Interfacial Characteristics of Nanocrystalline FeMoSiB Alloys

By using X-ray diffraction (XRD), transmission electronic microscopy (TEM) and transmissionMssbauer spectroseopy (TMES), the formation, structure and properties including microhardnessand electrical resistivity of nanocrystalline FeMoSiB alloys have been investigated. By annealing theas-quenched FeMoSiB sample at 833-1023K for 1 h, nanocrystalline materials with grain sizes of15 to 200 nm were obtained. Mssbauer spectroscopy results reveal a quasi-continuous distributionfeature of P(H)-H curves for 15 nm-and 20 nm-grained samples. Also, it was found that resistivityand microhardness of nanocrystalline Fe-Mo-Si-B alloys exhibit strong grain size effect.     

Structure and hardness of nanocrystalline silver

Nanocrystalline silver (Ag) was prepared by in situ compacting of ultra-fine silver particles. The structures of as-compacted and annealed specimens were analysed by high-resolution transmission electron microscopy and scanning electron microscopy. Vickers microhardness was measured on the specimens. The ultra-fine particles aggregate before compaction. It is found that the nanocrystalline specimens are obtained by the compaction of the aggregates. Microstructure inside the aggregates does not change as the compacting pressure increases from 0.25 to 2.00 GPa. The compacting pressure affects on the structure and density of the boundaries between the aggregates, i.e. the formation of the crack-type defects of about 1 μm at the boundaries. Thermal stability of nanocrystalline Ag is significantly low; grain coarsening starts below 200 °C. However, a nanometre-sized layered structure forms in local regions upon annealing and is stable up to 800 °C. Vickers microhardness of as-compacted specimens increases with increasing compacting pressure. The increase is attributed to the decrease of the number of crack-type defects. Vickers microhardness of nanocrystalline Ag begins to decrease due to grain coarsening upon annealing around 200 °C. The microhardness of nanocrystalline Ag deviates from the HalI–Petch relation. This revised version was published online in November 2006 with corrections to the Cover Date.     

Formation of nanocrystalline phases during thermal decomposition of amorphous Ni-P alloys by isothermal annealing

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a-->ni by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) ni-->nii by isothermal annealing of the nanocrystalline ni state; (3) ni-->nii by linear heating of the ni state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.     

Microstructure of Differently Annealed NanocrystallineFe_(72.7)Cu_1Nb_(1.8)Mo_2Si_(13)B_(9.5) Alloy

The microstructure of differently annealed nanocrystalline Fe72.7Cu1Nb1.8Mo2Si13B9.5 alloy was investigated by using Mssbauer spectroscopy and transmission electron microscope. The specimens were isochronally annealed at temperatures between 480℃ and 600℃ for 0.5 h. The experimental results show that the microstructure mainly consists of the nanoscale bcc α-Fe(Si) grains and the residual amorphous matrix phase. A trace paramagnetic phase was found for annealing about above 500℃. The volume fraction of cr-Fe(Si) grain increases with increasing annealing temperature, whereas the average size of grain is almost unchanged above 480℃ up to 580℃. The calculated thickness of the intergranular layer of the residual amorphous matrix clearly decreases with increasing annealing temperature.     

Structure and Magnetic Properties of Fe_(76.5)Si_(13.5)B_9Cu_1 Alloy with Nanoscale Grain Size

The structure and magnetic properties of Fe76.5Si13.5B9Cu1 alloys with a nanocrystalline (NC) bcc Fe(Si) phase trom about 23 to 46 nm in diameter, which were first formed into amorphous ribbons and then annealed at various temperatures between 703 and 773 K, have been investigated. At annealing temperatures from 703 to 748 K, the single NC bcc(Si) phase is obtained in the crystallized alloys. The grain size and the Si-content in the NC bcc Fe(Si) phase for the alloys annealed at different temperatures are presented. The soft magnetic properties and the saturation magnetostriction for the alloys with the NC bcc Fe(Si) phase are also measured. The results show that, the saturation magnetizotion and the permeability are improved for the alloys with only the NC bcc Fe(Si) phase and become better with decreasing of the NC bcc phase size, and the saturation magnetostriction declines for the alloys with increasing Si-content in the NC bcc Fe(Si) phase.     

