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Showing papers in "Materials Science and Engineering A-structural Materials Properties Microstructure and Processing in 2002"


Journal ArticleDOI
TL;DR: In this article, a tensile strength of 0.15 TPa was computed for individual carbon nanotubes in-situ in a transition electron microscope, based on the force required to break the tube.
Abstract: We have conducted pulling and bending tests on individual carbon nanotubes in-situ in a transition electron microscope. Based on our observation of the force required to break the tube, a tensile strength of 0.15 TPa was computed. From corresponding bending studies on such nanotubes, the Young's modulus was estimated to be 0.9 TPa (0.8 TPa after ‘sub continuum’ corrections). These results suggest a strength that is a large fraction of the elastic modulus, although previous measurements of their elastic stiffness have yielded higher modulus values, by as much as a factor of 2. The result does indicate that individual nanotubes can fail as essentially defect-free materials. Furthermore, we observed no obvious reduction in cross-sectional area prior to the failure. In addition, the bending experiments revealed a remarkable flexibility in these tubes. These unique properties support the potential of nanotubes as reinforcement fibers in structural materials.

1,069 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed and compared with carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX.
Abstract: Constitutive equations including an Arrhenius term have been commonly applied to steels with the objective of calculating hot rolling and forging forces. The function relating stress and strain rate is generally the hyperbolic-sine since the power and exponential laws lose linearity at high and low stresses, respectively. In austenitic steels, the equations have been used primarily for the peak stress (strain) associated with dynamic recrystallization (DRX) but also for the critical and steady state stresses (strains) for nucleation and first wave completion of DRX. Since the peak strain is raised by the presence of solutes and fine particles, the stress is raised more than by simple strain hardening increase, thus causing a marked rise in activation energy in alloy steels. In contrast, large carbides, inclusions or segregates, if hard, may lower the peak strain as a result of particle stimulated nucleation. Due to the linear relation between stress and strain at the peak, flow curves can be calculated from the constitutive data with only one additional constant. Maximum pass stresses can also be calculated from a sinh constitutive equation determined in multistage torsion simulations of rolling schedules. Comparison is made between carbon, micro-alloyed, tool and stainless steels and to ferritic steels which usually do not exhibit DRX. Parallels to the effects of impurities and dispersoids on the constitutive equations for Al alloys are briefly discussed.

892 citations


Journal ArticleDOI
TL;DR: In this article, the growth mechanism, morphology and mechanical properties of Fe-Al intermetallic compound layers on the surface of carbon steel were systematically evaluated for specimens diffused at temperatures ranging from 873 to 1323 K after hot dip aluminizing.
Abstract: The toughening of Fe–Al intermetallic compound coating formed by aluminizing of carbon steel was investigated. The growth mechanism, morphology and mechanical properties of Fe–Al intermetallic compound layers on the surface of carbon steel were systematically evaluated for specimens diffused at temperatures ranging from 873 to 1323 K after hot dip aluminizing. Fe2Al5 was mainly formed on the specimen surfaces at the usual diffusion temperatures from 873 to 923 K. However, FeAl and Fe3Al layers having relatively high fracture resistance and oxidation resistance properties were preferentially formed in the specimens diffused at temperatures greater than 1273 K. The activation energies required for the growth of the FeAl and Fe3Al layers were QFeAl=180 and QFe3Al=260 kJ mol−1, respectively. It was identified that the formation and growth of Fe–Al intermetallic compound layers is controlled by the diffusion of Fe atoms into the intermetallic compound layers.

497 citations


Journal ArticleDOI
TL;DR: In this paper, the formation of onion rings is found to be a geometric effect due to the fact that cylindrical sheets of material are extruded during each rotation of the tool and the cutting through the section of the material produces an apparent "Onion Rings".
Abstract: Onion rings are the most prominent features of most friction stir welds. The origin and the effect of these on properties are not clearly understood. In this paper, an attempt has been made to explain the formation of onion rings. The formation of onion ring is found to be a geometric effect due to the fact that cylindrical sheets of material are extruded during each rotation of the tool and the cutting through the section of the material produces an apparent ‘Onion Rings’. It is postulated that the tool appears to wait for a very short time to produce frictional heat and extrude a cylindrical shaped material around to the retreating side of the joint. The spacing of the markings has been found to be equal to the forward motion of the tool in one rotation.

