Effect Of Microstructure Size Research Articles

The Effect of Globular Microstructure Size on the Mechanical Properties in Reheating Process of Aluminum Alloys

One of the important steps in semi-solid forming is the process of reheating raw materials to the semi-solid state. This process is not only necessary to achieve the required semi-solid state of the billet, but also to control the microstructure of the billet. In the reheating process, the globule size is determined by the holding time of the final reheating step. Therefore, some experiments to investigate the relationship between the mechanical properties and the holding time in the last heating step were performed. The alloys used in this experiment were 357, 319, and A390 alloys. The experiments of reheating were performed using an induction heating system with a capacity of 50 kW. This article shows the evolution of the microstructure according to the holding time of the last reheating stage. Furthermore, to evaluate the effect of globule size as determined by holding time of the final reheating step, uniaxial tension tests were performed. The stress-strain curves were plotted according to the holding time, and a relationship between the microstructure and the flow stress of semi-solid material was formulated.

Failure Analysis of Miniature Solder Specimen

The effects of specimen geometry size on the behavior of 63Sn-37Pb solder are investigated both experimentally in the laboratory and analytically with finite-element simulations. The simulations are achieved by developing a constitutive model for solder which couples viscoplasticity with a unified damage theory. The unified damage theory is characterized by a damage surface in strain space which separates fatigue damage from inelastic damage. The damage evolution equations are derived within the framework of irreversible thermodynamics. A series of uniaxial tension, tensile creep, and strain-controlled fatigue experiments are performed to obtain material parameters for the solder damage model. The solder damage model is then implemented into a finite element code and used to simulate a uniaxial tension test on a miniature specimen and on a standard ASTM specimen (ASTM Standards, 1999, “Tension Testing of Metallic Materials,” ASTM E8-78). Predictions from these simulations are then compared with each other and with experimental results in order to examine microstructure size effects.

Multi‐scale constitutive modelling of heterogeneous materials with a gradient‐enhanced computational homogenization scheme

AbstractA gradient‐enhanced computational homogenization procedure, that allows for the modelling of microstructural size effects, is proposed within a general non‐linear framework. In this approach the macroscopic deformation gradient tensor and its gradient are imposed on a microstructural representative volume element (RVE). This enables us to incorporate the microstructural size and to account for non‐uniform macroscopic deformation fields within the microstructural cell. Every microstructural constituent is modelled as a classical continuum and the RVE problem is formulated in terms of standard equilibrium and boundary conditions. From the solution of the microstructural boundary value problem, the macroscopic stress tensor and the higher‐order stress tensor are derived based on an extension of the Hill–Mandel condition. This automatically delivers the microstructurally based constitutive response of the higher‐order macro continuum and deals with the microstructural size in a natural way. Several examples illustrate the approach, particularly the microstructural size effects. Copyright © 2002 John Wiley & Sons, Ltd.

Macroscopic modelling of multiperiodic composites

By a multiperiodic composite we mean a composite solid in which all constituents are periodically distributed in a matrix but a representative element (unit cell) may not exist. The aim of this Note is to propose a nonasymptotic approach to the formation of averaged (macroscopic) models of multiperiodic composites. The approach is based on the concept of tolerance averaging, which in [2] was applied to the modelling of periodic composites. The derived model, in contrast to homogenization, describes the effect of microstructure size on the overall solid behaviour and yields necessary conditions for the physical correctness of solutions to special problems. To cite this article: C. Woźniak, C. R. Mecanique 330 (2002) 267–272.

Microstructural size effects in high-strength high-conductivity Cu-Cr-Nb alloys

Microstructural refinement to further improve the strength and stability of high-strength high-conductivity Cu-Cr-Nb alloys was attained by mechanical milling (MM). Mechanically milled Cu-4Cr-2Nb and Cu-8Cr-2Nb exhibited an increase in hot-pressed Vickers hardness of 122 and 96 pct, respectively. Mechanical milling produced a corresponding decrease in electrical conductivity of ∼33 pct for both alloys. The increase in hardness was more due to Cu grain-size refinement than to second-phase particle-size refinement. The drop in conductivity was due to second-phase particle-size refinement, which both increased particle/matrix interfacial area and solute solubility. Mechanically processed Cu-4Cr-2Nb displayed an enhanced thermal stability. Hot-pressed 4-hour milled Cu-4Cr-2Nb experienced a 30 pct increase in conductivity with only a 22 pct drop in hardness when annealed at 1273 K for 50 hours. Such changes were largely due to an increase in dispersed-particle size (i.e., a decrease in solute and interfacial electron scattering) and Cu grain size (reduced Hall-Petch effect), respectively. The optimum hardness and conductivity combination was found in 4-hour milled and hot-pressed Cu-4Cr-2Nb material.

