Transmission electron microscopy studies of microstructure of Ti–Nb and Ti–Ta alloys after ball milling and hot consolidation

Two Ti alloys with compositions Ti?10Ta and Ti?10Nb (at.%) were milled in a high‐energy mill for a maximum of 80 h in an argon atmosphere. A nanocrystalline structure of α‐Ti(X) (X = Ta or Nb) solid solution was formed in both investigated alloys after milling, as shown by X‐ray diffraction. Transmission electron microscopy observations of powders milled for 80 h revealed chemical inhomogeneity of particles in nanometre‐scale regions and an average crystallite size of about 10 nm. The pulse plasma sintering method was applied for hot consolidation of milled powders. The mean density of pulse plasma sintering compacts of Ti–Nb alloy was about 99.5% of the theoretical value, whereas the density of the Ti?10Ta sample was lower, close to 92% of the theoretical value. Transmission electron microscopy observation of compacted samples showed that the sintering process caused the formation of a two‐phase α + β structure in both investigated alloys, with a mean grain size of 220 nm. The chemical inhomogeneity and high degree of deformation in nanometre‐scale regions of milled powders led to a martensitic transformation, resulting in formation of a 9R martensite structure.     

HRTEM studies of NiNbZr + Ag amorphous-nanocrystalline composites

Amorphous powder of composition corresponding to Ni60Ti20Zr20 (in at%) was obtained by ball milling in a high-energy mills starting from pure elements. Formation of the amorphous structure was observed already after 20 h of milling, although complete amorphization occurred after 40 h. The microhardness of powders increased from about 30 HV for pure elements to above 400 HV (1290 MPa) after 40 h of milling. Transmission electron microscopy (TEM) allowed to identify nanocrystalline inclusions of intermetallic phases of size 2–10 nm. Uniaxial hot pressing was performed in vacuum at temperature below the crystallization T_x it is 510°C and pressure of 600 MPa, Mixed amorphous powders and nanocrystalline silver powders were used to form a composite, in which microhardness was near 970 MPa HV and 400 HV for the amorphous phase and nanocrystalline silver, respectively. The compression strength of the composite containing 20 wt% of nanocrystalline Ag powder was equal to 600 MPa and plastic strain was 2%. Microstructure studies showed low porosity of composites of less than 1%, uniform distribution of the silver phase and a transition zone between both components, about 150 nm thick, where diffusion of nickel, niobium and zirconium into silver was observed. High-resolution TEM allowed identifying the structure of nanocrystalline inclusions in the amorphous matrix after hot pressing as either Ni₃Zr or Ni₁₇Nb₃. The identification was performed basing on measurements of angles and interatomic distances using inverse Fourier transformed images with enhanced contrast using Digital Micrograph computer program.     

Characterization of nanoscaled heterogeneities in mechanically alloyed and compacted CuFe.

Cu80Fe20 and Cu50Fe50 were mechanically alloyed from the pure elements by ball milling for 36 h. The alloy powder was compacted into tablets at room temperature by applying a pressure of 5 GPa. Characterization of the Cu80Fe20) and Cu50Fe50 alloys was carried out by high-resolution transmission electron microscopy (HREM), atom probe field ion microscopy and three-dimensional atom probe (3DAP). The grain size of the nanocrystalline microstructure of the ball-milled alloys observed with HREM varies between 3 and 50 nm.Atom probe and 3DAP measurements indicate that the as-prepared state is a highly supersaturated alloy, in which the individual nanocrystals have largely varying composition. Fe concentration in Cu was found to range from about 8 to 50 at%. It is concluded that by ball milling and compacting an alloy is produced which on a nanometer scale is heterogeneous with respect to morphology and composition.     

Microstructure of ball milled and compacted Co–Ni–Al alloys from the β range

Two powder alloys from the β phase region of compositions Co_28.5Ni_36.5Al₃₅ and Co₃₅Ni₃₀Al₃₅ were ball milled for 80 h in a high energy ball mill. The formation of amorphous structure was observed after 40 h of milling and further milling did not change their structure. The analytical and high-resolution transmission electron microscopy (TEM, HREM) examination of powder structure showed that nanoparticles of  L1₀ phase of size of about 5 nm were present within the amorphous matrix. The vacuum hot pressing of the milled powders under pressure of 400 MPa at 700°C for 12 min resulted in the formation of compacts with density of about 70% of the theoretical one. The additional heat treatment at 1300°C for 6 h followed by water quenching, led to significant improvement of density and induced the martensitic transformation manifested by a broad heat effect. The characteristic temperatures of the transformation were determined using DSC measurements, which revealed only small differences within the examined alloys compositions. TEM structure studies of heat-treated alloys allowed to identify the structure of an ordered β (B2) phase and L1₀ martrensite.     

