Browsing by Author "Harrison, RP"

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    Characterization of complex carbide–silicide precipitates in a Ni–Cr–Mo–Fe–Si alloy modified by welding
    (Elsevier B.V., 2015-07-01) Bhattacharyya, D; Davis, J; Drew, M; Harrison, RP; Edwards, L
    Nickel based alloys of the type Hastelloy-N™ are ideal candidate materials for molten salt reactors, as well as for applications such as pressure vessels, due to their excellent resistance to creep, oxidation and corrosion. In this work, the authors have attempted to understand the effects of welding on the morphology, chemistry and crystal structure of the precipitates in the heat affected zone (HAZ) and the weld zone of a Ni–Cr–Mo–Fe–Si alloy similar to Hastelloy-N™ in composition, by using characterization techniques such as scanning and transmission electron microscopy. Two plates of a Ni–Cr–Mo–Fe–Si alloy GH-3535 were welded together using a TiG welding process without filler material to achieve a joint with a curved molten zone with dendritic structure. It is evident that the primary precipitates have melted in the HAZ and re-solidified in a eutectic-like morphology, with a chemistry and crystal structure only slightly different from the pre-existing precipitates, while the surrounding matrix grains remained unmelted, except for the zones immediately adjacent to the precipitates. In the molten zone, the primary precipitates were fully melted and dissolved in the matrix, and there was enrichment of Mo and Si in the dendrite boundaries after solidification, and re-precipitation of the complex carbides/silicides at some grain boundaries and triple points. The nature of the precipitates in the molten zone varied according to the local chemical composition. © 2015 Elsevier Inc.
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    Characterization of complex carbide–silicide precipitates in a Ni–Cr–Mo–Fe–Si alloy modified by welding
    (Materials Australian and The Australian Ceramic Society, 2015-02-09) Bhattacharyya, D; Davis, J; Drew, M; Harrison, RP; Edwards, L
    Nickel based alloys of the type Hastelloy-N™ are ideal candidate materials for molten salt reactors, as well as for applications such as pressure vessels, due to their excellent resistance to creep, oxidation and corrosion. In this work, the authors have attempted to understand the effects of welding on the morphology, chemistry and crystal structure of the precipitates in the heat affected zone (HAZ) and the weld zone of a Ni–Cr–Mo–Fe–Si alloy similar to Hastelloy-N™ in composition, by using characterization techniques such as scanning and transmission electron microscopy. Two plates of a Ni–Cr–Mo–Fe–Si alloy GH-3535 were welded together using a TiG welding process without filler material to achieve a joint with a curved molten zone with dendritic structure. It is evident that the primary precipitates have melted in the HAZ and re-solidified in a eutectic-like morphology, with a chemistry and crystal structure only slightly different from the pre-existing precipitates, while the surrounding matrix grains remained unmelted, except for the zones immediately adjacent to the precipitates. In the molten zone, the primary precipitates were fully melted and dissolved in the matrix, and there was enrichment of Mo and Si in the dendrite boundaries after solidification, and re-precipitation of the complex carbides/silicides at some grain boundaries and triple points. The nature of the precipitates in the molten zone varied according to the local chemical composition. - Graphical abstract: Display Omitted - Highlights: • Ni-based alloy with Cr, Mo, Si, Fe and C was welded, examined with SEM, EBSD, and TEM. • Original Ni{sub 2}(Mo,Cr){sub 4}(Si,C) carbides changed from equiaxed to lamellar shape in HAZ. • Composition and crystal structure remained almost unchanged in HAZ. • Original carbides changed to lamellar Ni{sub 3}(Mo,Cr){sub 3}(Si,C) in some cases in weld metal. • Precipitates were mostly incoherent, but semi-coherent in some cases in weld metal.
