Computational modeling of metal matrix composite materials—II. Isothermal stress-strain behavior

The mechanical behavior of particulate reinforced metal matrix composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically using the computational micromechanics approach. In this, the second in a series of four articles, the isothermal overall stress-strain behavior and its relation to microstructural deformation is examined in detail. The macroscopic strengthening effect of the reinforcement is quantified in terms of a hardness increment. As seen in the first article for microscale deformation, inhomogeneous and localized stress patterns develop in the microstructures. These are predominantly controlled by the positions of the reinforcing particles. Within the particles stress levels are high, indicating a load transfer from matrix to reinforcement. The higher straining that develops in the matrix grains, relative to the unreinforced polycrystal, causes matrix hardness advancement. Hydrostatic stress levels in the composite are enhanced by constraints on plastic flow imposed by the particles. Constrained plastic flow and matrix hardness advancement are seen as major composite strengthening mechanisms. The latter is sensitive to the strain hardening nature in the matrix alloy. To assess the effects of constraint more fully, simulations using external confining loads were performed. Both strengthening mechanisms depend strongly on reinforcement volume fraction and morphology. In addition, texture development and grain interaction influence the overall composite behavior. Failure mechanisms can be inferred from the microscale deformation and stress patterns. Intense strain localization and development of high stresses within particles and in the matrix close to the particle vertices indicate possible sites for fracture.     

Computational modeling of metal matrix composite materials—III. Comparisons with phenomenological models

The mechanical behavior of particulate reinforced metal matrix composites, in particular an SiC reinforced Al-3 wt% Cu model system, was analyzed numerically and analytically. In this, the third article in a series, the results of the computational micromechanics are compared with those of simpler and more approximate analytical/numerical models. The simplerapproaches considered use phenomenological theories of plasticity and power-law strain hardening. Models that predict overall composite behavior make use of a result, valid for incompressible materials in small strain, that both pure matrix material and composite harden with the same hardening exponent. Results of micromechanical simulations, with power-slip system hardening, show that in a very approximate sense over restricted strain regions, power-law slip hardening is preserved with the power-law exhonent tending to increase with volume fraction. The results of the computations presented in the previous article are compared with the predictions of one such analytical/numerical model, where the matrix hardening function is fitted to the unreinforced poyycrystal stress-strain response. This model employs the self-consistent method to quantify strengthening. There is good agreement between the computed and predicted results. Simulations are performed using existing reinforcement geometry but replacing the physically based crystal plasticity theory with the phenomenologically based J₂ flow theory. The results are in good qualitative agreement with those of the original crystal plasticity simulations at both the microscale and the macroscale. Deformation patterns in the J₂ flow theory composites are smoother and tend to be less localized than those in the crystal plasticity composites; however, these features depend strongly on volume fraction and morphology. The J₂ flow theory composites display power-law exponents whose dependence on overall strain, volume fraction and morphology are much more easily characterized than in the crystal plasticity case.     

Plastic deformation of the plastic matrix–hard inclusion composite system during high-temperature gas extrusion

The results of investigations of the production of abrasive rod or wire diamond tools by high-temperature gas extrusion are presented. The versions of preparation of an initial metallic workpiece filled with a mixture of abrasive diamond grains are considered. The forming of plastic materials with hard inclusions, which is caused by deformation redistribution in the volume and is accompanied by the formation of pores and discontinuities adjoined to the hard inclusions, is considered. The results obtained demonstrate the prospects of application of high-temperature gas extrusion for the production of diamond tools for various purposes.     

Oxidation resistance and strength of a molybdenum fiber–oxide matrix composite material

The oxidation kinetics of a composite material, which consists of an Al₂O₃–Al₅Y₃O₁₂ matrix and molybdenum fibers and has a high cracking resistance, is studied. The mass loss of the composite material during oxidation is shown to be several orders of magnitude lower than that of molybdenum. Oxidation in quiet air at 1250°C for several hours weakly changes the strength of the composite material at temperatures from room temperature to 1300°C. It is also shown that the strength of the composite material as a function of the oxide matrix composition (Al: Y ratio) changes nonmonotonically. The maximum strength shifts from the Al₂O₃–Al₅Y₃O₁₂ eutectic point toward garnet.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—II. Haz microstructure and strength evolution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part II the heat and material flow models presented in the first paper (Part I) are utilized for prediction of the HAZ subgrain structure and strength evolution following welding and subsequent natural ageing. The modelling is done on the basis of well established principles from thermodynamics, kinetic theory and simple dislocation mechanics. The models are validated by comparison with experimental data, and are illustrated by means of novel mechanism maps. These show the competition between the different process variables that contribute to microstructural changes and strength losses during friction welding of AlMgSi alloys and AlSiC metal matrix composites.     

