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Showing papers in "Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science in 1995"


Journal ArticleDOI
TL;DR: In this paper, a two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures.
Abstract: A two-phase alloy of composition Ti-47.5Al-2.5Cr has been studied under two heat-treated conditions in order to obtain different microstructures. These consisted of lamellar and equiaxed distributions of y grains in which the α2 phase was distributed as long lamellae or smaller globules, respectively. The specific rotation relationships between γ/γ and γ/α2 grains have been measured, and these have been used to understand their effect on the compatibility of deformation across adjacent grains. For this, detailed analysis of active slip systems has been carried out by transmission electron microscopy (TEM) observations of deformed samples. A theoretical calculation of a geometric compatibility factor characterizing the best slip transfer across adjacent grains has been used in such a way that it has been possible to deduce the role played by the type of orientation relationship between grains in producing active deformation systems that allow the maximum compatibility of deformation.

358 citations


Journal ArticleDOI
TL;DR: In this article, a liquid mixing and casting process that can be used to produce SiC particulate-reinforced magnesium metal matrix composites via conventional foundry processes is presented.
Abstract: Magnesium metal matrix composites (MMCs) have been receiving attention in recent years as an attractive choice for aerospace and automotive applications because of their low density and superior specific properties. This article presents a liquid mixing and casting process that can be used to produce SiC particulate-reinforced magnesium metal matrix composites via conventional foundry processes. Microstructural features, such as SiC particle distribution, grain refinement, and particle/matrix interfacial reactions of the cast magnesium matrix composites, are investigated, and the effects of solidification-process parameters and matrix alloys (pure Mg and Mg-9 pct Al-1 pct Zn alloy AZ91) on the microstructure are established. The results of this work suggest that in the solidification processing of MMCs, it is important to optimize the process parameters both to avoid excessive interfacial reactions and simultaneously achieve wetting, so that a good particle distribution and interfacial bonding are obtained. The tensile properties, strain hardening, and fracture behavior of the AZ91/SiC composites are also studied and the results are compared with those of the unreinforced AZ91 alloy. The strengthening mechanisms for AZ91/SiC composite, based on the proposed SiC particle/matrix interaction during deformation, are used to explain the increased yield strength and elastic modulus of the composite over the magnesium matrix alloy. The low ductility found in the composites is due to the early appearance of localized damages, such as particle cracking, matrix cracking, and occasionally interface debonding, in the fracture process of the composite.

270 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that the grain-size dependence of shock response can significantly affect the performance of shaped charges, leading to a prediction of the shock threshold stress for twinning as a function of grain size.
Abstract: Copper with four widely differing grain sizes was subjected to high-strain-rate plastic deformation in a special experimental arrangement in which high shear strains of approximately 2 to 7 were generated. The adiabatic plastic deformation produced temperature rises in excess of 300 K, creating conditions favorable for dynamic recrystallization, with an attendant change in the mechanical response. Preshocking of the specimens to an amplitude of 50 GPa generated a high dislocation density; twinning was highly dependent on grain size, being profuse for the 117- and 315-μm grain-size specimens and virtually absent for the 9.5-μm grain-size specimens. This has a profound effect on the subsequent mechanical response of the specimens, with the smaller grain-size material undergoing considerably more hardening than the larger grain-size material. A rationale is proposed which leads to a prediction of the shock threshold stress for twinning as a function of grain size. The strain required for localization of plastic deformation was dependent on the combined grain size/shockinduced microstructure, with the large grain-size specimens localizing more readily. The experimental results obtained are rationalized in terms of dynamic recrystallization, and a constitutive equation is applied to the experimental results; it correctly predicts the earlier onset of localization for the large grain-size specimens. It is suggested that the grain-size dependence of shock response can significantly affect the performance of shaped charges.

257 citations


Journal ArticleDOI
TL;DR: In this article, the age-hardening response of two cast Mg-Zn-RE alloys was investigated and compared with that of a binary MgZn alloy, and microstructures of the aged specimens were examined by analytical electron microscopy.
Abstract: The age-hardening response of two cast Mg-Zn-RE alloys has been investigated and compared with that of a binary Mg-Zn alloy. The microstructures of the aged specimens were examined by analytical electron microscopy. Formation of a fine dispersion of rodlikeβ1′ precipitates is the main cause for age hardening, while extensive precipitation of disc-shapedβ2′ coincides with the onset of overaging. Rare earth additions retard the formation ofβ2′ precipitates and thus postpone overaging. Four different orientation relationships betweenβ2′ precipitates and matrix were found and explained in terms of the near-CSL model.