Fracture and erosion of CVD diamond

Abstract The fracture and erosion behaviour of free-standing specimens of CVD diamond is investigated in this paper. Surface erosion through high velocity impact with quartz particles has been studied and measured, and a mechanism for the process is discussed. The effect of microstructure and grain size is considered, and anisotropy in the observed fracture is related to the different crystalgraphic orientation of specimens. Importance is placed on the relative size of impinging particle to the grain size, since the response of the surface to damage can range from that observed in single crystal diamond to that expected of a bulk polycrystal.

On the description of dynamic behavior for microperiodic solids

A new micromechanical approach to the description of dynamic problems for periodic composite materials is outlined. We can mention two characteristic features of the proposed approach. First, it takes into account the microstructural size effect on the dynamic behavior of a composite. Second, it has an adaptive form which enables us to evaluate the degree of introduced approximations and to carry out the analysis on various levels of accuracy. General considerations are restricted to linear elastic materials but the proposed modeling procedure can be applied to both elastic and inelastic composites in the framework of small and large deformation theories. An overview of related recent contributions closes the paper.

Microstructural Size and Alignment Effects on the Dielectric Response of Inhomogeneous Media

ABSTRACTThe dielectric response of inhomogeneous media presents a complex boundary problem dependent upon dielectric susceptibility and volume fraction of its components, as well as the physical size of the heterogeneities. In this paper, we have used the LOCALF method to examine the size dependent problem for cubic and columnar defects for ZrO2 dielectric films of varying microstructure size and volume fraction. Both the alignments of low dielectric components with respect to the applied field and their size can effectively modulate the composite response. Comparison of these results with the conventional effective medium approximation methods reinforces the need to include microstructural detail in the modeling approach.

Key reaction steps and active surface phase of vanadyl pyrophosphate for selective oxidation of butane

Among several crystalline V-P oxides, (VO) 2P 2O 7 was the most selective for maleic anhydride production from butane oxidation. This is because (VO) 2P 2O 7 was selective In the key step of the oxidation, i.e., the dehydrogenation of butane to butene. Effect of microstructure (shape and size of catalyst particle) was studied using (VO) 2P 2O 7 prepared with four different methods. Phenomenologically, the microstructure had a great influence on the selectivity especially at high conversions; a plate-like particle was more selective than a particle having rose-like shape. Over the former catalyst, the consecutive oxidation of product maleic anhydride was much slower, resulting in the high selectivity at high conversions. The surface P/V ratios of these (VO) 2P 2O 7 were determined to be close to that of the bulk (P/V = 1) by XPS with V-P-O glasses as standard materials, suggesting that the active sites are (V 4+=O)-O-V 4+ pair sites on (100) plane. By isotopic studies, e.g., butane + 18O 2, it was revealed that reduction-oxidation only near the surface was involved in the catalysis.

Visible Photoluminescence of Porous Si and Its Related Optical Properties

Visible photoluminescence (PL) characteristics of porous silicon (PS) layers have been studied in relation to photoconduction and optical absorption spectra. The PS layers are formed on p- and n-type Si wafers by electrochemical and photoelectrochemical anodization in HF solutions. Some intrinsic correlations are observed between the peak wavelength of PL spectra and the anodization parameters. Other macroscopic observations show that both the photoconductivities and the fundamental absorption edges of PS layers are present in the visible region. This evidence suggests that the optoelectronic properties of PS are strongly modulated by the quantum size effect in microstructures of the PS.

A theoretical study of micro-structural size effect on crack extension in composite materials

Effect of micro-structural size on the crack extension in composites is theoretically studied by the use of a simplified two-dimensional crack model in uni-directionally reinforced composites. The model assumes that the plastic deformation occurs in the matrix layer ahead of the main crack and that micro-crack initiation in the plastic region leads to the global fracture of the composites. The continuous dislocations method is introduced, and used for the analysis of the crack and plastic fields in composites. The micro-structural size effect is discussed in terms of the ratio of reinforcement thickness to the possible in situ crack size.