Composition design study of strong and ductile Mo-alloyed CoCrNi medium-entropy alloys

The assistance of alloying elements provides enormous opportunities for the discovery of high-performance face-centered cubic(FCC)medium-entropy alloys(MEAs).In this work,the influence of al-loying element Mo on the phase stability,stacking fault energy(SFE),deformation mechanisms,lattice distortion,and mechanical properties of(CoCrNi)100-x Mox(0≤x≤10)MEAs was synthetically explored with the first-principles calculations.It indicates that the FCC phase remains metastable at 0 K,and its stability degenerates with increasing Mo content.The monotonous decrease of SFE is revealed with the rise of Mo content,which promotes the activation of stacking faults,deformation twinning,or martensitic transformation.Raising Mo content also causes the aggravation of lattice distortion and thus triggers in-tense solid solution strengthening.Significantly,the essential criterion for the composition design of FCC(CoCrNi)100-x Mo MEAs with superior strength-ductility combination was established based on the syner-gistic effects between multiple deformation mechanisms and solid solution strengthening.According to the criterion,the optimal composition is predetermined as(CoCrNi)93 Mo7 MEA.The criterion is proved to be effective,and it can provide valuable inspiration for the development of alloying-element reinforced FCC multi-principal element alloys.

Effects of nanostructuring on mechanical and tribological behaviors of FeCoNi medium-entropy alloy

The effects of nanostructuring on the mechanical and dry-sliding wear behaviors of a FeCoNi medium-entropy alloy(MEA)were systematically investigated through nano-indentation and ball-on-disc wear tests.The results show that reducing the grain size down into the nano-meter regime,on the one hand,significantly elevates the hardness of the FeCoNi alloy,and on the other hand,facilitates the formation of a surface oxide layer.As a result,the wear rate of the nanocrystalline(NC)FeCoNi alloy is one order of magnitude lower than its coarse-grained counterpart.The NC FeCoNi alloy also exhibits obviously enhanced wear resistance compared with conventional NC Ni and Ni-based alloys in terms of both lower wear rate and friction coefficient.Such enhancement in tribological properties mainly stems from the improved strain hardening ability,owing to the inevitable concentration heterogeneity in MEA that imposes extra resistance to dislocation motion.

Design and optimization of the composition and mechanical properties for non-equiatomic CoCrNi medium-entropy alloys

The development of multi-principal element alloys(MPEAs,also called as high-or medium-entropy al-loys,HEAs/MEAs)provides tremendous possibilities for materials innovation.However,designing MPEAs with desirable mechanical properties confronts great challenges due to their vast composition space.In this work,we provide an essential criterion to efficiently screen the CoCrNi MEAs with outstanding strength-ductility combinations.The negative Gibbs free energy difference△E_(FCC-BCC)between the face-centered cubic(FCC)and body-centered cubic(BCC)phases,the enhancement of shear modulus G and the decline of stacking fault energy(SFE)γ_(isf)are combined as three requisites to improve the FCC phase stability,yield strength,deformation mechanisms,work-hardening ability and ductility in the criterion.The effects of chemical composition on△E_(FCC-BCC),G andγisf were investigated with the first principles calculations for Co_(x)Cr_(33)Ni_(67-x),Co_(33)Cr_(y)Ni_(67-y)and Co_(z)Cr_(66-z)Ni_(34)(0≤x,y≤67 and 0≤z≤66)alloys.Based on the essential criterion and the calculation results,the composition space that displays the neg-ative Gibbs free energy difference△E_(FCC-BCC),highest shear modulus G and lowest SFEγ_(isf)was screened with the target on the combination of high strength and excellent ductility.In this context,the optimal composition space of Co-Cr-Ni alloys was predicted as 60 at.%-67 at.%Co,30 at.%-35 at.%Cr and 0 at.%-6 at.%Ni,which coincides well with the previous experimental evidence for Co_(55)Cr_(40)Ni_(5)alloys.The valid-ity of essential criterion is further proved after systematic comparison with numerous experimental data,which demonstrates that the essential criterion can provide significant guidance for the quick exploitation of strong and ductile MEAs and promote the development and application of MPEAs.