212 citations


Journal ArticleDOI
TL;DR: In this paper, the authors derived models for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example.
Abstract: Models were derived for monolayer and bilayer growth into a substrate in which diffusion of the solute governs the growth kinetics, as in gas-solid reactions, for example. In the models, the composition dependence of the solute diffusivity in the phases constituting the layers was accounted for by appropriate definition of an effective diffusion coefficient for a (sub)layer. This effective diffusion coefficient is the intrinsic diffusion coefficient weighted over the composition range of the (sub)layer. The models were applied for analyzing the growth kinetics of a γ′-Fe4N1-x monolayer on an α-Fe substrate and the growth kinetics of an e-Fe2N1-z/γ′-Fe4N1-x bilayer on an α-Fe substrate, as observed by gaseous nitriding in an NH3/H2-gas mixture at 843 K. The kinetics of layer development and the evolution of the microstructure were investigated by means of thermogravimetry, layer-thickness measurements, light microscopy, and electron probe X-ray microanalysis (EPMA). The effective and self-diffusion coefficients were determined for each of the nitride layers. The composition dependence of the intrinsic (and effective) diffusion coefficients was established. Re-evaluating literature data for diffusion in γ′-Fe4N1-x on the basis of the present model, it followed that the previous and present data are consistent. The activation energy for diffusion of nitrogen in γ′-Fe4N1-x was determined from the temperature dependence of the self-diffusion coefficient. The self-diffusion coefficient for nitrogen in e-Fe2N1-z was significantly larger than that for γ′-Fe4N1-x. This was explained qualitatively, considering the possible mechanisms for interstitial diffusion of nitrogen atoms in the close-packed iron lattices of the e and γ′ iron nitrides.

211 citations


Journal ArticleDOI
TL;DR: In this article, the formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel is simulated by simultaneously solving macroscopic mass, momentum, energy, and species conservation equations with full coupling of the temperature and concentrations through thermodynamic equilibrium at the solid/liquid interface.
Abstract: The formation of macrosegregation by multicomponent thermosolutal convection during the solidification of steel is simulated by simultaneously solving macroscopic mass, momentum, energy, and species conservation equations with full coupling of the temperature and concentrations through thermodynamic equilibrium at the solid/liquid interface. The flow field, solid fraction evolution, and macrosegregation patterns for four cases are presented. The results show both the formation of channel segregates and the formation of islands of mush surrounded by bulk melt. In examining the solidification of a ten-element steel, the global extent of macrosegregation of an element is found to be linearly dependent on its partition coefficient (more severe segregation for small partition coefficient), although such scaling is not possible locally. Results for the solidification of a binary Fe-C alloy (with the same carbon content as the ten-element alloy) are similar to those for the ten-element alloy due solely to the large contribution of carbon to buoyancy driven flow in the ten-element steel chosen for study. While including only those elements that make significant contributions to buoyancy driven flow reproduces the global extent of macrosegregation seen in the ten-element alloy, local differences in the predictions are visible. Finally, comparison of results for the solidification of the same ten-element steel using two different sets of data to describe the partition coefficients and change in liquidus temperature with concentration of the elements shows completely opposite behavior,i.e., upward flow through the mushy zone for one case and downward flow for the other. Thus, the need to have accurate phase-equilibrium data when modeling multicomponent macrosegregation is illustrated. Together, the results give an indication of what areas require more careful examination if accurate modeling of multicomponent solidification is to be accomplished.

203 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of cold work prior to aging on precipitation hardening in selected Al-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys were investigated.
Abstract: A study has been made of the effects of cold work prior to aging on precipitation hardening in selected Al-Cu-Mg-(Ag) and Al-Cu-Li-(Mg-Ag) alloys. General aging characteristics have been determined by differential scanning calorimetry, and response to hardening has been correlated with microstructure using transmission electron microscopy (TEM), selected area electron dif-fraction (SAED), and quantitative stereology. Particular attention has been given to the phases Ω andT 1 that form on the {111 }α planes, although information on the precipitates θ′,S′ (orS), and δ′ is also reported. Although Ω andT 1, have similar morphologies and habit planes, their response to cold work prior to aging is different. Deformation promotesT 1 formation at the expense of the δ′ phase in Al-Cu-Li alloys and at the expense of δ′, θ′, andS′ in Al-Cu-Li-Mg-Ag alloys. On the other hand, in Al-Cu-Mg-Ag alloys, deformation assists precipitation of θ′ at the expense of Ω phase, and some decrease is recorded in the hardening response. Prior cold work is also found to reduce the response during natural aging in most alloys. These results are discussed in terms of the role of particular alloying additions.

