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Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 2001"


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TL;DR: In this article, a numerical 3D heat flow model for friction stir welding (FSW) is developed, based on the method of finite differences, which is implemented in MATLAB 5.2 and validated by comparison with in-situ thermocouple measurements and experimental hardness profiles.
Abstract: In the present investigation, a numerical three-dimensional (3-D) heat flow model for friction stir welding (FSW) has been developed, based on the method of finite differences. The algorithm, which is implemented in MATLAB 5.2, is provided with a separate module for calculation of the microstructure evolution and the resulting hardness distribution. The process model is validated by comparison with in-situ thermocouple measurements and experimental hardness profiles measured at specific time intervals after welding to unravel the strength recovery during natural aging. Furthermore, the grain structure within the plastically deformed region of the as-welded materials has been characterized by means of the electron backscattered diffraction (EBSD) technique in the scanning electron microscope (SEM). Some practical applications of the process model are described toward the end of the article.

497 citations


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TL;DR: In this article, the authors have shown that the microstructural evolution follows a universal pattern of grain subdivision from the macroscale to the nanometer scale, and that the similarity between the behavior of materials undergoing different deformation patterns forms the basis for future research and development encompassing traditional as well as new materials and processes.
Abstract: Microstructural analysis by advanced and automated methods has allowed deformation microstructures to be quantified in terms of common structural parameters. This quantification has shown for a variety of materials and processing conditions that the microstructural evolution follows a universal pattern of grain subdivision from the macroscale to the nanometer scale. This microstructural evolution has been described empirically and in theoretical models based on general principles for the formation of dislocation structures during plastic deformation by slip. The similarity between the behavior of materials undergoing different deformation patterns forms the basis for future research and development encompassing traditional as well as new materials and processes.

406 citations


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TL;DR: In this article, the modified quasi-chemical model in the pair approximation for short-range ordering (SRO) in liquids is extended to multicomponent solutions and an improved general functional form for "ternary terms" in the excess Gibbs energy expression is proposed, related to the effect of a third component upon the binary pair interaction energies.
Abstract: Further improvements to the modified quasi-chemical model in the pair approximation for shortrange ordering (SRO) in liquids are extended to multicomponent solutions. The energy of pair formation may be expanded in terms of the pair fractions or in terms of the component fractions, and coordination numbers are permitted to vary with composition. The model permits complete freedom of choice to treat any ternary subsystem with a symmetric or an asymmetric model. An improved general functional form for “ternary terms” in the excess Gibbs energy expression is proposed. These terms are related to the effect of a third component upon the binary pair interaction energies. It is shown how binary subsystems that have been optimized with the quasi-chemical model can be combined in the same multicomponent Gibbs energy equation with binary subsystems that have been optimized with a random-mixing Bragg-Williams model and a polynomial expression for the excess Gibbs energy. This is of much practical importance in the development of large databases for multicomponent solutions. The model also applies to SRO in solid solutions as a special case, when the number of lattice sites and coordination numbers are constant.

353 citations


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TL;DR: In this article, the role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu and a series of Cu-Al solid-solution alloys.
Abstract: The role of stacking fault energy (SFE) in deformation twinning and work hardening was systematically studied in Cu (SFE ∼78 ergs/cm2) and a series of Cu-Al solid-solution alloys (0.2, 2, 4, and 6 wt pct Al with SFE ∼75, 25, 13, and 6 ergs/cm2, respectively). The materials were deformed under quasi-static compression and at strain rates of ∼1000/s in a Split-Hopkinson pressure bar (SHPB). The quasi-static flow curves of annealed 0.2 and 2 wt pct Al alloys were found to be representative of solid-solution strengthening and well described by the Hall-Petch relation. The quasi-static flow curves of annealed 4 and 6 wt pct Al alloys showed additional strengthening at strains greater than 0.10. This additional strengthening was attributed to deformation twins and the presence of twins was confirmed by optical microscopy. The strengthening contribution of deformation twins was incorporated in a modified Hall-Petch equation (using intertwin spacing as the “effective” grain size), and the calculated strength was in agreement with the observed quasi-static flow stresses. While the work-hardening rate of the low SFE Cu-Al alloys was found to be independent of the strain rate, the work-hardening rate of Cu and the high SFE Cu-Al alloys (low Al content) increased with increasing strain rate. The different trends in the dependence of work-hardening rate on strain rate was attributed to the difference in the ease of cross-slip (and, hence, the ease of dynamic recovery) in Cu and Cu-Al alloys.