Exceptional strength-ductility synergy of additively manufactured CoCrNi medium-entropy alloy achieved by lattice defects in heterogeneous microstructures

The selective laser melting(SLM)with subsequent cold rolling and annealing is used to produce high-density lattice defects and grain refinement in the CoCrNi medium-entropy alloys(MEAs).The superior comprehensive mechanical properties have been achieved in the as-SLMed CoCrNi alloy after rolling and annealing.The as-SLMed alloys delivered the yield strength of 693.4 MPa,the ultimate tensile strength of 912.7 MPa and the fracture strain of 54.4%.After rolling with 70%reduction in thickness and annealing at 700℃for 2 h.the yield strength,ultimate tensile strength and fracture strain reached 1161.6 MPa,1390.8 MPa and 31.5%,respectively.The exceptional strength-ductility synergy is mainly attributed to the refined hierarchical microstructures with coarsening grains at a level of 30μm and ultrafine grains at a level of 1μm,and the heritage of dislocation-formed sub-grains and other lattice defects.This investigation demonstrates that the SLM with subsequent rolling and annealing is beneficial to fabricate high strength and ductile MEAs with single face-centered cubic(fcc)structure.

W-CoCrNi合金的烧结致密化及其组织与性能

本文采用粉末冶金技术制备以CoCrNi中熵合金为黏结相的W-CoCrNi高密度钨合金,研究黏结相含量和烧结温度(1300~1500℃)对合金显微组织和力学性能的影响。结果表明:烧结态合金组织由W相、CoCrNi相、Co_(7)W_(6)相和亚微米富Cr颗粒组成。随着烧结温度的升高,合金致密度随之升高。合金在1500℃以下烧结时,致密化机制主要为固相烧结机制,固相烧结驱动力主要来自于表面能的降低和Co_(7)W_(6)新相的形成。当烧结温度达1500℃时,合金致密化机制主要为液相烧结机制,致密化过程包含W向CoCrNi熔体中的溶解和扩散、Co_(7)W_(6)相析出和钨颗粒球化三个阶段。1500℃烧结温度下制备的75W-CoCrNi和95W-CoCrNi合金的压缩屈服强度分别为1060 MPa和1602 MPa,75W-CoCrNi合金在压缩应变超过50%时仍未断裂。随着CoCrNi黏结相含量的增加,合金致密度随之升高,压缩变形能力增加,但压缩屈服强度降低。

Effect of Al on microstructure and mechanical properties of cast CrCoNi medium-entropy alloy

Cast CrCoNiAIx （x=0-1.2） medium-entropy alloys （MEAs） were produced by arc melting and flip cast to investigate the alloying effect of AI addition on the microstructure, phase constituent and mechanical properties. The crystal structure changes from an initial face-centered cubic （FCC） to duplex FCC and body-centered cubic （BCC） and finally a single BCC with increasing AI content. In the duplex region, FCC and BCC phases form under a eutectic reaction in the interdendrite region. In the single BCC region, the dendrites transform to ordered B2 and disordered A2 BCC phases resulting from spinodal decomposition. Corresponding to their phase constituents, yield strength increases accompanied with an elongation reduction with increasing AI addition. A very interesting phenomenon of very weak ordered FCC （001） spots appearing in AI-0.4 alloy was observed, indicating a local ordering of FCC phase. The changes of fracture surfaces after the tensile deformation are also corresponding to the variations in mechanical properties.