191 citations


Journal ArticleDOI
TL;DR: In this paper, the stress-strain behavior of pure Zr was studied systematically at various temperatures and strain rates, and the transition from slip to twinning in the stress strain behavior was linked to differing strain-hardening rates and temperature sensitivities of the two deformation modes.
Abstract: The stress-strain behavior of pure Zr was studied systematically at various temperatures and strain rates. At 76 K, Zr deforms predominantly by twinning, whereas above room temperature (RT), slip is the controlling deformation mode. A transition in the rate-controlling deformation mode from slip to twinning has been observed to occur at intermediate temperatures during the course of plastic deformation. Above 373 K, slip dominates the entire course of deformation. The transition from slip to twinning in the stress-strain behavior is linked to differing strain-hardening rates and temperature sensitivities of the two deformation modes.

184 citations


Journal ArticleDOI
TL;DR: In this article, the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves.
Abstract: Microscopic observations and the results of static strain aging, stress relaxation, and strain rate change tests on 310s stainless steel foils, with and without hydrogen, have been presented to complement the stress-strain curves in a previous article. The hydrogen-free specimens showed minute yield points during static strain aging, while the hydrogen-containing specimens demonstrated “preyield microstrain. ” Thermal activation analysis of the strain rate change and stress relaxation plots led to the conclusion that the activation area for dislocation motion is decreased by hydrogen. Microstructural examination with the scanning electron microscope (SEM) revealed extensive strain localization, while transmission electron microscopy (TEM) studies showed microtwinning and austenite faulting in hydrogenated specimens tested at room temperature. The relation of hydrogen-induced changes in plastic deformation to hydrogen embrittlement is discussed.

182 citations


Journal ArticleDOI
TL;DR: In this article, the effect of grain size on the deformation-induced martensite (α′) in 304 and 316 stainless steels during room-temperature rolling has been studied.
Abstract: The effect of grain size on the deformation-induced martensite (α′) in 304 and 316 stainless steels (SS) during room-temperature rolling has been studied. Samples of four grain sizes of 52, 180, 229, and 285 μ in 304 and three grain sizes of 77, 125, and 200 /μm in 316 SS have been rolled from 16 to 63 pct reduction in thicknesses to characterize the microstructures during the rolling deformation. The amount of α′ formed increases with increase in the amount of deformation in both SS for a given grain size. The volume fraction of martensite formed increases with a decrease in grain size in 304 SS, while the α’ martensite formation has been found to be grain size insensitive in 316 SS. The volume fraction of α’ formed in 304 SS is always higher than that in 316 SS for a fixed percent reduction in thickness and grain size. This is attributed to the higher number of shear band intersections observed in 304 SS, which are considered to be the nucleation sites for the α’ embryos. The lath martensite obtained at small true rolling strains changes to blocky type at higher true strains. The morphology of α′ formed has been discussed and its characteristics obtained from rolling deformation have been compared with those earlier reported from the room-temperature tensile deformation.

152 citations


Journal ArticleDOI
TL;DR: In this article, a set of thermodynamic parameters more consistent with most of the selected experimental data than previous assess-ments has been obtained by a computerized least-squares method using phenomenological models for the Gibbs energy of various phases.
Abstract: Thermochemical and phase diagram data in the Fe-Cu system have been critically evaluated by using phenomenological models for the Gibbs energy of various phases A set of thermodynamic parameters more consistent with most of the selected experimental data than previous assess-ments has been obtained by a computerized least-squares method Stable and metastable phase equilibria,T 0 curves, and thermodynamic properties are calculated with the optimized param-eters The calculated liquid/face-centered cubic (fcc)T 0 curve and metastable liquid spinodal seem to permit an accurate prediction of maximum solid solubility obtained upon melt quenching in this system