427 citations


Journal ArticleDOI
TL;DR: The development of creep resistant alloys over the past 20 years is discussed, ranging from the WE series developed early in the 1980s and which represents the present state of the art to MgSc and MgGd ternary alloys that show an improvement in creep resistance of two orders of magnitude.
Abstract: The development of creep-resistant alloys over the past 20 years is discussed, ranging from the WE series developed early in the 1980s and which represents the present state of the art to MgSc and MgGd ternary alloys that show an improvement in creep resistance of two orders of magnitude.

414 citations


Journal ArticleDOI
TL;DR: In this paper, the microstructures and tensile properties of three typical Sn-Ag-Cu alloys, Sn-30wt%Ag-05wt%Cu and Sn-39Ag-06Cu, were evaluated after casting under three different cooling conditions.
Abstract: The microstructures and tensile properties of three typical Sn–Ag–Cu alloys, Sn–30wt%Ag–05wt%Cu, Sn–35wt%Ag–07wt%Cu and Sn–39wt%Ag–06wt%Cu, prepared under three different cooling conditions were evaluated after casting The microstructures of all rapidly cooled specimens consisted of the eutectic phase of β-Sn with fine fibrous Ag3Sn dispersion surrounding primary β-Sn grains The slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys exhibited additional large primary Ag3Sn platelets, while the Sn–30Ag–05Cu did not For all alloys, both ultimate tensile strength and 02% proof stress increased with increasing strain-rates in tensile tests Lowering cooling speed decreased tensile strength Elongation increased with an increasing strain rate from 10−5 to 10−2 s−1, and decreased slightly at 10−1 s−1 for the rapidly cooled specimens Elongation remarkably decreased for the slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys, a degradation attributable to the formation of large primary Ag3Sn platelets

370 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of hot working parameters on deformation localization, width of α platelets, phase transformation, and metadynamic recrystallization were systematically examined by optical microscopy and scanning electron microscopy.
Abstract: Variation of microstructure of a Ti–6A1–4V alloy during thermomechanical processing at a range of hot working conditions (temperature: 850–1050 °C, strain rate: 0–1.0 s−1 has been investigated for further understanding and control of the processing–structure–property relationships. The influences of hot working parameters on deformation localization, width of α platelets, (α→β) phase transformation, and (meta)dynamic recrystallization were systematically examined by optical microscopy and scanning electron microscopy. The experimental results showed that the lamellar morphology of the α phase can be sub-divided into five different categories for samples processed in the (α+β) phase field, and these represent three stages of the (α→β) phase transformation. It was also noted that a certain degree of the (α→β) phase transformation occurred concurrently with the mechanical deformation in the (α+β) phase field. The experimental evidence suggests that dynamic and/or metadynamic recrystallization has occurred in the β phase field, but was not involved in the (α+β) phase field under the experimental conditions.

364 citations


Journal ArticleDOI
TL;DR: In this article, the authors summarized the principles of microstructure and texture evolution during the main steps of the thermomechanical processing of age-hardenable Al-Mg-Si sheets (6 xxx series alloys).
Abstract: The properties of Al-alloys for car body applications are largely controlled by microstructure and crystallographic texture of the final sheets. In this paper, the impact of texture on formability and, in particular, on surface appearance of the sheets is reviewed. The paper summarizes the principles of microstructure and texture evolution during the main steps of the thermomechanical processing of age-hardenable Al–Mg–Si sheets (6 xxx series alloys). The most important parameters that may be used to modify the textures and hence to improve the resulting properties are outlined.

