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Type Galtrey, M.J.; Oliver, R.A.; Kappers, M.J.; McAleese, C.; Zhu, D.; Humphreys, C.J.; Clifton, P.H.; Larsen, D.; Cerezo, A.
  Publication Atom probe revels the structure of InxGe1-xN based quantum wells in three dimensions Volume Journal Article
Pages 2008
  Abstract Physica Status Solidi B-Basic Solid State Physics  
  Corporate Author Phys. Status Solidi B-Basic Solid State Phys.  
Publisher 245  
Editor (up) 5
  Summary Language 861-867 Series Editor  
Abbreviated Series Title The three-dimensional atom probe has been used to characterize InxGa1-xN based multiple quantum well structures emitting from the green to the ultra-violet with sub-nanometre resolution over a 100 nm field of view. The results show gross discontinuities and compositional variations within the UV-emitting quantum well layers on a 20-100 nm length scale. We propose that these may contribute to the high efficiency of this structure: In addition, the analysis shows the presence of indium in the barrier layers of all the samples, whether or not there was an indium precursor present during barrier growth. The distribution of indium within the blue- and green-emitting InxGa1-xN quantum wells is also analyzed, and we find no evidence that InxGa1-xN with a range of compositions is not a random alloy, and so rule out indium clustering as the cause of the reported carrier localization in these structures. The upper interface of each quantum well layer is shown to be rougher and more diffuse than the lower. and the existence of monolayer steps in the interfaces that could effectively localize carriers at room temperature is revealed. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
  Series Issue Univ Cambridge, Dept Mat Sci & Met, Cambridge CB2 3QZ, England, Email: mjg73@cam.ac.uk ISSN  
Medium
  Expedition Wiley-V C H Verlag Gmbh Notes  
Call Number  
Contribution Id English  
Serial URL ISBN  
0370-1972 ISI:000256242300017 no NU @ m-krug @ 10392
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Lee, W.-S.; Chen, T.-H. Mechanical and microstructural response of aluminum-scandium (Al-Sc) alloy as a function of strain rate and temperature Journal Article 2009 Materials Chemistry and Physics 113 2-3 734-745 Al-Sc alloy; Strain rate sensitivity; Activation energy; Shearing; Dislocation; Precipitates This study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of aluminum-scandium (Al-Sc) alloy at temperatures ranging from -100 °C to 300 °C and strain rates of 1.2 × 103 s-1, 3.2 × 103 s-1 and 5.8 × 103 s-1. The relationship between the dynamic mechanical behaviour of the Al-Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress-strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. However, at room temperature under a low strain rate of 1.2 × 103 s-1 and at a high experimental temperature of 300 °C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. The Zerilli-Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al-Sc alloy. Comparing the predicted flow stress values with the experimental values over all the considered strain rate and temperature conditions, the maximum error between the two sets of results is found to be less than 4%. SEM observations show that the specimens fracture predominantly as a result of a shearing mechanism. Moreover, the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of a ductile fracture mode. Fine Al3Sc precipitates are found to be distributed in the matrix and at the grain boundaries. Finally, the TEM analysis results reveal that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size. no NU @ karnesky @ 10512
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Lee, W.S.; Chen, T.H. Effects of strain rate and temperature on dynamic mechanical behaviour and microstructural evolution in aluminium-scandium (Al-Sc) alloy Journal Article 2008 Materials Science and Technology 24 10 1271-1282 AL-SC ALLOY; STRAIN RATE SENSITIVITY; ACTIVATION ENERGY; ADIABATIC SHEARING; DISLOCATION; AL3SC PRECIPITATES The present study applies a compressive split Hopkinson bar to investigate the mechanical response, microstructural evolution and fracture characteristics of an aluminium-scandium (Al-Sc) alloy at temperatures ranging from − 100 to 300°C and strain rates of 1·2 × 103, 3·2×103 and 5·8 × 103 s−1. The relationship between the dynamic mechanical behaviour of the Al-Sc alloy and its microstructural characteristics is explored. The fracture features and microstructural evolution are observed using scanning and transmission electron microscopy techniques. The stress-strain relationships indicate that the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, but decrease with increasing temperature. Conversely, the activation volume and activation energy increase as the temperature increases or the strain rate decreases. Additionally, the fracture strain reduces with increasing strain rate and decreasing temperature. The Zerilli-Armstrong fcc constitutive model is used to describe the plastic deformation behaviour of the Al-Sc alloy, and the error between the predicted flow stress and the measured stress is found to be less than 5%. The fracture analysis results reveal that cracks initiate and propagate in the shear bands of the Al-Sc alloy specimens and are responsible for their ultimate failure. However, at room temperature, under