热处理对AlCrFeNiTi高熵合金组织和硬度的影响

在氩气保护下用真空电弧炉制备了AlCrFeNiTi高熵合金,采用X射线衍射仪、金相显微镜和维氏硬度计等研究了加热温度、保温时间和冷却方式对AlCrFeNiTi高熵合金组织和硬度的影响。结果表明,铸态AlCrFeNiTi高熵合金由两个体心立方结构固溶体构成,形成了由枝晶、枝晶间和α+β共晶组织组成的典型高熵合金枝晶组织。随着加热温度的升高,合金枝晶相先粗化,当温度升至900℃时再细化;随着保温时间增加和冷却速度减慢,枝晶组织和共晶组织均粗化,而合金硬度呈上升趋势。合金经过热处理后的硬度取决于枝晶相含量和形貌,在高速冷却方式下残余内应力对合金的硬度也起关键作用。     

激光原位合成MC增强AlCrFeNb_3MoTiW高熔点高熵合金基复合涂层的高温组织演变

利用激光熔覆技术在工具钢表面成功制备出MC/AlCrFeNb_3MoTiW涂层,研究了其在750,850,950℃下组织转变行为及硬度变化。利用扫描电子显微镜(SEM)、能谱分析仪(EDS)、示差扫描量热仪(DSC)、X射线衍射仪(XRD)、透射电子显微镜(TEM)以及显微硬度计分析了高熵合金的显微组织结构及性能。结果表明,高熵合金涂层室温下由BCC相、复合碳化物及Fe_2Nb相组成,其中,复合碳化物由NbC,TiC,MoC,W_2C所组成;其组织为共晶组织及颗粒状碳化物构成。随着退火温度升高,碳化物稳定存在,共晶组织逐渐溶解,BCC相逐渐增多。750℃退火时,涂层内部共晶组织基本未发生改变,在黑色基体组织中析出细小的颗粒状Fe_2Nb相;850℃时,涂层中共晶组织发生长大,间距变大;当在950℃退火4h后,共晶组织基本消失,并在晶界处聚集。涂层在750℃以下具有良好的抗软化能力,能够很好地满足工具钢的工作要求。     

激光熔覆(FeCrCo/Ni)_5-SiTiNbMoW高熵合金涂层组织及性能研究

通过激光熔覆技术在工具钢表面成功制备(FeCrCo/Ni)_5-SiTiNbMoW高熵合金涂层,并采用带有能谱仪的扫描电镜(SEM/EDS)、X射线衍射仪(XRD)、显微硬度计对涂层组织、成分、物相和硬度进行分析测试。结果表明,(FeCrCo)_5-SiTiNbMoW涂层含有BCC主相、少量MC相及Laves相,(FeCrNi)_5-SiTiNbMoW涂层含有FCC主相及少量MC相、Laves相;两种涂层组织均以树枝晶为主,(FeCrNi)_5-SiTiNbMoW涂层组织中同时还出现共晶组织。(FeCrCo)_5-SiTiNbMoW高熵合金涂层的显微硬度略高于(FeCrNi)_5-SiTiNbMoW高熵合金涂层。     

间隙原子对高熵合金组织及性能影响的研究现状

高熵合金作为一种新型的合金体系,虽然其组成元素复杂,但能形成简单的固溶体,具有许多异于传统合金的结构和性能特征,其研究近年来成为热点。间隙原子可以溶入基体晶格间隙产生固溶强化,与合金元素结合形成细小弥散强化相,以及降低层错能,改变位错运动方式等,从而改善高熵合金性能。文章在论述高熵合金组织结构特性的基础上,分析了间隙原子C、N、O、B对高熵合金相形成规律、强化机理、塑性变形机制的影响,总结了间隙原子含量及其产生的固溶强化、晶粒细化、第二相强化作用对高熵合金组织性能等方面影响的研究进展,最后提出了含间隙原子的高强高韧高熵合金组织结构设计研究的新方向。      