Structure and mechanical properties of nanocrystalline Ag/MgO composites

Nanocrystalline Ag/MgO composites were prepared by the ultrafine-powder-compaction method. The structure was investigated for the first time by high-resolution electron microscopy. Nanometre-sized Ag grains and MgO grains in the composites bonded directly without any intermediate phase layer. Certain preferred orientation relationships were observed between the Ag and MgO grains. The nanocrystalline Ag/MgO composites retained their grain size during annealing up to 873 K. Vickers microhardness measurements were performed on the as-compacted and annealed specimens. Generation and propagation of cracks were less active in the nanocrystalline Ag/MgO composites than in a single-phase nanocrystalline MgO. The Vickers microhardness of the nanocrystalline Ag/MgO composites remained up to 1073 K. Hot-pressing deformation tests showed that the nanocrystalline Ag/MgO composites deformed plastically at 1073 K.     

退火对电沉积纳米晶Ni-Mn合金性能的影响

采用脉冲电沉积法制备了纳米晶Ni-Mn合金,测定了退火前后纳米晶Ni-Mn合金的显微硬度和抗拉强度.结果表明:退火后电沉积纳米晶Ni-Mn合金的显微硬度和抗拉强度值均有提高.分析了退火处理对电沉积纳米晶Ni-Mn合金性能的影响.     

Microstructural evolution of FINEMET type alloys with chromium: An electron microscopy study

Microstructural changes in Fe_73.5–xCr_xCu₁Nb₃Si_13.5B₉ (0x5) alloys with thermal treatment were studied by electron microscopy. In a first stage, around 800 K, an Fe(Si) nanocrystalline phase is formed in the amorphous residual matrix. Crystallization onset is enhanced with the Cr content of the alloy. In a second stage, around 950 K, full crystallization of the samples leads to the formation of a body centred cubic (b.c.c.) boride-type unknown crystal phase with a lattice parameter of a=1.52 nm, and recrystallization of the previous Fe(Si) nanophase also occurs. No qualitative differences were found between dynamic and isothermal crystallization. The size effect for thin samples is limited to a lowering of crystallization temperatures. For isothermal nanocrystallization in the temperature range 775–900 K, the mean grain size of the nanocrystals increases for short annealing times to stabilize at a constant value of about 10–15 nm for long annealing times. The stabilized grain size increases with increasing annealing temperature and slightly decreases with the Cr content of the alloy.     

Microstructure and microhardness of high entropy alloys with Zn addition: AlCoFeNiZn and AlCoFeNiMoTiZn

High entropy alloys were designed from equiatomic multicomponent systems using powder metallurgy including mechanical alloying and sintering. The structure and morphology of the resulting alloys were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy techniques and their hardness values were also determined in the Vickers scale. The results indicate under the milling conditions used, the AlCoFeNiZn, AlCoFeNiMoTi and AlCoFeNiMoTiZn alloys crystallized forming BCC structures whereas the AlCoFeNi alloy presented two different phases, one with FCC structure and the other one with BCC. The synthesis method resulted in alloys with grain sizes in the nano scale having values between 4.1 and 9.4 nm on the powder form up to 40.1 nm after sintering phenomenon which lead to phase transformations which were more evident in the Mo-containing alloys. In addition, the AlCoFeNiZn and AlCoFeNiMoTiZn alloys did not show Zn traces after sintering as it was suggested by chemical analyses using energy dispersive spectroscopy, suggesting it is lost by evaporation during sintering process. Mo-containing systems exhibited the highest microhardness in both milled and sintered conditions.     

Nanocrystalline Fe-P-Si alloys

Annealing Fe-P-Si amorphous alloys was found to produce nanocrystalline particles and raise the microhardness of the alloys by a factor of 2 to 3. The most significant strengthening was observed in the alloys containing the smallest amounts of Si and P and the largest amount of the α-Fe-based phase. As shown by x-ray diffraction and electron microscopy, the alloy consisting entirely of nanocrystalline phases with a particle size of about 25 nm crystallizes in three steps.     

纳米晶Ba_（1－x）Pb_xTiO_3的合成与表征

以醋酸铅、硬脂酸钡和钛酸丁酯为原料，用硬脂酸凝胶法（ＳＡＧ）合成了粒度均匀、粒径１０－２０ｎｍ的Ｂａ（１－ｘ）ＰｂｘＴｉＯ３纳米晶粉末．利用红外光谱（ＩＲ）、热重（ＴＧ）和差热分析（ＤＴＡ）研究了纳米晶粉末的合成过程．用ＴＥＭ、ＸＲＤ观察和研究纳米晶的形貌及晶体结构，并用发射光谱测定样品的纯度．     

Pressure-induced preferential growth of nanocrystals in amorphous Nd(9)Fe(85)B(6)

Control over the growth and crystallographic orientation of nanocrystals in amorphous alloys is of particular importance for the development of advanced nanocrystalline materials. In the present study, Nd(2)Fe(14)B nanocrystals with a strong crystallographic texture along the [410] direction have been produced in Nd-lean amorphous Nd(9)Fe(85)B(6) under a high pressure of 6?GPa at 923?K. This is attributed to the high pressure inducing the preferential growth of Nd(2)Fe(14)B nanocrystals in the alloy. The present study demonstrates the potential application of high-pressure technology in controlling nanocrystalline orientation in amorphous alloys.     