Journal ArticleDOI
TL;DR: In this article, the authors studied the general stress-strain behaviors due to the thermoelastic martensitic transformation induced by a combination of external forces of axial load and torque.
Abstract: Combined tension and torsion experiments with thin wall specimens of Cu-Al-Zn-Mn polycrystalline shape memory alloy (SMA) were performed at temperatureT =A f + 25 K. The general stress-strain behaviors due to the thermoelastic martensitic transformation, induced by a combination of external forces of axial load and torque, were studied. It is shown that the progress of martensitic transformation (MT) at general stress conditions can be well considered as triggered and controlled by the supplied mechanical work (a kind of equivalent stress) in the first approximation. Pseudoelastic strains in proportional as well as nonproportional combined tension-torsion loadings were found fully reversible, provided that uniaxial strains were reversible. The axial strain can be controlled by the change of torque andvice versa due to the coupling among tension and torsion under stress, not only in forward transformation, but also in reverse transformation on unloading. The pseudoelastic strains of SMA polycrystal are path dependent but well reproducible along the same stress path. The evolution of macroscopic strain response of SMA polycrystal, subjected to the nonproportional pseudoelastic loading cycles with imposed stress path, was systematically investigated. The results bring qualitatively new information about the progress of the MT in SMA polycrystal, subjected to the general variations of external stress.

Journal ArticleDOI
TL;DR: In this article, a model for predicting the evolution of microstructure in Waspaloy during thermomechanical proc-essing was developed in terms of dynamic recrystallization (DRX), metadynamic recrystization, and grain growth phenomena.
Abstract: A model for predicting the evolution of microstructure in Waspaloy during thermomechanical proc-essing was developed in terms of dynamic recrystallization (DRX), metadynamic recrystallization, and grain growth phenomena. Three sets of experiments were conducted to develop the model: (1) preheating tests to model grain growth prior to hot deformation; (2) compression tests in a Gleeble testing machine with different deformation and cooling conditions to model DRX, metadynamic recrystallization, and short time grain growth during the post deformation dwell period and cooling; and (3) pancake and closed die forging tests conducted in a manufacturing environment to verify and refine the model. The microstructural model was combined with finite element modeling (FEM) to predict microstructure development during forging of Waspaloy. Model predictions showed good agreement with microstructures obtained in actual isothermal and hammer forgings carried out at a forging shop.

Journal ArticleDOI
TL;DR: In this article, the authors compared the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al, and found that the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy.
Abstract: High-temperature strengthening mechanisms in discontinuous metal matrix composites were examined by performing a close comparison between the creep behavior of 30 vol pct SiC-6061 Al and that of its matrix alloy, 6061 Al. Both materials were prepared by powder metallurgy techniques. The experimental data show that the creep behavior of the composite is similar to that of the alloy in regard to the high apparent stress exponent and its variation with the applied stress and the strong temperature dependence of creep rate. By contrast, the data reveal that there are two main differences in creep behavior between the composite and the alloy: the creep rates of the composite are more than one order of magnitude slower than those of the alloy, and the activation energy for creep in the composite is higher than that in the alloy. Analysis of the experimental data indicates that these similarities and differences in creep behavior can be explained in terms of two independent strengthening processes that are related to (a) the existence of a temperature-dependent threshold stress for creep, τ0, in both materials and (b) the occurrence of temperature dependent load transfer from the creeping matrix (6061 Al) to the reinforcement (SiC). This finding is illustrated by two results. First, the high apparent activation energies for creep in the composite are corrected to a value near the true activation energy for creep in the unreinforced alloy (160 kJ/mole) by considering the temperature dependence of the shear modulus, the threshold stress, and the load transfer. Second, the normalized creep data of the composite fall very close to those of the alloy when the contribution of load transfer to composite strengthening is incorporated in a creep power law in which the applied stress is replaced by the effective stress, the stress exponent,n, equals 5, and the true activation energy for creep in the composite,Q c , is equal to that in the alloy.