激光熔覆高熔点AlCrFeMoNb_xTiW高熵合金涂层组织及耐磨性能

为了提高高速切削刀具的表面耐磨性能,设计了一种高熔点高熵合金成分体系,采用激光熔覆技术制备了AlCrFeMoNb_xTiW高熵合金涂层。利用光学显微镜(OM),X射线衍射(XRD),扫描电镜(SEM)等手段对其显微组织进行表征分析;利用显微硬度计、摩擦磨损试验机对涂层硬度及耐磨性能进行了检测。结果表明:熔覆层相组成主要由BCC,(Nb,Ti)C及Laves相所组成;当Nb原子分数为1时,涂层显微组织为胞状树枝晶及颗粒状碳化物;随着Nb原子比增加,涂层组织逐渐由放射状共晶向长条树枝状过共晶组织转变,但弥散分布的颗粒状(Nb,Ti)C始终存在于涂层内部。涂层硬度逐渐上升,当Nb原子分数为7时,涂层的最高硬度可达HV0.21017。磨损实验表明,各个Nbx涂层的平均摩擦系数差别不大;磨损量随着Nb含量的升高而降低;涂层磨损机制主要以磨粒磨损为主。     

均匀化处理对Cu-Cr-Zr合金显微组织的影响

采用OM、SEM和TEM分析手段对Cu-Cr-Zr合金的铸态和均匀化组织进行研究.结果表明,Cu-Cr-Zr合金的铸态组织呈典型的枝晶状组织,主要由网状的Cr枝晶、共晶组织和基体组成,其中共晶组织是由基体和层片状的Cu5Zr相组成.在Cu-Cr-Zr合金的均匀化退火过程中,发生共晶组织的溶解和Cr相的析出.随着均匀化退火温度和时间的升高和延长,共晶组织逐渐溶解,Cr相的析出体积分数逐渐减小.合理的均匀化退火制度为900℃×12 h.     

Nb含量对Al0.3CoCrFeNi高熵合金组织与性能的影响

采用真空电弧炉制备了Al_0.3CoCrFeNiNb_x(x为摩尔比,x=0, 0.1, 0.3, 0.5, 0.7, 1.0)高熵合金棒材,用X射线衍射(XRD)、光学显微镜(OM)、扫描电镜(SEM)、能谱仪(EDS)、显微维氏硬度计和万能试验机对合金的微观组织和性能进行了研究。结果表明,Al_0.3CoCrFeNi合金的微观组织为单相面心立方(fcc);添加Nb后,合金由fcc相和Laves相组成,并随着Nb含量的增加,Laves相体积分数逐渐增多;至Al_0.3CoCrFeNiNb_1.0时,基体相从fcc相变为Laves相。Al_0.3CoCrFeNiNb_x合金组织随Nb含量变化经历了亚共晶到过共晶的转变,同时衍射峰(111)fcc先向小角度移动后向大角度移动,晶格常数先增大后减小。Nb元素的增加可以有效提高Al_0.3CoCrFeNiNb_x合金的强度和硬度,硬度呈近似线性增加,从Al_...     

铸态Cr_xNbTiZr高熵合金的组织与性能研究

根据相形成准则设计了混合强化型Cr_xNbTiZr高熵合金,利用X射线衍射仪、扫描电子显微镜、万能材料试验机等研究了Cr对Cr_xNbTiZr高熵合金的显微组织与力学性能的影响。结果表明,Cr_xNbTiZr高熵合金由BCC相和部分Cr_2M(M=Nb,Ti,Zr)型Laves相金属间化合物组成,合金具有典型的树枝晶形貌,第二相不连续分布于枝晶间。Cr的固溶强化和Laves相第二相强化使得Cr1.0合金屈服强度提高到1 401 MPa,抗压强度提高到1 677 MPa,合金断裂应变降至13.30%。断口形貌分析显示该系列合金具有延性与脆性混合型断裂特征。     