337 citations


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TL;DR: In this article, the material flow in solid-state, friction-stir, butt-welded AA2195-T8 was investigated using a marker insert technique (MIT).
Abstract: The material flow in solid-state, friction-stir, butt-welded AA2195-T8 was investigated using a marker insert technique (MIT). Markers made of AA5454-H32 were embedded in the path of the rotating friction stir welding (FSW) tool and their final position after welding was detected by metallographic means. Changes in material flow due to welding parameter and tool geometry variations were examined. The method provides a semiquantitative, three-dimensional view of the material transport in the welded zone. Because of the placement of markers at different positions at the weld centerline, the material transport in the longitudinal, transverse, and the vertical directions could be studied. Markers embedded in the path of the tool remain continuous after welding. The material transport, which is not symmetrical about the weld centerline, was such that the bulk of the material was transported to a position behind its original position. Superimposed on the primary motion of material in the horizontal plane of the weld is a circulation about the longitudinal axis of the weld. This circulation is found to increase with increasing weld energy.

335 citations


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TL;DR: In this article, a commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size, which was subsequently degassed, hot isostatically pressed, and extruded.
Abstract: A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al2O3 particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.

302 citations


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TL;DR: In this article, the authors measured the variation of the volume fraction of untransformed retained austenite as a function of uniaxial plastic strain and found that the increase of the mechanical stability of retained Austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.
Abstract: The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.

290 citations


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TL;DR: In this article, the global tensile properties of a friction-stir-welded joint of 6063 aluminum were examined by estimating distribution of local tensile property corresponding to local microstructure and hardness.
Abstract: Dominant microstructural factors governing the global tensile properties of a friction-stir-welded joint of 6063 aluminum were examined by estimating distribution of local tensile properties corresponding to local microstructure and hardness. Yield and ultimate tensile strengths of the as-welded weld were significantly lower than those of the base material. Postweld aging and postweld solution heat-treatment and aging (SHTA) restored the strengths of the weld to the levels of the base material. Elongation was found to increase with increasing strength. Hardness tests showed that the as-welded weld was soft around the weld center and that the aged weld and the SHTA weld had relatively homogeneous distributions of high hardness. Hardness profiles of the welds were explained by precipitate distributions and precipitation sequences during the postweld heat treatments. The strengths of the welds were related to each minimum hardness value. In a weld having a heterogeneous hardness profile, the fracture occurred in the region with minimum hardness. When a weld had a homogeneous hardness profile, its fracture site depended on both crystallographic-orientation distribution of the matrix grains and strain tensor of the imposed deformation, i.e., it fractured in the region with a minimum average Taylor factor.

275 citations


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TL;DR: In this paper, the effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles and the weld of Al alloy 5083 had a slightly greater hardness in the stir zone than the base material.
Abstract: Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al6(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.

249 citations


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TL;DR: In this paper, a simple analytical model of microsegregation for the solidification of multicomponent steel alloys is presented, based on the Clyne-Kurz model and extended to take into account the effects of multiple components, a columnar dendrite microstructure, coarsening, and the δ/γ transformation.
Abstract: A simple analytical model of microsegregation for the solidification of multicomponent steel alloys is presented. This model is based on the Clyne-Kurz model and is extended to take into account the effects of multiple components, a columnar dendrite microstructure, coarsening, and the δ/γ transformation. A new empirical equation to predict secondary dendrite arm spacing as a function of cooling rate and carbon content is presented, based on experimental data measured by several different researchers. The simple microsegregation model is applied to predict phase fractions during solidification, microsegregation of solute elements, and the solidus temperature. The predictions agree well with a range of measured data and the results of a complete finite-difference model. The solidus temperature decreases with either increasing cooling rate or increasing secondary dendrite arm spacing. However, the secondary dendrite arm spacing during solidification decreases with increasing cooling rate. These two opposite effects partly cancel each other, so the solidus temperature does not change much during solidification of a real casting.

