The Hall–Petch breakdown in nanocrystalline metals: A crossover to glass-like deformation

The deformation behavior of nanocrystalline Ni–W alloys is evaluated by nanoindentation techniques for grain sizes of 3–150 nm, spanning both the range of classical Hall–Petch behavior as well as the regime where deviations from the Hall–Petch trend are observed. The breakdown in strength scaling, observed at a grain size of 10–20 nm, is accompanied by a marked transition to inhomogeneous, glass-like flow (i.e. shear banding) at the finest grain sizes approaching the amorphous limit. As a consequence of this mechanistic crossover, additional inflections arise in the mechanical properties; maxima are observed in both the rate and pressure dependence of deformation at approximately the same grain size as the onset of the Hall–Petch breakdown. These data experimentally connect the mechanical properties of nanocrystalline alloys to the well-known behavior of amorphous metals.     

纳米晶(Ag-Cu28)-25Sn合金粉末的制备及表征

采用机械合金化法制备纳米晶(Ag-Cu28)-25Sn合金粉末.用X射线衍射(XRD)仪、扫描电镜(SEM)、高分辨透射电镜(HRTEM)和差示扫描量热分析仪(DSC)等分析手段,对合金化过程中物相组成、微观结构及熔化特性进行表征.结果表明:(Ag-Cu28)-25Sn纳米晶合金粉末的物相组成为Cu3Sn和Ag4Sn.球磨 40 h,合金化完全,其熔化温度为548.5 ℃;球磨至60 h,合金明显非晶化,其熔化温度为554.0 ℃,熔程变小且在186.3和399.5 ℃处出现明显放热峰.HRTEM表明,纳米晶的尺寸约为5～10 nm,合金中有非晶态物质出现和晶格缺陷产生.200和400 ℃退火后,合金的平均晶粒尺寸分别为21.3和33.9 nm.     

Al-TiB2纳米晶薄膜中的反Hall-Petch效应

多种微结构因素作用的相互交织使纳米晶合金中是否存在与纯金属类似的反Hall-Petch现象难以得到实验证实。选用Al-TiB_2体系,采用二维结构纳米多层膜的方法,实现了对晶粒尺寸因素的孤立和使其独立地改变,研究了晶粒尺寸对薄膜力学性能的作用规律。结果表明:Al-TiB_2过饱和固溶纳米晶薄膜也与纳米晶纯金属Al一样,存在硬度随晶粒尺寸减小从遵从Hall-Petch关系提高转变为偏离Hall-Petch关系,并进一步出现反Hall-Petch效应的3个阶段,实验得到了偏离Hall-Petch关系为32 nm,产生反Hall-Petch现象的临界晶粒尺寸为8 nm,这2个临界晶粒尺寸与分子动力学方法对纳米晶纯金属A1计算的结果相当。     

机械合金化W-Ni-Fe纳米复合粉的制备及结构研究

W,Ni,Fe粉末按照91.16W6．56Ni2.26Fe和95W5Ni的成分配比进行了机械合金化(MA)．通过调整球磨转速、球磨时间等工艺参数研究了其对粉末结构的影响,并对机械合金化粉末的物相、合金化特性、晶粒尺寸、点阵畸变及粉末形貌和颗粒度作了测定和分析讨论.机械合金化使晶粒细化并产生孪晶和位错．有利于原子扩散形成过饱和固溶体和非晶；高的球磨能有利于形成非晶相、晶粒细化和点阵畸变，350r／min球磨20h后晶粒尺寸可达25nm；输入的球磨能不同．粉末粒度的变化路径不同，但都会经历长大，变小和稳定三个不同阶段.     

Thermal behavior and structure of Fe84Nb7B9 nanocrystalline powders

1 Introduction Fe84Nb7B9 nanocrystalline alloy is an excellent type of soft magnetic materials, characterized by its high saturation induction, permeability and good noise degeneration property[1, 2]. It can effectively promote the miniature, light, energ…     

机械合金化制备W-Ni-Fe纳米-非晶材料

按照80．7W-13．2Ni-6．1Fe的原子分数．采用机械合金化(MA)方法，制备了W-Ni-Fe合金纳米晶和非晶相的混晶结构。结合XRD，利用近似内标法计算了球磨不同时间球磨粉中残留晶体W的体积分数和非晶相中的W含量，并分析了球磨过程中非晶形成的机制。结果表明：随球磨时间的延长，W晶粒不断细化．球磨60h，钨晶粒尺寸可达到10nm-20nm，非晶相的形成过程主要是Ni(Fe)首先溶入W中形成过饱和固溶体，球磨20h后形成W-Ni(Fe)非晶。过饱和固溶体的形成是由于携带较大晶界存储能的小粒子不断溶入W中，计算得到可固溶的临界Ni粒子尺寸约为3nm。由于Fe污染不断溶入W中，在球磨过程中，残留晶体W的体积分数不断减少．而非晶相中的W-Ni(Fe)比例基本保持恒定，为63W-37Ni(Fe)。     

