guideline to design the GNT metallic multilayers with required mechanical properties.

The wrinkling and buckling of graphene induced by nanotwinned copper matrix:A molecular dynamics study

The three-dimensional(3D)graphene-based materials have raised significant interest due to excellent catalytic performance and unique electronic properties,while the preparation of uniform and stable 3D graphene structures remains a challenge.In this paper,using molecular dynamics simulations,we found that the nanotwinned copper(nt-Cu)matrix with small twin spacing can induce the wave-shaped wrinkling and sawtooth-shaped buckling graphene structures under uniaxial compression.The nt-Cu matrix possesses a symmetrical lattice structure for the lattice rotation with the dislocation annihilation,resulting in the transition of sandwiched graphene from 2D to 3D structures with good uniformity.The newly formed twin boundaries(TBs)in the nt-Cu matrix improve the resistance of graphene against the out-of-plane deformation so that graphene can maintain a stable wrinkling or buckling morphology in a wide strain range.These 3D texturing structures show great flexibility and their micro parameters can be controlled by applying different compressive strains.Furthermore,we propose a simple sliding method for decoupling graphene from the nt-Cu matrix without any damage.This work provides a novel strategy to induce and transfer the uniform wrinkling and buckling of graphene,which may expand the application of graphene in energy storage and catalysts.

Influences of nanotwin volume fraction on the ballistic performance of coarse-grained metals

Coarse-grained（CG） metals strengthened by nanotwinned（NT） regions possess high strength and good ductility. As such, they are very suitable for applications in bullet-proof targets. Here, a numerical model based on the conventional theory of strain gradient plasticity and the Johnson–Cook failure criterion is employed to study the influences of volume fraction of NT regions on their ballistic performance.The results show that in general a relatively small twin spacing（4–10 nm） and a moderate volume fraction（7%–20%） will lead to excellent limit velocity and that the influences of volume fraction on limit displacement change with the category of impact processes.

Achieving high strength and high ductility in nanostructured metals: Experiment and modelling

Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong, Chinagrains to a^1-martensite nanograins with bimodal grain size distribution for lower strain rates to nanotwins in the ultrafine/coarse grained austenite phase for higher strain rates. Meanwhile, we will further address the mechanism-based plastic models to describe the yield strength, strain hardening and ductility in nanostructured metals with bimodal grain size distribution and nanotwinned polycrystalline metals. The proposed theoretical models can comprehensively describe the plastic deformation in these two kinds of nanostructured metals and excellent agreement is achieved between the numerical and experimental results. These models can be utilized to optimize the strength and ductility in nanostructured metals by controlling the size and distribution of nanostructures.

Simultaneous enhancement of strength and ductility via microband formation and nanotwinning in an L1_(2)-strengthened alloy

Metallic alloys with high strength and large ductility are required for extreme structural applications.However,the achievement of ultrahigh strength often results in a substantially decreased ductility.Here,we report a strategy to achieve the strength-ductility synergy by tailoring the alloy composition to control the local stacking fault energy(SFE)of the face-centered-cubic(fcc)matrix in an L1_(2)-strengthened superlattice alloy.As a proof of concept,based on the thermodynamic calculations,we developed a non-equiatomic CoCrNi_(2)(Al_(0.2)Nb_(0.2))alloy using phase separation to create a near-equiatomic low SFE disordered CoCrNi medium-entropy alloy matrix with in situ formed high-content coherent Ni_(3)(Al,Nb)-type ordered nanoprecipitates(∼12 nm).The alloy achieves a high tensile strength up to 1.6 GPa and a uniform ductility of 33%.The low SFE of the fcc matrix promotes the formation of nanotwins and parallel microbands during plastic deformation which could remarkably enhance the strain hardening capacity.This work provides a strategy for developing ultrahigh-strength alloys with large uniform ductility.

Improving Fatigue Properties of 316L Stainless Steel Welded Joints by Surface Spinning Strengthening

The surface spinning strengthening(3S)mechanism and fatigue life extension mechanism of 316L stainless steel welded joint were systematically elucidated by microstructural analyses and mechanical tests.Results indicate that surface gradient hardening layer of approximately 1 mm is formed in the base material through grain fragmentation and deformation twin strengthening,as well as in the welding zone composed of deformedδ-phases and nanotwins.The fatigue strength of welded joint after 3S significantly rises by 32%(from 190 to 250 MPa),which is attributed to the effective elimination of surface geometric defects,discrete refinement ofδ-Fe phases and the appropriate improvement in the surface strength,collectively mitigating strain localization and surface fatigue damage within the gradient strengthening layer.The redistributed fineδ-Fe phases benefited by strong stress transfer of 3S reduce the risk of surface weak phase cracking,causing the fatigue fracture to transition from microstructure defects to crystal defects dominated by slip,further suppressing the initiation and early propagation of fatigue cracks.