348 citations


Journal ArticleDOI
TL;DR: In this paper, a plate-shaped ultra-fine grained metallic materials without changing their initial dimensions were fabricated using the constrained groove pressing (CGP) technique. But the results of the grain refinement sequences during pressing were examined by transmission electron microscopy.
Abstract: The new intense plastic straining technique, named ‘constrained groove pressing’ (CGP), was developed for fabrication of plate-shaped ultrafined grained metallic materials without changing their initial dimensions. The principle of CGP is that a material is subjected to the repetitive shear deformation under the plane strain deformation condition by utilizing alternate pressing with the asymmetrically grooved die and flat die constrained tightly by the cylinder wall. A submicrometer order grain structure was obtained in pure aluminum by utilizing this technique. The grain refinement sequences during pressing were examined by transmission electron microscopy. The enhancement of the mechanical properties of submicrometer order grained pure aluminum fabricated by this technique was comparable to that produced by other intense plastic straining techniques at the similar accumulated strains.

339 citations


Journal ArticleDOI
TL;DR: In this paper, a banded microstructure consisting of alternating hard particle rich and hard particle poor regions is developed for friction stir welds in 7 mm thick, 2024-T351 aluminum rolled sheet material.
Abstract: Friction stir welds in 7 mm thick, 2024-T351 aluminum rolled sheet material have been completed. Metallurgical, hardness and quantitative energy dispersive X-ray measurements have been performed which demonstrate that a segregated, banded, microstructure consisting of alternating hard particle rich and hard particle poor regions is developed. Mixed-mode I/II monotonic fracture experiments confirm that the observed banded microstructure affects the macroscopic fracture process. Since the band spacing is directly correlated with the welding tool advance per revolution, our results indicated that the opportunity exists to manipulate the friction stir weld process parameters in order to modify the weld microstructure and improve a range of material properties, including fracture resistance.

331 citations


Journal ArticleDOI
TL;DR: In this paper, a hot deformation behavior of commercial grade Ti-6Al-4V with a lamellar starting microstructure is studied in the temperature range 750-1100°C and strain rate range 3×10 −4 −10 s −1 with a view to model the microstructural evolution.
Abstract: The hot deformation behavior of commercial grade Ti–6Al–4V with a lamellar starting microstructure is studied in the temperature range 750–1100 °C and strain rate range 3×10 −4 –10 s −1 with a view to model the microstructural evolution. On the basis of flow stress data obtained as a function of temperature and strain rate in compression, a processing map for hot working has been developed. In the ranges 800–975 °C and 3×10 −4 –10 −2 s −1 , globularization of lamellae occurs for which an apparent activation energy of 455 kJ mol −1 has been estimated using the kinetic rate equation. Stress-dependent thermal activation analyses proposed by Schock and Cocks et al. have shown that the apparent activation energies are in the range 160–245 kJ mol −1 and the normalized activation volumes are in the range 20–80, which suggest that cross-slip is the rate controlling process during globularization. The variation of primary α grain size with Zener–Hollomon parameter ( Z ) in the globularization region exhibited a linear relationship on a log–log scale. At strain rates slower than 10 −1 s −1 and temperatures below 900 °C, cracking at the prior β grain boundaries/triple junctions occurs, which sets the lower limits for globularization. At strain rates higher than 10 −1 s −1 in the α+β range, the material exhibited flow instabilities manifested as adiabatic shear bands. These bands are intense below 800 °C and above 1 s −1 and caused cracking along the bands. In the β phase field, dynamic recrystallization (DRX) occurs at about 1100 °C and in the strain rate range 10 −3 –10 −1 s −1 . The apparent activation energy for DRX of β is about 172 kJ mol −1 which is close to that for self-diffusion in β phase (153 kJ mol −1 ). The application of these results in the design of bulk metalworking processes for achieving microstructural control is discussed.

Journal ArticleDOI
TL;DR: In this paper, a series of aging treatments at the temperature range of 650-975°C for different time intervals in a solution-treated 2205 duplex stainless steel were carried out.
Abstract: A series of aging treatments at the temperature range of 650–975 °C for different time intervals in a solution-treated 2205 duplex stainless steel were carried out. The corresponding microstructure of aged specimens was observed and the impact toughness was measured. More attention was paid to the secondary phase precipitation and the transition of ductile-to-brittle fracture. The results indicated that the impact toughness of duplex stainless steel was sensitive to the precipitation of σ phase even at the initial stage of aging. Two kinds of Cr-, Mo-enriched intermetallic phases, σ and χ , were found to precipitate preferentially at δ / γ interface boundary and within δ -ferrite grain after 5 min of aging at the temperature range of 875–900 °C. The volume fraction of σ phase was continuously increased with the time of aging and σ phase developed into a coarse particle due to the high diffusibility of solute atoms at high temperatures. The precipitation of Mo-enriched χ phase at the initial stage of aging is presumably enhanced due to the low interfacial energy of highly coherent χ / δ interface with a characteristic cubic-to-cubic orientation relationship. However, this pre-formed χ phase was re-dissolved eventually into the σ phase when the specimen aged above 750 °C. Accompanied with the growth of secondary phases, the γ phase nearby the interface of σ or χ phase was induced to grow into the δ -ferrite region which was depleted in Cr and Mo. The δ -ferrite in original duplex structure would be completely decomposed into the σ phase and secondary γ after long-term aging.

