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.

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。

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。

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。

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.

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。

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.

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

Cast Cr Co Ni Alx(x=0-1.2) medium-entropy alloys(MEAs) were produced by arc melting and flip cast to investigate the alloying effect of Al 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 Al 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 Al addition. A very interesting phenomenon of very weak ordered FCC(001) spots appearing in Al-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.

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.

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。

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.

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.

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.

CoCrNi基中熵合金研究进展

对于近年来CoCrNi基中熵合金在加工方法、组织、性能等方面的研究进展进行了综述。介绍了CoCrNi基中熵合金的具体情况,包括高熵合金、中熵合金的相关概念及CoCrNi基中熵合金的应用前景和应用领域。重点对其在加工工艺、显微组织和力学性能等方面的研究进行了介绍。总结了该合金在研究中存在的问题,展望了CoCrNi基中熵合金未来的研究方向。

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.

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。

两种不同加工状态CrCoNi中熵合金的微观组织和力学性能研究

中熵合金是近年来材料研究的热点,一般对中熵合金采用“铸造-轧制-热处理”的加工工艺。采用XRD、金相显微镜、万能试验机、扫描电镜对铸态、轧制+热处理的CoCrNi中熵合金的金相组织、硬度、室温、低温(77 K)力学性能以及拉伸断口进行分析。结果表明:对铸态CoCrNi合金轧制+热处理,两种不同状态合金均稳定保持面心立方(FCC)相,其中铸态组织为典型的树枝晶,但是无明显第二相存在,轧制+热处理对铸态组织有明显的细化和均匀化作用,对材料的拉伸强度和硬度有明显提升。两种不同加工状态的合金在低温下拉伸性能均发生明显提升,其中轧制后热处理中熵合金在液氮温度下抗拉强度达到1262.9 MPa、拉伸延伸率能达到89.6%,且两种状态下中熵合金在不同温度下均发生塑性断裂。在室温下CoCrNi合金的塑性变形行为以滑移为主,低温下CoCrNi合金塑性变形行为以孪生为主。

Wear-resistant CoCrNi multi-principal element alloy at cryogenic temperature

Traditional high strength engineering alloys suffer from serious surface brittleness and inferior wear performance when servicing under sliding contact at cryogenic temperature.Here,we report that the recently emerging CoCrNi multi-principal element alloy defies this trend and presents dramatically enhanced wear resistance when temperature decreases from 273 to 153 K,surpassing those of cryogenic austenitic steels.The temperature-dependent structure characteristics and deformation mechanisms influencing the cryogenic wear resistance of CoCrNi are clarified through microscopic observation and atomistic simulation.It is found that sliding-induced subsurface structures show distinct scenarios at different deformation temperatures.At cryogenic condition,significant grain refinement and a deep plastic zone give rise to an extended microstructural gradient below the surface,which can accommodate massive sliding deformation,in direct contrast to the strain localization and delamination at 273 K.Meanwhile,the temperature-dependent cryogenic deformation mechanisms(stacking fault networks and phase transformation)also provide additional strengthening and toughening of the subsurface material.These features make the CoCrNi alloy particularly wear resistant at cryogenic conditions and an excellent candidate for safety–critical applications.

Ultrastrong and ductile(CoCrNi)_(94)Ti_(3)Al_(3)medium-entropy alloys via introducing multi-scale heterogeneous structures

The coarsening-grained single-phase face-centered cubic(fcc)medium-entropy alloys(MEAs)normally exhibit insufficient strength for some engineering applications.Here,superior mechanical properties with ultimate tensile strength of 1.6 GPa and fracture strain of 13.1%at ambient temperature have been achieved in a(CoCrNi)_(94)Ti_(3)Al_(3)MEA by carefully architecting the multi-scale heterogeneous structures.Electron microscopy characterization indicates that the superior mechanical properties mainly originated from the favorable heterogeneous fcc matrix(1-40μm)and the coherent sphericalγ’precipitates(10-100 nm),together with a high number density of crystalline defects(2-10 nm),including dislocations,small stacking faults,Lomer-Cottrell locks,and ultrafine deformation twins.

Effects of post annealing on the microstructure,precipitation behavior,and mechanical property of a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy fabricated by laser powder bed fusion

The additive manufacturing of multi-principal element alloys has remarkable potential for industrial ap-plications.In this study,a(CoCrNi)_(94)Al_(3)Ti_(3)medium-entropy alloy(MEA)with adequate strength-ductility synergy was prepared via laser powder bed fusion.The microstructural evolution,mechanical property,and deformation mechanisms of the MEA were investigated after post annealing for a short period(0.5 h)at a temperature range of 773-1373 K using various microstructural characterization techniques and quantitative analysis.The static recrystallization temperature of the(CoCrNi)_(94)Al_(3)Ti_(3)MEA ranged from 973 to 1073 K.The average grain size first decreased and then increased,while the dislocation den-sity persistently decreased and texture gradually weakened with increasing annealing temperature.Cr-richσ-phase precipitates formed after 1073 K and then gradually dissolved at 1373 K,while Ni,Al,and Ti elements were aggregated to form a small amount of fine L1_(2)coherent precipitates with an aver-age diameter of approximately 70 nm at 1373 K.The evolution of the dislocation density,grain size,and precipitates significantly influenced the propensity of deformation twins and stacking faults,which consequently affected the strain hardening behavior and mechanical properties.The quantitative calcu-lation of strengthening mechanisms showed that dislocation strengthening played a dominant role at annealing temperatures below 1073 K,and it significantly weakened at 1373 K.Precipitation and grain boundary strengthening both markedly increased owing to the formation of precipitation particles and recrystallization-induced grain refinement after annealing at 1073 K.