242 citations


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TL;DR: In this article, the microtextures in a friction-stir weld of the precipitation-hardened aluminum alloy 6063 have been analyzed by orientation imaging microscopy (OIM).
Abstract: In order to characterize plastic flow during friction-stir welding, the microtextures in a friction-stir weld of the precipitation-hardened aluminum alloy 6063 have been analyzed by orientation imaging microscopy (OIM). The base-material plate has a Goss orientation. The weld center region, except for the upper surface, takes a typical shear texture component with two types of orientations. The orientations have a pair of common {111} and 〈110〉 parallel to the cylindrical pin surface and transverse direction of the plate, respectively. The typical texture component is also observed around the weld center on the midsection, although it rotates about the plate normal direction. A microtexture analysis after postweld heat treatment has suggested that dynamic recrystallization during friction-stir welding generates the recrystallized grains at the weld center.

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TL;DR: In this paper, the authors examined the susceptibility to hydrogen embrittlement of martensitic steels by means of a delayed-fracture test and hydrogen thermal desorption analysis.
Abstract: The susceptibility to hydrogen embrittlement (HE) of martensitic steels has been examined by means of a delayed-fracture test and hydrogen thermal desorption analysis. The intensity of a desorptionrate peak around 50 °C to 200 °C increased when the specimen was preloaded and more remarkably so when it was loaded under the presence of hydrogen. The increment appeared initially at the low-temperature region in the original peak. As hydrogen entry proceeded, the increment then appeared at the high-temperature region, while that in the low-temperature region was reduced. The alteration occurred earlier in steels tempered at lower temperatures, with a higher embrittlement susceptibility. A defect acting as the trap of the desorption in the high-temperature region was assigned to large vacancy clusters that have higher binding energies with hydrogen. Deformation-induced generation of vacancies and their clustering have been considered to be promoted by hydrogen and to play a primary role on the HE susceptibility of high-strength steel.

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TL;DR: In this article, the effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutective Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM).
Abstract: The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.

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TL;DR: In this paper, the authors discussed the locally measured textures in the context of the material flow required to produce such textures, ultimately resulting in an estimate of the flow field present during FSW.
Abstract: Over the past decade, friction stir welding (FSW) has rapidly become an important industrial joining process, particularly in the aluminum industry Included among the advantages of FSW are such important attributes as improved weld strength and the elimination of cracking and porosity During the friction stir process, the metal undergoes a tortuous deformation path that is not yet fully understood The crystallographic texture that evolves during FSW contains sharp spatial gradients that undoubtedly influence the integrity of the weld and surrounding region in subsequent performance The locally measured textures are discussed in the context of the material flow required to produce such textures, ultimately resulting in an estimate of the flow field present during FSW

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TL;DR: In this paper, the precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wtpct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing.
Abstract: The precipitation behavior of a commercial high-strength low-alloy (HSLA) steel microalloyed with 0.086 wt pct Nb and 0.047 wt pct Ti has been investigated using transmission electron microscopy (TEM) and mechanical testing. The emphasis of this study is to compare an industrially hot-rolled steel and samples from a laboratory hot torsion machine simulation. From TEM observations, the Ti and Nb containing precipitates could be grouped according to their size and shape. The precipitates in order of size were found to be cubic TiN particles with sizes in the range of 1 µm, grain boundary precipitates with diameters of approximately 10 nm, and very fine spherical or needle-shaped precipitates with sizes on the order of 1 nm. The needlelike precipitates were found on dislocations in ferrite and constituted the dominant population in terms of density. Thus, they appear to be responsible for the precipitation strengthening observed in this steel. Aging tests were carried out at 650°C to evaluate the precipitate strengthening kinetics in detail. The strengthening mechanisms can be described with a nonlinear superposition of dislocation and precipitation hardening. The mechanical properties of torsion-simulated material and as-coiled industrial material are similar; however, there are some microstructural differences that can be attributed to the somewhat different processing routes in the laboratory as compared to hot strip rolling.

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TL;DR: In this paper, an exponential equation describing the influence of carbon concentration on the martensite start (Ms) temperature has been determined, and a function describing the Ms temperature and the energy change of the system has been found.
Abstract: Three stabilization mechanisms—the shortage of nuclei, the partitioning of alloying elements, and the fine grain size—of the remaining metastable austenite in transformation-induced plasticity (TRIP) steels have been studied by choosing a model alloy Fe-0.2C-1.5Mn-1.5Si. An examination of the nucleus density required for an athermal nucleation mechanism indicates that such a mechanism needs a nucleus density as large as 2.5 · 1017 m−3 when the dispersed austenite grain size is down to 1 µm. Whether the random nucleation on various heterogeneities is likely to dominate the reaction kinetics depends on the heterogeneous embryo density. Chemical stabilization due to the enrichment of carbon in the retained austenite is the most important operational mechanism for the austenite retention. Based on the analysis of 57 engineering steels and some systematic experimental results, an exponential equation describing the influence of carbon concentration on the martensite start (Ms) temperature has been determined to be Ms (K)=273+545.8 · e−1.362wc(mass pct). A function describing the Ms temperature and the energy change of the system has been found, which has been used to study the influence of the grain size on the Ms temperature. The decrease in the grain size of the dispersed residual austenite gives rise to a significant decrease in the Ms temperature when the grain size is as small as 0.1 µm. It is concluded that the influence of the grain size of the retained austenite can become an important factor in decreasing the Ms temperature with respect to the TRIP steels.