Electron crystallography applied to the structure determination of Nb(Cu,Al,X)2 Laves phases

The presence of primary precipitates of the Laves phases considerably improves the mechanical properties and the resistance to thermal degradation of the high‐temperature shape memory Cu–Al–Nb alloys. The structure analysis of the Laves phases was carried out on particles contained in the ternary and quaternary alloys as well on synthesized compounds related to the composition of the Nb(Cu,Al,X)₂ phase, where X = Ni, Co, Cr, Ti and Zr. The precise structure determination of the Laves phases was carried out by the electron crystallography method using the crisp software.     

Combining Ar ion milling with FIB lift-out techniques to prepare high quality site-specific TEM samples

Focused ion beam (FIB) techniques can prepare site‐specific transmission electron microscopy (TEM) cross‐section samples very quickly but they suffer from beam damage by the high energy Ga⁺ ion beam. An amorphous layer about 20–30 nm thick on each side of the TEM lamella and the supporting carbon film makes FIB‐prepared samples inferior to the traditional Ar⁺ thinned samples for some investigations such as high resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). We have developed techniques to combine broad argon ion milling with focused ion beam lift‐out methods to prepare high‐quality site‐specific TEM cross‐section samples. Site‐specific TEM cross‐sections were prepared by FIB and lifted out using a Narishige micromanipulator onto a half copper‐grid coated with carbon film. Pt deposition by FIB was used to bond the lamellae to the Cu grid, then the coating carbon film was removed and the sample on the bare Cu grid was polished by the usual broad beam Ar⁺ milling. By doing so, the thickness of the surface amorphous layers is reduced substantially and the sample quality for TEM observation is as good as the traditional Ar⁺ milled samples.     

Microstructural characterization of mechanicallyactivated ZnO powders

In this paper, changes of microstructural characteristics of disperse systems during mechanical activation of zinc oxide (ZnO) have been investigated. ZnO powder was activated by grinding in a planetary ball mill in a continuous regime in air during 300 min at the basic disc rotation speed of 320 rpm and rotation speed of bowls of 400 rpm but with various balls‐to‐powder mass ratios. During ball milling in a planetary ball mill, initial ZnO powder suffered high‐energy impacts. These impacts are very strong, and large amounts of microstructural and structural defects were introduced in the milled powders. The morphology and dispersivity of particles and agglomerates of all powders were investigated by scanning electron microscopy and scanning transmission electron microscopy. The specific surface area of initial ZnO powder was determined as 3.60 m² g^?1 and it increased to 4.42 m² g^?1 in mechanically activated powders. An increase of the ball‐to‐powder mass ratio led to a decrease of particle dimensions as well as increased the tendency for joining into quite compact agglomerates, that is aggregates, compared with the very loose, soft initial agglomerates. The obtained results pointed out that activation of ZnO powders produces a highly disperse, nano‐scaled mixture of small particles, that is crystallites with sizes in the range of 10–40 nm. Most of these particles are in the form of aggregates with dimensions of 0.3–0.1 μm. The crystallite and aggregate size strongly depend on milling conditions, that is ball‐to‐powder mass ratio, as shown in this investigation.     

Microstructure and properties of hot compacted powders of aluminium alloys

Atomized 6061 aluminium alloy powders with and without the addition of 2 wt% Zr were milled for 80 h in a planetary ball mill and hot pressed in vacuum. The milled powders showed microhardness of about 170 HV, which increased after hot pressing up to 260 HV and up to 280 HV for powders without and with the Zr additions, respectively. Compression tests showed the high yield stress of 300 MPa obtained for the hot-pressed sample produced from the initial powders compared with ultimate compression strength of above 800 MPa for that of the milled sample and slightly higher for that with Zr additions. The effect of hot pressing on the structure of powders was investigated using a conventional analytical and high-resolution electron microscopy and high angle annular dark-field scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis. The samples of initial powders hot pressed in vacuum showed a cell structure with particles of the Mg₂Si and AlFeSi phases in intercell areas. In the milled and hot-pressed sample, the homogeneous structure of small grains of size below 200 nm was observed. The AlFeSi and Mg₂Si particles with size 20–100 nm were uniformly distributed as well as the Zr rich particles in the Zr containing alloy. The Zr-rich particles containing up to 80 at% Zr were identified as a metastable fcc cubic phase with lattice parameter a = 0.48 nm.     