Effect of carbon on microstructure,mechanical properties and wear resistance of non-equiatomic Fe_(70)Co_(7.5)Cr_(7.5)Ni_(7.5)V_(7.5) medium-entropy alloys fabricated by powder metallurgy

In this study,non-equiatomic Fe_(70)Co_(7.5)Cr_(7.5)Ni_(7.5)V_(7.5) medium-entropy alloys(MEAs)with different carbon contents were prepared via mechanical ball-milling,cold pressing and vacuum sintering.The microstructural evolution,mechanical properties and wear resistance of the MEAs were investigated.Fe_(70)Co_(7.5)Cr_(7.5)Ni_(7.5)V_(7.5) exhibited a bodycentered cubic(bcc)structure withσphase precipitation.After adding 4 at%and 8 at%carbon,the phase composition of the alloys was transformed to bcc+MC+σand bcc+MC+M_(23)C_(6),respectively.The mechanical properties and wear resistance were observed to be significantly enhanced by the formation of carbides.Increasing the carbon content,the corresponding bending strength and hardness increased from 1520 to 3245 MPa and HRC 57.2 to HRC 61.4,respectively.Further,the dominant wear mechanism changed from the adhesion wear to the abrasion wear.Owing to the evenly distributed carbides and precipitated nanocarbides,Fe_(64.4)Co_(6.9)Cr_(6.9)Ni_(6.9)V_(6.9)C_(8) revealed an extremely low specific wear rate of 1.3×10^(−6) mm_(2)/(N·m)under a load of 10 N.

Microstructure and mechanical properties of as-cast(CuNi)100-xCox medium-entropy alloys

Microstructure and mechanical properties of non-equiatomic(CuNi)_(100-x)Co_(x)(x=15,20,25 and 30,at.%)medium-entropy alloys(MEAs)prepared by vacuum arc-melting were investigated.Results show that all the as-cast MEAs exhibit dual face-centered cubic(fcc)solid-solution phases with identical lattice constant,showing typical dendrite structure consisting of(Ni,Co)-rich phase in dendrites and Cu-rich phase in inter-dendrites.The positive enthalpy of mixing among Cu and Ni-Co elements is responsible for the segregation of Cu.With the increase of Co content,the volume fraction of(Ni,Co)-rich phase increases while the Cu-rich phase decreases,resulting in an increment of yield strength and a decrement of elongation for the(CuNi)_(100-x)Co_(x) MEAs.Nano-indentation test results show a great difference of microhardness between the two fcc phases of the MEAs.The measured microhardness value of the(Ni,Co)-rich phase is almost twofold as compared to that of the Cu-rich phase in all the(CuNi)_(100-x)Co_(x) MEAs.During the deformation of the MEAs,the Cu-rich phase bears the main plastic strain,whereas the(Ni,Co)-rich phase provides more pronounced strengthening.

Effects of Si Addition on Microstructure,Properties and Serration Behaviors of Lightweight Al-Mg-Zn-Cu Medium-entropy Alloys

A series of as-cast lightweight multicomponent alloys Al(86-x)Mg10Zn2Cu2Six(x=0,0.3,0.6,0.9,1.2 at.%)were prepared by a vacuum induction furnace with a steel die.With the addition of Si,the reticular white Al-Cu phase deposited were gradually replaced by the gray eutectic Mg-Si phase,while the compressive strength of the alloys increases first and then decreases slowly.It is particularly noteworthy that the compression plasticity also exhibits this trend.When the Si content is 0.9 at.%,the compressive strength reaches its maximum at 779.11 MPa and the compressive plasticity reaches 20.91%.The effect of the addition of Si on the serration behavior of alloy was also studied;we found that the addition of Si introduces a new MgSi phase,and with the change of Si is significantly affects the morphology of the precipitated phase,which affects the serration behavior of the alloys.The comprehensive mechanical properties of the alloy are optimal at the critical point where the serration behavior disappears.In this work,we have provided a method and a composition for the preparation of a low-cost,high-strength,lightweight medium-entropy alloys.