Journal ArticleDOI
TL;DR: In this article, the wear behavior of A356 aluminum alloy (Al-7 Pct Si-0.3 Pct Mg) matrix composites reinforced with 20 vol pct SiC particles and 3 or 10 vol Pct graphite was investigated.
Abstract: The wear behavior of A356 aluminum alloy (Al-7 Pct Si-0.3 Pct Mg) matrix composites reinforced with 20 vol Pct SiC particles and 3 or 10 vol Pct graphite was investigated. These hybrid composites represent the merging of two philosophies in tribological material design: soft-particle lubrication by graphite and hard-particle reinforcement by carbide particles. The wear tests were performed using a block-on-ring (SAE 52100 steel) wear machine under dry sliding conditions within a load range of 1 to 441 N. The microstructural and compositional changes that took place during wear were characterized using scanning electron microscopy (SEM), Auger electron spectroscopy (AES), energy-dispersive X-ray spectroscopy (EDXA), and X-ray diffractometry (XRD). The wear resistance of 3 Pct graphite-20 Pct SiC-A356 hybrid composite was comparable to 20 Pct SiC-A356 without graphite at low and medium loads. At loads below 20N, both hybrid and 20 Pct SiC-A356 composites without graphite demonstrated wear rates up to 10 times lower than the unreinforced A356 alloy due to the load-carrying capacity of SiC particles. The wear resistance of 3 Pct graphite 20 Pct SiC-A356 was 1 to 2 times higher than 10 Pct graphite-containing hybrid composites at high loads. However, graphite addition reduced the counterface wear. The unreinforced A356 and 20 Pct SiC-A356 showed a transition from mild to severe wear at 95 N and 225 N, respectively. Hybrid composites with 3 Pct and 10 Pct graphite did not show such a transition over the entire load range, indicating that graphite improved the seizure resistance of the composites. Tribolayers, mainly consisting of a compacted mixture of graphite, iron oxides, and aluminum, were generated on the surfaces of the hybrid composites. In the hybrid composites, the elimination of the severe wear (and hence the improvement in seizure resistance) was attributed to the reduction in friction-induced surface heating due to the presence of graphite- and iron-oxide-containing tribolayers.

Journal ArticleDOI
TL;DR: In this article, tensile tests on 310s stainless steel foils, with and without hydrogen, were conducted at temperatures from 77 to 295 K and strain rates from 10-3 to 10-6/s.
Abstract: Tensile tests on 310s stainless steel foils, with and without hydrogen, were conducted at temperatures from 77 to 295 K and strain rates from 10-3 to 10-6/s. Cathodic charging at elevated temperatures and at very low current densities was used to produce homogeneous solid solutions of hydrogen in this material. The yield stress and flow stress were found to increase with hydrogen content. Discontinuous yielding was observed at room temperature for specimens with hydrogen contents greater than 5 at. pct. The ductility, as measured by the strain to failure, was not critically dependent on hydrogen concentration at 77 and 295 K but was reduced at intermediate temperatures. The changes in mechanical behavior are discussed in terms of hydrogen-dislocation interactions.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the damping behavior and mechanisms that are present in discontinuously reinforced MMCs, with particular emphasis on particulate-reinforced Al alloy MMC, processed using spray atomization and deposition.
Abstract: High damping materials allow undesirable mechanical vibration and wave propagation to be passively suppressed. This proves valuable in the control of noise and the enhancement of vehicle and instrument stability. Accordingly, the scientific community is continually working toward the development of high damping metals (hidamets) and high damping metal-matrix composites (MMCs). The MMCs are particularly attractive in weight-critical applications when the matrix and reinforcement phases are combined to provide desirable property combinations, such as high damping and low density. Inspection of the available scientific literature, however, reveals that an understanding of the precise correlation between the presence of secondary phases (either reinforcements or precipitates) and material damping has eluded investigators, partly as a result of the superposition of multiple mechanisms. As a step toward the clarification of damping phenomena in discontinuously reinforced MMCs, this article describes the damping behavior and mechanisms that are present in discontinuously reinforced MMCs, with particular emphasis on particulate-reinforced Al alloy MMCs processed using spray atomization and deposition. The operative damping mechanisms in the particulate-rein-forced MMCs are discussed in light of the data obtained from microstructural studies and damping capacity measurements.

Journal ArticleDOI
TL;DR: In this paper, the constitutive behavior of the same material was established through quasi-static and dynamic compression tests at ambient and elevated temperatures, and it was proposed that thermal energy is sufficient to produce a significant reorganization of the deformation substructure, leading to a recovered structure.
Abstract: Tantalum plate produced by a forging-rolling sequence was subjected to high plastic shear strains(γ = 1 → 5.5) at high strain rates (∼4 × 104 s-1) in two experimental configurations: (a) a special hat-shaped geometry and (b) thin disks deformed in a split Hopkinson bar. In parallel experiments, the constitutive behavior of the same material was established through quasi-static and dynamic compression tests at ambient and elevated temperatures. The microstructure generated at high strain rates and retained by rapid cooling from a narrow (200-μm) deformation band progresses from dislocated, to elongated cells, to banded structures, and finally, to subgrains as the shear strain increases from 0 to 5.5. The temperature rise predictions from the constitutive description of the material indicate that the temperature reaches values of 800 K, and it is proposed that thermal energy is sufficient to produce a significant reorganization of the deformation substructure, leading to a recovered structure.