车用不锈钢表面激光原位合成FeCrCoNi涂层组织及耐磨性

为提升车用316不锈钢的表面硬度及耐磨性,采用激光熔覆原位合成法在316不锈钢表面制备FeCrCoNi高熵合金涂层。分别对FeCrCoNi高熵合金涂层的相组成、元素分布、显微硬度及耐磨性进行研究。结果表明：原位合成制备的FeCrCoNi高熵合金涂层无裂纹、气孔等缺陷,与基材呈现良好的冶金结合。FeCrCoNi高熵合金涂层由单一的FCC相组成,涂层组织为树枝晶。各元素分布均匀,无明显偏析。FeCrCoNi高熵合金涂层的截面平均显微硬度约为283.7 HV,相较于316不锈钢基体提升了约50%。FeCrCoNi高熵合金涂层的平均摩擦因数分别为0.377和0.438,比磨损率分别为2.27×10^-5mm³/N·m和6.17×10^-5mm³/N·m,FeCrCoNi高熵合金涂层的磨损率降低了70%。FeCrCoNi高熵合金涂层的磨损机制为磨粒磨损。     

时效处理对Al0.5CoCrFeNi高熵合金微观组织结构和力学性能的影响

对Al0.5CoCrFeNi高熵合金经不同温度时效处理24 h后的微观组织和力学性能进行研究。结果表明:时效处理前后,Al0.5CoCrFeNi高熵合金均由简单的体心立方相和面心立方相组成,组织主要为树枝晶形貌。但是,随着时效温度的升高(800℃和1 000℃),树枝晶析出弥散分布的针状第二相,且第二相数量不断增大。由于第二相的弥散强化作用,时效处理能显著提高合金的抗拉强度。     

Overcoming the Strength-Plasticity Trade-Off in a Multi-phase Equiatomic CrCuNiTiV 3d Transition Metal High-Entropy Alloy

The phase components, microstructures, and mechanical properties of a multi-phase equiatomic 3d transition metal high entropy alloy, CrCuNiTiV, were investigated. In the as-cast condition, the alloy was mainly composed of a body-centered cubic (BCC) dendritic phase and a face-centered cubic (FCC) interdendritic phase, with the BCC dendrite uniformly distributed in the FCC matrix. Through annealing, three phases decomposed from the FCC parent phase, and small particles and stripes were derived from the elongated stripes of the as-cast alloy. Outstanding plasticity was acquired with only a small sacrifice in the yield strength after annealing. Specifically, the plasticity increased from 12.7 pct in the as-cast condition to 23.4 pct in the annealed condition while high yield strengths of 965 and 877 MPa were retained in the as-cast and annealed alloys, respectively. Overcoming the strength-plasticity trade-off in the annealed CrCuNiTiV alloy was mainly achieved via the large volume fraction of FCC interdendritic phases, together with the precipitation of one BCC phase and two FCC phases.

     

The Role of Carbon in Grain Refinement of Cast CrFeCoNi High-Entropy Alloys

As a promising engineering material, high-entropy alloys (HEAs) CrFeCoNi system has attracted extensive attention worldwide. Their cast alloys are of great importance because of their great formability of complex components, which can be further improved through the transition of the columnar to equiaxed grains and grain refinement. In the current work, the influence of C contents on the grain structures and mechanical properties of the as-cast high-entropy alloy CrFeCoNi was chosen as the target and systematically studied via a hybrid approach of the experiments and thermodynamic calculations. The alloys with various C additions were prepared by arc melting and drop cast. The as-cast macrostructure and microstructure were characterized using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The cast HEAs transform from coarse columnar grains into equiaxed grains with the C level increased to ≥ 2 at. pct and the size of equiaxed grains is further decreased with the increasing C addition. It is revealed that the interdendritic segregation of Cr and C results in grain boundary precipitation of M₂₃C₆ carbides. The grain refinement is attributed to the additional constitutional supercoiling from the C addition. The yield stress and tensile strength at room temperature are improved due to the transition of columnar to equiaxed grains and grain refinement.     