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    A comparison of microstructural strengthening for thermal creep and radiation damage resistance of titanium aluminide alloys
    (Elsevier, 2013-07-01) Zhu, HL; Wei, T; Carr, DG; Harrison, RP; Edwards, L; Seo, DY; Maruyama, K
    Titanium aluminide (TiAl) alloys were initially developed for moderate temperature (600–850 °C) applications in the aerospace and automotive industries because they have high specific strength, low density, good corrosion, oxidation and creep resistance at elevated temperatures [1]. TiAl alloys have also received much attention as potential candidate materials for high temperature nuclear structural applications because of excellent radiation resistance and low neutron activation [2], [3], [4] and [5]. Moreover, the microstructure of TiAl alloys can be developed to be more complex than the up-to-now reported microstructures of other advanced structural materials. Various microstructures allow different combinations of properties for various extreme environments in advanced nuclear systems. The effects of microstructural features on creep behaviour of TiAl alloys have been intensively investigated over the last two decades [6], [7] and [8]. However, the effects of microstructural features on irradiation behaviour of TiAl alloys have rarely been studied. In the present short note, the microstructural strengthening for thermal creep and irradiation damage of TiAl alloys is compared. This provides useful guidance for further experiment work necessary to understand the irradiation behaviour of TiAl alloys. © 2013, Elsevier B.V.
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    Defect dynamics in polycrystalline zirconium alloy probed in situ by primary extinction of neutron diffraction
    (American Institute Physics, 2013-02-13) Kabra, S; Yan, K; Carr, DG; Harrison, RP; Dippenaar, RJ; Reid, M; Liss, KD
    After alpha+beta-zirconium has fully transformed into beta-phase upon heating, the intensities of all beta-Zr Bragg reflections decrease simultaneously as a function of time. It is shown that this effect represents a transition from the kinematic to the dynamic theory of diffraction due to the ever increasing crystal perfection driven by thermal recovery of the system. The best fitting coherent crystallite size of 30 mu m and other microstructural features are verified by in situ laser scanning confocal microscopy. This effect of primary extinction in neutron diffraction has been employed to further investigate the crystal perfection kinetics. Upon further heating, crystal recovery is identified as a process of dislocation annihilation, suffering from lattice friction. Upon cooling, precipitating alpha-Zr induces strain into the perfect beta-crystallites, re-establishing the kinematic diffraction intensities. An Avrami analysis leads to the estimations of nucleation time, consumption of nucleation sites and lower-dimensional growth. Such technique bears great value for further investigation on all metal systems annealed close to the melting temperature. © 2013, American Institute of Physics.
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    Effect of double ion implantation and irradiation by Ar and He ions on nano-indentation hardness of metallic alloys
    (Elsevier, 2013-07-01) Dayal, P; Bhattacharyya, D; Mook, WM; Fu, EG; Wang, YQ; Carr, DG; Anderogluc, O; Mara, NA; Misra, A; Harrison, RP; Edwards, L
    In this study, the authors have investigated the combined effect of a double layer of implantation on four different metallic alloys, ODS steel MA957, Zircaloy-4, Ti–6Al–4V titanium alloy and stainless steel 316, by ions of two different species – He and Ar – on the hardening of the surface as measured by nano-indentation. The data was collected for a large number of indentations using the Continuous Stiffness Method or “CSM” mode, applying the indents on the implanted surface. Careful analysis of the data in the present investigations show that the relative hardening due to individual implantation layers can be used to obtain an estimate of the relative hardening effect of a combination of two separate implanted layers of two different species. This combined hardness was found to lie between the square root of the sum of the squares of individual hardening effects, (ΔHA2 + ΔHB2)0.5 as the lower limit and the sum of the individual hardening effects, (ΔHA + ΔHB) as the upper limit, within errors, for all depths measured.© 2013, Elsevier B.V.