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TL;DR: In this article, a study of fatigue-crack propagation behavior in Nitinol, a 50Ni-50Ti (at. pct) superelastic/shape-memory alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance was presented.
Abstract: This article presents a study of fatigue-crack propagation behavior in Nitinol, a 50Ni-50Ti (at. pct) superelastic/shape-memory alloy, with particular emphasis on the effect of the stress-induced martensitic transformation on crack-growth resistance. Specifically, fatigue-crack growth was characterized in stable austenite (at 120 °C), superelastic austenite (at 37 °C), and martensite (at −65 °C and − 196 °C). In general, fatigue-crack growth resistance was found to increase with decreasing temperature, such that fatigue thresholds were higher and crack-growth rates slower in martensite compared to stable austenite and superelastic austenite. Of note was the observation that the stress-induced transformation of the superelastic austenite structure, which occurs readily at 37 °C during uniaxial tensile testing, could be suppressed during fatigue-crack propagation by the tensile hydrostatic stress state ahead of a crack tip in plane strain; this effect, however, was not seen in thinner specimens, where the constraint was relaxed due to prevailing plane-stress conditions.

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TL;DR: In this article, the modified quasi-chemical model is further extended, in the quadruplet approximation, to treat, simultaneously, first-nearest-neighbor (FNN) and second-NEIGHbor (SNN) short-range ordering (SRO) in solutions with two sublattices.
Abstract: The modified quasi-chemical model is further extended, in the quadruplet approximation, to treat, simultaneously, first-nearest-neighbor (FNN) and second-nearest-neighbor (SNN) short-range ordering (SRO) in solutions with two sublattices. When one sublattice is occupied by only one species, or is empty, the model reduces to the modified quasi-chemical model for one sublattice in the pair approximation. The coordination numbers and the ratio of the number of sites on the two sublattices are permitted to vary with composition, thereby making the model ideally suited to the treatment of liquid solutions such as molten salts. The model also applies to solid solutions if the number of sites and coordination numbers are held constant and may be combined with the compound-energy formalism to treat SRO in a wide variety of types of solutions. A significant computational simplification is achieved by formally treating the quadruplets as the “components” of the solution.

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TL;DR: In this article, the progress of Mg enrichment at grain boundaries was measured for increasing thermal-treatment times, using auger electron spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by analytical electron microscopy of thin foils.
Abstract: The corrosion and stress corrosion cracking (SCC) susceptibility of an Al-Mg alloy, AA5083, has been shown to depend on the precipitation of the Mg-rich β phase, (Al3Mg2), but not the enrichment of elemental Mg at grain boundaries to an enrichment ratio of 1.4. These results were determined by measuring the progress of Mg enrichment at grain boundaries, for increasing thermal-treatment times, using auger electron spectroscopy (AES) of grain boundaries exposed by fracture within the spectrometer and by analytical electron microscopy (AEM) of thin foils. The progress of the β phase precipitation was followed by AEM and scanning electron microscopy (SEM), for the same thermal-treatment times. The lack of a Mg-segregation effect on SCC was demonstrated by results obtained with X-ray photoelectron spectroscopy (XPS) analysis of Mg-implanted Al following in-situ electrochemical tests and SCC tests, while the dominance of β phase precipitation was demonstrated by electrochemical analysis and SCC testing. Crack-growth tests of alloy AA5083 demonstrated faster cracking at potentials anodic to the open circuit potential (OCP) with no increase at potentials cathodic to the OCP.

