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Li N.,Beijing Institute of Technology | Wang Y.D.,University of Science and Technology Beijing | Liu W.J.,Argonne National Laboratory | An Z.N.,University of Tennessee at Knoxville | And 4 more authors.
Acta Materialia | Year: 2014

Abstract The traditional phenomenological crystallographic theory of martensitic transformations can only explain the change in the shape and crystallographic orientation of a martensitic plate within a single parent crystal. It cannot predict the detailed transformation scenario for preferred selections of martensitic variants or the contributions of partial slip/twinning to local lattice distortion, especially in polycrystalline metals/alloys that exhibit grain-to-grain interactions throughout deformation-induced phase transformation. In this work, synchrotron-based X-ray microdiffraction was used to characterize changes in the local orientation, morphology and strain distribution inside individual martensitic plates, as well as the effect of parent orientation on variant selection in bulk polycrystalline 304 stainless steel (SS) during in situ uniaxial tensile loading at the low temperature of 210 K. It was directly verified that the martensitic phase transformation in the studied 304 SS has two stages, transformation first from γ to ε in the nanoscaled lamella, and then from ε to α′ in the microbands. The selection of martensitic variants was predicted well by the minimum strain work criterion. Phase transformation-induced stress relaxation was evidenced by fluctuations in the (1 1 1) plane lattice strain accompanied by a strain gradient inside the martensitic plate, indicating a load transfer from the transformed grain to its neighbor. This leads to good stress/strain accommodation, as stresses can dissipate from the matrix into martensitic plates and nearby grains. Our experimental observations and theoretical analysis provide an in-depth understanding of the micromechanical behavior, particularly phase transformation-induced plasticity enhancement, of metals containing the metastable phase. © 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Source

Ma S.M.,Shanghai JiaoTong University | Zhang P.,Shanghai JiaoTong University | Ji G.,CNRS Materials and Transformations Unit of UMET | Chen Z.,Shanghai JiaoTong University | And 4 more authors.
Journal of Alloys and Compounds | Year: 2014

In this paper, we reported microstructure and mechanical properties of the in-situ TiB2/Al-Mg-Si composites processed by friction stir processing (FSP). Compared with the initial state, the proper FSP conditions can enhance both the ultimate strength and the elongation of the composite samples obtained from the nugget zone. Detailed microstructure investigation has been performed by synchrotron X-ray diffraction, scanning and transmission electron microscopy and associated electron backscattered diffraction in order to reveal the mechanisms being responsible for the unusual mechanical behaviors. The results show that the initial composite has a grain size of 50-100 μm and the synthesized nanosized TiB2 particles are almost agglomerated to form micrometer sized clusters at grain boundaries. Comparatively, after FSP, the nugget zone is characterized by fine and equiaxed recrystallized grains (1-5 μm in average grain size). The initial clusters are also broken up, while the nanosized TiB2 particles are distributed much more uniformly in the matrix and act as effective pins to interact with dislocations. Hence, the significantly refined grains and the uniform distribution of the nanosized TiB2 particles mainly contribute to the increase of both strength and ductility of the FSPed composites. The strengthening mechanisms are also discussed. © 2014 Elsevier Ltd. All rights reserved. Source

Tian Q.,Lanzhou University | Tian Q.,Institute of Nuclear Physics and Chemistry | Li J.,Lanzhou University | Xie Q.,Lanzhou University | Wang Q.,Lanzhou University
Materials Chemistry and Physics | Year: 2012

This paper reports morphology-tuned synthesis of arrayed one-dimensional ZnO nanostructures on Agcoated glass substrates from Zn(NO 3) 2 and dimethylamine borane (DMAB) aqueous solutions and their photoluminescence and photocatalytic properties. By adjusting the Zn(NO 3) 2 concentration in the solutions, the ZnO nanoneedle arrays, nanorod arrays, and polycrystal films can be obtained. Low Zn(NO 3) 2 concentrations result in ZnO nanoneedle arrays. The ZnO nanoneedles grow along the 0 {0 0 1} directions. The growth process of the ZnO nanoneedles at low Zn(NO 3) 2 concentrations goes through the formation of the equiaxial ZnO granules, the growth of the equiaxial granules into small nanorods, the growth of the small nanorods into nanoneedles, the growth of the nanoneedles in diameter and length, the growth of the nanoneedles into nano-obelisks, and the growth of the nano-obelisks into thick hexangular rods. So the morphologies of the arrayed 1D ZnO nanostructures can be tuned from arrayed nanoneedles through arrayed nanorods and nano-obelisks to arrayed thick hexagonal rods by controlling the Zn(NO 3) 2 concentration in the DMAB and Zn(NO 3) 2 solutions and/or the deposition time in our process. The ZnO nanoneedle arrays have the largest ratio of the visible to ultraviolet emission and the highest photocatalytic activity. © 2011 Elsevier B.V. All rights reserved. Source

Ke J.,Institute of Nuclear Physics and Chemistry | Zhou C.,Institute of Nuclear Physics and Chemistry
IEEE Transactions on Plasma Science | Year: 2014

An empirical method has been used in this paper to estimate the sheath edge of plasma ion sources. Based on Kovaleski's hypothesis in 2006 that the sheath edge from an unmagnetized plasma is coincided with a critical electric field magnitude contour, the sheath edges simulated by a trajectory code were used to calculate Ep and to fit Ep as a function of Te and ne0. A test shows that only half iterations are needed when the initial sheath is set by the empirical method of this paper. This indicates that the using of the empirical method can save much computing time. The deviation of the expression of Ep in Kovaleski's paper and this paper is also discussed. The deviation may be due to the different type of used codes and that the plasma distribution will influence the electric field distribution in sheath edge calculations. © 2014 IEEE. Source

Wu E.,CAS Shenyang Institute of Metal Research | Sun G.,Institute of Nuclear Physics and Chemistry | Chen B.,Institute of Nuclear Physics and Chemistry | Zhang J.,CAS Shenyang Institute of Metal Research | And 3 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2014

A single crystal superalloy with initial sample axis 10 deg deviated from [001] was creep deformed at 1273 K (1000 C) 235 MPa and its triaxial strain/stress state and subgrain defects were studied by neutron diffraction. Normal internal stresses with their directions close to the loading axis and their scales smaller than those perpendicular to the axis were observed and attributed to a lattice rotation toward [001] pole. The internal stress at a level approaching to the loading stress and mostly in the state of interphase stress was induced during the first stage of creep prior to rafting and associated to lattice rotation, microstrain relaxation and line-up of misoriented γ′-precipitates. The internal stress was diminished and released at final stage of creep associated with a reduction in unit-cell volume and a transition of strain/stress state between the two phases. The observation was explained by development of dislocations and raft structure during creep. © 2013 The Minerals, Metals & Materials Society and ASM International. Source

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