Analytical electron microscopy and focused ion beam: complementary tool for the imaging of copper sorption onto iron oxide aggregates

Nanometre‐scale electron spectroscopic imaging has been applied to characterize the operation of a copper filtration plant in environmental science. Copper washed off from roofs and roads is considered to be a major contributor to diffuse copper pollution of urban environments. A special adsorber system has been suggested to control the diffusion of copper fluxes by retaining Cu with a granulated iron hydroxide. The adsorber was tested over an 18‐month period on facade runoff. The concentrations range of Cu in the runoff water was measured between 10 and 1000 p.p.m. and could be reduced by between 96% and 99% in the adsorption ditch. Before the analysis of the adsorber, the suspended material from the inflow was ultracentrifuged onto TEM grids and analysed by energy‐filtered transmission electron microscopy (EFTEM). Copper was found either as small precipitates 5–20 nm in size or adsorbed onto organic and inorganic particles. This Cu represents approximately 30% of the total dissolved Cu, measured by atomic emission spectrometry. To locate where the copper sorption takes place within the adsorber, the granulated iron oxide was analysed by analytical electron microscopy after exposure to the roof run‐off water. A section of the granulated iron hydroxide was prepared by focused ion beam milling. The thickness of the lamina was reduced to 100 nm and analysed by EFTEM. The combination of these two techniques allowed us to observe the diffusion of Cu into the aggregate of Fe. Elemental maps of Fe and Cu revealed that copper was not only present at the surface of the granules but was also sorbed onto the fine particles inside the adsorber.     

Identification and quantification of microstructures formed during nanocrystallization of amorphous (Fe, Co)-Nb-(Si, B) systems

The effect of addition of Si and variation of the Fe/Co ratio on the evolution of the nanostructure was studied in a modification of the Fe–Nb–B system. The entire system (Fe, Co)₇₃Nb₇(Si, B)₂₀ was prepared in an amorphous state by rapid quenching using the planar flow casting method over a wide range of Fe/Co atomic ratios, ranging from 0 to 1. Nanocrystallization was investigated by evolution of the electrical resistivity with time and temperature. The microstructural analysis was performed using transmission electron microscopy as well as electron and X‐ray diffraction. The results from microscopy observations were used to determine the distribution of grain size, which in these alloys attain very small dimensions of ～5–8 nm. New algorithms of microscope image analysis were used for grain size determination, crucial for quantifying the microprocesses controlling nucleation and growth from the amorphous rapidly quenched phase.     

Pecularities of nanocrystal formation in rapidly quenched (FeCo)MoCuB amorphous alloys

The effect of the substitution of Fe by Co on the enhancement of glass‐forming ability limits and subsequent nanocrystallization was studied in a rapidly quenched amorphous system (Fe_xCo_y)₇₉Mo₈Cu₁B₁₂ for y/x ranging from 0 to 1. The effect of Cu on nanocrystallization was investigated by comparison with Cu‐free amorphous Fe₈₀Mo₈B₁₂. Systems partially crystallized at the surface layer were prepared for y/x = 0 using different quenching conditions. The effect of heat treatment of master alloys used for ribbon casting was also assessed. The microstructure and surface/bulk crystallization effects were analysed using transmission electron microscopy and electron and X‐ray diffraction in relation to the expected enhancement of high‐temperature soft magnetic properties, drastically reduced grain sizes (～5 nm) and Co content. Unusual surface phenomena were observed, indicating the origin of possible nucleation sites for preferential crystallization in samples with low Co content.     

TEM and HREM study of Al3Zr precipitates in an Al-Mg-Si-Zr alloy

The structure of Al₃Zr precipitates in Al‐1.0Mg‐0.6Si‐0.5Zr (in wt.%) alloy was investigated using conventional transmission electron microscopy (TEM) and high‐resolution TEM (HREM). After annealing of the alloy in the temperature range 450–540 °C, spherical precipitates of metastable L1₂‐Al₃Zr phase appeared nearly homogeneously within the matrix, and elongated particles were found at grain boundaries. L1₂‐structured Al₃Zr were about 20–30 nm in diameter and coherent with the matrix. Inside some of them, planar faults parallel to {100} planes were revealed by use of HREM. Most probably, these faults are an indication of the transition stage of transformation to the stable D0₂₃‐type Al₃Zr phase. The elongated precipitates (about 100 nm) were identified as D0₂₂‐type Al₃Zr. Energy‐dispersive X‐ray analysis showed that they contain, apart from Al, mainly Zr with small amounts of Si. The substitution of Al by Si increased the stability of the D0₂₂‐Al₃Zr as compared with D0₂₃‐Al₃Zr.     

Preparation of TEM samples for hard ceramic powders

It is challenging to prepare a good sample for high-resolution electron microscopy of polycrystalline ceramic powders containing hard particles or particles with a strong preferential cleavage. Here we demonstrate that embedding the particles in a Cu matrix in a pressed pellet allows for straightforward conventional ion milling. The method is applied to powders of Mg10Ir19B16 and Na0.5CoO2 to show its feasibility, whereby transmission electron microscopy (TEM) samples with crystalline areas thinner than 10 nm can be obtained easily.     