Journal ArticleDOI
TL;DR: A metallographic study of porosity and fracture behavior in unidirectionally solidified end chill castings of aluminum alloy was carried out using optical microscopy and scanning electron microscopy (SEM) as discussed by the authors.
Abstract: A metallographic study of the porosity and fracture behavior in unidirectionally solidified end chill castings of 319.2 aluminum alloy (Al-6.2 pct Si-3.8 pct Cu-0.5 pct Fe-0.14 pct Mn-0.06 pct Mg-0.073 pct Ti) was carried out using optical microscopy and scanning electron microscopy (SEM) to determine their relationship with the tensile properties. The parameters varied in the production of these castings were the hydrogen (∼0.1 and ∼0.37 mL/100 g Al), modifier (0 and 300 ppm Sr), and grain refiner (0 and 0.02 wt pct Ti) concentrations, as well as the solidification time, which increased with increasing distance from the end chill bottom of the casting, giving dendrite arm spacings (DASs) ranging from ∼15 to ∼95 /im. Image analysis and energy dispersive X-ray (EDX) analysis were employed for quantification of porosity/microstructural constituents and fracture surface analysis (phase identification), respectively. The results showed that the local solidification time(viz. DAS) significantly influences the ductility at low hydrogen levels; at higher levels, however, hydro-gen has a more pronounced effect (porosity related) on the drop in ductility. Porosity is mainly observed in the form of elongated pores along the grain boundaries, with Sr increasing the porosity volume percent and grain refining increasing the probability for pore branching. The beneficial effect of Sr modification, however, improves the alloy ductility. Fracture of the Si, β-Al5FeSi, α- Al15(Fe,Mn)3Si2, and Al2Cu phases takes place within the phase particles rather than at the particle/Al matrix interface. Sensitivity of tensile properties to DAS allows for the use of the latter as an indicator of the expected properties of the alloy.

Journal ArticleDOI
K. S. Raghavan1
TL;DR: In this paper, a simple technique to generate in-plane forming limit curves has been developed, based on the Marciniak biaxial stretch test using a single punch/die configuration, but the specimen and washer geometries have been modified to achieve failure in both drawing and stretching deformation modes.
Abstract: A simple technique to generate in-plane forming limit curves has been developed. This technique is based on the Marciniak biaxial stretch test using a single punch/die configuration, but the specimen and washer geometries have been modified in order to achieve failure in both drawing and stretching deformation modes. The experimental technique is described, and the advantages of using this inplane method over the conventional out-of-plane dome method are discussed. It is shown that (a) sheet thickness has an intrinsic influence on forming limits that is not related to small bending strain variations with thickness or to deformation in the presence of friction and curvature, (b) plastic anisotropy (\(\bar r\) value) does not substantially affect forming limits, and (c) in-plane forming limits are slightly lower (5 to 6 pct) than out-of-plane forming limits near plane strain; these differences are smaller than previously reported values (12 to 15 pct) in the literature.

Journal ArticleDOI
TL;DR: In this paper, the solidification path and microstructure of cast Mg-9Zn and Mg8Zn-1.5MM alloys have been investigated by a combination of thermal analysis and analytical electron microscopy.
Abstract: The solidification path and microstructure of cast Mg-9Zn and Mg-8Zn-1.5MM (misch metal) alloys have been investigated by a combination of thermal analysis and analytical electron microscopy. The addition of 1.5 wt pct MM had a strong influence on the as-cast microstructure with the introduction of new ternary interdendritic phases and structural modification of known binary phases. The temperature ranges for formation of these phases from the melt were identified, their crystal structures determined, and their compositions analyzed. Products from eutectoidal decomposition of the interdendritic phase in the binary Mg-9Zn alloy were also identified.