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    Effect of welding thermal cycle on the microstructure of Zircaloy-4
    (International Institute of Welding, 2009-07-16) Thorogood, KJ; Li, HJ; Carr, DG; Harrison, RP; Nolan, D
    Zirconium and its alloys are important materials commonly utilised in the nuclear industry, primarily due to their low neutron absorption cross-section and excellent corrosion resistant properties. Zircaloy-4 is one of the most widely used nuclear grade zirconium alloys and contains primary alloying elements of tin, iron, chromium and oxygen. Typical applications include structural core components and fuel cladding. Fully welded structures such as heavy water reflector vessels have been fabricated from Zircaloy-4. Although the structure-property relationships of Zircaloy-4 parent metal and weld metal is well understood, there is limited understanding of the same relationship for the narrow weld heat-affected zone. The work reported in this paper is an investigation of the influence of the weld thermal cycles on the microstructure and texture of Zircaloy-4 heat-affected zone material. Discrete regions within the heat-affected zone were simulated using a thermo-mechanical simulator (Gleeble 3500). Peak temperatures of the thermal cycles studied were 1000,1200, 1400 and 1600°C. Weld simulation has been shown to produce a volume of material sufficient for analysis and which represents the individual sub-zones. © 2010, International Institute of Welding
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    Hardness-tensile property relationships for HAZ in 6061-T651 aluminum
    (American Welding Society, 2014-08) Stathers, PA; Hellier, AK; Harrison, RP; Norrish, J
    High-strength aluminum is used extensively in industry, with welding being a widely used fabrication method. This work focuses on welding of 6061-T651 aluminum and establishment of the hardness–tensile properties relationship in the heat-affected zone (HAZ) of a gas metal arc weld using 4043 filler material. Test welds were prepared from 12.7-mm-thick plate with a single-V weld preparation. Base plate temperatures were measured with an array of eight embedded thermocouples during welding, relating temperature to properties at intervals from the weld. Through-thickness slices 1.7 mm thick were removed, by electric discharge machining, from the plate parallel to the weld at 2-mm intervals and extending from the weld centerline to 40 mm. into the HAZ and base plate. Tensile samples were prepared from these slices, and tensile properties and hardness values measured to establish a relationship between these two parameters. Both EQUOTIP (portable hardness tester) and Vickers microhardness measurements were conducted and related to tensile properties. Although a significant body of work exists relating tensile properties to hardness, no previous study was found that used this approach. Most work appeared to use cross-weld tensile tests, which only give the point of lowest strength. Sections of base plate material having a different thickness (31.75 mm) from that of the welded samples, and from a different source, were thermally aged to four hardness values and the hardness–tensile relationship was also established for this material. These results were compared with those of the HAZ samples; the results were found to fall within the scatter band of HAZ results.
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    Hydride precipitation and its influence on mechanical properties of notched and unnotched Zircaloy-4 plates
    (Elevier Science BV., 2013-05-01) Wang, ZY; Garbe, U; Li, HJ; Harrison, RP; Toppler, K; Studer, AJ; Palmer, T; Planchenault, G
    The hydride formation and its influence on the mechanical performance of hydrided Zircaloy-4 plates containing different hydrogen contents were studied at room temperature. For the unnotched plate samples with the hydrogen contents ranging from 25 to 850 wt. ppm, the hydrides exerted an insignificant effect on the tensile strength, while the ductility was severely degraded with increasing hydrogen content. The fracture mode and degree of embrittlement were strongly related to the hydrogen content. When the hydrogen content reached a level of 850 wt. ppm, the plate exhibited negligible ductility, resulting in almost completely brittle behavior. For the hydrided notched plate, the tensile stress concentration associated with the notch tip facilitated the hydride accumulation at the region near the notch tip and the premature crack propagation through the hydride fracture during hydriding. The final brittle through-thickness failure for this notched sample was mainly attributed to the formation of a continuous hydride network on the thickness section and the obtained very high hydrogen concentration (estimated to be 1965 wt. ppm). © 2013, Elsevier Ltd.