Tailoring and patterning the grain size of nanocrystalline alloys

Nanocrystalline alloys that exhibit grain boundary segregation can access thermodynamically stable or metastable states with the average grain size dictated by the alloying addition. Here we consider nanocrystalline Ni–W alloys and demonstrate that the W content controls the grain size over a very broad range: ∼2–140 nm as compared with ∼2–20 nm in previous work on strongly segregating systems. This trend is attributed to a relatively weak tendency for W segregation to the grain boundaries. Based upon this observation, we introduce a new synthesis technique allowing for precise composition control during the electrodeposition of Ni–W alloys, which, in turn, leads to precise control of the nanocrystalline grain size. This technique offers new possibilities for understanding the structure–property relationships of nanocrystalline solids, such as the breakdown of Hall–Petch strength scaling, and also opens the door to a new class of customizable materials incorporating patterned nanostructures.     

机械合金化制备纳米晶Ti-6Al-4V合金

采用机械合金化制备Ti-6Al-4V粉末。结果表明:采用机械合金化可以制备纳米晶Ti-6Al-4V合金粉,其反应机理以扩散为主,该固态反应是缺陷能和碰撞能共同作用的结果;随球磨时间延长,部分V固溶于Ti中形成置换固溶体Ti(V),球磨过程中没有中间相生成。球磨40 h后都能获得纳米晶,60 h的粉末为纳米晶和非晶的混合物,晶粒尺寸小于60 nm;60 h后晶粒尺寸变化缓慢。球磨后Ti、Al、V的原子比近似为90:6:4,与Ti-6Al-4V元素成分一致。     

Grain boundary segregation,chemical ordering and stability of nanocrystalline alloys: Atomistic computer simulations in the Ni–W system

Atomistic computer simulations are used to investigate the equilibrium solute distribution and alloying energetics in nanocrystalline Ni–W. Composition and grain size-dependent trends in grain boundary segregation and chemical ordering behavior are evaluated and we find the equilibrium state to be significantly influenced by the nanostructure. The energetics of alloying are assessed through computation of the segregation, formation, and grain boundary energy, and these quantities are linked to previous thermodynamic models of nanostructure stability. With comparison to experiments, we conclude that nanocrystalline Ni–W alloys are synthesized in a metastable state. These findings have important consequences for theories of nanostructure control in general and particularly for the thermal stability of nanocrystalline Ni–W.     

The correlation of microstructural development and thermal stability of mechanically alloyed multicomponent Fe-Co-Ni-Zr-B alloys

The microstructural evolution of multicomponent Fe_70-x-yCo_xNi_yZr₁₀B₂₀ (x = 0, 7, 21; y = 7, 14, 21, 28) alloys during mechanical alloying (MA) has been studied using XRD, SEM and TEM. Mixtures of elemental and pre-alloyed powders have been transformed initially into the single supersaturated bcc α-Fe solid solution phase for the alloys investigated. Subsequently, an amorphous phase has been obtained in Co-free alloys and Co-containing alloys with high Ni/Co ratios of 1 and 3. However, no amorphous phase was detected in another Co-containing alloy with a lower Ni/Co ratio (e.g. 0.33). The thermal stability of the as-milled powders has been investigated by a combination of DSC and the Pendulum magnetometer experiments. The DSC studies provide information on the thermodynamics and kinetics of crystallization of amorphous structure as a function of alloying contents. The Pendulum magnetometer studies reveal the phase transformation from nanocrystalline bcc α-Fe solid solution to amorphous structure during MA and the thermomagnetization behavior of the as-milled powder.     