梯度异质结构CoCrNi中熵合金的设计及变形机理研究

CoCrNi是一种典型的中熵合金,其具有塑性高但拉伸强度较低的特点,这种强塑性权衡问题限制了其工程应用范围。本文采用轧制退火和旋转加速喷丸工艺,在CrCoNi中熵合金中引入梯度晶粒尺寸和孪晶的异质结构。结果表明,这种设计实现了优良的强塑性,其屈服强度和抗拉强度分别为665 MPa和950 MPa,同时断裂塑性达到40.6%。在拉伸变形过程中,梯度晶粒异质结构能够产生非均匀变形诱导硬化,提升合金屈服强度的同时,保持高的加工硬化率和优异的延展性。因此,基于轧制退火和旋转喷丸工艺,引入梯度晶粒异质结构是克服合金强度-延展性失衡问题的有效途径。

Comparison of formation and evolution of radiation-induced defects in pure Ni and Ni-Co-Fe medium-entropy alloy

High-entropy alloys(HEAs)and medium-entropy alloys(MEAs)have attracted a great deal of attention for developing nuclear materials because of their excellent irradiation tolerance.Herein,formation and evolution of radiation-induced defects in Ni Co Fe MEA and pure Ni are investigated and compared using molecular dynamics simulation.It is observed that the defect recombination rate of ternary Ni Co Fe MEA is higher than that of pure Ni,which is mainly because,in the process of cascade collision,the energy dissipated through atom displacement decreases with increasing the chemical disorder.Consequently,the heat peak phase lasts longer,and the recombination time of the radiation defects(interstitial atoms and vacancies)is likewise longer,with fewer deleterious defects.Moreover,by studying the formation and evolution of dislocation loops in Ni-Co-Fe alloys and Ni,it is found that the stacking fault energy in Ni-Co-Fe decreases as the elemental composition increases,facilitating the formation of ideal stacking fault tetrahedron structures.Hence,these findings shed new light on studying the formation and evolution of radiation-induced defects in MEAs.

异质结构Ti(C,N)-(Fe)CoCrNi基金属陶瓷及其电化学腐蚀行为

采用(Fe)CoCrNi高/中熵合金雾化粉末原料,制备Ti(C_(0.5),N_(0.5))-20%WC-8%TaC-5%Mo_(2)C-22%FeCoCrNi(A)和Ti(C_(0.5),N_(0.5))-20%WC-8%TaC-5%Mo_(2)C-22%CoCrNi(B)金属陶瓷,采用具有相同黏结金属体积分数的Ti(C_(0.5),N_(0.5))-20%WC-8%TaC-5%Mo_(2)C-12%Co-12%Ni(C)金属陶瓷作为对比合金。电化学实验结果表明,尽管金属陶瓷A和B中存在脱碳相(η相)和微观结构均质性较差等问题,在H_(2)SO_(4)(pH=1)、Na_(2)SO_(4)(pH=7)和NaOH(pH=13)溶液中,相较于无微观组织结构缺陷的金属陶瓷C,金属陶瓷A和B在3种介质中平均自腐蚀电流密度J_(corr)分别降低33%和88%,平均电荷转移电阻Rct分别提高97%和219%,微观组织结构缺陷不影响其耐蚀性改善;在H_(2)SO_(4)中,金属陶瓷B的J_(corr)降低89%,Rct提高750%,CoCrNi中熵合金黏结金属能显著改善金属陶瓷在强腐蚀性介质中的耐蚀性。从高、中熵合金本征特性,合金体系润湿性等视角分析了金属陶瓷A和B中η相形成伴随的异质结构形成机理。

Structure evolution in undercooled CoCrNi medium entropy alloys by glass fluxing method

Undercooling of ternary CoCrNimediumentropy alloy(MEA)was achieved by molten glass fluxing method.The influence of undercooling on microstructure and mechanical properties of was investigated.The microstructure changes during the undercooling process identified by transmission electron microscope and scanning electron microscope shows that the grain size and intergranular phase all change after the undercooling treatment.The yield strength of the ternary MEAincreased significantly after undercooling treatment,which attribute to the refined grain size and the formation of the new phase.Undercooling method can be used as a potential method to modify the microstructure and improve the mechanical properties of MEAs.