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    Hydrogen-induced microstructure, texture and mechanical property evolutions in a high-pressure torsion processed zirconium alloy
    (Elsevier Ltd., 2012-11-01) Wang, ZY; Garbe, U; Li, HJ; Studer, AJ; Harrison, RP; Callaghan, MD; Wang, Y; Liao, XZ
    The gaseous hydriding-induced evolutions of the microstructure, texture and mechanical properties of Zircaloy-4 processed by high-pressure torsion (HPT) were assessed. Much delta-ZrH(1.66) precipitation at 15 atm (21%) incurred significant hardening of vacuum-annealed HPT samples, and pure epsilon-ZrH(2) obtained at 20 atm showed a superior microhardness of 470 HV(0.3) and a low fracture toughness of 0.63 MPa m(1/2). The delta-hydrides presented strong (1 1 1) texture and followed the (0 0 0 1)(alpha-Zr)//{1 1 1}(delta-ZrH1.66) orientation relationship with the alpha-Zr matrix. During hydriding, alpha-Zr recrystallization texture was developed from the initial deformation texture. Copyright © 2012 Acta Materialia Inc.
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    In situ micro tensile testing of He+ ion irradiated single crystal nickel film
    (Australian Microscopy and Microanalysis Society, 2015-11-01) Bhattacharyya, D; Reichardt, A; Ionescu, M; Davis, J; Hosemann, P; Harrison, RP; Edwards, L
    Introduction : Radiation damage can cause increase in strength and decrease in ductility, thus reducing the service life of structural parts in reactors. Ion beam irradiation has been a method of choice to simulate the effects of neutron irradiation in a reactor for some time now [1], since it enables the attainment of reasonable doses within hours, instead of years inside a reactor. A major problem in this method is that the damaged region is very shallow, and mechanical testing of such thin layers is extremely difficult. In this study, we have used in situ micro-tensile testing in the scanning electron microscope (SEM) to understand the effects of high energy ion beam irradiation on the mechanical properties of a single crystal nickel thin film. Experiments Single crystal Nickel foils, ~12.8-13.1 μm thick, were irradiated with 6 MeV He+ ions in the Tandetron “STAR” accelerator at ANSTO. The samples were irradiated to two different fluences – (i) 2 x 1017 ions/cm2 (peak damage of ~ 10 displacements per atom or dpa), and (ii) 3.8 x 10 17 ions/cm2 (peak damage of ~19 dpa). Damage profiles calculated using the SRIM software [2], showed that there is a long, low tail of the profile beginning at the entry face and extending to approximately 9-10 μm depth, after which the damage rises sharply (Fig. 1(a)). Micro-tensile samples of approximate dimensions 25-30 μm (l) x 10 μm (w) x 12-13 μm (h) were fabricated using a Zeiss® Auriga 60™ Cross-Beam™ instrument. The free end of the sample was milled to obtain a rectangular hole which was used as a grip. The end of the tensile device, shaped as an L shaped hook, was inserted into the aforementioned rectangular hole. The sample was then subjected to tension by applying a voltage to a piezo-electric device attached to the tensile head, causing it to move at a rate of ~20 nm/sec. SEM images were taken at regular intervals, and the strain measured using two fiducial markers, one on each side of the gauge length. Results : An image of a typical tensile sample used in these tests is shown in Figure 1(b), before the start of the test. The SEM image in Figure 1(c) shows the unirradiated sample after a tensile strain of e ~ 56%. The sample had a Y.S. of ~70- 100 MPa, and an U.T.S. of ~240 MPa (see Fig. 2). There was significant strain hardening up to the U.T.S., and subsequently it underwent plastic strain with large slip bands passing on two major sets of planes in an alternate manner. The formation of these slip bands was accompanied by small drops in the stress and increases in strain. A post- test SEM image of a sample irradiated with 6 MeV He+ ions to a fluence of ~2e17 ions/ cm2 and a peak damage of ~ 10 dpa is presented in Fig. 1(d), showing slip bands passing through the whole thickness of the sample and fracture at the lower surface, which in this case is the “exit surface” of the ions. This sample had a Y.S. of ~ 195-230 MPa, and a peak strength of ~358 MPa before first rupture at the surface near peak damage, at a strain of about 1.9% (Fig. 2). A post-test SEM image of the sample fabricated from the foil irradiated with He+ ions to a total fluence of 3.8e17 ions/ cm2 and a peak damage of ~ 19 dpa is shown in Figure 1(e). This sample showed a Y.S. of ~ 400 MPa and a peak strength of ~ 500 MPa before first rupture at the exit surface of the ions, which is the top surface in this case. Conclusions: The effect of He+ ion irradiation on the tensile strength of Ni single crystals was measured successfully by in situ micro- tensile testing of FIB-fabricated samples which included the damaged layers. The results showed increase in average strength of up to ~118 MPa for a total fluence of 2e17 ions/ cm2 and ~260 MPa for a peak damage of ~3.8e17 ions/cm2. Brittle fracture was observed in the irradiated samples at the surface nearer to the peak damage layer.