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TL;DR: In this article, metal-intermetallic (aluminide) laminate (MIL) composites have been fabricated in air using dissimilar metal foils, which exhibit a very high degree of microstructural design and control.
Abstract: Metal-intermetallic (aluminide) laminate (MIL) composites have been fabricated in air using dissimilar metal foils. Foils of varying Al thickness were reacted with foils of Ti-3Al-2.5V resulting in microstructures of well-bonded metal-intermetallic layered composites with either Ti or Al residual metal layers alternating with the Al3Ti intermetallic layers. The MIL composites exhibit a very high degree of microstructural design and control. Microstructure characterization by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffractometry (XRD) has been performed, and basic physical properties of the Ti-Al composites have been determined. The Ti-Al reaction has been studied by interrupting the reaction processing, in steps, to observe the microstructural changes. An oxide layer between the Ti and Al foils initially controls the reaction kinetics. After breakdown of the oxide layer, a two-phase Al+Al3Ti layer (∼10 µm thickness) is formed. After formation of the two-phase layer, liquid phases are continuously present, and Al3Ti spherules (∼10 µm diameter) are formed through interfacial tension, solidify (in times of 2 to 4 µs), and are expelled into the liquid. This mechanism allows for a continuous reaction interface and higher reaction rates. Both reaction regimes, diffusion through the oxide, and, subsequently, the intermetallic phase reaction mechanism result in linear kinetics.

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TL;DR: In this article, the microstructural evolution during age hardening of a Cu-bearing Al-Mg-Si alloy has been investigated by the three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques, in order to clarify the effect of Cu on the initial age-hardening response.
Abstract: The microstructural evolution during age hardening of a Cu-bearing Al-Mg-Si alloy has been investigated by the three-dimensional atom probe (3DAP) and transmission electron microscope (TEM) techniques, in order to clarify the effect of Cu on the initial age-hardening response. After 30 minutes of artificial aging at 175 °C, the alloy shows a significant increase in hardness. The TEM observations have revealed that very fine, needle-shaped β″ precipitates are formed in addition to spherical Guinier-Preston (GP) zones, whereas only the spherical GP zones are observed in the Al-Mg-Si ternary alloy using the same aging condition. The number density of the precipitates is significantly affected by the preaging conditions. The 3DAP analysis shows that the distribution of Cu atoms is uniform after 30 minutes of artificial aging at 175 °C, whereas Cu atoms are incorporated into the needle-shaped β″ precipitates after 10 hours of aging at 175 °C. Based on these microanalytical results, the effect of Cu additions on the age-hardening response of Al-Mg-Si alloys is discussed.

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TL;DR: In this paper, the authors used differential scanning calorimetry (DSC) to trace and model the transformation kinetics in continuous cooling conditions, and derived the JMA kinetic parameters.
Abstract: The relationship between heat-treatment parameters and microstructure in titanium alloys has so far been mainly studied empirically, using characterization techniques such as microscopy. Calculation and modeling of the kinetics of phase transformation have not yet been widely used for these alloys. Differential scanning calorimetry (DSC) has been widely used for the study of a variety of phase transformations. There has been much work done on the calculation and modeling of the kinetics of phase transformations for different systems based on the results from DSC study. In the present work, the kinetics of the β ⇒ α transformation in a Ti-6Al-4V titanium alloy were studied using DSC, at continuous cooling conditions with constant cooling rates of 5 °C, 10 °C, 20 °C, 30 °C, 40 °C, and 50 °C/min. The results from calorimetry were then used to trace and model the transformation kinetics in continuous cooling conditions. Based on suitably interpreted DSC results, continuous cooling-transformation (CCT) diagrams were calculated with lines of isotransformed fraction. The kinetics of transformation were modeled using the Johnson-Mehl-Avrami (JMA) theory and by applying the “concept of additivity.” The JMA kinetic parameters were derived. Good agreement between the calculated and experimental transformed fractions is demonstrated. Using the derived kinetic parameters, the β ⇒ α transformation in a Ti-6Al-4V alloy can be described for any cooling path and condition. An interpretation of the results from the point of view of activation energy for nucleation is also presented.

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TL;DR: In this article, the influence of the casting filling technique and hot isostatic pressing (hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has been studied.
Abstract: The influence of the casting filling technique and hot isostatic pressing (hipping) on the fatigue-life distribution of Al-7Si-Mg alloy castings has been studied. To vary the number density and size of oxide-film defects in the castings, test bars were cast using bottom-gated filling systems with and without filtration. Some unfiltered castings were subjected to a hipping treatment of 100 MPa at 500 °C for 6 hours. Test pieces were machined from the castings and were fatigue tested in pull-pull sinusoidal loading, at maximum stresses of 150 and 240 MPa under a stress ratio of R=+0.1. The fatigue lives at any probability of failure and Weibull statistical parameters of the filtered castings were higher than those of the unfiltered and nonhipped castings, illustrating the importance of the casting technique. However, the unfiltered but hipped castings exhibited higher performance. It is proposed that the significant improvement in fatigue life after hipping is due to the deactivation of entrained double oxide-film defects as fatigue-crack initiators.