Journal ArticleDOI
TL;DR: In this article, the early stages of θ′ precipitation in an Al-Cu-Sn alloy were studied by APFIM and transmission electron microscopy (TEM) and it was found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ-′ phase.
Abstract: We have studied the early stages of θ′ precipitation in an Al-Cu-Sn alloy by atom probe field ion microscopy (APFIM) and transmission electron microscopy (TEM). Clusters of Sn atoms were observed in as-quenched samples, and their formation clearly precedes the formation of the θ′ phase. Transmission electron microscopy also revealed that elevated temperature aging leads to the formation of spherical particles. Atom probe analysis and microbeam electron diffraction suggested that these particles were β-Sn (I41/amd, α = 0.583 nm, and c = 0.318 nm). The TEM observations showed that θ′ nucleated heterogeneously on these particles. We have also examined the θ′/α interface following further precipitate growth to check for possible Sn segregation. Atom probe analysis of both the broad face and the narrow rim of the platelike θ′ precipitates has shown no evidence of Sn segregation at θ′/α interfaces. It was also found that cold work prior to aging inhibits the formation of Sn particles resulting in a lower number of these types of nucleation sites for the θ′ phase. Atom probe analysis has also revealed solute depletion at grain boundaries during the early stages of aging in the Al-Cu-Sn alloy, and this, in conjunction with vacancy depletion, explains the formation of narrow precipitate-free zones observed following further aging.

Journal ArticleDOI
TL;DR: In this paper, a theoretical approach has been developed to account for the effects of the alloy system and solute concentration on the dendrite coherency in the alloy, and the grain sizes of the alloys were evaluated using the parameters at co-herency point.
Abstract: Dendrite coherency, or dendrite impingement, is important to the formation of the solidification structure and castability of alloys. Dendrite coherency in the systems Al-xMn, Al-xCu, Al-xFe, and Al-xSi(x = 0 to 5 wt pct) has been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendrite coherency point, fs*, varies with the alloy system and the solute concentration in the alloy, from 18 to 56 pct for the present alloys investigated. An increase in solute concentration decreases the coherency fraction solid,fs*. An alloy system with a large slope of the liquidus line has a high coherency fraction solid. A theoretical approach has been developed to account for the effects of the alloy system and solute concentration on the dendrite coherency in the alloy. The grain sizes of the alloys were evaluated using the parameters at coherency point.

Journal ArticleDOI
TL;DR: In this paper, a modified route involving hot extrusion followed by two pilgering operations with an intermediate annealing step was examined in detail, with the main aim of this work being to produce a microstructure and texture which are known to yield a lower irradiation growth.
Abstract: Microstructural changes occurring during the fabrication of Zr-2.5 pct Nb alloy pressure tubes by a modified route, involving hot extrusion followed by two pilgering operations with an intermediate annealing step, have been examined in detail. In the conventional fabrication route, the hot extrusion step is followed by a single cold drawing operation in which the cold work to the extent of 25 pct is imparted to the material for achieving the required mechanical properties. Tensile properties obtained at each stage of fabrication have been evaluated and compared between the two processes. The main aim of this work has been to produce a microstructure and texture which are known to yield a lower irradiation growth. Additionally, suitable annealing conditions have been optimized for the intermediate annealing which annihilates the cold work introduced by the first cold pilgering operation without disturbing the two-phase elongated microstructure. This elongated α+ β I microstructure is required for obtaining the desired level of strength at 310 °C. The final microstructure and the crystallographic texture of the finished pressure tube have been compared with those reported for the conventionally processed material.

Journal ArticleDOI
TL;DR: In this paper, the Gibbs energy of the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases of the Fe-Mn system has been analyzed using dilatometry and electrical resistivity measurements, which are applied to the whole composition range where the transformation can be detected.
Abstract: A new experimental study ofA s andM s in the Fe-Mn system has been performed by using two complementary experimental techniques,viz., dilatometry and electrical resistivity measurements, which are applied to the whole composition range where the transformation can be detected,i.e., between 10 and 30 pct Mn. We used theA s andM s temperatures as input information in an analysis based on thermodynamic models for the Gibbs energy of the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. In these models, the magnetic contribution to Gibbs energy is accounted for, which allows us to study, by calculation, the influence of the entropy of magnetic ordering upon the relative stability of the phases. The picture of magnetic effects upon the fcc/hcp transformation that emerges from our work is as follows. At low Mn contents, the martensitic transformation temperatures are larger than the Neel temperature of the fcc phase, and bothA s andM s decrease linearly with increasing Mn. This encourages an extrapolation to zero Mn content, and we use that to critically discuss the available information on the fcc/hcp equilibrium temperature for Fe at atmospheric pressure. At sufficiently large Mn contents, we haveM s