Journal ArticleDOI
TL;DR: The AZ31 Mg alloy was hot torsion tested from 180 to 450°C and from 0.01 to 1.0 s −1. The flow curves showed a peak and a decline towards a steady-state regime which were lower as temperature T rose and strain rate declined as mentioned in this paper.
Abstract: The AZ31 Mg alloy was hot torsion tested from 180 to 450 °C and from 0.01 to 1.0 s −1 . The flow curves showed a peak and a decline towards a steady-state regime which were lower as temperature T rose and strain rate declined; however, the fracture strain increased to about 1.9 at 0.1 s −1 . In transmission electron microscopy, twins were observed from 180 to 360 °C (in declining numbers). At low T , they had sharp walls and contrasting transverse bands; while the matrix showed indistinct linear streaks. As T rose, the twin bands developed cells with tangled walls and finally subgrains (∼360 °C), while the twin walls became tangles of dislocations and finally serrated boundaries. The matrix developed elongated dislocation walls and subgrains at higher T . The twin intersections at 180 and 240 °C consisted of diamond-shaped cells with a duplex set of orientations but at 300 and 360 °C, these had developed into polygonal cells with high misorientations in dark field. The first very small dynamically recrystallized grains were observed at these intersections, slightly larger than the cells. At 360–450 °C, as observed by optical microscopy, small dynamically recrystallized grains formed at the original grain boundaries, probably related to multiple slip. Since twinning and other features described at low T were also found at high ones, albeit with decreasing frequency, the microstructures showed severe heterogeneity which accounted for the limited ductility.

Journal ArticleDOI
TL;DR: In this article, a linear regression analysis was used to estimate the diameter of the primary dendrite arm, the secondary dendritic arm spacing, and the radius of the dendritrite tip radius.
Abstract: Directional solidification experiments have been carried out on different Al–Cu alloys as a function of solidification parameters, temperature gradient G, growth rate V, and composition C0. The specimens were solidified under steady state conditions with a constant temperature gradient (7.4 K mm−1) at a wide range of growth rates (9–490 μm s−1) and with a constant growth rate of 9.5 μm s−1 at a wide range of temperature gradients (1.0–7.4 K mm−1). Microstructural parameters, the primary dendrite arm spacing λ1, secondary dendrite arm spacing λ2, dendrite tip radius R, mushy zone depth d were measured and expressed as functions of solidification parameters, G, V and C0 by using a linear regression analysis. The results were in good agreement with previous experimental work and current theoretical models suggested for dendritic growth.

Journal ArticleDOI
TL;DR: Magnesium-gadolinium binary alloys exhibit good mechanical properties and high creep resistance comparable to or better than commercial WE type (Mg-Y-Nd-Zr) alloys as mentioned in this paper.
Abstract: Magnesium–gadolinium binary alloys exhibit good mechanical properties and high creep resistance comparable to or better than commercial WE type (Mg–Y–Nd–Zr) alloys. Combining scandium and manganese with a particular rare earth element (R.E.–Gd, Y, Ce) has a beneficial effect on the creep behaviour of complex Mg–R.E. alloys, at lower R.E. contents than in WE type alloys. They stabilise high creep resistance up to high temperatures (above 300°C) by precipitation of the stable phase Mn2Sc and by precipitation of basal plates of a Mn and R.E.-containing hexagonal phase.