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TL;DR: In this article, the effects of Pt incorporation on the isothermal oxidation and diffusion behavior of low-sulfur aluminide bond coatings were investigated, and it was concluded that the major beneficial effect of the incorporation of Pt was the elimination of void growth at the oxide-metal interface.
Abstract: The effects of Pt incorporation on the isothermal oxidation and diffusion behavior of low-sulfur aluminide bond coatings were investigated. Aluminide (NiAl) coatings and Pt-modified aluminide (Ni,Pt)Al coatings were synthesized on a low-sulfur, yttrium-free single-crystal Ni-based superalloy by a high-purity, low-activity chemical vapor deposition (CVD) aluminizing procedure. The isothermal oxidation kinetics and scale adhesion behavior of CVD NiAl and (Ni,Pt)Al coatings before and after isothermal oxidation were determined by electron microprobe analysis. Platinum did not reduce oxide-scale growth kinetics. No significant differences in bulk refractory metal (W, Ta, Re, and Mo) distributions were observed as a result of Pt incorporation. Spallation of the alumina scale and the formation of large voids along the oxide-metal interface were commonly observed over the NiAl coating grain boundaries after 100 hours at 1150 °C. In contrast, no spallation of Al2O3 scales occurred on (Ni,Pt)Al coating surfaces or grain boundaries, although the sulfur content in the CVD (Ni,Pt)Al coatings was higher than that of the CVD NiAl coatings. Most significantly, no voids were observed at the oxide-metal interface on (Ni,Pt)Al coating surfaces or cross sections after 200 hours at 1150 °C. It was concluded that a major beneficial effect of Pt incorporation on an aluminide coatings oxidation resistance is the elimination of void growth at the oxide-metal interface, likely by mitigation of detrimental sulfur effects.


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TL;DR: In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques.
Abstract: In this article, amorphous and nanocomposite thermally deposited steel coatings have been formed by using both plasma and high-velocity oxy-fuel (HVOF) spraying techniques. This was accomplished by developing a specialized iron-based composition with a low critical cooling rate (≈104 K/s) for metallic glass formation, processing the alloy by inert gas atomization to form micron-sized amorphous spherical powders, and then spraying the classified powder to form coatings. A primarily amorphous structure was formed in the as-sprayed coatings, independent of coating thickness. After a heat treatment above the crystallization temperature (568 °C), the structure of the coatings self-assembled (i.e., devitrified) into a multiphase nanocomposite microstructure with 75 to 125 nm grains containing a distribution of 20 nm second-phase grain-boundary precipitates. Vickers microhardness testing revealed that the amorphous coatings were very hard (10.2 to 10.7 GPa), with further increases in hardness after devitrification (11.4 to 12.8 GPa). The wear characteristics of the amorphous and nanocomposite coatings were determined using both two-body pin-on-disk and three-body rubber wheel wet-slurry sand tests. The results indicate that the amorphous and nanocomposite steel coatings are candidates for a wide variety of wear-resistant applications.

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TL;DR: In this paper, a modified quasi-chemical model in the pair approximation for short-range ordering (SRO) in liquids is extended to solutions with two sublattices, and a significant computational simplification is achieved by formally treating the nearest-neighbor pairs as the "components" of the solution.
Abstract: The modified quasi-chemical model in the pair approximation for short-range ordering (SRO) in liquids is extended to solutions with two sublattices. Short-range ordering of nearest-neighbor pairs is treated, and the effect of second-nearest-neighbor (SNN) interactions upon this ordering is taken into account. The model also applies to solid solutions, if the number of lattice sites and coordination numbers are held constant. It may be combined with the compound-energy formalism to treat a wide variety of solution types. A significant computational simplification is achieved by formally treating the nearest-neighbor pairs as the “components” of the solution. The model is applied to an evaluation/optimization of the phase diagram of the Li,Na,K/F,Cl,SO4 system.