Pressure-composition-isotherm behavior of nano-/amorphous Mg<Subscript>2</Subscript>NiH<Subscript>x</Subscript>

It has recently been shown that the hydriding properties of the nanocrystalline metal hydrides are far superior to those of the polycrystalline ones. Especially in the case of the Mg-based hydrogen storage alloys, nanostructural modifications have been studied for the purpose of improving their hydrogenation kinetics. In previous studies, I reported on the successful fabrication of Mg₂NiH_x from Mg and Ni chips with hydrogen induced mechanical alloying (HIMA). Observation of the microstructure showed that the synthesized particles (processed with a 66:1 ball to chips mass ratio and 96 hr HIMA) are composed of amorphous and nanocrystalline composite phases with a grain size of less than 10 nm. The aim of the present work was to examine the hydriding/dehydriding behavior of nanocrystalline metal hydrides using a Sieverts type automatic pressure-composition-isotherm (PCI) apparatus at 393, 423, 453, 483, 513 and 543 K. The specimen was characterized by X-ray diffraction after PCI measurement. The influence of hydrogenation behavior on the phase transition of nano-/amorphous Mg₂Ni is a key factor in commercial application. The particles synthesized at 66:1 BCR and 96 hr HIMA revealed a good hydrogen capacity of 2.25 mass% at 483 K.     

Effects of doping route on microstructure and mechanical properties of W−1.0wt.%La2O3 alloys

A comparative study was conducted by using solution combustion synthesis with three different doping routes (liquid−liquid (WL10), liquid−solid (WLNO) and solid−solid (WLO)) to produce nanoscale powders and further fabricate the ultrafine-grained W−1.0wt.%La₂O₃ alloys by pressureless sintering. Compared with pure tungsten, W−1.0wt.%La₂O₃ alloys exhibit ultrafine grains and excellent mechanical properties. After sintering, the average grain size of the WLO sample is larger than that of WL10 and WLNO samples; the microhardness values of WL10 and WLNO samples are similar but larger than the value of WLO sample. The optimized La₂O₃ particles are obtained in the WL10 sample after sintering at 1500 °C with the minimum mean size by comparing with WLNO and WLO samples, which are uniformly distributed either at grain boundaries or in the grain interior with the sizes of (57±29.7) and (27±13.1) nm, respectively. This study exhibits ultrafine microstructure and outperforming mechanical properties of the W−1.0wt.%La₂O₃ alloy via the liquid−liquid doping route, as compared with conventionally-manufactured tungsten materials.     

Crystallization kinetics of （Ni0.75Fe0.25）78-xNbxSi10B12 （x = 0, 5） amorphous alloys

Amorphous ribbons of （Ni0.75Fe0.25）78-xNbxSi10B12 （x = 0, 5） were prepared by a single roller melt-spinning technique in air atmosphere. The crystallization kinetics of the alloys were investigated by means of continuous heating, and the activation energies of the alloys were calculated using Kissinger plot method and Ozawa plot method on the basis of differential thermal analysis data. The crystallization products were analyzed by X-ray diffraction. After the （Ni0.75Fe0.25）78Si10B12 amorphous alloy was annealed at the temperatures 715 and 745 K, a single phase of γ-（Fe, Ni） solid solution with grain sizes of about 10.3 and 18.5 nm, respectively, precipitates in the amorphous matrix. The crystallized phases are γ-（Fe, Ni） solid solution, Fe2Si, Ni2Si, and Fe3B after annealing at 765 K. The （Ni0.75Fe0.25）73NbsSi10 B12 amorphous alloy was annealed at 720, 750, and 800 K; and the crystallization phases, γ（Fe, Ni） solid solution, （Fe, Ni）23B6. Ni31Si12 and Nb2NiB0.16 form simultaneously.     

碳化帆颗粒尺寸对纳米晶硬质合金组织和性能的影响

利用原位还原碳化反应制备纳米尺度的WC-Co复合粉体,应用放电等离子烧结(SPS)技术制备出纳米晶WC-Co硬质合金块体材料。分析了晶粒长大抑制剂碳化钒(VC)颗粒尺寸对纳米晶硬质合金的显微组织、晶粒尺寸及分布和力学性能的影响。结果表明:当VC的粒径减小到100 nm以下时,利用快速烧结技术可制备得到平均晶粒尺寸约为70 nm的致密WC-Co硬质合金块体材料,其物相纯净,晶粒尺寸分布均匀,维氏硬度为19.84 GPa,断裂韧性达到12.10 MPa·m1/2。     

Formation of Fe-19 wt%Cr-9 wt%Ni Nanocrystalline Alloy with Excellent Corrosion Resistance: Phase Transition and Microstructure

