Showing papers in "Acta Metallurgica Et Materialia in 1995"
TL;DR: In this paper, a comprehensive model for solving the heat and solute diffusion equations during solidification that avoids tracking the liquid-solid interface is developed, where the bulk liquid and solid phases are treated as regular solutions and an order parameter (the phase field) is introduced to describe the interfacial region between them.
Abstract: A comprehensive model is developed for solving the heat and solute diffusion equations during solidification that avoids tracking the liquid—solid interface. The bulk liquid and solid phases are treated as regular solutions and an order parameter (the phase field) is introduced to describe the interfacial region between them. Two-dimensional computations are performed for ideal solutions and for dendritic growth into an isothermal and highly supersaturated liquid phase. The dependence upon various material and computational parameters, including the approach to conventional sharp interface theories, is investigated. Realistic growth patterns are obtained that include the development, coarsening, and coalescence of secondary and tertiary dendrite arms. Microsegregation patterns are examined and compared for different values of the solid diffusion coefficient.
653 citations
TL;DR: In this article, lead lanthanum zirconate titanate (PLZT) was loaded with compressive stress parallel to the polarization and the stress vs strain curve was recorded.
Abstract: Ferroelectric and ferroelastic switching cause ferroelectric ceramics to depolarize and deform when subjected to excessive electric field or stress. Switching is the source of the classic butterfly shaped strain vs electric field curves and the corresponding electric displacement vs electric field loops [1]. It is also the source of a stress—strain curve with linear elastic behavior at low stress, non-linear switching strain at intermediate stress, and linear elastic behavior at high stress [2, 3]. In this work, ceramic lead lanthanum zirconate titanate (PLZT) is polarized by loading with a strong electric field. The resulting strain and polarization hysteresis loops are recorded. The polarized sample is then loaded with compressive stress parallel to the polarization and the stress vs strain curve is recorded. The experimental results are modeled with a computer simulation of the ceramic microstructure. The polarization and strain for an individual grain are predicted from the imposed electric field and stress through a Preisach hysteresis model. The response of the bulk ceramic to applied loads is predicted by averaging the response of individual grains that are considered to be statistically random in orientation. The observed strain and electric displacement hysteresis loops and the nonlinear stress—strain curve for the polycrystalline ceramic are reproduced by the simulation.
651 citations
TL;DR: In this paper, the principles of fail-safe thermomechanical design, based on the damage mechanisms known to occur in these systems, are discussed, including delamination and crazing of brittle layers.
Abstract: Thin films and multilayers comprised of different classes of material are often used for various functional requirements. As these become relatively large in section and geometrically more complex, thermomechanical integrity is a major concern. It influences performance, yield and reliability. A methodology for thermomechanical design is needed that complements procedures used for circuit design. This article elaborates the principles of fail-safe thermomechanical design, based on the damage mechanisms known to occur in these systems. Among the important mechanisms are delamination and crazing of brittle layers. thermomechanical fatigue of metallic constituents and interface decohesion. The damage mechanisms are generally activated by residual stress, both thermal and ‘intrinsic’. The origins of these stresses are discussed, as well as stress redistribution effects that arise because of bending, discontinuities, etc. Emphasis is given to measurement methods which provide those data needed for implementation of the fail-safe design methodology.
407 citations
TL;DR: In this article, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique, and experimental findings indicate that TiB 2 crystallites alone do not nucleate α-Al.
Abstract: Despite the commercial importance of grain refinement and the volume of scientific studies on this topic, its mechanism is still unclear. There are several theories as to how and why commercial grain refiners (Al-Ti and Al-Ti-B) work, but careful analysis shows that no clear consensus has yet emerged. In the present study, the commercial grain refining practice of aluminium has been experimentally simulated by introducing synthetic TiB2 crystallites into melts by a specially developed technique. Experimental findings indicate that TiB2 crystallites alone do not nucleate α-Al. On the other hand, in the presence of dissolved Ti even below the peritectic level, an interfacial TiAl3 layer is formed at the TiB2/melt interface which subsequently nucleates the α-Al. The theoretical and practical implications of grain refinement phenomena are discussed in the light of the present experimental findings. A ‘duplex’ nucleation mechanism is proposed based on solute segregation to the substrate/melt interface.
405 citations
TL;DR: In this article, the possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystine materials.