a low strain rate of 1·2 × 103 s−1 and at a high experimental temperature of 300°C under all three tested strain rates, the specimens do not fracture, even under large strain deformations. Scanning electron microscopy observations show that the surfaces of the fractured specimens are characterised by transgranular dimpled features, which are indicative of ductile fracture. The depth and density of these dimples are significantly influenced by the strain rate and temperature. The transmission electron microscopy structural observations show the precipitation of Al3Sc particles in the matrix and at the grain boundaries. These particles suppress dislocation motion and result in a strengthening effect. The transmission electron microscopy analysis also reveals that the dislocation density increases, but the dislocation cell size decreases, with increasing strain rate for a constant level of strain. However, a higher temperature causes the dislocation density to decrease, thereby increasing the dislocation cell size. no NU @ m-krug @ 10531
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Lagow, B.W.; Robertson, I.M.; Jouiad, M.; Lassila, D.H.; Lee, T.C.; Birnbaum, H.K. Observation of dislocation dynamics in the electron microscope Journal Article 2001 Materials Science And Engineering A Mater. Sci. Eng. A 309-310 445-450 Deformation experiments performed in situ in the transmission electron microscope have led to an increased understanding of dislocation dynamics. To illustrate the capability of this technique two examples will be presented. In the first example, the processes of work hardening in Mo at room temperature will be presented. These studies have improved our understanding of dislocation mobility, dislocation generation, and dislocation-obstacle interactions. Zn the second example, the interaction of matrix dislocations with grain boundaries will be described. From such studies predictive criteria for slip transfer through grain boundaries have been developed. 0921-5093 WOS:000169044600089 no NU @ m-krug @ 10533
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Muraleedharan, K.; Balamuralikrishnan, R.; Das, N. TEM and 3D atom probe characterization of DMS4 cast nickel-base superalloy Journal Article 2009 Journal of Materials Science J. Mater. Sci. 44 9 2218-2225 Cast nickel-base superalloys possess the required mechanical properties (creep resistance and stress rupture life) at elevated temperatures that make them suitable for turbine blades in aero-engines. The origin of these properties lies in the presence of a simple two phase [gamma]-[gamma]' microstructure (with cuboidal [gamma]' particles dispersed in a [gamma] matrix), in spite of the presence of several alloying elements. The cuboidal nature of the [gamma]' particles arises from an optimal misfit between the two phases, which is a function of the composition of [gamma] and [gamma]' phases. In addition, several microstructural issues arising out of the partitioning of the alloying elements influences directly the deformation mechanisms in the [gamma] and [gamma]', and therefore the mechanical properties of the alloy. In this article, we discuss how some of these microstructural issues have been investigated in DMS4, a cast single crystal superalloy, experimentally using TEM and 3DAP techniques. no NU @ karnesky @ 10534
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Knipling, Keith E.; Karnesky, Richard A.; Lee, Constance P.; Dunand, David C.; Seidman, David N. Precipitation Evolution in Al-0.1Sc, Al-0.1Zr, and Al-0.1Sc-0.1Zr (at.%) Alloys during Isochronal Aging Journal Article 2010 Acta Materialia 58 15 5184-5195 Aluminum alloys, Precipitation, Scandium, Zirconium, Atom-probe tomography; Al-Sc-Zr Precipitation strengthening is investigated in binary Al-0.1Sc, Al-0.1Zr, and ternary Al-0.1Sc-0.1Zr (at.%) alloys aged isochronally between 200 and 600 °C. A pronounced synergistic effect is observed when both Sc and Zr are present. Above 325 °C, where peak microhardness (670 MPa) occurs in the binary Al-Sc alloy due to Al[sub:3]Sc (L1[sub:2]) nanometer-scale precipitates, Zr additions result in a secondary increase in strength due to additional precipitation of Zr-enriched outer shells onto these precipitates. The ternary alloy reaches a peak microhardness of 780 MPa at 400 °C, delaying overaging by >100 °C compared with the binary Al-Sc alloy and increasing strength compared with the binary Al-Zr alloy (peak microhardness of 420 MPa at 425–450 °C). Compositions, radii, volume fractions, and number densities of the Al[sub:3](Sc[sub:1-x]Zr[sub:x]) precipitates are measured directly using atom-probe tomography. This information is used to quantify the observed strengthening increments, attributed to dislocation shearing of the Al[sub:3](Sc[sub:1-x]Zr[sub:x]) precipitates. no NU @ karnesky @ 10705