Journal ArticleDOI
TL;DR: The cumulative roll-bonding (ARB) process is an intense plastic deformation process that has been performed for a 6061 aluminum alloy to develop ultra-fine grains below 1 μm in diameter and to improve mechanical properties as discussed by the authors.
Abstract: Accumulative roll-bonding (ARB) process is an intense plastic deformation process that has been performed for a 6061 aluminum alloy to develop ultra-fine grains below 1 μm in diameter and to improve mechanical properties. The ARB process up to eight cycles is performed at ambient temperature under unlubricated conditions. The ultra-fine grains surrounded by clear boundaries begin to appear at the third cycle, and the specimen after eight cycles shows a microstructure covered with ultra-fine grains with an average diameter of 310 nm. The tensile strength of the ARB processed 6061 alloy increases with the number of ARB cycles (equivalent total strain), and after eight cycles it reaches the maximum of 363 MPa, which is about three times of the initial. On the other hand, the elongation drops abruptly at the first cycle, above which it decreases progressively with the number of ARB cycles. The hardness of the specimens ARBed by one, three and five cycles varies inhomogeneously in the thickness direction; having peak values near the surface and the center. This is due to the redundant shear strain and wire brushing. The results show that the ARB process is effective for grain refinement and strengthening of 6061 alloy.

Journal ArticleDOI
V.M. Segal1
TL;DR: In this paper, the effect of deformation mode on structure evolution under severe plastic deformation was analyzed from a continuum standpoint, all possible strain states range from pure shear to simple shear and can be described by a single parameter.
Abstract: The paper analyzes an effect of deformation mode on structure evolution under severe plastic deformation. From a continuum standpoint, all possible strain states range from pure shear to simple shear and can be described by a single parameter. The microstructure evolution at large strains is linked with successive steps of continuous flow and flow localization. It is shown that simple shear conforms to the optimal deformation mode for development of spatial networks of high angle boundaries and fine grains during flow localization both for monotonic loading and cross loading. Using this approach, different deformation techniques for simple shear processing are considered with the emphasis on equal channel angular extrusion.

Journal ArticleDOI
TL;DR: In this paper, the secondary dendrite arm spacing (SDAS) within the cast plates of a commercial AZ91C alloy have been used for the study of sand-cast plates.
Abstract: Sand-cast plates of a commercial AZ91C alloy have been used for the study. Varying the solidification rate by placing large cast-iron chills in the mould produced a range of secondary dendrite arm spacing (SDAS) within the cast plates. The plates were solution heat-treated. quenched and aged at 165 degreesC for up to 350 h. The SDAS (mum) varied with the solidification time, t(f) (s), as SDAS = 5.3 t(f)(0.43). The tensile ductility in the as-quenched (T4) condition did not depend on the solidification rate whilst in the T6 condition it tended to decrease for slowly solidified material (SDAS > 50 mum). The yield strength and hardness increased and the ductility decreased with ageing. The fracture mode changed from predominantly transgranular in the T4 condition to predominantly intergranular in the T6 condition. The properties of the sand-castings are compared with those of high-pressure diecastings and the possible strengthening mechanisms are discussed. A number of areas that require more research are pointed out. (C) 2002 Elsevier Science B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, the tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size.
Abstract: Nanocomposite materials consisting of a nanocrystalline Ni matrix (grain size 10–15 nm) reinforced with sub-micron size SiC particulates (average particle size: 0.4 μm) up to 10.5 vol.% have been produced by pulse electrodeposition. Substantial improvements in mechanical properties including hardness, yield and tensile stress were obtained for the nanocomposite material, as compared with conventional Ni–SiC composites with a matrix grain size in the micrometer range. Tensile strengths up to four times that for conventional polycrystalline Ni and two times that for conventional polycrystalline Ni–SiC of comparable SiC content was measured. The tensile and yield strengths of the nanocomposite material with SiC content less than 2 vol.% were higher than those for pure nanocrystalline Ni of comparable grain size. For these nanocomposites an unexpected increase in tensile ductility was also observed when compared to pure nanocrystalline nickel. At higher SiC content (>2 vol.%) the strength and ductility were found to decrease to the detriment of the nanocomposite. Particle clustering was considered the main cause of this decrease.