Abstract: The possibility of a dislocation mechanism in the deformation process of nanocrystalline materials is reviewed and analyzed. The present theoretical calculation, by taking the anisotropic characteristic of crystallographic symmetry and different choices of critical shear strength into account, results in a reasonable limit in grain size for applying dislocation pile-up theory to nanocrystalline materials. The deviation from the Hall—Petch relationship is rationalized in terms of a small number dislocation pile-up mechanism. A composite model is proposed to evaluate the strength of nanocrystalline materials. It is shown that this model can be used for interpreting the various cases observed in Hall—Petch studies. An analytical expression for assessing the creep rate of nanocrystalline materials by a diffusion mechanism, including triple line diffusion, is derived. It is predicted that the creep rate due to triple line diffusion will exhibit a stronger grain size dependence than that due to grain boundary diffusion.
297 citations
TL;DR: In this article, the kinematic equations giving the velocity and acceleration of a ball in a vial in a planetary ball mill are given, the kinetic energy transferred at the collision event, the shock frequency, and the injected shock power are also calculated.
Abstract: Based on a kinematic modeling of the planetary ball mill, the kinematic equations giving the velocity and the acceleration of a ball in a vial in a planetary ball mill are given. The kinetic energy transferred at the collision event, the shock frequency, and the injected shock power are also calculated. The confrontation of the calculated to some experimental results documented in the material literature, show that neither the shock energy nor the shock frequency separately taken into account, govern the end product but only the injected shock power is responsible for the ball milled end product.
258 citations
TL;DR: The addition of Ca into Mg-(3-9)wt%Al alloys increased the hardness at the high temperatures up to 350°C as mentioned in this paper, probably owing to the precipitation of the Mg2 Ca phase in the alloys.
Abstract: The addition of Ca into Mg-(3–9)wt%Al alloys increased the hardness at the high temperatures up to 350°C. In particular, this was remarkable when the mass ratio of Ca to Al was higher than 0.8, probably owing to the precipitation of the Mg2 Ca phase in the alloys. Such alloys with the Ca/Al mass ratio higher than 0.8 showed excellent heat resistance at a moderate temperature. Therefore, they were considered to be the new promising light-weight heat-resistant alloys for interior automotive applications.
239 citations
TL;DR: In this article, the growth of the intermetallic compounds of the Cu and Ni systems were studied in thin films at temperatures from 513 to 673 K. The results showed that the Ni3Sn phase does not nucleate below 623 K in specimens with clean Ni/Sn interfaces, and the influence of the small grain size of the Ni thin films were studied by performing similar experiments with Cu or Ni single crystals.
Abstract: Fundamental investigations were carried out to examine a new interconnection technology which is based on the rapid formation of intermetallic compounds composed of a high melting component (e.g. Cu or Ni) and a low melting component (e.g. Sn) between two layers of the high melting component at temperature just above the melting point of Sn. This reaction is known as isothermal solidification. The growth of the intermetallic compounds of the CuSn and NiSn systems were studied in thin films at temperatures from 513 to 673 K. At the beginning of interdiffusion, the Sn-rich intermetallic compounds ν (Cu6Sn5) and Ni3Sn4 grow fastest with a non-parabolic time dependence. The Cu3Sn phase grows parabolically, however, not the Ni3Sn2 phase. The Ni3Sn phase does not nucleate below 623 K in specimens with clean Ni/Sn interfaces. For the first time, the influence of the small grain size of the Cu or Ni thin films were studied by performing similar experiments with Cu or Ni single crystals. In the CuSn system the interdiffusion coefficients for Cu3Sn obtained from the thin film experiments are twice those obtained from the single crystal experiments. In the NiSn system there are no differences between thin film and single crystal results.
233 citations
TL;DR: In this paper, a methodology is developed that enables fatigue life predictions to be made, based on a minimum number of experimental measurements, which relies on analysis of hysteresis loops.
Abstract: Fatigue in ceramic matrix composites typically occurs when matrix cracks are present. It proceeds by cyclic degradation of the sliding resistance of the interface. The basic mechanisms are discussed and a methodology is developed that enables fatigue life predictions to be made, based on a minimum number of experimental measurements. The methodology relies on analysis of hysteresis loops. Changes in modulus upon cyclic loading as well as the permanent strains are predicted, as well as the fatigue threshold and the S-N curve.
231 citations
TL;DR: A detailed analysis of superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis as discussed by the authors, where the role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined.