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    In situ micro tensile testing of He+2 ion irradiated and implanted single crystal nickel film
    (Elsevier B.V., 2015-11-01) Reichardt, A; Ionescu, M; Davis, J; Edwards, L; Harrison, RP; Hosemann, P; Bhattacharyya, D
    The effect of ion irradiation on the tensile properties of pure Ni single crystals was investigated using an in situ micro-mechanical testing device inside a scanning electron microscope. A 12.8 μm-thick Ni film with {0 0 1} plane normal was irradiated with 6 MeV He+2 ions to peak damage of 10 and 19 displacements per atom (dpa). Micro-tensile samples were fabricated from the specimens parallel to the plane of the film using a focused ion beam (FIB) instrument, and tested in tension along [1 0 0] direction, up to fracture. The peak strength increased from ∼230 MPa for the unirradiated material to about 370 MPa and 500 MPa for the 10 dpa and 19 dpa samples respectively, while the ductility decreased with increasing dose. The surface near the peak damage regions fractured in a brittle manner, while the regions with smaller dose underwent significant plastic deformation. Slip bands extended to the peak-damage zone in the sample with a dose of 19 dpa, but did not propagate further. Transmission electron microscopy confirmed the stopping of the slip bands at the peak-damage region, just before the high He concentration region with voids or bubbles. By removing the peak damage region and the He bubble region with FIB, it was possible to attain propagation of slip bands through the entire remaining thickness of the sample. This material removal also made it possible to calculate the irradiation hardening in the region with peak hardness – thus enabling the separation of hardening effects in the high and low damage regions. © 2015 Elsevier Ltd.
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    In-situ characterization of lattice structure evolution during phase transformation of Zr-2.5Nb
    (Wiley-Blackwell, 2011-09-01) Yan, K; Carr, DG; Kabra, S; Reid, M; Studer, AJ; Harrison, RP; Dippenaar, RJ; Liss, KD
    The alpha-beta phase transformation behavior of Zr-2.5Nb (in mass%) has been characterized in real time during an in situ neutron diffraction experiment. The Zr-2.5Nb material in the current study consists, at room temperature, of alpha-Zr phase (hcp) and two beta phases (bcc), a Nb rich beta-Nb phase and retained, Zr rich, beta-Zr(Nb) phase. It is suggested that this is related to a quench off the equilibrium solubility of Nb atoms in the Zr bcc unit cells. Vegard's law combined with thermal expansion is applied to calculate the composition of the beta-phase, which is compared with the phase diagram, revealing the system's kinetic behavior for approaching equilibrium. © 2011, Wiley-Blackwell.