Microstructures and properties of CoCrCuFeNiMox high-entropy alloys fabricated by mechanical alloying and spark plasma sintering

CoCrCuFeNiMox (x values in molar ratio, x?=?0, 0.2, 0.4 and 0.8) high-entropy alloys were prepared by mechanical alloying and spark plasma sintering method. The effects of Mo addition on microstructure and mechanical properties were investigated. The X-ray diffraction (XRD) result showed that the addition of Mo into CoCrCuFeNi high-entropy alloy (HEA) changed the original phase constitution from FCC to FCC?+?σ?+?μ and the peak intensity of (1 1 1) shifted to the left and decreased steadily. The field emission scanning electron microscope confirmed that the Cu-rich second FCC phase disappeared and the σ phase with a tetragonal structure expanded as the Mo content was increased. Additionally, a high density of dimple-like features were seen in CoCrCuFeNi HEA while typical quasi-cleavage facets could be observed from the fracture surfaces of the HEAs with the addition of Mo. The Mo0.8 alloy showed a good wear resistant and appropriate strength with fracture strain 22.70%, fraction coefficient 0.65, hardness 530?HV and compressive strength 1448?MPa.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

Influence of zirconium addition on the microstructure,thermodynamic stability,thermal stability and mechanical properties of mechanical alloyed spark plasma sintered (MA-SPS) FeCoCrNi high entropy alloy

ABSTRACT

Equiatomic FeCoCrNi (Zr₀) and non-equiatomic FeCoCrNiZr_0.4 (Zr_0.4) high-entropy alloys (HEAs) were synthesised by mechanical alloying and spark plasma sintering. XRD analysis verified the formation of FCC and BCC solid solution phases in both alloys after 30?h of ball milling. While the SPS FeCoCrNi alloy contains both FCC and BCC solid solution phases, the FeCoCrNiZr_0.4 presents an FCC solid solution. The thermodynamic analysis showed that FeCoCrNiZr_0.4 is more stable with respect to the FeCoCrNi alloy. The phase stability of FeCoCrNiZr_0.4 was revealed up to ～800°C. The shear strength and hardness of the FeCoCrNi HEA improved with Zr addition. Failure analysis of the shear punch tested samples revealed a ductile fracture with dimple structure for FeCoCrNi and a brittle fracture with a smooth featureless surface for FeCoCrNiZr_0.4.     

Tribological properties of FeCoCrNiAlBx high-entropy alloys coating prepared by laser cladding

FeCoCrNiAlBx(x=0, 0.25, 0.50, 0.75) coatings were prepared by laser cladding to study the effects of boron on the structure and properties of high-entropy alloys coatings.The microstructure, microhardness, and wear resistance properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, metallographic micro-hardness test, and friction-wear test, respec-tively, and the mechanism of the wear behavior was also analyzed.The results showed that the high-entropy alloys consisted of BCC phase and eutectic structure, which contained FCC phase and M2 B. With boron addition, the content of BCC phase increased while that of eutectic structure decreased. The wear resistance of the high-entropy coatings was considerably improved with increasing addition of boron, and accordingly, the FeCoCrNiAlB0.75 coating showed the best wear resistance.     

Strength and ductile-phase toughening in the two-phase Nb/Nb5Si3 alloys

The effect of heat treatment on the mechanical properties of Nb-Nb₅-Si₃ two-phase alloys having compositions Nb-10 and 16 pct Si (compositions quoted in atomic percent) has been investigated. This includes an evaluation of the strength, ductility, and toughness of as-cast and hot-extruded product forms. The two phases are thermochemically stable up to ∼1670 °C, exhibit little coarsening up to 1500 °C, and are amenable to microstructural variations, which include changes in morphology and size. The measured mechanical properties and fractographic analysis indicate that in the extruded condition, the terminal Nb phase can provide significant toughening of the intermetallic Nb₅Si₃ matrix by plastic-stretching, interface-debonding, and crack-bridging mechanisms. It has been further shown that in these alloys, a high level of strength is retained up to 1400 °C.     