Abstract: Superplasticity in powder metallurgy aluminum alloys and composites has been reviewed through a detailed analysis. The stress-strain curves can be put into four categories: a classical well-behaved type, continuous strain hardening type, continuous strain softening type and a complex type. The origin of these different types of stress-strain curves is discussed. The microstructural features of the processed material and the role of strain have been reviewed. The role of increasing misorientation of low angle boundaries to high angle boundaries by lattice dislocation absorption is examined. Threshold stresses have been determined and analyzed. The parametric dependencies for superplastic flow in modified conventional aluminum alloys, mechanically alloyed alloys and aluminum alloy matrix composites is determined to elucidate the superplastic mechanism at high strain rates. The role of incipient melting has been analyzed. A stress exponent of 2, an activation energy equal to that for grain boundary diffusion and a grain size dependence of 2 generally describes superplastic flow in modified conventional aluminum alloys and mechanically alloyed alloys. The present results agree well with the predictions of grain boundary sliding models. This suggests that the mechanism of high strain rate superplasticity in the above-mentioned alloys is similar to conventional superplasticity. The shift of optimum superplastic strain rates to higher values is a consequence of microstructural refinement. The parametric dependencies for superplasticity in aluminum alloy matrix composites, however, is different. A true activation energy of 313 kJ mol−1 best describes the composites having SiC reinforcements. The role of shape of the reinforcement (particle or whisker) and processing history is addressed. The analysis suggests that the mechanism for superplasticity in composites is interface diffusion controlled grain boundary sliding.
203 citations
TL;DR: In this article, three-dimensional reconstructions of the atomic structure of a series of thermally aged Fe-Cr alloys are shown, and two methods for computer simulation of the decomposition process are described.
Abstract: A three-part series of papers is presented concerning the atomic scale analysis of spinodal decomposition in Fe-Cr alloys. This first part deals with the experimental techniques and computer simulations, the second part discusses the dynamics of early stage phase separation, and the third part describes the morphological and structural characterization of spinodal microstructures. In this first paper, three-dimensional reconstructions of the atomic structure of a series of thermally aged Fe-Cr alloys are shown. Two methods for computer simulation of the decomposition process are described. The first is an atomistic simulation based on the Monte Carlo algorithm and the second is a numerical solution to the Cahn—Hilliard—Cook theory. The three-dimensional atomic scale structures resulting from decomposition within the low temperature miscibility gap are reconstructed. It is shown that both models generate microstructures which are qualitatively similar to those observed experimentally.
TL;DR: In this article, tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1-86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10−4s−1 to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development.
Abstract: Tensile specimens of Type 316L stainless steel having grain sizes in the range 3.1–86.7 μm were deformed to 34% strain at temperatures 24, 400 and 700°C and strain rate 1 × 10−4s−1 to investigate the Hall-Petch (H-P) relationship, the nature of stress-strain curves and the substructure development. Upto ∼5% strain the H-P relationship exhibits bi-linearity whereas the single Hall-Petch relation is exhibited at larger strains. The presence of bi-linearity is explained by the back stress associated with the difference in the dislocation densities in the vicinity of grain boundary and in the grain interior. The log stress (σ)-log strain (e) plots depict three regimes and follow the relationship σ = Ken in each regime, but with varying magnitudes of the strength coefficient (K) and strain-hardening exponent (n).
TL;DR: In this article, the cyclic thermal response in multi-layered materials which comprise layers of fixed compositions of a metal and a ceramic, and a compositionally graded interface is analyzed.
Abstract: Elastopllastic analyses are presented for the cyclic thermal response in multi-layered materials which comprise layers of fixed compositions of a metal and a ceramic, and a compositionally graded interface. Analytical solutions for the characteristic temperature at which the onset of thermally induced plastic deformation occurs are derived for the layered composite. Solutions for the evolution of curvature and thermal strains, and for the initiation of plastic yielding are also obtained for different combinations of the geometry, physical properties and compositional gradation for both thermoelastic and thermoplastic deformation. Finite-element formulations incorporating continuous and smooth spatial variations in the composition and properties of the graded layer are used to simulate the evolution of thermal stresses, the accumulation of plastic strains, and the development of monotonic and cyclic plastic zones at the interfaces, edges and free surfaces of different layers during thermal cycling. Engineering diagrams detailing the effects of compositional gradients are also presented for optimizing thermal residual stresses, layer geometry, and plastic strain accumulation.