Journal ArticleDOI
TL;DR: In this paper, the shearing patterns for equal-channel angular pressing (ECAP) under three different conditions were calculated: rotation of a sample around the X-axis when using a die angle of 90°, rotation around the Y-and Z-axes when using plate samples with an angle of 120°, and rotation around a plate sample with a 90° die angle.
Abstract: The shearing patterns are calculated for equal-channel angular pressing (ECAP) under three different conditions: (i) rotation of a sample around the X-axis when using a die angle of 90°, (ii) rotation around the X-axis when using a die angle of 120° and (iii) rotation around the Y- and Z-axes when using plate samples with a die angle of 90°. Separate sets of shearing patterns are constructed for the four standard processing routes: route A with no rotation of the sample between consecutive passes, routes BA and BC with rotations of 90° in alternate directions or the same direction between each pass, and route C with a rotation of 180° between passes. It is concluded that the development of a uniform microstructure of equiaxed grains, separated by high-angle grain boundaries, is favored using route BC because (i) shearing occurs over large angular ranges on the three orthogonal planes within the sample, (ii) there is a regular and periodic restoration of an equiaxed structure during consecutive pressings and (iii) deformation occurs on each orthogonal plane.

Journal ArticleDOI
TL;DR: In this paper, microstructure and microtexture evolution during dynamic recrystallization (DRX) was investigated in compression of polycrystalline copper in the temperature range from 473 K to 723 K and at strain rates from 10−3s−1 to 10−1 s−1.
Abstract: Microstructure and microtexture evolution during dynamic recrystallization (DRX) was investigated in compression of polycrystalline copper in the temperature range from 473 K to 723 K and at strain rates from 10−3s−1 to 10−1s−1. A compression texture of near 〈101〉 direction, evolved by low temperature deformation, is gradually weakened and randomized by the progress of DRX at higher temperature, where 〈101〉 component still exists. New DRX grains are evolved by the operation of bulging of serrated grain boundaries, which is accompanied either by rotation of a bulged portion or twinning at the back of the migrating boundary. The mechanisms of dynamic nucleation and necklace DRX are discussed.

Journal ArticleDOI
TL;DR: In this article, the authors investigated the effect of the thermo-mechanical control process (TMCP) on the microstructures and mechanical properties of pipeline steels, and they found that the acicular ferrite dominated microstructure obtained by the optimized TMCP has optimum mechanical properties.
Abstract: The thermo-mechanical control process (TMCP) was investigated for its effect on microstructures and mechanical properties of a commercial pipeline steel. Regression equations describing the relationship between processing parameters and mechanical properties were obtained through analyses of the experimental results: yield strength (YS)=0.508T(s)-0.23IT(f)-0.334T(c)+ 1.905V(c) +323.6, EL (elongation)= -0.002T(s)-0.064T(f)-0.086T(c)+0.325V(c)+121.8, where T-s, T-f and T-c are the start rolling, final rolling and final cooling temperature (degreesC), respectively, and V-c is the cooling rate (degreesC s(-1)). The processing parameters of TMCP for obtaining better mechanical properties were given, i.e. T-s is approximately 1100 degreesC, T-f 890 degreesC, T-c 520 degreesC and V-c 30 degreesC s(-1). It was found that the acicular ferrite dominated microstructure obtained by the optimized TMCP has optimum mechanical properties, and the microstructural characteristics of acicular ferrite play important roles in enhancement of both strength and toughness of pipeline steels. (C) 2002 Elsevier Science B.V. All rights reserved.

Journal ArticleDOI
TL;DR: In this paper, Yttria-stabilized zirconia (YSZ) electrolytes were tested by AC impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity.
Abstract: Yttria-stabilized zirconia (YSZ) electrolytes with diverse microstructures were prepared by using nano-size (Y 2 O 3 ) 0.08 (ZrO 2 ) 0.92 powders as precursors through conventional sintering in air. The electrolytes were tested by AC impedance spectroscopy to elucidate the contribution of intragranular and intergranular conductivity to the total ionic conductivity. The intragranular conductivity and intergranular conductivity were correlated with the microstructures of the electrolyte to interpret the transportation of oxygen ions through the electrolyte. The intragranular conductivity was found to be dominated mainly by the relative density while the intergranular conductivity strongly depended on the grain size and grain boundary area of the electrolyte. The sintering temperature and isothermal time dependence of ionic conductivity reached a maximum value of 0.105 S/cm at a sintering temperature of 1350 °C for 4 h and 0.112 S/cm at a holding time of 8 h at 1250 °C when measured at 1000 °C, respectively. Concepts for improving the ionic conductivity of YSZ electrolyte were reviewed.

