A full rate-dependent constitutive theory for dynamic strain aging is developed based on two key ideas. The first idea is that both solute strengthening and forest strengthening must exist and must ...exhibit aging phenomena. The second idea is that a single physical aging mechanism, cross-core diffusion within a dislocation core, controls the aging of both the solute and forest strengthening mechanisms. All the material parameters in the model, apart from forest dislocation density evolution parameters, are derivable from atomistic-scale studies so that the theory contains essentially no adjustable parameters. The model predicts the steady-state stress/strain/strain-rate/temperature/concentration dependent material response for a variety of Al–Mg alloys, including negative strain-rate sensitivity, in qualitative and quantitative agreement with available experiments. The model also reveals the origin of non-additivity of solute and forest strengthening, and explains observed non-standard transient stress behavior in strain-rate jump tests.
A full thermal activation rate theory for dynamic strain aging is developed for the case where a single rate dependent strengthening mechanism controls dislocation motion in a material (e.g. solute ...diffusion). The analysis shows that negative strain-rate sensitivity (SRS) cannot be obtained within such a framework, a conclusion previously reached by Hähner Hähner P. Mater Sci Eng A 1996;207:208. However, the SRS can be greatly reduced over a range of strain rates, making the inverse behavior more accessible by other mechanisms. In addition, the aging mechanism naturally gives rise to an instantaneous positive SRS and stress relaxation behavior under strain-rate jump conditions, putting the concepts advanced by McCormick McCormick PG. Acta Metall 1988;36:3061; Estrin Y, McCormick PG. Acta Metall Mater 1991;39:2977 on a quantitative footing. The results here set the stage for subsequent work wherein consideration of multiple strengthening mechanisms (solute and forest hardening) operating together can predict negative SRS in quantitative agreement with data in Al–Mg alloys.
•We study the effective properties of composites with matrix and inclusion properties sampled from distributions.•Fluctuations of the elastic modulus of phases about given mean lead to the reduction ...of the mean composite elastic modulus.•Fluctuations of the matrix yield stress about given mean lead to the reduction of the mean yield stress of the composite.•The strain hardening of the composite decreases as fluctuations of the matrix strain hardening increase.
This article presents a numerical study of the mechanical behavior of particulate composites with stochastic composition. Two types of such materials are considered: composites with homogeneous elastic–plastic matrix and randomly distributed inclusions of stiffness sampled from a distribution function, and composites with matrix having spatially varying elastic–plastic material parameters with no inclusions as well as with randomly distributed identical inclusions. We observe that the presence of fluctuations in either inclusions or matrix material properties leads to smaller effective modulus, smaller strain hardening and a reduction of the yield stress of the composite. Fluctuations of the yield stress of the matrix leads to a significant reduction of the mean yield stress of the composite. Fluctuations of the elastic modulus and of the strain hardening are associated with the reduction of the mean of the distributions of elastic modulus and strain hardening of the composite. For the range of parameters considered, fluctuations lead to maximum principal stress fields with narrow distribution of values, which implies enhanced resistance to damage initiation. Increasing the variance of the distribution functions from which local material properties are sampled, while keeping the mean constant, renders these effects more pronounced. This study is motivated by the growing interest in additive manufacturing technologies which open new possibilities for designing composite materials.
This article proposes an efficient method for solving mechanics boundary value problems formulated for domains with multiscale self-similar microstructure. In particular, composite materials for ...which one of the phases has a fractal-like structure with scale cut-offs are considered. The boundary value problems are solved using a finite element procedure with enriched shape functions that incorporate information about the geometric complexity. The use of these shape functions makes possible the definition of a unique, parametrically defined model from which the solution for configurations with an arbitrary number of scales can be derived. The proposed method is primarily useful for structures with a large number of self-similar scales for which using the usual finite element method would be too expensive. In order to exemplify the method, a 2D composite with fractal microstructure is considered and several boundary value problems are solved.
Curcumin, an important bioactive compound and natural photosensitizer, which exhibits various biological properties, was loaded into inorganic carriers MgAl/ZnAl layered double hydroxides (LDH). The ...supporting matrices were obtained by co-precipitation, and a surfactant was used for including the organic drug. The structural, morphological, and spectral properties of the obtained composites were followed by X-ray diffraction, FTIR, SEM, UV–Vis, and fluorescence spectroscopy in solid state and in different solvents. The prepared formulations, the curcumin-loaded carriers, were introduced in alcohol-aqueous mixtures and the effect of solvent on the spectral properties of the formulations was investigated. Interactions between the keto-enol group of the pigment and the metals in LDHs laminates as well as inter-molecular bonds between alcohols and curcumin were evidenced. As a consequence, it was concluded that the composition of the carrier as well as the nature of the solvent determine the structural changes of the formulations. MgAl-LDH in powder has better spectral characteristics in the low-energy range (UV–Vis and fluorescence) than ZnAl-LDH. Their spectra are altered when methanol and water are present in the solvent mixture. The released curcumin concentration is in the range for photosensitization applications. The results have potential utilizations in many fields such as food, pharmaceutical, and biology.
The development of a new test chip is presented, containing structures for the direct measurement of stress in metallic interconnect layers associated with silicon integrated circuit technology. The ...rotation of the structures provides a simple method of differentiating between tensile and compressive stress. This test chip design has been used to fabricate working structures allowing the study of stresses in aluminum layers before and after sample sintering. The results are presented together with the design, fabrication, and measurement considerations that have arisen during the research. The problems experienced in removing the sacrificial layer material, necessary to release the structures, are discussed along with potential solutions. The sensor structure is suitable for fabrication within a CMOS facility and its inherent scalability makes it potentially suitable for in-line testing of state-of-the-art processes.
A method is developed by which the field generated by a source (measured or numerically evaluated) is decomposed in a series of singular self-equilibrating linear elastic field components. These ...elementary sources form an elastic equivalent of the real source, the set of their magnitudes representing a 'spectral decomposition' of the investigated field. The decomposition is performed using path-independent interaction integrals computed through the field of interest, far from the source. The method may be used to investigate internal field sources, such as dislocations, as well as sources of field perturbation, such as crack tips or wedges. The method is applied to the analysis of the core structure of an edge dislocation in aluminium. The dislocation is represented in an atomistic simulation and a model core is sought in the form of a series of multipoles or elementary linear elastic field sources. The field of the dislocation is composed from the Volterra solution and higher-order components induced by the nonlinear behaviour of the material in the core region. The magnitude of the component elementary sources is determined from the numerically evaluated field far from the core. This expansion characterizes the core structure. Its variation associated with the core deformation as the dislocation is loaded against the Peierls barrier is investigated.
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