TL;DR: In this article, a kinematic model for kink band formation, propagation and band width broadening was proposed based on a high-resolution video camera, where the easy modes of deformation were identified and these formed the basis of a new kinematics model.
Abstract: Various stages of kink band formation, propagation and band width broadening were recorded by a high resolution video camera. Based on these observations, the easiest modes of deformation have been identified and these form the basis of a new kinematic model for kinking. Theoretical predictions for kink band orientation and compression strength under steady-state band broadening are made. The conditions at incipient kinking and the incipient kinking stress are investigated. The relevance of the incipient kinking stress and the band broadening stress are discussed.
TL;DR: In this paper, the average growth rates and misorientations between recrystallization nuclei (or grains) and neighbouring deformed matrix material have been studied for partially recrystized samples by the electron back scattering pattern (EBSP) technique in heavily cold rolled aluminium and copper.
Abstract: Average growth rates and misorientations between recrystallization nuclei (or grains) and neighbouring deformed matrix material have been studied for partially recrystallized samples by the electron back scattering pattern (EBSP) technique in heavily cold rolled aluminium and copper It was studied how the annealing time and the crystallographic orientation of nuclei/grains affects the growth rates and distribution of misorientations The two materials, aluminium and copper, develop a weak and a strong recrystallization cube texture respectively Information about effects of cube texture strength was therefore also obtained It was found that grains of cube orientation grow faster than grains of other orientations A wide distribution of misorientation relationships was observed to exist between the growing grains and the neighbouring deformed matrix, and this distribution was not significantly affected by the annealing time The faster growth of the cube oriented grains may be ascribed to a larger misorientation between cube grains and deformed matrix than that between other grains and the matrix
TL;DR: In this article, the Si microstructure of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix.
Abstract: Heterogeneous nucleation of solidification in melt spun Al-Si and Al-Si-P has been studied using differential scanning calorimetry, and transmission, scanning transmission and high resolution electron microscopies. The microstructures of the heat treated melt spun alloys all consist of an Al matrix, Al-Si eutectic distributed along the Al grain boundaries, and Si embedded in the Al matrix. The Si microstructure depends on the level of P: coarse faceted Si particles are nucleated by AlP particles in Al-Si containing 2 ppm P and Al-Si-P containing 35 ppm P whereas eutectic droplets of fine Si particles are nucleated by the surrounding Al matrix at a high undercooling in Al-Si containing 0.25 ppm P. The Si nucleation onset temperature remains approximately constant while the peak and end temperatures both decrease with increasing cooling rate, in agreement with classical nucleation theory. Kinetic analysis, using the spherical cap model gives contact angles of 10°, 43° and 10° for Si nucleation in low and high purity Al-Si and Al-Si-P respectively.
TL;DR: In this article, the influence of the misfit strain on two-phase microstructural evolution during coarsening is examined in a series of five low-volume-fraction Ni-Al-Mo alloys using TEM and SAXS.
Abstract: The influence of the sign and magnitude of the misfit strain on two-phase (γ—γ′) microstructural evolution during coarsening is examined in a series of five low-volume-fraction Ni-Al-Mo alloys using TEM and SAXS. The evolution of the symmetry-conserving, sphere-to-cube transition and the symmetry-breaking cube-to-cuboid transition with increasing particle size were quantified and the general trends were found to be in excellent agreement with two-dimensional calculations of particle shapes. Both types of particle shape transitions and particle alignment occur at smaller particle sizes the larger the magnitude of the misfit strain. A decrease in the coarsening rate constant was measured with increasing Mo content and was attributed to Mo diffusion and partitioning rather than the increasingly negative misfit strain of the alloys with increasing Mo content.
TL;DR: In this article, the authors considered the problem of a crack approaching a bimaterial interface and showed that the near-tip "driving force" for fracture is strongly influenced by whether the crack approaches the interfaces from the lower strength or the higher strength materials.