汽车用不锈钢表面激光熔覆CoCrNi中熵合金熔覆层微观组织及耐蚀性研究

为提升车用304不锈钢的表面硬度及耐蚀性,采用激光熔覆技术在304不锈钢表面制备了CoCrNi中熵合金熔覆层,并研究了熔覆层的相组成、元素分布及耐蚀性能。结果表明,CoCrNi中熵合金熔覆层与基体形成了良好的冶金结合,无裂纹、气孔等缺陷;熔覆层由单一FCC相组成,组织为枝晶、枝晶间结构,各元素在熔覆层中均匀分布,无明显偏析现象,Cr元素的均匀分布对提升涂层耐蚀性具有积极作用;CoCrNi中熵合金熔覆层的极化电阻(101.65 kΩ)高于304基体的极化电阻(90.56 kΩ),耐蚀性明显高于304基体。

PBS人工模拟体液中纤维蛋白原对CoCrNi合金的腐蚀行为的影响(英文)

采用电化学测试技术研究了在PBS人工模拟体液中纤维蛋白原对CoCrNiMo和CoCrNiW合金腐蚀行为的影响。结果表明,CoCrNi合金具有良好的耐孔蚀性能,其钝化电位区较宽,且电位扫描曲线滞后环面积小。用EDX能谱对试样进行分析发现,在CoCrNi合金表面形成了Co,Cr,Ni,Mo,W等的钝性金属氧化物膜。纤维蛋白原在CoCrNi合金表面发生了吸附,且吸附的蛋白质分布不连续。用紫外吸收法测量CoCrNiMo和CoCrNiW对纤维蛋白原的吸附量分别为10.628μg/cm2和9.561μg/cm2。含有纤维蛋白原的PBS人工模拟体液中CoCrNi合金的腐蚀行为可用吸附理论解释。其反应步骤为:(1)氧原子吸附在合金能量较高的活性点上,形成较为稳定的吸附氧化膜;(2)纤维蛋白原扩散到合金表面以形成络合物的形式与氧原子构成竞争吸附;(3)形成的金属络合物向溶液产生电化学迁移,破坏了合金表面的钝性氧化膜,使icorr值显著增大,加剧了合金的腐蚀。

Enhancing Strength-Ductility Synergy of CoCrNi-Based Medium-Entropy Alloy Through Coherent L1_(2)Nanoprecipitates and Grain Boundary Precipitates

The L1_(2)-strengthened Co_(34)Cr_(32)Ni_(27)Al_(4)Ti_(3)medium-entropy alloy(MEA)with precipitations of grain boundaries has been developed through selective laser melting(SLM)followed by cold rolling and annealing,exhibiting excellent strength-ductility synergy.The as-printed alloy exhibits low yield strength(YS)of~384 MPa,ultimate tensile strength(UTS)of~453 MPa,and uniform elongation(UE)of 1.5%due to the existence of the SLM-induced defects.After cold rolling and annealing,the YS,UTS,and UE are significantly increased to~739 MPa,~1230 MPa,and~47%,respectively.This enhancement primarily originates from the refined grain structure induced by cold rolling and annealing.The presence of coherent sphericalγ'precipitates(L1_(2)phases)and Al/Ti-rich precipitates at the grain boundaries,coupled with increased lattice defects such as dislocations,stacking faults,and ultrafine deformation twins,further contribute to the property’s improvement.Our study highlights the potential of SLM in producing high-strength and ductile MEA with coherent L1_(2)nanoprecipitates,which can be further optimized through subsequent rolling and annealing processes.These findings offer valuable insights for the development of high-performance alloys for future engineering applications.