Journal ArticleDOI
TL;DR: In this paper, the morphology, composition and crystallinity of both precipitator and cenosphere fly ash particles of different sizes were studied with scanning electron microscopy (SEM), EDX, and X-ray diffraction (XRD).
Abstract: The morphology, composition and crystallinity of both precipitator (solid) and cenosphere (hollow) fly ash particles of different sizes were studied with scanning electron microscopy (SEM), EDX, and X-ray diffraction (XRD). Bulk density, tap density and real density of both precipitator and cenosphere particles of different sizes as well as the wall thickness to diameter ratio of cenosphere particles were measured. The microhardness of individual fly ash particles embedded in the matrix of aluminum alloy was also measured. The crystalline to amorphous ratio weight percentage in fly ash particles, and the weight or volume fraction of each crystalline component varies with the particle size. The crystallinity of precipitator particles increased as the particle size increases, whereas the crystallinity of cenosphere decreased as the particle size increases. The elastic modulus of fly ash was estimated from the crystallinity of fly ash and the volume fraction of each component, using the rule of mixtures. The calculated upper limits for Young's modulus of precipitator particles were 126 GPa for particles in the size range 150–250 μm and 98 GPa for particles in the size range 5–10 μm. Young's modulus of cenosphere particles was estimated to be approximately in the range of 13–17 GPa in all particle size ranges. The hardness of the larger precipitator fly ash particles (120 μm) exhibited a wide scatter in the range of 160–400 kg mm−2, while the hardness of the smaller size precipitator particles (20 μm) were in a narrow range from 250 to 270 kg mm−2.

Journal ArticleDOI
TL;DR: The constitutive relation that links the stress-strain rate-grain size-temperature relation (Mukherjee-Bird-Dorn, MBD correlation) was presented in 1968/1969 to describe the elevated temperature crystalline plasticity has held up well during the intervening quarter of a century.
Abstract: It was 25 years ago that the symposium on rate processes in plasticity was organized. Since then, advances in transmission electron microscopy, large-scale computation as well as molecular dynamics simulation, etc. have contributed much to our understanding of elevated temperature plasticity. The constitutive relation that links the stress–strain rate–grain size–temperature relation (Mukherjee–Bird–Dorn, MBD correlation) was presented in 1968/1969 to describe the elevated-temperature crystalline plasticity. This equation has held up well during the intervening quarter of a century. It has been applied to metals, alloys, intermetallics, ceramics, and tectonic systems, and it has worked equally well. It made the depiction of deformation mechanism maps in normalized coordinates a reality and provided a rationale for estimating life prediction by giving a quantitative estimate of the steady-state creep rate in creep damage accumulation relationship. In the case of particle-dispersed systems as well as metal matrix composites, the introduction of the concept of a threshold stress was a substantial improvement in creep studies. One of the significant applications of the MBD relation has been in superplasticity. The concept of scaling with either temperature or with strain rate, inherent in this relationship, seems to be obeyed as long as the rate-controlling mechanism is unchanged. The application of this relation to high strain-rate superplasticity and also to low-temperature superplasticity has been illustrated. Experimental data demonstrate that superplasticity of nanocrystalline metals and alloys follows the general trend of the constitutive relation but with important differences in the level of stress and strain hardening rates. It is shown that in the nanocrystalline range, molecular dynamics simulation has the potential to yield data on stress–grain size–temperature dependencies at very low grain size ranges where experimentalists cannot conduct their studies yet.