Abstract: The problem of a crack approaching a bimaterial interface is considered in this paper. Attention is focused on an interface between two elastoplastic solids whose elastic properties are identical and whose plastic properties are different. For the case of a crack approaching a bimaterial interface perpendicularly, it is shown by recourse to detailed finite element analyses that the near-tip “driving force” for fracture is strongly influenced by whether the crack approaches the interfaces from the lower strength or the higher strength materials. Specifically, it is demonstrated that the crack-tip is “shielded” from the remote loads when it approaches the interface from the weaker material, and that the effective J-integral at the crack tip is greater than the remote J when it approaches the interface from the stronger material. This plasticity effect determines whether a crack approaching the bimaterial interface continues to advance through the interface or is arrested before penetrating the interface. These theoretical findings are substantiated using controlled experiments of fatigue crack growth perpendicular to a ferrite—austenite bimaterial interface. The effect of the non-singular T-stress, acting parallel to the crack plane, on shielding and amplification of the stress fields is also discussed.
TL;DR: In this paper, a new numerical technique for investigating the failure of fiber reinforced composites is presented, which utilizes 3D lattice Green's functions to calculate load transfer from broken to unbroken fibers, and also includes the important effects of fiber/matrix sliding.
Abstract: A new powerful numerical technique for investigating the failure of fiber reinforced composites is presented. The technique utilizes 3D lattice Green's functions to calculate load transfer from broken to unbroken fibers, and also includes the important effects of fiber/matrix sliding. The inherent flexibility of the technique in adjusting the spatial extent of load transfer allows for the study of many aspects of real composite failure processes which have been unobtainable to date. Using this technique, composite reliability, the influence of manufacturing defects on performance, and the overall optimization of composite performance can all be investigated in detail. Initial results using this approach show that load transfer and the existence of spatially-staggered fiber breaks play an important role in determining strength and toughness of composites. Furthermore, the critical configurations of fiber breaks that initiate catastrophic failure are complicated 3D objects and any single spatial plane is composed mainly of sliding fibers rather than broken fibers, with a few strong fibers intact within the critical defect.
TL;DR: In this article, the authors show that dislocation loops can be initiated at loads well below those previously thought to represent elastic loading only, by considering the equilibrium of forces associated with tip, image and friction stresses acting on dislocations emitted from the indenter tip.
Abstract: Diamond indentation of a surface with a thin passive film require s loads an order of magnitude smaller for Ni〈100〉 crystals than for Fe-3 wt% Si〈100〉 crystals. The load bearing capacity of the Fe-3 wt% Si can be reduced by two orders of magnitude by removing the 1- nm thick native oxide film. These phenomena can be explained by considering the equilibrium of forces associated with tip, image and friction stresses acting on dislocations emitted from the indenter tip. The key ingredient to this model is the nucleation and growth of dislocation loops at loads of only tens of micronewtons. Three types of critical contact experiments demonstrate that dislocations can be initiated at loads well below those previously thought to represent elastic loading only.
TL;DR: In this paper, a microstructural and textural evolution during rolling was investigated in single crystals of Al, Cu and homogenous supersaturated All and the results showed that after a rolling degre of 30% the initial C-orientation (112)[11 1 ] of all three materials has rotated towards the so-called D-directional shear bands.
Abstract: Microstructural and textural evolution during rolling were investigated in (112)[11 1 ] single crystals of Al, Cu and homogenous supersaturated All.9wt%Cu. After a rolling degre of 30% the initial C-orientation (112)[11 1 ] of all three materials has rotated towards the so called D-orientation (4411)[1111 8 ]. While in the non-shear banding Al the D-orientation remains stable up to high rolling degrees, in the shear banding materials Cu and AlCu it rotates back to the initial C-orientation simultaneously with the formation of shear bands. This orientation change is explained by a rigid body rotation due to the special geometry of a deformation with unidirectional shear bands. With the onset of shear band formation also strong orientation scatterings about tthe transverse direction appear in the pole figures. These scatterings are located inside the shear bands as well as in their vicinity. They are due to the strong shear deformation and the resulting reaction stresses occurring in the shear bands and in their vicinity, respectively.
TL;DR: In this article, the evolution of two-dimensional shapes to equilibrium shapes is investigated for two kinetic mechanisms, surface diffusion and surface attachment limited kinetics, and qualitative differences are found that may be used in experiments for easy distinction among the two mechanisms and find topological changes not expected for the corresponding isotropic problems.
Abstract: The evolution of two-dimensional shapes to equilibrium shapes is investigated for two kinetic mechanisms, surface diffusion and surface attachment limited kinetics. Qualitative differences are found that may be used in experiments for easy distinction among the two mechanisms, and find topological changes not expected for the corresponding isotropic problems. We take advantage of the mathematical developments for surface evolution and equilibration problems when surface energy anisotropy is “crystalline”, so extreme that crystals are fully faceted. We confirm the prediction that with this anisotropy these problems are more easily solvable than for lesser anisotropies, and the techniques developed may even be useful for approximating isotropic problems.