45钢表面激光熔覆CoCrNi中熵合金涂层组织与性能

为探究CoCrNi中熵合金在激光熔覆领域中的应用,以CoCrNi合金粉末作为熔覆粉末,在45钢表面采用同轴送粉法制备合金涂层。利用扫描电镜、X射线衍射仪、显微硬度仪、摩擦磨损实验机和电化学工作站等设备研究了熔覆层微观组织、硬度、耐磨性和耐腐蚀性能。结果表明:熔覆层成形良好,组织均匀致密,组成相主要为FCC单相固溶体;熔池与基体交界处为平面晶,底部靠近中心为柱状晶,顶部分别为胞状晶和等轴晶,3种元素在熔覆层深度方向上的比例几乎相同;熔覆层平均硬度为250HV,摩擦系数、磨损量较基体分别降低了11.7%和36.7%;自腐蚀电流密度略有降低,CoCrNi熔覆层的钝化区域为-150到1100 mV,表明熔覆层显著提高45钢的耐腐蚀性能。

One-Dimensional Magnetic FeCoNi Alloy Toward Low-Frequency Electromagnetic Wave Absorption

Rational designing of one-dimensional(1D)magnetic alloy to facilitate electromagnetic(EM)wave attenuation capability in low-frequency(2-6 GHz)microwave absorption field is highly desired but remains a significant challenge.In this study,a composite EM wave absorber made of a FeCoNi medium-entropy alloy embedded in a 1D carbon matrix framework is rationally designed through an improved electrospinning method.The 1D-shaped FeCoNi alloy embedded composite demonstrates the high-density and continuous magnetic network using off-axis electronic holography technique,indicating the excellent magnetic loss ability under an external EM field.Then,the in-depth analysis shows that many factors,including 1D anisotropy and intrinsic physical features of the magnetic medium-entropy alloy,primarily contribute to the enhanced EM wave absorption performance.Therefore,the fabricated EM wave absorber shows an increasing effective absorption band of 1.3 GHz in the low-frequency electromagnetic field at an ultrathin thickness of 2 mm.Thus,this study opens up a new method for the design and preparation of high-performance 1D magnetic EM absorbers.

Strength and ductility synergy of Nb-alloyed Ni_(0.6)CoFe_(1.4) alloys

Designing strong, yet ductile, and body-centered cubic(BCC) medium-entropy alloys(MEAs) remains to be a challenge nowadays.In this study, the strength–ductility trade-off of Ni_(0.6)CoFe_(1.4)MEAs was tackled via introducing a BCC + face-centered cubic(FCC) dual-phase microstructure. Ni_(0.6)CoFe_(1.4)Nbx(x = 0, 0.05, 0.08, 0.10, and 0.15, in molar ratio) MEAs were prepared using vacuum induction melting. Results show that the new alloy is composed of BCC plus FCC dual phases featuring a network-like structure, and the BCC phase is the main phase in this alloy system. Moreover, the Nb0.10 MEA shows high strength and respectable tensile ductility, representing the best combination of the strength and fracture elongation among the alloys studied here. The remarkable strength of the Nb0.10 MEA is attributed to the combined effect of the solid solution strengthening, the precipitation hardening effect and the interface strengthening effect.

热锻态CoCrNi中熵合金组织及力学性能的研究

通过力学性能测试以及OM、XRD、SEM和EBSD分析,研究了热锻对CoCrNi三主元中熵合金组织和力学性能的影响。结果表明:热锻后合金的晶粒得到明显的细化,晶体结构为面心立方结构(FCC)。晶粒内部的大量退火孪晶与扩展层错的共同作用使得合金具有良好的室温力学性能。屈服强度达到380 MPa,抗拉强度达到850 MPa,伸长率达到92%,合金表现出极高的强塑性(强塑积为76.5 GPa·%)。