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TL;DR: In this article, the authors reviewed the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C.
Abstract: This article reviews the strengthening and fracture mechanisms that operate in carbon and low-alloy carbon steels with martensitic microstructures tempered at low temperatures, between 150 °C and 200 °C. The carbon-dependent strength of low-temperature-tempered (LTT) martensite is shown to be a function of the dynamic strain hardening of the dislocation and transition carbide substructure of martensite crystals. In steels containing up to 0.5 mass pct carbon, fracture occurs by ductile mechanisms of microvoid formation at dispersions of second-phase particles in the matrix of the strain-hardened tempered martensite. Steels containing more than 0.5 mass pct carbon with LTT martensitic microstructures are highly susceptible to brittle intergranular fracture at prior austenite grain boundaries. The mechanisms of the intergranular fracture are discussed, and approaches that have evolved to minimize such fracture and to utilize the high strength of high-carbon hardened steels are described.

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TL;DR: In this article, the effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti52Al48-xM alloys were investigated.
Abstract: The effects of beta stabilizers such as Fe, Cr, V, and Nb on the microstructures and phase constituents of Ti52Al48-xM (x=0, 1.0, 2.0, 4.0, or 6.0 at. pct and M=Fe, Cr, V, and Nb) alloys were studied. The dependence of the tensile properties and creep resistance of TiAl on the alloying elements, especially the formation of B2 phase, was investigated. Fe is the strongest B2 stabilizer, Cr is second, V is an intermediate stabilizer, and Nb is the weakest stabilizer. The composition partitioning of Fe, Cr, V, and Nb in the γ phase is affected by the formation of B2 phase. The peaks of the tensile strengths and creep rupture life of Ti52Al48-xM generally occur at the maximum solid solution of these elements in the γ phase, which is just before the formation of B2 phase. Ti52Al48-0.5Fe shows an attractive elongation of 2.5 pct at room temperature, and the Ti52Al48-1V, Ti52Al48-Cr, and Ti52Al48-2Nb alloys have about 1.1 to 1.3 pct elongation at room temperature. The increase of tensile strengths and creep resistance with increasing Fe, Cr, V, and Nb contents is chiefly attributed to the solid-solution strengthening of these elements in the γ phase. The appearance of B2 phase deteriorates the creep resistance, room-temperature strengths, and ductility. With respect to the maximum solid-solution strengthening, an empirical equation of the Cr equivalent [Cr] is suggested as follows: [Cr]=Cr+Mn+3/5V+3/8Nb+3/2 (W+Mo)+3Fe=1.5 to 3.0. The solid-solution strengthening mechanism of Fe, Cr, V, and Nb at room temperature arises from the increase of the Ti 3s and Al 2s binding energies in Ti-Ti and Al-Al bonds, and the retention of the strength and creep resistance at elevated temperatures in Ti52Al48-xM is mainly attributed to the increase of the Ti 3s and Al 2s binding energies in Ti-Al bonds in γ phase. The decrease of the Ti 3p and Al 2p binding energies in Ti-Ti, Ti-Al, and Al-Al bonds benefits the ductility of TiAl.

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TL;DR: In this paper, the effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was determined by conducting isothermal, constant-strainrate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transverse direction (TD), 45 deg, and normal) from hot-rolled plate.
Abstract: The effect of crystallographic texture and slip mode on the plastic flow of Ti-6Al-4V with either a colony- or globular-alpha microstructure was determined by conducting isothermal, constant-strainrate, hot-compression tests on specimens cut at various orientations (rolling direction (RD), transverse direction (TD), 45 deg, and normal) from hot-rolled plate. Testing was performed using a fixed strain rate (0.1 s−1) and various temperatures below the beta transus. The flow curves from all of the experiments exhibited a peak flow stress followed by a large and a small amount of flow softening for the colony and globular microstructures, respectively. Although the flow softening response did not depend noticeably on test direction for a given microstructure and test temperature, the peak flow stress and development of sample ovality did. This orientation dependence was interpreted using both lower-bound (isostress-type) and upper-bound (isostrain, Taylor/Bishop-Hill) models to deduce the operative slip systems in the alpha phase. These analyses suggested that prism 〈a〉 and basal 〈a〉 slip are considerably easier than pyramidal 〈c+a〉 or 〈a〉 slip at hot-working temperatures. A comparison of the flow curves for the colony and globular alpha microstructures suggested that slip transfer across alpha/beta interfaces and loss of Hall-Petch boundary strengthening can account for a substantial portion of the flow softening observed during hot working.