Journal ArticleDOI
TL;DR: Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere as mentioned in this paper.
Abstract: Particulate TiC-reinforced aluminum composite specimens were processed by compacting a mixture of titanium, carbon, and aluminum powders into preforms that were infiltrated with molten aluminum and subsequently heated in a differential thermal analyzer to about 1573 K under argon atmosphere. The onset of formation of TiC particles began at about 1150 K by reaction of TiAl3 with Al4C3. Subsequent formation of TiC particles at higher temperatures to approx-imately 1265 K occurred by direct reaction of carbon with TiAl3. Above this temperature, the TiC particles coarsened with increasing temperature from an initial size of about 0.15μm. TiC particles were also produced in preforms that were not infiltrated; however, the presence of liquid aluminum in infiltrated specimens inhibited particle agglomeration and sintering. Infil-trated preforms could, therefore, serve as excellent "master alloys" for subsequent dilution in an aluminum melt and processing of metal-matrix composites (MMCs) reinforced with sub-micron TiC particulates.

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TL;DR: The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe was investigated using X-ray diffraction (XRD) and electron microscopic techniques as discussed by the authors.
Abstract: The structural evolution in mechanically alloyed binary aluminum-iron powder mixtures containing 1, 4, 7.3, 10.7, and 25 at. pct Fe was investigated using X-ray diffraction (XRD) and electron microscopic techniques. The constitution (number and identity of phases present), microstructure (crystal size, particle size), and transformation behavior of the powders on annealing were studied. The solid solubility of Fe in Al has been extended up to at least 4.5 at. pct, which is close to that observed using rapid solidification (RS) (4.4 at. pct), compared with the equilibrium value of 0.025 at. pct Fe at room temperature. Nanometer-sized grains were observed in as-milled crystalline powders in all compositions. Increasing the ball-to-powder weight ratio (BPR) resulted in a faster rate of decrease of crystal size. A fully amorphous phase was obtained in the Al-25 at. pct Fe composition, and a mixed amorphous phase plus solid solution of Fe in Al was developed in the Al-10.7 at. pct Fe alloy, agreeing well with the predictions made using the semiempirical Miedema model. Heat treatment of the mechanically alloyed powders containing the supersaturated solid solution or the amorphous phase resulted in the formation of the Al3Fe intermetallic in all but the Al-25 at. pct Fe powders. In the Al-25 at. pct Fe powder, formation of nanocrystalline Al5Fe2 was observed directly by milling. Electron microscope studies of the shock-consolidated mechanically alloyed Al-10.7 and 25 at. pct Fe powders indicated that nanometer-sized grains were retained after compaction.

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TL;DR: In this paper, the dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical properties studies, and the optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.
Abstract: Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.

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TL;DR: In this article, the Monte Carlo (MC) algorithm was modified to simulate curvature-driven grain growth, which results in an acceleration of the simulated grain growth and an early estimate of the grain growth exponent that is close to the theoretical value of 0.5.
Abstract: The Monte Carlo (MC) algorithm that currently exists in the literature for simulating curvature-driven grain growth has been modified. The modified algorithm results in an acceleration of the simulated grain growth and an early estimate of the grain growth exponent that is close to the theoretical value of 0.5. The upper limit of grain size distributions obtained with the new algorithm is significantly lower than that obtained with the old, because the new algorithm eliminates grain coalescence during grain growth. The log-normal function provides an excellent fit to the grain size distribution data obtained with the new algorithm, after taking into account the anisotropy in grain boundary energy.

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TL;DR: In this paper, a theoretical model is developed for the formation of a layered structure in which two phases form alternate layers that are oriented parallel to the interface, and the morphology of the transition interface is shown to depend upon the relative effects of nucleation and growth of the two phases.
Abstract: Two-phase growth in a peritectic system has been examined, and a theoretical model is developed for the formation of a layered structure in which two phases form alternate layers that are oriented parallel to the interface. The width of each layer and the spacing between layers have been shown to be inversely proportional to velocity, and the proportionality constants depend upon the nucleation temperatures for the two phases. The nucleation temperatures for the two phases are shown to play a critical role in the formation and stability of layered structures. The range of compositions over which layered structures can form is identified, and it is shown that convection effects in the liquid will destabilize the layered structure. Several key experimental studies have been identified to evaluate minimum undercoolings required for the nucleation of each of the two phases. The morphology of the transition interface is shown to depend upon the relative effects of nucleation and growth of the two phases.