TL;DR: In this paper, the fracture resistance in a lamellar TiAl-base alloy was studied as a function of lamellae spacing and colony size, and the results revealed that fracture toughness appears to depend upon the colony size in a more complex manner.
Abstract: The fracture resistance in a lamellar TiAl-base alloy was studied as a function of lamellae spacing and colony size. Fully-lamellar microstructures with several lamellae spacings and colony sizes were obtained by heat-treatment. Fracture resistance curves were generated by performing standard J-testing at ambient temperature, while the corresponding fracture and toughening mechanisms were identified by characterizing the crack-tip region and crack-wake ligaments using optical and scanning electron microscopies. Results of this study were compared with published data to elucidate the role of lamellae spacing and colony size in fracture of lamellar TiAl alloys. The comparison revealed that both the initiation toughness, KIC, and crack growth toughness. Ks of lamellar TiAl alloys increase with decreasing lamellae spacing, but depend upon the colony size in a more complex manner. In general, fracture toughness (KIC or Ks) increases with colony size, but may become independent of colony size when the crack-tip plastic zone is embedded within an individual colony, which occurs at ≈600 μm in the alloys used for present experiments. For colony sizes exceeding ≈600 μm, fracture toughness appears to be controlled by the lamellae spacing and the colony orientation through their influence on the size of shear ligaments formed between the main crack and the microcracks.
TL;DR: In this article, the evolution of texture and microstructure during cold rolling of different Ti and Zr alloys was modeled using a Taylor theory, and the results of the modelling, both for the Pole Figures (PF) and for the Orientation Density Function (ODF), agree well with experiment in the range from 0 to 80% reduction.
Abstract: This work describes the evolution of texture and microstructure during cold rolling of different Ti and Zr alloys. These alloys accommodate deformation with prismatic glide and with “secondary” mechanisms (gliding and/or twinning) which are different according to the type of alloys and may vary with deformation degree. We have modelled texture evolution during cold rolling of two Ti and Zr alloys, using a Taylor theory. The choice or relevance of the model variant (FC = full Constrained, RC = Relax Constrained) are discussed. In order to account for the changes in the secondary systems during deformation, we have decided to work by steps, since there are no defined hardening laws accepted for each system. The results of the modelling, both for the Pole Figures (PF) and for the Orientation Density Function (ODF), agree well with experiment in the range from 0 to 80% reduction. Therefore, a good knowledge of the microstructure evolution and of the deformation mechanisms is required.
TL;DR: In this article, a finite element model was used to simulate grain deformation and crystal orientation evolution in a small region at the interior of polycrystal, where the initial shapes and orientations of the grains and the crystal hardening relations were determined from polycrystalline aluminum samples.
Abstract: Detailed simulations of grain deformation and crystal orientation evolution in a small region at the interior of polycrystal have been performed. The finite element model accounts for deformation by crystallographic slip and for crystal lattice rotation with deformation. The initial shapes and orientations of the grains and the crystal hardening relations were determined from polycrystalline aluminum samples. The results clearly demonstrate the effects of grain interaction on local deformation and texture evolution. A comparison of the predicted lattice orientations with results from plane strain compression experiments shows good agreement for some of the grains and little agreement for others. Part of the discrepancy results from kinematic restrictions which were necessary to model the 3D microstructure with 2D models. The model shows very nonuniform strain fields and provides detailed information on grain interactions.
TL;DR: In this paper, the effects of reinforcement shape, size and spatial distribution on the overall stress-strain response of metal-matrix composites were analyzed using axisymmetric and plane strain unit cell formulations, where the metal matrix is characterized as an isotropically hardening elastic-plastic solid and the ceramic reinforcement is taken to be isotropic elastic.