Microstructure and mechanical properties of Mg-Zn-Mn-Sn-Nd wrought alloys

The microstructure and mechanical properties of as-cast and extruded Mg-6Zn-1Mn-4Sn-xNd(x=0, 0.5, 1.0, 1.5) alloys were investigated by means of optical microscopy(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM) and tensile test. The results showed that the dendrites sizes of these alloys were decreased by the addition of Nd. The phase compositions of the as-cast Mg-Zn-Mn-Sn-Nd alloys were dendritic α-Mg, MgZn2, Mg2 Sn, T phase and MgSnNd ternary phase. The mechanical properties of the as-extruded alloys were improved due to the refined equiaxed grains and dispersive Mg2 Sn and MgSnNd second phases. The comparison of the theoretical yield strength with the experimental yield strength revealed that the yield strength model of the as-extruded alloys should be modified as σys =σMg +σgb +σss +σsp.     

Solid solution decomposition mechanisms in as-cast and microcrystalline Al-Sc alloys: IV. Effect of the decomposition of a solid solution on the mechanical properties of the alloys

The effect of the decomposition of the solid solution in Al-X wt % Mg-0.22 wt % Sc-0.15 wt % Zr (X = 0, 1.5, 4.5 wt %) alloys on their mechanical properties in the as-cast and microcrystalline (MC) states is studied. The contribution of the particles that precipitate during the decomposition to the mechanical properties of the as-cast and MC alloys is determined. For this purpose, the dependences of the volume fraction and the size of the particles precipitating during the decomposition on the annealing time and temperature and the magnesium content in the as-cast and MC alloys are calculated. An additional contribution to the hardening of the MC alloys is shown to be related to the internal stress fields created by the defects that accumulate at the disperse particles distributed in grain boundaries during grain-boundary migration. The calculated macroelasticity limits in the as-cast and MC alloys are compared with the experimental data.     

Calculation-experimental study of the phase composition of Al-Zn-Mg-(Cu)-Ni-Fe aluminum alloys

To optimize the compositions of new high-strength aluminum ATs7NZh and ATs6N0.5Zh alloys (economically alloyed nikalins), the thermodynamic optimization of the Al-Zn-Mg-Cu-Ni-Fe system is performed via the construction of polythermal sections and the calculation of the chemical composition and volume fraction of phases at characteristic temperatures. The concentrations of the matrix elements (Zn, Mg, Cu) that determine the high level of mechanical properties are shown to be 6–7 wt % Zn, 2–3 wt % Mg, and up to 0.3 wt % Cu. The concentrations of the eutectic-forming elements (Ni, Fe) that ensure the solidification of (Al) + Al₉FeNi eutectic are determined. This eutectic favors an increase in the manufacturing properties of the alloys during casting, metal forming, and welding along with a retained high level of the mechanical properties. In general, experimental results confirm the calculated data.     

Order and Disorder in Amorphous and High-Entropy Materials

Order and disorder are important principles in materials science in which entropy is a measure of disorder in a system. For example, recently developed high-entropy alloys and amorphous alloys have drawn interest based on the ability to design their disorder to bring out different material characteristics. High-entropy alloys are controlled by chemical disorder, whereas amorphous alloys are governed by topological disorder. There is often a need to increase disorder or entropy in these materials to satisfy certain complex performance requirements. Here, we examine the roles of order, disorder, and entropy in amorphous and high-entropy alloys. Several key research topics are summarized, including high-entropy films, high-entropy ceramics, and high-entropy alloys. Moreover, there remain questions about the role of entropy stabilization in high-entropy ceramics. Here, we also report three novel porous light-weight high-entropy nitrides based on the NbTiAlSi system. Our findings clarify the general role of entropy in high-entropy ceramics.