机械合金化合成TiB2/Fe3Al纳米复合粉体

采用铁、铝、钛、硼四元粉体机械合金化与后续热处理的方法合成纳米TiB_2／Fe_3Al复合粉体,并利用XRD、DSC、SEM和TEM等对粉体进行了表征。结果表明：在球磨过程中,四元粉体形成了Fe(Al,Ti,B)过饱和固溶体,有序度不断降低,逐渐向非晶态转变,同时粉体晶粒尺寸逐渐细化,球磨40h后Fe(Al,Ti,B)的晶粒尺寸为9．6nm；并在热处理过程中Fe(Al,Ti,B)分解生成纳米Fe_3Al和TiB_2复合粉体,同时发生组成相晶粒生长,结构有序度提高等转变。     

Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (～0.5 nm) interfacial SiO_x layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO₂‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.     

机械合金化结合冷压烧结制备铜-锆合金的组织与性能

采用机械合金化结合冷压烧结的方法制备了不同锆含量的铜-锆合金,用X射线衍射仪和扫描电镜等对制备的复合粉体进行了表征,并研究了球磨时间、烧结温度和保温时间、锆含量等因素对合金电阻率、硬度及抗弯强度的影响.结果表明:随着球磨时间的延长,复合粉体的主衍射峰强度逐渐降低,而铜-锆金属间化合物的衍射峰强度逐渐增加,颗粒由片状转变为近球状,而烧结后制得合金的电阻率和硬度逐渐升高;提高烧结温度和保温时间可以使合金的导电性能提高,但硬度下降;随着锆含量的增加,合金的电阻率、硬度及抗弯强度均不断增加.     

机械合金化法制备Finemet非晶粉体的研究

利用高能卧式搅拌球磨机,研究了利用机械合金化法制备Finemet非晶粉体的工艺,研究结果表明,通过高速球磨可以得到部分非晶粉体。非晶化机制是局域熔体的快速冷却,粉体结块和粘壁,是机械合金化法制备高纯度Finemet非晶粉体的主要阻碍因素。     

High-resolution scanning electron microscopy of immunogold-labelled cells by the use of thin plasma coating of osmium

The feasibility of plasma coating of a thin osmium layer for high‐resolution immuno‐scanning electron microscopy of cell surfaces was tested, using Drosophila embryonic motor neurones as a model system. The neuro‐muscular preparations were fixed with formaldehyde and labelled with a neurone‐specific antibody and 10 or 5 nm colloidal gold‐conjugated secondary antibodies. The specimens were post‐fixed with osmium tetroxide and freeze‐dried. Then they were coated with a 1–2 nm thick layer of osmium using a hollow cathode plasma coater. The thin and continuous coating of amorphous osmium gave good signals of gold particles and fine surface structures of neurites in backscattered electron images simultaneously. This method makes it possible to visualize the antigen distribution and the three‐dimensionally complex surface structures of cellular processes with a resolution of several nanometres.     

Evolution of rapidly solidified NiAlCu(B) alloy microstructure

This study concerned phase transformations observed after rapid solidification and annealing at 500, 700 and 800 °C in 56.3 Ni‐39.9 Al‐3.8 Cu‐0.06 B (E1) and 59.8 Ni‐36.0 Al‐4.3 Cu‐0.06 B (E2) alloys (composition in at.%). Injection casting led to a homogeneous structure of very small, one‐phase grains (2–4 µm in size). In both alloys, the phase observed at room temperature was martensite of L1₀ structure. The process of the formation of the Ni₅Al₃ phase by atomic reordering proceeded at 285–394 °C in the case of E1 alloy and 450–550 °C in the case of E2 alloy. Further decomposition into NiAl (β) and Ni₃Al (γ′) phases, the microstructure and crystallography of the phases depended on the path of transformations, proceeding in the investigated case through the transformation of martensite crystallographic variants. This preserved precise crystallographic orientation between the subsequent phases, very stable plate‐like morphology and very small β + γ′ grains after annealing at 800 °C.     

Structure of nanometre-sized Xe particles embedded in Al crystals

The structure and lattice parameters of Xe particles about 1 nm to about 6 nm in size embedded in Al were investigated with off‐Bragg condition high‐resolution transmission electron microscopy. An Xe particle about 1 nm in size had different structural properties from those 2–6 nm in sizes. Some 1‐nm Xe particles had an face‐centred cubic (f.c.c.) structure with the same orientation as the Al matrix, whereas others of the same size had a non‐f.c.c. structure. The lattice parameters of a 1‐nm f.c.c. Xe particle were about 20% smaller than the average value obtained from electron diffraction, i.e. the particle was compressed by about 80%. The lattice parameters of Xe crystals about 2 nm to about 6 nm in size were almost the same as those obtained from diffraction results. One of the reasons for the extra compression seen with a 1‐nm Xe particle is the increase in pressure inside an Xe particle with decreasing particle size.