Fabrication and characterization of nano/amorphous dual-phase FINEMET microwires

A nano/amorphous dual-phase FINEMET microwire was fabricated directly from molten alloy without any interstage annealing by a home-built melt extraction technique (MET). The microstructure, mechanical and pronounced electromagnetic interference shielding (EMI) effectiveness of this dual-phase microwire has been systematically evaluated. The structural analysis reveals that the as-cast FINEMET microwire consists of two distinct structures, i.e., amorphous and nanocrystalline phase due to their different cooling characteristics. Compared with other reported FINEMET alloys, the extracted microwire exhibits a superior high tensile strength of 1800 MPa. These nanocrystals enabled dual-phase microwires also exhibit large EMI SE values in the frequency range of 8–12 GHz (X-band) due to the multiple magnetic loss mechanisms associated with their intrinsic structural characteristics. The combination of excellent mechanical properties and electromagnetic properties make this kind of melt-extracted dual-phase FINEMET microwire promising for a range of structure and multifunctional applications.     

Fabrication and characterization of nanocrystalline oxides by crystallization of amorphous precursors

Amorphous zirconia and alumina powders were produced by the electrochemical deposition. Hot-pressing of the zirconia powder at 2.5 GPa and 360°C for 30 min caused simultaneous consolidation of the powder to a dense body and crystallization of a nanocrystalline tetragonal phase. Cold-pressing of the same powder at 2.5 GPa and sintering at 600°C or 800°C for 1 hr resulted in large tetragonal crystallites within the highly porous compacts. Cold-pressing of alumina powder at 2.5 GPa and sintering at 500°C, or 900°C for 2 hrs resulted in partial crystallization of the amorphous phase to a mixture of various polymorphs of alumina. The microstructure was inhomogeneous and composed of both nanometer and submicrometer grains. Microhardness of the hot-pressed partially crystallized nanocrystalline tetragonal zirconia was comparable to that of single crystal monoclinic phase, and decreased after annealing at 450°C for 30 min. Microhardbess of the nanocrystalline alumina was lower by on order of magnitude than that of conventional polycrystalline alumina. The decrease in the microhardness of the sintered specimens was related both to the grain size and the porosity.     

Grain size dependence of tensile behavior in nanocrystalline Ni–Fe alloys

The tensile behaviors of FCC Ni–Fe alloys were investigated within three grain size regimes: >100 nm, 15–100 nm, and <15 nm. The results show that the nanocrystalline metals demonstrated large strain hardening rates, which increase with decreasing the grain size. With the similar grain size, lowing the stacking-fault energy (SFE) by addition of alloying element increases the yield strength and strain hardening ability. The “low” tensile elongation of nanocrystalline metals is due to the basic tradeoff between the strength and tensile elongation, i.e. nanostructured metals are not inherently brittle. Both the tensile results and fracture surface observations suggest that the tensile ductility increases with increasing the grain size. Furthermore, within the large grain size regime, the fracture surface exhibited the real void structure; while the fracture surface showed the concave and convex features when the grain size is less than the critical value.     

Formation of nanocrystalline structure during electron irradiation induced crystallization in amorphous Fe-Zr-B alloys

Effect of electron irradiation on the crystallization and phase stability of Fe₈₈Zr₉B₃ and Fe₇₁Zr₉B₂₀ amorphous alloys was examined. Electron irradiation at an accelerated voltage of 2000 kV was performed at room temperature. The Fe₇₁Zr₉B₂₀ alloy showed a wide supercooled liquid region and the ΔT_x value was 71 K, while no glass transition was observed in Fe₈₈Zr₉B₃ alloy. The amorphous phase in Fe–Zr–B alloys was not stable under irradiation and crystallization from the amorphous phase was accelerated by the irradiation. Nanocrystalline structure composed of α-Fe and cubic-Fe₂Zr was formed in Fe₈₈Zr₉B₃ alloy by irradiation induced crystallization, while no nanoscale precipitates of intermetallic compounds were formed during annealing. In Fe₇₁Zr₉B₂₀ alloy, the formation of nanocrystalline precipitates was also confirmed by irradiation induced crystallization, although the formation of nanocrystalline structure had not been realized in high B concentration Fe–Zr–B alloys by annealing. These new results show that electron irradiation is effective in producing a new nanocrystalline structure.     