In this paper, microstructure characteristics and phase transitions of Fe-19 wt%Cr-9 wt%Ni nanocrystalline alloy are comprehensively studied during the mechanical alloying and hot pressing sintering processes. Corrosion resistance of the sintered Fe-19 wt%Cr-9 wt%Ni nanocrystalline alloy samples is further analyzed. During the mechanical alloying process, Fe-19 wt%Cr-9 wt%Ni nanocrystalline alloy powders mainly composed of metastable ferrite phase are obtained after mechanical alloying for 8, 16 and 24 h, respectively. In the subsequent hot pressing sintering process, the phase transitions(from ferrite to austenite) occur from 650 to 750 °C for Fe-19 wt%Cr-9 wt%Ni alloy powders milled for 24 h. When the sintering temperature is raised to 1050 °C for 1 h, the ferrite phase has transformed into austenite phase completely, and the obtained grain size of sintered Fe-19 wt%Cr-9 wt%Ni alloy is around 40 nm. Electrochemistry test of the sintered Fe-19 wt%Cr-9 wt%Ni alloy has been operated in 0.5 mol L~(-1) H_2SO_4 solution to show the corrosion resistance properties. Results show that the sintered Fe-19 wt%Cr-9 wt%Ni alloy exhibits excellent corrosion resistance, which is proved by higher self-corrosion potential, lower self-corrosion current density and larger capacitive reactance, compared with that of commercial 304 stainless steel.     

Mechanical alloying and magnetic saturation of tungsten–nickel powders

Alloying mechanism and magnetic saturation of tungsten and W-40 wt.% Ni milled powders were investigated using XRD, SEM and saturation magnetisation techniques. Mechanical alloying was proceeded by deformation of FCC Ni toward FCT phase and BCC to BCT in W, hence formation of supersaturated tetragonal Ni(W) solid solution. Milling of pure W yielded a product comprised of magnetic BCT and non-magnetic nanocrystalline BCC W powders. The magnetic saturation of W increased at the early milling stage and decreased later due to the transition of the BCC W structure toward anisotropic close packed crystal structure and formation of nanograins with high specific surface. Magnetic saturation of W–Ni powders decreased with milling time but increased after forming a metastable tetragonal solid solution.     

Synthesis of TiB2 nanocrystalline powder by mechanical alloying

1 Introduction TiB2 has been widely used in some industrial fields owing to its high melting temperature, hardness, elastic modulus, electro-conductibility and thermal diffusivity, and excellent refractory properties and chemical inertness. Usually, TiB2…     

Effect of Sn on synthesis of nanocrystalline Ti-based alloy with fcc structure

The effect of the amount of Sn on the formation of fcc phase in Ti−13Ta−xSn (x=3, 6, 9 and 12, at.%) alloys was studied. The alloys were synthesized by mechanical alloying using a planetary mill, jar and balls of stabilized yttrium. Using Rietveld refinement, it was found that the obtained fcc phase has crystallite size smaller than 10 nm and microstrain larger than 10⁻³. Both conditions are required to form an fcc phase in Ti-based alloys. For all samples, the microstructure of the fcc phase consists of equiaxial crystallites with sizes smaller than 10 nm. The largest presence of fcc phase in the studied Ti alloy was found with 6 at.% Sn, because this alloy exhibits the largest microstrain (1.5×10⁻²) and crystallite size of 6.5 nm. Experimental data reveal that a solid solution and an amorphous phase were formed during milling. The necessary conditions to promote the formation of solid solution and amorphous phases were determined using thermodynamic calculations. When the amount of Sn increases, the energy required to form an amorphous phase varies from approximately 10 to approximately −5 kJ/mol for 3 and 12 at.% Sn, respectively. The thermodynamic calculations are in agreement with XRD patterns analysis and HRTEM results.     

高能球磨制备PS304纳米复合粉

将Ni，Cr，Cr2O3，Ag和BaF2／CaF2按PS304成分配比进行机械合金化（MA）。研究了不同的球磨时间对粉末性能的影响，并对机械合金化粉末的物相，晶粒尺寸及粉末形貌作了测定和分析讨论。研究发现，机械合金化使晶粒细化，其中Cr2O3，Ag的晶粒尺寸（球磨15h）可达50nm以下。CaF2和BaF2的X射线衍射峰随球磨时间的延长而逐渐地减弱直至消失，同时在整个球磨过程中，并没有发现CaF2或BaF2的新相生成，氟化物可能形成了过饱和固溶体。EDX分析表明，得到的粉为复合粉。     

铁基纳米软磁合金的制备

介绍机械合金化方法制备纳米软磁合金的试验中,不同的球磨时间与晶粒尺寸之间的变化关系,并分析了晶粒尺寸,镍、钼等元素含量对磁导率、矫顽力、单位质量功耗等软磁性能的影响,认为在铁基纳米软磁合金中适量加入钼有利于改善综合软磁性能。