Journal ArticleDOI
TL;DR: Impression creep and impression fatigue, both using cylindrical indenters, are reviewed in this paper, and a steady state per cycle is shown and the power law dependence of maximum stress is presented.
Abstract: Impression creep and impression fatigue, both using cylindrical indenters, are reviewed in this paper. For impression creep, analytical solutions and computer simulations for different situations are presented. Materials tested include metals and alloys, superplastic materials, weldments, glasses, ceramics and polymers. Viscosity measurements using indentation techniques and impression creep of thin films are discussed also. For impression fatigue, a steady state per cycle is shown and the power law dependence of maximum stress is presented. Underloading and overloading effects, as well as delayed retardation, are described. Other localized tests, such as nanoindentation, stress relaxation, impression recovery and the adhesion energy determined by impression testing, are briefly discussed also.

Journal ArticleDOI
TL;DR: In this paper, the global and local mechanical response of friction stir welded AA2024 is examined experimentally and numerically, assuming an iso-stress configuration, local constitutive data were determined for the various weld regions and used as input for a 2D finite element model.
Abstract: The mechanical response of heterogeneous structures, such as weldments, is largely governed by the response of the local constituents. In the present paper, the global and local mechanical response of friction stir welded AA2024 is examined experimentally and numerically. Full field strain measurements are obtained on transversely loaded tensile specimens via the digital image correlation technique. Assuming an iso-stress configuration, local constitutive data were determined for the various weld regions and used as input for a 2-D finite element model. The simulation results were compared with the experimental results to assess the viability of the modeling approach and the validity of the iso-stress assumption

Journal ArticleDOI
TL;DR: In this paper, a physically-based model for the deformation of Ti-6%−Al-4%V is proposed, and a strategy by which the relevant strengthening effects are captured in the model is proposed.
Abstract: A physically-based model for the deformation of Ti–6%–Al-4%V is proposed. The various deformation mechanisms active in this material over the whole range of temperatures of industrial interest are discussed, and a strategy by which the relevant strengthening effects are captured in the model is proposed. The flow stress contains a thermal and an athermal component. The thermally activated processes are modeled based on the Kocks–Mecking formalism, while the athermal processes are simulated using an internal state variable. The deformation of the α-and β-phases is captured separately. The model is calibrated based on experimental results obtained from tests performed in the temperature range (77–1400 K) and at strain rates between 10 −3 and 10 s −1 . The model predictions are extrapolated to strain rates as high as 2000 s −1 . The experimental findings are presented in the companion paper.

Journal ArticleDOI
TL;DR: In this article, the mechanical behavior of nanostructured partially stabilized zirconia (PSZ) coatings was evaluated via Knoop microhardness via Weibull statistics.
Abstract: The mechanical behavior of nanostructured partially stabilized zirconia (PSZ) coatings was evaluated via Knoop microhardness. The distribution of the microhardness values of the feedstock particles and coatings under a 10 g load were analyzed via Weibull statistics. The percentage of non-molten material was determined using scanning electron microcopy and image analysis. It was observed that the nanostructured coatings present a bimodal distribution in their Weibull plots, indicating the presence of two phases which are described as molten and non-molten. The presence of the bimodal distribution in the mechanical properties allows the prediction of microhardness values of these nanostructured coatings.

Journal ArticleDOI
TL;DR: In this article, hot isostatic pressing (HIP) and metal injection molding (MIM) were used for the fabrication of NiTi compacts, and the mechanical properties of HIPed samples were measured by tensile tests at room temperature.
Abstract: NiTi components with reproducible and stable shape memory properties are attractive for various technical applications (e.g. couplings). With the aim of producing NiTi components on an industrial scale, near-net-shape fabrication routes are preferred considering the limited machinability of NiTi alloys. Powder metallurgy (PM) is known to provide the possibility of material-saving and automated fabrication of at least semi-finished products as well as net-shape components. As promising PM routes hot isostatic pressing (HIP) and metal injection moulding (MIM) were used for the fabrication of NiTi compacts. Microstructural investigations, chemical analysis, X-ray diffraction (XRD) measurements and also differential scanning calometry (DSC) measurements were performed in order to characterize the produced parts. Additionally, the mechanical properties of HIPed samples were measured by tensile tests at room temperature. The components from both fabrication routes show reversible austenite↔martensite transformations which are a prerequisite for shape memory effects.