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Zhou, F.; Lee, J.; Dallek, S.; Lavernia, E.J. High grain size stability of nanocrystalline Al prepared by mechanical attrition Journal Article 2001 Journal of Materials Research J. Mater. Res. 16 12 3451-3458 Grain growth in nanocrystalline (nc) Al with a grain size of 26 nm produced by cryogenic mechanical milling was studied through x-ray diffraction, transmission electron microscopy, and differential scanning calorimetry. Grain growth kinetics resembled those of ball-milled nc Fe. For homologous temperatures (T/TM) of 0.61-0.83, the time exponent n from D1/n - D01/n = kt was 0.04-0.28, tending toward 0.5 as T/TM increased. Two grain-growth regimes were distinguished: below T/TM = 0.78 growth ceased at an approximate grain size of 50 nm while at higher temperatures, grain growth proceeded steadily to the submicrometer range. Grain growth over the range of temperatures studied cannot be explained in terms of a single thermally activated rate process. The observed high grain size stability was attributed primarily to impurity pinning drag associated with the grain growth process. 0884-2914 no NU @ karnesky @ 2757 10753
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Rogozhkin, S.; Ageev, V.; Aleev, A.; Zaluzhnyi, A.; Leont’eva-Smirnova, M.; Nikitin, A. Tomographic atom-probe analysis of temperature-resistant 12%-chromium ferritic-martensitic steel EK-181 Journal Article 2009 The Physics of Metals and Metallography 108 6 579-585 Abstract At present, the temperature-resistant steels with a rapid reduction of induced radioactivity appear to be a perspective structural material for new-generation nuclear and thermonuclear reactors. Special attention is paid to the nanostructural state of the elaborated materials. In this work, for the first time, there have been carried out tomographic atom-probe studies of the chromium ferritic-martensitic steel EK-181 (RUSFER EK-181) with 12% Cr. Spatial distributions of chemical elements in the investigated volumes of the material with an atomic resolution have been obtained. The dimensions of the investigated portions of the material are on the order of 10 × 10 × 30 nm3. There have been observed nanosized preprecipitates (nanoclusters), i.e., regions enriched in V, Cr, and N atoms, with characteristic sizes of about 3 nm. no NU @ karnesky @ 10767
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Lee, Woei-Shyan; Chen, Tao-Hsing; Lin, Chi-Feng; Chen, Ming-Shiang Impact deformation behaviour and dislocation substructure of Al-Sc alloy Journal Article 2010 Journal of Alloys and Compounds 493 1-2 580-589 Al-Sc alloy; Strain rate sensitivity; Activation volume; Adiabatic shear band; Dislocation; Precipitates This paper employs a compressive split-Hopkinson pressure bar to investigate the impact deformation behaviour of Al-Sc alloy under high strain rates of 1.2103s-1, 3.2103s-1 and 5.8103s-1, respectively, and temperatures of -100C, 25C and 300C. It is shown that for a constant temperature, the flow stress, work hardening rate and strain rate sensitivity increase with increasing strain rate, while the activation volume decreases. Conversely, for a constant strain rate, the flow stress, work hardening rate and strain rate sensitivity decrease with increasing temperature, while the activation volume increases. It is found that the impact deformation behaviour of Al-Sc alloy can be accurately described using the Zerilli-Armstrong constitutive equation. Optical microscopy (OM) observations reveal that the specimens fail principally as the result of an adiabatic shearing mechanism. Furthermore, scanning electron microscopy (SEM) observations show that the fracture surfaces are characterised by a dimple-like structure, which indicates a ductile failure mode. Transmission electron microscopy (TEM) observations indicate that the dislocation density and cell size are related to the strain rate, flow stress and temperature. Finally, the TEM observations suggest that the strengthening effect observed in the deformed Al-Sc alloy is the result of Al3Sc precipitates within the matrix and at the grain boundaries, which suppress dislocation motion and prompt an increase in the work hardening stress. 0925-8388 no NU @ karnesky @ 10784
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Lee, Woei-Shyan; Chen, Tao-Hsing; Lin, Chi-Feng; Lu, Ging-Ting Adiabatic Shearing Localisation in High Strain Rate Deformation of Al-Sc Alloy Journal Article 2010 Materials Transactions 51 7 1216-1221 aluminium-scandium alloy, strain rate sensitivity, adiabatic shearing, precipitates Aluminium-scandium (Al-Sc) alloy is subjected to shear deformation at high strain rates ranging from 3.0×105 s−1 to 6.2×105 s−1 using a compressive-type split-Hopkinson pressure bar (SHPB). The effects of the strain rate on the shear stress, adiabatic shear band characteristics, and fracture features of the Al-Sc alloy are systematically examined. The results show that both the shear stress and the strain rate sensitivity increase with an increasing strain rate. In addition, it is shown that an adiabatic shear band is formed within the deformed specimens for all values of the strain rate. As the strain rate is increased, the width of the shear band decreases, but the microhardness increases. Moreover, the distortion angle and the magnitude of the local shear strain near the shear band both increase with an increasing strain rate. At a strain rate of 3.0×105 s−1, the fracture surface is characterised by multiple transgranular clearage fractures. However, for strain rates greater than 4.4×105 s−1, the fracture surface has a transgranular dimple-like characteristic, and thus it is inferred that the ductility of the Al-Sc alloy improves with an increasing strain rate. no NU @ karnesky @ 10917
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