Abstract: Finite element analyses of the overall stress-strain response of metal-matrix composites are carried out using axisymmetric and plane strain unit cell formulations. The metal matrix is characterized as an isotropically hardening elastic-plastic solid and the ceramic reinforcement is taken to be isotropic elastic. Perfect bonding between the matrix and the reinforcement is assumed. The focus is on the effects of reinforcement shape, size and spatial distribution. Under monotonic loading, the stress-carrying capacity in the plastic range increases in the following order for the reinforcement shapes considered: double-cone → sphere → truncated cylider → unit cylinder → whisker. The extent of the Bauschinger effect under reversed loading increases in the same order for particle reinforced composites. The effects of reinforcement size and distribution are analyzed by considering a plane strain model with two sizes of reinforcing particles. For certain distributions, it is found that the smaller family of particles plays virtually no role in affecting the stress-strain response. Thermal residual stresses are also considered and their effects are seen to persist far into the plastic range. The predicted plastic stress-strain behavior can be rationalized in terms of the evolution of matrix field quantities and, in particular, in terms of the effect of the constraint on plastic flow.
TL;DR: In this paper, the mechanical properties of a 6061-T6 aluminum alloy reinforced with a 20 vol.% fraction of alumina particles and of an unreinforced 6061T6 alloy are studied over a range of strain rates (10-4to 6 x 105s-1) using quasistatic compression, compression and torsion Kolsky bars, and high strain rate pressure-shear plate impact.
Abstract: The mechanical properties of a 6061-T6 aluminum alloy reinforced with a 20 vol.% fraction of alumina particles and of an unreinforced 6061-T6 alloy are studied over a range of strain rates (10-4to 6 x 105s-1) using quasistatic compression, compression and torsion Kolsky Bars, and high strain rate pressure-shear plate impact. At a given strain rate the composite displays increased strength but essentially the same strain hardening as the matrix. However, the composite displays a stronger rate-sensitivity than does the unreinforced alloy at high rates of deformation (>103s-1). The rate-sensitivity of the unreinforced alloy is shown to be largely the result of the imposed strain rate rather than of the rate history. For quasistatic deformations, a model proposed by Bao et al. (1991) describes the behavior of the composite fairly accurately given the behavior of the unreinforced alloy. This paper presents an extension of the model that is able to predict the dynamic behavior of the composite given the dynamic response of the monolithic alloy.
TL;DR: In this article, the problem of diffusional growth of a proeutectoid constituent in a ternary steel is considered, taking into account the interfacial diffusion of a slow-diffusing substitutional solute, under conditions which do not permit its long-range redistribution between parent and daughter phases.
Abstract: The problem of diffusional growth of a proeutectoid constituent in a ternary steel is considered, taking into account the interfacial diffusion of a slow-diffusing substitutional solute, under conditions which do not permit its long-range redistribution between parent and daughter phases. It is assumed that the faster diffusing interstitial solute (carbon) controls the rate of the transformation. The substitutional solute profile within (across) the interface is estimated as a function of interface velocity; the interstitial chemical potential difference is allowed to vary with, and balance, the solute drag due to the substitutional component. A transition to paraequilibrium is found at high interface velocities, and a variety of behaviour is predicted for intermediate states, depending on the relative rates of diffusion of the two solutes and their energetic interactions with each other and with the interphase boundary.
TL;DR: In this paper, the grain boundary diffusion was investigated in ultrafine grained copper with an initial grain size of 160 nm and it was revealed that its kinetics follow to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T m ).
Abstract: The static grain growth has been investigated in ultrafine grained copper with an initial grain size of 160 nm. It has been revealed that its kinetics follows to normal grain growth behaviour, but a grain growth starts at a relatively low temperature (0.32 T m ). Good fits with experimental data for several ultrafine grained metals have been obtained if the activation energy for grain boundary diffusion is assumed to be lower than for coarse grained materials, but increases during grain growth. It is suggested that this unusual behaviour of the activation energy is caused by the presence of non-equilibrium grain boundaries in ultrafine grained materials and their recovery during heating.
TL;DR: In this paper, a model describing the interplay of precipitation and dynamic strain ageing is proposed, and it is shown that as a result of precipitation in concentrated solid solutions the critical strains for the occurrence of the Portevin-Le Chatelier effect should exhibit an "inverse" behaviour with temperature and strain rate as compared to the behaviour observed in dilute solid solutions.
Abstract: A model describing the interplay of precipitation and dynamic strain ageing is proposed. Both homogeneous and heterogeneous precipitation is considered. It is shown that as a result of precipitation in concentrated solid solutions the critical strains for the occurrence of the Portevin—Le Chatelier effect should exhibit an “inverse” behaviour with temperature and strain rate as compared to the behaviour observed in dilute solid solutions.