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    Irradiation behaviour of α2 and γ phases in He ion implanted titanium aluminide alloy
    (Elsevier, 2014-07-01) Zhu, HL; Wei, T; Blackford, MG; Short, KT; Carr, DG; Harrison, RP; Edwards, L; Seo, DY; Maruyama, K
    A Ti–45Al–2Nb–2Mn + 0.8 vol.% TiB2 (at.%) alloy with fully lamellar microstructure consisting of hexagonal-close-packed (hcp) α2 and face-centred-tetragonal (fct) γ phases was irradiated by implanting helium ions to different fluences. Microstructural examination showed that helium cavities are formed in both the α2 and γ phases after He-ion irradiation. However, the helium cavities and their size change with fluence are much larger in the α2 phase than those in the γ phase, indicating that the γ phase exhibits better tolerance to the He-ion irradiation than the α2 phase. Since α2 and γ phases have different crystal structures, they possess differences in helium solubility and interstitial migration. These differences are responsible for the variation in radiation damage behaviour between the two phases. © 2014, Elsevier Ltd.
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    Materials surveillance program for the OPAL research reactor
    (European Nuclear Society, 2007-03-11) Harrison, RP; Carr, DG; Wei, T; Stathers, PA
    The OPAL research reactor has recently achieved full power and will begin normal operation in the early part of 2007. One aspect of the design of OPAL has been the inclusion of a surveillance program for the materials used in the reactor core regions. These materials are exposed to a high neutron flux and their properties, such as tensile strength, fracture toughness and physical dimensions (through radiation-induced growth), are expected to change through the life of the reactor. Estimates of these changes have been obtained from literature data and have been accommodated in the design. However, data at the operating temperature of OPAL is limited. In order to guarantee safe operation of these materials, a surveillance program was developed during the detailed design phase of the project. The program involves the placement of miniature samples in high flux regions close to the reactor core. These samples will be removed at intervals and will be subjected to extensive mechanical testing to determine any changes compared with samples in the unirradiated condition. Additional samples will be sectioned from other high-fluence components that will be removed well before the 40 year design life. © The Authors
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    Materials surveillance program for the OPAL Research Reactor
    (International Group On Research Reactors, 2007-03-11) Harrison, RP
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    Microstructural design for thermal creep and radiation damage resistance of titanium aluminide alloys for high-temperature nuclear structural applications
    (Elsevier, 2014-10) Zhu, HL; Wei, T; Carr, DG; Harrison, RP; Edwards, L; Seo, DY; Maruyama, K; Dargusch, MS
    Microstructure plays an important role in strengthening of metallic materials. Various microstructures can be developed in titanium aluminide (TiAl) alloys, which can enable different combinations of properties for various extreme environments in advanced nuclear systems. In the present paper the mechanisms for microstructural strengthening and the effects of various microstructural features on thermal creep and radiation damage resistance of TiAl alloys are reviewed and compared. On the basis of the results, the evidence-based optimum microstructure for the best combination of thermal creep and radiation damage resistance of TiAl alloys is proposed. The heat treatment processes for manufacturing the optimal microstructure are also discussed. © 2014, Elsevier Ltd.
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    Microstructure and texture analysis of δ-hydride precipitation in Zircaloy-4 materials by electron microscopy and neutron diffraction
    (International Union of Crystallography, 2014-01-01) Wang, ZY; Garbe, U; Li, HJ; Wang, YB; Studer, AJ; Sun, GG; Harrison, RP; Liao, XZ; Vicente Alvarez, MA; Santisteban, JR; Kong, C
    This work presents a detailed microstructure and texture study of various hydrided Zircaloy-4 materials by neutron diffraction and microscopy. The results show that the precipitated δ-ZrH1.66 generally follows the δ (111)//α(0001) and δ [110]//α[1120] orientation relationship with the α -Zr matrix. The δ-hydride displays a weak texture that is determined by the texture of the α-Zr matrix, and this dependence essentially originates from the observed orientation correlation between α-Zr and δ-hydride. Neutron diffraction line profile analysis and high-resolution transmission electron microscopy observations reveal a significant number of dislocations present in the δ-hydride, with an estimated average density one order of magnitude higher than that in the α-Zr matrix, which contributes to the accommodation of the substantial misfit strains associated with hydride precipitation in the α -Zr matrix. The present observations provide an insight into the behaviour of δ-hydride precipitation in zirconium alloys and may help with understanding the induced embrittling effect of hydrides.© 2014 International Union of Crystallography.