Densification Behavior and Wear Response of Spark Plasma Sintered Iron‐Based Bulk Amorphous Alloys

In the present study, spark plasma sintering (SPS) process is used to sinter Fe‐based bulk amorphous alloys from starting amorphous powder. The sintering was performed in supercooled liquid region (at 630 °C with uniaxial pressure of 70 MPa) where plastic flow of the amorphous powder results in better densification without crystallization. To study the crystallization behavior and its influence on mechanical behavior, the sintered discs were de‐vitrified by annealing at 700 and 800 °C. Detailed characterization of phase development, microhardness at different loads, and wear behavior of the as‐sintered and annealed alloys is presented.     

Effect of Grain Size on Phase Transformation and Photoluminescence Property of the Nanocrystalline ZrO_2 Powders Prepared by Sol-Gel Method

Nanocrystalline zirconia （ZrO2） powders were prepared by a sol-gel process followed by annealing treatments from 500 to 1200 ℃. Phase transformation, microstructural features and photoluminescence properties were characterized by X-ray diffraction, transmission election microscopy and photoluminescence spectra, respectively. The results show that both monoclinic phase and tetragonal phase exist in the nanocrystalline ZrO2 powders at annealing temperature in the range of 500-900 ℃, and the concentration of monoclinic phase increases with increasing the annealing temperature. Tetragonal phase is totally transformed to monoclinic phase when annealing temperature is up to 900 ℃. The average grain size of the powders also increases when annealing temperature increases. Two emission peaks centered at 390 nm （named as /390） and 470 nm （named as /470） exist in the photoluminescence spectra, and the intensity ratio of /390 to /470 decreases with increasing annealing temperature. The grain size is proposed to be responsible for the phase transformation in the nanocrystalline ZrO2 powders.     

热处理对(Fe_(0.52)Co_(0.30)Ni_(0.18))_(73)Cr_(17)Zr_(10)非晶合金的组织结构及磁性能的影响

采用单辊急冷法制备(Fe_(0.52)Co_(0.30)Ni_(0.18))_(73)Cr_(17)Zr_(10)非晶薄带,并对该合金进行等温退火。采用XRD,AFM,VSM研究退火温度对(Fe_(0.52)Co_(0.30)Ni_(0.18))_(73)Cr_(17)Zr_(10)非晶合金的组织结构和磁性能的影响。结果表明:非晶合金晶化过程为Am→α-Fe(Co)+Am′→α-Fe(Co)+Cr_2Ni_3+Fe_3Ni_2+Cr_2Zr+未知相。当退火温度Ti玻璃转变温度Tg时,由于结构弛豫、内应力的释放,合金的饱和磁化强度Ms有所提高;当晶化起始温度TxTi第一晶化峰值温度Tp1时,由于铁磁性α-Fe(Co)相的析出,Ms显著提升;当TiTp1时,由于晶粒长大和第二相的析出,Ms急剧恶化,565℃退火能够获得最好磁性能(Ms=106.8A·m~2·kg~(-1))。490℃和565℃退火后薄带表面的AFM观察表明,AFM图片所呈现的颗粒尺寸要比用Scherrer法测得的α-Fe(Co)纳米晶尺寸大得多,这是典型的包裹晶粒现象。     

TEM Investigations of Thermal Stability of Nanocrystamne Co_(81)Cr_(19)

In situ transrnission electron microscopic observations were carried out to study the thermal stability of nanocrystalline Co81 Cr19 (n-Co81 Cr19) alloy prepared by d.c. sputtering, The TEM results show that the originally existed nanocrystalline phase hcp-CoCr is thermally stable, no apparent grain growth was observed until annealing to the final stage at.800℃. But the evolution of .microstructure of amorphous oxide Co2Cro4. which is formed duting in situ annealing. is drastically affected by the electron irradiation. In the electron irradiated area. the newly formed amorphous oxide Co2CrO4crystallizes at about 550℃. and grows into fine crystals finally. However, in the electron nonirradiated area, the amorphous oxide Co2CrO4 crystallizes at about 630℃. and grows into large strip shaped crystals during afterward annealing. the space between strip shaped crystals are also fine grained hcp-CoCr alloy. XRD analysis result of the thick film Co81 Cr19 is in good agreement with that of in situ TEM for the excellent stability of hcp-CoCr