High strength and deformation stability achieved in CrCoNi alloy containing deformable oxides

Hard secondary phases usually strengthen alloys at the expense of ductility.In this work,we made a dual-phase CrCoNi-O alloy containing a face centered cubic matrix and chromium oxide.On one side,the dispersed chromium oxide nano-particles impeded dislocation movement and increased the strength of the alloy.On another side,the spreading lattice distortion in CrCoNi-O high entropy solution locally relieved the severe interfacial mismatch and led to nanoscale variation of interfacial strain at the matrix-oxide interface,which facilitated dislocations’transmission from one phase to another.Consequently,unlike the strong but brittle oxide nanoparticles used before,the oxide phase here can afford significant dislocation activities during material’s plastic deformation.Comparing the mechanical properties of CrCoNi-O alloys with and without chromium oxide particles,it was found that the yield strength of the dual-phase samples was twice of the single phase CrCoNi-O alloy and strong strain hardening was obtained with ultra-high deformation stability.High density of nanotwins formed in dual-phase samples under high stress,resulting in significant strain hardening according to the well-known twinning-induced plasticity(TWIP)effect.Our results shed light on optimizing the combination of strength and plasticity of compounds by modulating the variation of interfacial strain field based on the spreading lattice distortion.

块体择优取向纳米孪晶铜的优异抗疲劳裂纹扩展性能

本文研究了具有不同微观结构(如晶粒尺寸和孪晶厚度)的块体择优取向纳米孪晶Cu(NT-Cu)的疲劳裂纹扩展(FCG)特性,着重讨论了裂纹闭合效应和近门槛值区域的本征FCG阻力.与传统无孪晶的粗晶Cu相比,NT-Cu样品表现出显著提升的疲劳裂纹扩展抗力(有效门槛值应力强度因子范围,△K_(th,eff)).这种增强的裂纹扩展阻力与NT-Cu中特殊的跨孪晶位错滑移模式有关.该位错主导着NT-Cu裂纹尖端的循环变形,有效降低了裂纹尖端循环滑移不可逆水平,从而提高了纳米孪晶材料损伤扩展的本征抗力.

Hardness-thermal stability synergy in nanograined Ni and Ni alloys:Superposition of nanotwin and low-energy columnar boundary

Refining grains into nanoscale can significantly strengthen and harden metallic materials;however,nanograined metals generally exhibit low thermal stability,hindering their practical applications.In this work,we exploit the superposition of the contribution of nanotwins,low-angle grain boundaries,and microalloying to tailor superior combinations of high hardness and good thermal stability in Ni and Ni alloys.For the nanotwinned Ni having a twin thickness of∼2.9 nm and grain size of 28 nm,it exhibits a hardness over 8.0 GPa and an onset coarsening temperature of 623 K,both of which are well above those of nanograined Ni.Re/Mo microalloying can further improve the onset coarsening temperature to 773 K without comprising hardness.Our analyses reveal that high hardness is achieved via strengthen-ing offered by extremely fine nanotwins.Meanwhile,the superior thermal stability is mainly ascribed to the low driving force for grain growth induced by the low-angle columnar boundary architecture and to the additional pinning effect on the migration of twin/columnar boundaries provided by minor Re/Mo solutes.The present work not only reveals a family of nanotwinned metals possessing the combination of ultra-high hardness and high thermal stability but also provides a strategy for tailoring properties of metallic materials by pairing low-angle grain boundaries and twin boundaries.

Nanotwin-assisted nitridation of quenched FeNi alloy nanopowders for rare-earth-free magnets

L1_(0)-ordered FeNi alloy with a high uniaxial magnetic anisotropy and large magnetic moment is a promising candidate for rare-earth-free permanent magnets applications.However,the synthesis of this chemically ordered phase remains a longstanding challenge because of its low chemical order-disorder transition temperature(200-320℃).Although a non-equilibrium synthetic route based on a nitrogen topotactic reaction has been proposed as a valid approach,the volume fraction and degree of chemical ordering of the product phase are limited.Herein,we propose a promising approach that promotes the efficient formation of L1_(0)-ordered nitride phase in FeNi nanopowders by introducing a quenching treatment during a low-oxygen induction thermal plasma process.The quenched FeNi nanopowders possessed much smaller powder sizes(40.4 vs 74.0 nm),exhibited higher number densities of nanotwins(39.8%vs 24.1%)and formed much larger volume fraction(33.6 wt.%vs 0.6 wt.%)of ordered phase than the unquenched nanopowders.Notably,quenching-induced high-density nanotwins led to the dominant coverage of serrated{001}crystal facets over the surfaces of the FeNi nanopowders.Such unique features substantially accelerated the formation of the L1_(0)-ordered nitride phase in the FeNi nanopowders because the{001}crystallographic orientation had the highest nitrogen diffusivity.This work provides not only a valid synthetic approach for mass production of the L10-ordered nitride phase in FeNi nanopowders but also novel insights into the crystal-defect-assisted nitridation of nanomaterials.

