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Characterization of Mechanically Alloyed Nanocrystalline Fe(Al): Crystallite Size and Dislocation Density  [PDF]
M. Mhadhbi,M. Khitouni,L. Escoda,J. J. Su?ol,M. Dammak
Journal of Nanomaterials , 2010, DOI: 10.1155/2010/712407
Abstract: A nanostructured disordered Fe(Al) solid solution was obtained from elemental powders of Fe and Al using a high-energy ball mill. The transformations occurring in the material during milling were studied with the use of X-ray diffraction. In addition lattice microstrain, average crystallite size, dislocation density, and the lattice parameter were determined. Scanning electron microscopy (SEM) was employed to examine the morphology of the samples as a function of milling times. Thermal behaviour of the milled powders was examined by differential scanning calorimetry (DSC). The results, as well as dissimilarity between calorimetric curves of the powders after 2 and 20?h of milling, indicated the formation of a nanostructured Fe(Al) solid solution. 1. Introduction Mechanical alloying (MA) is a technique commonly used to obtain supersaturated solid solutions, compounds with high energy of mixing, and alloys of combinations of elements which do not show appreciable solubility in their equilibrium phase diagrams [1–3]. The Fe-Al system is an example of a binary system with a low solid miscibility at room temperature; however, by means of MA Fe(Al) solid solutions have been obtained in almost all the compositional range. In recent years, a number of studies have been reported on mechanical alloying of FeAl. It is established that the formation of the supersaturated solid solution (SSS) α-Fe(Al) as a final product of MA takes place with at % Al though in a number of papers the formation of SSS was found with at % Al [4–9]. For instance, Zeng and Baker [9] investigated the effect of milling time on the crystallite size, lattice strain, and lattice parameter of Fe-40 at % Al powder mixture. According to their results, the crystallite size is less than 7?nm after 4?h, indicating a rapid decrease of size, but milling for more than 6?h results in some increase in the nanocrystalline size. They attributed this fact to the dynamic recrystallization and/or nanocrystalline grain growth. The same results were shown in the case of MA Fe-45 at % Al [10]. Moreover, the lattice parameter also increased at first up to 10?h milling, then decreased. This decrease has attributed to the oxidation during milling, which decreases the Al content. In addition, Wolski et al. [11] studied the effect of milling conditions on the Fe-35 at % Al intermetallic formation by MA. According to their investigation, this process occurred in two steps: a nanocrystallization step and an FeAl formation step. FeAl formation started rapidly by the creation of Al-rich and Fe-rich solid solutions; the
ZHANG Cheng,WANG Yongzhong,JIANG Ming,ZENG Dechang,TANG Hong,LI Zhongrun,QIAO Guiwen,CHUANG Yuzhi,
,WANG Yongzhong,JIANG Ming,ZENG Dechang,TANG Hong,LI Zhongrun,QIAO Guiwen,CHUANG Yuzhi

金属学报(英文版) , 1995,
Abstract: Bi-2212 superconductive oxides with nanometer grains have been prepared by means of crystallization from amorphous materials.The structure and grain-size of samples were examined by.X-ray.diffraction,scanning electron microscopy.and high resolution electron microscopy.The temperature dependence of the resistivity.and a.c.susceptibility was measured by the standard four-probe method and the standard mutual-induction measurement,respectively.With the decrease of temperature,the resistivity of the Bi-2212 phase with grain size in nanometer was found to diminish to zero without sudden drop.No diamagnetic transition was detected from room temperature to 4.2 K.The formation of the nanocrystalline 2212 phase and the effect of grain size on the superconductivity were discussed.
Deformation behaviour of dispersion hardened nanocrystalline copper  [PDF]
J.P. Stobrawa,Z.M. Rdzawski
Journal of Achievements in Materials and Manufacturing Engineering , 2006,
Abstract: Purpose: The aim of this work was to describe deformation behaviour of nanocrystalline copper dispersion-hardened with nanoparticles of tungsten carbide and yttria.Design/methodology/approach: Tests were made with the Cu, Cu-WC and Cu-Y2O3 micro-composites containing up to 3 % of a hardening phase. These were obtained by powder metallurgy techniques, i.e. milling the input powders in the planetary ball mills, compacting and sintering. The mechanical properties (hardness, 0,2 YS, elongation during compression test) and microstructure were examined by the optical, scanning and transmission electron microscopy.Findings: Analysis of the initial nanocrystalline structure of these materials was made, and its evolution during deformation process was investigated with an account of the hardening effect and the changes in the mechanical and plastic properties. Results of this analysis have been discussed based on the existing theories related to hardening of nanocrystalline materials.Research limitations/implications: The powder metallurgy techniques make it possible to obtain copper-based bulk materials by means of milling input powders in the planetary ball, followed by compacting and sintering. Additional operations of hot extrusion are also often used. There is some threat, however, that during high-temperature processing or using these materials at elevated or high temperatures this nanometric structure may become unstable. The studies have shown the importance of “flows” in the consolidated materials such as pores or regions of poor powder particles joining which significantly deteriorate mechanical properties of compacted and sintered powder micro composites.Practical implications: A growing trend to use new copper-based functional materials is observed recently world-wide. Within this group of materials particular attention is drawn to those with nanometric grain size of a copper matrix, which exhibit higher mechanical properties than microcrystalline copper.Originality/value: The paper contributes to the elucidation of deformation behaviour of high-porosity nanocrystalline copper dispersion hardened with tungsten carbide and yttria.
Structural and magnetic properties of nanocrystalline BaFe12O19 synthesized by microwave-hydrothermal method
K. Sadhana,K. Praveena,S. Matteppanavar,B. Angadi
Applied Nanoscience , 2012, DOI: 10.1007/s13204-012-0100-1
Abstract: Nanocrystalline BaFe12O19 powders were prepared by microwave-hydrothermal method at 200 °C/45 min. The as-synthesized powders were characterized by using X-ray diffraction (XRD), thermogravimetry (TG) and differential thermal analysis (DTA). The present powders were densified at different temperatures, i.e., 750, 850, 900 and 950 °C for 1 h using microwave sintering method. The phase formation and morphology studies were carried out using XRD and field emission scanning electron microscopy (FE-SEM). The average grain sizes of the sintered samples were found to be in the range of 185–490 nm. The magnetic properties such as saturation magnetization and coercive field of sintered samples were calculated based on magnetization curves. A possible relation between the magnetic hysteresis curves and the microstructure of the sintered samples was investigated.
Modeling the dependence of strength on grain sizes in nanocrystalline materials
Wei He, Sanjeev D Bhole and DaoLun Chen
Science and Technology of Advanced Materials , 2008,
Abstract: A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall–Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall–Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ 1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.
Magnetothermal Analysis of Nanocrystallization of Amorphous and Relaxations of Nanocrystalline Materials
材料科学技术学报 , 1997,
Abstract: For a few years it has been realized that nanocrystalline phases can be formed during crystallization of amorphous alloys annealed isothermally below the crystallization temperature of usual heating experiments. Data of this transformation monitored by the measurement of magnetic susceptibility are presented. A method using a magnetic balance with electronic stabilisation and combined computer facilities is applied. Constant heating and cooling rates as well as isothermal heat treatments are used. Magnetic measurements are able to detect the onset of the transformation of amorphous Ni-P alloys much earlier than was possible with differential scanning calorimetry. The transformation kinetics can be analyzed by means of the Avrami plot based on the Johnson-Mehl-Avrami equation.The kinetics of solid state reactions in the nanostructured material can be investigated similarly. Formation of a Ni-phase in a nanostructured Hf-Ni alloy could be detected in a very early stage, where calorimetric methods are not sensitive. Segregation phenomena could be detected from the experiments even after long time. The sensitivity of the applied method is not dependent on the heating rate as the sensitivity of scanning calorimetry is
The mechanism of the anomalous variation of grain size for Fe-based nanocrystalline alloys