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    The observation of slip phenomena in single crystal Fe samples during in situ micro-mechanical testing through orientation imaging
    (Cambridge University Press, 2014-05-28) Bhattacharyya, D; Wheeler, RW; Harrison, RP; Edwards, L
    This paper reports a study of local orientation change occurring within micro-scale tensile samples as a function of strain. These samples were fabricated from a thin film of single crystal bcc Fe and deformed in tension using an in situ micro-mechanical testing device inside a scanning electron microscope. Samples were loaded along the <110> direction parallel to the specimen axis, strained to different levels, and then subjected to electron backscatter diffraction scans over the entire area of the gauge section. Analysis of the surface orientation data shows that, within a necked zone of the micro-sample gauge section, there are two distinct regions of significant orientation change, in which local crystal rotations occur in opposite directions. These two regions are separated by an intermediate band that shows minimal misorientation from the original state. Crystal rotations within the two regions that develop opposite orientations are found to be consistent with classic single crystal slip, where the slip direction rotates toward the tensile axis. It is shown that increasing tensile strain causes an increasing degree of rotation away from the starting orientation. The tests also illustrate the occurrence of slip on at least two different slip systems, based on the slip traces and orientation change. © 2014 Microscopy Society of America.
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    Observations on the zirconium hydride precipitation and distribution in zircaloy-4
    (Springer Link, 2014-05-14) Wang, ZY; Garbe, U; Li, HJ; Harrison, RP; Kaestner, A; Lehmann, EH
    Hydride precipitation and distribution in hot-rolled and annealed Zircaloy-4 plate samples artificially induced by gaseous hydrogen charging were studied primarily by neutron tomography, scanning electron microscopy (SEM), and SEM-based electron backscattered diffraction techniques. The precipitated hydride platelet (d-ZrH1.66) at a hydrogen pressure of 20 atm was found following the {111}d-ZrH1.66//(0001)a-Zr with the surrounding a-Zr matrix. The microstructural characterization indicated that hydrides with a relatively uniform distribution were precipitated on the rolling-transverse section of the plate, whereas, on the normal-transverse section, a hydride concentration gradient was present with a dense hydride layer near the surface. Further, the neutron tomography investigations clearly identified the nonuniform spatial distribution of hydrides. Thin hydride layers preferentially formed on the sample surface, and the concentrated hydrides precipitating at the edges/corner of the sample were observed. The causes for the localized hydride accumulation werealso discussed. © 2014, The Minerals, Metals & Materials Society and ASM International 2013.
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    Radiation effects on microstructure and hardness of a titanium aluminide alloy irradiated by helium ions at room and elevated temperatures
    (Elsevier B.V., 2015-04) Wei, T; Zhu, HL; Ionescu, M; Dayal, P; Davis, J; Carr, DG; Harrison, RP; Edwards, L
    A 45XD TiAl alloy possessing a lamellar microstructure was irradiated using 5MeV helium ions to a fluence of 5×1021ionm−2 (5000appm) with a dose of about 1dpa (displacements per atom). A uniform helium ion stopping damage region about 17μm deep from the target surface was achieved by applying an energy degrading wheel. Radiation damage defects including helium-vacancy clusters and small helium bubbles were found in the microstructure of the samples irradiated at room temperature. With increasing irradiation temperature to 300°C and 500°C helium bubbles were clearly observed in both the α2 and γ phases of the irradiated microstructure. By means of nanoindentation significant irradiation hardening was measured. For the samples irradiated at room temperature the hardness increased from 5.6GPa to 8.5GPa and the irradiation-hardening effect reduced to approximately 8.0GPa for the samples irradiated at 300°C and 500°C. © 2015 Elsevier B.V.