Papers by Keyword: Nanocrystalline Material

Abstract: Equal channel angular pressing (ECAP) is the one of the promising methods of severe plastic deformation to obtain bulk ultrafine grain structures. However, ECAP can also be used for powder consolidation. In the present study, fully dense bulk AA 4032 alloy was consolidated from nanocrystalline and microcrystalline powders. These materials were processed by ECAP until four passes at ambient temperature. It is observed that hardness and densification increased significantly with increase in number of ECAP passes. Transmission electron microscopic and scanning electron microscopic examinations evidenced that crystallite size of the nanopowders are unaltered, however a significant crystallite size reduction from around 50 µm down to submicron size is observed. Moreover, higher densification is achieved in microcrystalline powders than nano powders, whereas higher hardness in the case of nanopowders compared to microcrystalline powders.

Abstract: In this work Ni-free austenitic stainless steels with nanostructure and their nanocomposites were synthesized by mechanical alloying (MA), heat treatment and nitriding of elemental microcrystalline Fe, Cr, Mn and Mo powders with addition of hydroxyapatite (HA). Microhardness and corrosion tests' results of obtained materials are presented. Mechanical alloying and nitriding are very effective technologies to improve the corrosion resistance of stainless steel. Decreasing the corrosion current density is a distinct advantage for prevention of ion release and it leads to better cytocompatibility. Similar process in case of nanocomposites of stainless steel with hydroxyapatite helps achieve even better mechanical properties and corrosion resistance. Hence nanocrystalline nickel-free stainless steels and nickel-free stainless steel/hydroxyapatite nanocomposites could be promising bionanomaterials for use as a hard tissue replacement implants, e.g. orthopedic implants.

Abstract: The nanocrystalline Ni3Fe powders were obtained via wet mechanical alloying route in argon atmosphere. As process control agent (PCA) the benzene (C6H6) was used. In order to release the internal stresses an annealing at 350 °C for 4 hours was performed. Alternatively, the nanocrystalline Ni3Fe powder was subjected to a process of incipient recrystallization at 600 °C for 30 minutes in vacuum. The recrystallised powders have a crystallite size of 30 nm. The magnetic properties (the frequencies dependence of permeability and magnetic losses) were investigated in AC field of 0.05, 0.1 and 0.2 T. It was found that the polymerization process has a strong influence over the magnetic properties of compacts. If the polymerization was performed by heating the powders into a mould, generally we observed a positive influence over the permeability but a negative influence over the magnetic losses.

Abstract: The Hipernik alloy (50Ni50Fe wt. %) was obtained by mechanical alloying. The milling was performed in argon atmosphere, with a ball/powder mass ration of 8:1 for times ranging from 2 up to 20h. The alloy formation was studied by X-ray diffraction. The obtained structure is face cantered cubic, indicating the extension of the γ domain for the Ni-Fe alloys by mechanical alloying. The mean crystallite size was calculated with the Williamson – Hall method. Using scanning electron microscopy (SEM) the morphology and the chemical homogeneity of the powders was analysed. The technological properties of the powders as particle size distribution and flowability are determined as a function of the milling time. The magnetic behaviour of the samples was studied by magnetic measurements under high magnetic fields.

Abstract: The effect of grain size on the deformation twinning and de-twinning in a nanocrystalline Ni-Fe alloy was investigated using transmission electron microscopy. Specimens with different grain sizes were obtained by severely deforming an electrochemically deposited nanocrystalline Ni-20wt.% Fe alloy using high-pressure torsion, which resulted in continuous grain growth from an average grain size of ~ 21 nm in the as-deposited material to ~ 72 nm for the highest strain applied in this study. Results show that deformation de-twinning occurs at very small grain sizes while deformation twinning takes place when the grain size is larger than ~ 45 nm. The mechanism of the observed grain size effect on twinning and de-twinning is briefly discussed.

Abstract: Molecular dynamics simulations have show that nanocrystalline (NC) materials can be treated as composite materials consisting of two phases of grain and grain boundary. In this paper, the incremental stress-strain relation is derived based on deformation mechanism of NC materials and internal variable theory from micromechanics point of view. The developed model is exemplified by the pure copper subjected to uniaxial tension. Implicated iteration algorithm is then employed to obtain the stress-strain relation. Moreover, the effects of grain shape and statistical distribution of grain sizes are also discussed, and predicted results are compared with experimental values to verify the model.

Abstract: Atom probe tomography provides compositional information in three dimensions at the atomic scale, and is therefore extremely suited to the study of nanocrystalline materials. In this paper we present atom probe results from the investigation of nanocomposite TiSi¬Nx coatings and nanocrystalline Al. We address some of the major challenges associated with the study of nanocrystalline materials, including specimen preparation, visualisation, common artefacts in the data and approaches to quantitative analysis. We also discuss the potential for the technique to relate crystallographic information to the compositional maps.

Abstract: The problem of the crystallite size determination for nanomaterials from X-ray diffraction data obtained in asymmetrical GIXD geometry was analyzed. The studies were performed on nanocrystalline MgO powder prepared by sol-gel synthesis. The nanopowder was preliminary characterized from X-ray diffraction pattern registered in classical Bragg-Brentano geometry and electron microscope observation. The estimated crystallite size, calculated form Williamson-Hall method, equals to 5 nm whereas the lattice distortion is negligible (0.1%). The X-ray diffraction patterns were registered in 30-135º 2θ range using tunnel GIXD technique for the incident α angle: 0.25; 0.5; 1; 2.5 and 5 degrees, respectively. Additional broadening of diffraction lines originated from applied geometry was observed. The calculated crystallite size deviate significantly in comparison to results obtained from classical Bragg-Brentano data. Corrections for additional line broadening were determined, which should be applied for accurate crystallite size calculation in studies of thin nanocrystalline layers using GIXD technique.

Abstract: Recent advances in Line Profile Analysis of powder diffraction patterns must be paralleled by increasing attention to the quality and quantity of experimental data. The analysis of simulated data with different noise levels demonstrates the importance of statistical quality to reveal fine details of interest in the analysis of nanocrystalline materials, like the crystallite shape. It is also shown how synchrotron radiation diffraction can improve data quality with respect to laboratory measurements, both in terms of statistical quality and in terms of accessible information.

Abstract: In this paper, examples of some of the most challenging features of GB diffusion are considered covering selected problems, strongly related to the research activity at our Laboratories and to the scientific interest of Boris Bokstein too. The following problems (and still open questions related to them) are addressed: i) Diffusion in a random network of grain boundaries with different structures and diffusion coefficients in polycrystalline materials; ii) Segregation effects; iii) Stress effects and iv) Effect of the presence of moving and/or non-equilibrium grain boundaries.