Yang Wei-Ming,Liu Hai-Shun,Dun Chao-Chao,Zhao Yu-Cheng,Dou Lin-Ming,

物理学报 , 2012,
Abstract: The magnetic properties of the Fe-based nanocrystalline alloys are determined mainly by their grain sizes, and the mechanism of the variation of grain size with annealing temperature is an important issue in the study of nanocrystalline alloys. In this paper, the relationships between grain size and annealing temperature for these alloys within the primary crystallization temperature (Tx1) and the secondary crystallization temperature (Tx2) for 1 h are investigated, and a corresponding model is proposed. The physical mechanism of the fact that the grain size first decreases and then increases with the increase of annealing temperature is explained by using this model. It is found that the grain size has a minimum value when these alloys are isochronally annealed at the temperature near 0.6 times that of the melting point. Theoretical analysis results are found to be in agreement with the experiments data within the investigated temperature range. This investigation provides a means to obtain the smallest grain size quickly.
Grain fragmentation and property modification of nanocrystalline ZnO under high pressure

Shao Guang-Jie,Qin Xiu-Juan,Liu Ri-Ping,Wang Wen-Kui,Yao Yu-Shu,

物理学报 , 2006,
Abstract: Grain fragmentation and property modification of nanocrystalline ZnO under high pressure were studied on CS-1B 6×8000 kN cubic high pressure apparatus. Grain size and microstructure of the samples have been characterized by X-ray diffraction and field emission scanning electron microscopy. The results show that the significant grain fragmentation phenomenon occurs inside the crystallites. Experimental results on hardness and the current-voltage characteristics indicate that micro-hardness of the samples after high pressure is 2.3 times that of the samples sintered at atmospheric pressure and the nonlinear current-voltage characteristics is obviously better than that of the latter.
Distortion Regions near the Grain Boundary and Their Effects on Nanocrystalline Materials
Chiwei LUNG,KANG Long,Enke TIAN,Zongjun LIANG,

材料科学技术学报 , 2000,
Abstract: The distortion regions near grain boundaries in cr-Fe-C solid solution were studied by use of internal friction method. It was found that the total thickness of these regions is quite large though the thickness of real grain boundaries is usually very thin. It was also found that the smaller the grain size, the thicker the total distortion region. A model for the structure of distortion regions near grain boundaries is proposed. Their effects on nanocrystalline materials are discussed.
Grain boundary character distributions in nanocrystalline metals produced by different processing routes  [PDF]
David B. Bober,Amirhossein Khalajhedayati,Mukul Kumar,Timothy J. Rupert
Physics , 2015,
Abstract: Nanocrystalline materials are defined by their fine grain size, but details of the grain boundary character distribution should also be important. Grain boundary character distributions are reported for ball milled, sputter deposited, and electrodeposited Ni and Ni-based alloys, all with average grain sizes of ~20 nm, to study the influence of processing route. The two deposited materials had nearly identical grain boundary character distributions, both marked by a {\Sigma}3 length percentage of 23-25%. In contrast, the ball milled material had only 3% {\Sigma}3-type grain boundaries and a large fraction of low angle boundaries (16%), with the remainder being predominantly random high angle (73%). These grain boundary character measurements are connected to the physical events that control their respective processing routes. Consequences for material properties are also discussed with a focus on nanocrystalline corrosion. As a whole, the results presented here show that grain boundary character distribution, which has often been overlooked in nanocrystalline metals, can vary significantly and influence material properties in profound ways.
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