Fundamental strengthening mechanisms of ordered gradient nanotwinned metals

Ordered structures with functional units offer the potential for enhanced performance in metallic materials.Among these structures,gradient nanotwinned(GNT)microstructures demonstrate excellent controllability.This paper provides a comprehensive review of the current state-of-the-art studies on GNT structures,encompassing various aspects such as design strategies,mechanical properties characterization,spatially gradient strain evolution analysis,and the significant role of geometrically necessary dislocations(GNDs).The primary objective is to systematically unravel the fundamental strengthening mechanisms by gaining an in-depth understanding of the deformation behavior of nanotwinned units.Through this work,we aim to contribute to the broader field of materials science by consolidating knowledge and providing insights for the development of novel metallic materials with enhanced properties and tailored performance characteristics.

Adiabatic shear instability in a titanium alloy:Extreme deformation-induced phase transformation,nanotwinning,and grain refinement

Increasingly harsh service conditions place higher requirements for the high strain-rate performance of titanium alloys.Adiabatic shear band(ASB),a phenomenon prone to dynamic loading,is often accom-panied by catastrophic damage.Yet,it is unclear how the internal nanostructures are related to shear instability.Here we report detailed microstructural evolution in the ASB of a titanium alloy via in-depth focused ion beam(FIB),transmission Kikuchi diffraction(TKD),and high-resolution transmission electron microscope(HRTEM)analyses,with the deformation instability phenomenon discussed from the energy perspective.The ASB interior undergoes multifaceted changes,namely deformation-induced beta-to-alpha transformation and deformation-induced martensitic transformation to form substantially refined and heterogeneous structures.Meanwhile,two types of extremely fine twins are identified to occur within both nano-sized martensite and alpha phase.The critical plastic work representing the onset of adiabatic shear instability and dynamic equilibrium is observed to be constant for a specific structure in the same deformation mode.The energy analysis could be extended to other materials subjected to high strain-rate dynamic deformation.

Nanostructures and nanoprecipitates induce high strength and high electrical conductivity in a CuCrZr alloy

The mixed nanostructure mainly consisting of nanotwins and nanograins was obtained in a solid solution CuCrZr alloy by means of dynamic plastic deformation at cryogenic temperature.After subsequent aging treatments,the precipitation of Cr at nanometer scale provided further strengthening and brought substantial recovery of electrical conductivity.The aged nanostructured CuCrZr alloy exhibited a high tensile strength of 832MPa and a high electrical conductivity of 71.2%IACS.The details of precipitation tuned by nanotwin boundaries were demonstrated in this work.The combined strengthening of nanostructures and nanoprecipitates was discussed.

Selective laser melting of bulk immiscible alloy with enhanced strength:Heterogeneous microstructure and deformation mechanisms

To overcome the dimension limits of immiscible alloys produced by traditional techniques and enhance their mechanical properties,bulk Cu-Fe-based immiscible alloy with abundant nanotwins and stacking faults was successfully produced by selective laser melting(SLM).The SLM-produced bulk immiscible alloy displays a heterogeneous microstructure characterized by micro-scaledγ-Fe particles dispersed in fineε-Cu matrix with a high fraction(~92%)of high-angle grain boundaries.Interestingly,abundant nanotwins and stacking faults are generated in the interior of nano-scaledγ-Fe particles embedded withinε-Cu matrix.The heterogeneous interface of soft domains(ε-Cu)and hard domains(γ-Fe)not only induces the geometrically necessary dislocations(GNDs)but also affects the dislocation propagation during plastic deformation.Therefore,the bimodal heterogeneous interface,and the resistance of nanotwins and stacking faults to the propagation of partial dislocation make the bulk immiscible alloy exhibit an enhanced strength of~590 MPa and a good ductility of~8.9%.

Twin and twin intersection phenomena in a creep deformed microalloyed directionally solidified high Nb containing TiAl alloy

The creep behavior of a directionally solidified TiAl alloy with a high Nb content after microalloying by the addition of small amounts of W,Cr,and B elements was investigated by scanning electron microscopy and transmission electron microscopy.By means of directional solidification and microalloying,a TiAl alloy with a fine and uniform microstructure and continuous columnar crystals was obtained.High-density dislocations,deformation nanotwins,and twin intersections were observed inγlamellae.The results show that,in comparison with the ternary TiAl alloy,the microalloyed high Nb containing TiAl alloy exhibited better creep properties at 760℃and 275 MPa.The decrease in stacking fault energy can promote dislocation dissociation and the formation of deformation twins,and the twin intersections can hinder the movement of dislocation to enhance the creep performance of the TiAl alloy.