On the tensile properties of a fiber reinforced titanium matrix composite—II. Influence of notches and holes

The effects of holes and notches on the ultimate tensile strength of a unidirectionally reinforced titanium matrix composite have been examined. During tensile loading, a narrow plastic strip forms ahead of the notch or hole prior to fracture, similar to that observed in thin sheets of ductile metals. Examination of the fibers following dissolution of the matrix indicates that essentially all the fibers within such a strip are broken prior to catastrophic fracture of the composite. The trends in notch-strength have been rationalized using a fracture mechanics-based model, treating the plastic strip as a bridged crack. The observations suggest that the bridging traction law appropriate to this class of composite is comprised of two parts. In the first, the majority of fibers are unbroken and the bridging stress corresponds to the unnotched tensile strength of the composite; in the second, the fibers are broken and the bridging stress is governed by the yield stress of the matrix, with some contribution derived from fiber pullout. This behavior has been modeled by a two-level rectilinear bridging law. The parameters characterizing the bridging law have been measured and used to predict the notch strength of the composite. A variation on this scheme in which the fracture resistance is characterized by an intrinsic toughness in combination with a rectilinear bridging traction law has also been considered and found to be consistent with the predictions based on the two-level traction law.     

A process model for friction welding of AlMgSi alloys and AlSiC metal matrix composites—I. Haz temperature and strain rate distribution

The present investigation is concerned with the development of an overall process model for the microstructure and strength evolution during continuous drive friction welding of AlMgSi alloys and AlSiC metal matrix composites. In Part I the different components of the model are outlined and analytical solutions presented which provide quantitative information about the HAZ temperature distribution for a wide range of operational conditions. Moreover, a general procedure for modelling the HAZ strain rate distribution has been developed by introducing a series of kinematically admissible velocity equations which describe the material flow fields in the radial, the rotational, and the axial direction, respectively. Calculations performed for both types of materials show that the effective strain rate may exceed 1000 s⁻¹ in positions close to the contact section due to the high rotational velocities involved. Application of the model for evaluation of the response of AlMgSi alloys and AlSiC metal matrix composites to the imposed heating and plastic deformation is described in an accompanying paper (Part II).     

Critical materials selection for 700℃clean and high-efficiency coal-fired boiler

One of the pathways for achieving the goal of increasing the efficiency of coal-fired power plants and reducing the emission of pollutants and CO₂ is by improving the steam temperature and pressure.But the increase of steam parameters depend on the development of new high-temperature metal materials.This paper presented the constructive suggestions on critical materials selection for China’s new generation of 700℃clean and high-efficiency coal-fired boiler by summarizing the research and development situation of critical materials for coal-fired boilers operating at 700℃/35 MPa of the Europe,Japan and USA.     

On the tensile properties of a fiber reinforced titanium matrix composite—I. Unnotched behavior

An investigation of the ultimate tensile strength and fracture strain of a fiber-reinforced Ti-matrix composite has been conducted. Comparisons have been made between experimental measurements and predictions of two micromechanical models: one assumes that the fibers behave independently of the matrix, i.e. as in a dry fiber bundle, and the other assumes frictional coupling between the fibers and the matrix, characterized by a constant interfacial sliding stress. To conduct such comparisons, a number of constituent properties have been measured, including the fiber strength distribution, the thermal residual stress and the interfacial sliding stress. In addition, the effects of gauge length on the tensile properties of the composite have been studied. The comparisons indicate that the model prediction based on frictional coupling provide a good representation of the experimental results. In constrast, predictions based on the dry fiber bundle approach strongly underestimate both the ultimate strength and the fracture strain and predict a gaunge length dependence that is inconsistent with the experiments.     

Effective interface model for design and tailoring of WC–Co microstructures

Interface structures are a key feature in developing modern composite material solutions with ever improved performance. We present a nano-microstructural modelling approach for the tungsten carbide (WC)–Co system which can include the interface structures of WC–Co and various other phases present in the microstructure, utilising a methodology which combines imaging-based and synthetically generated nano-microstructures into an effective interface model for predicting the behaviour and properties of the resulting composite material. The effective model comprises of a local model of the WC/Co interface interacting with a larger-scale model of the WC–Co microstructure. The results provide a linkage between the interface character of cemented carbide microstructures and their properties, for example with respect to compressive strength, fracture toughness and wear resistance. The methodology presents a multiscale formalism for carrying out performance and application-driven evaluation and tailoring of composite interfaces and mesostructures, carried out on the basis of the emerging engineering material properties.     

A potentiostatic double-step method for measuring hydrogen atom diffusion and trapping in metal electrodes—II. Experimental

A potentiostatic double-step technique was used to study hydrogen atom ingress into a planar iron electrode. The technique involves generating hydrogen atoms at a constant cathodic potential, then stepping the potential to a more positive value and recording the anodic current and charge associated with the removal of hydrogen atoms from the electrode. It was shown that a model for the diffusion and trapping of hydrogen atoms under the conditions imposed by this technique could quantitatively account for the experimental results. Accordingly, parameters such as the rate constant for trapping have been evaluated for iron (annealed and non-annealed) in aqueous H₂S and H₂S-free solutions. In H₂S solutions, equilibrium is established between the adsorbed hydrogen layer and the hydrogen atoms just below the iron surface, so that the hydrogen flux is controlled solely by diffusion and trapping in the metal. In H₂S-free solutions, the rate of hydrogen ingress in iron is controlled by the rate of transfer of hydrogen atoms across the surface into the metal.     

A potentiostatic double-step method for measuring hydrogen atom diffusion and trapping in metal electrodes—I. Theory

Hydrogen atom ingress into a planar metal electrode can be measured using a potentiostatic method. Hydrogen atoms are generated during a defined period at a constant cathodic potential. The potential is then stepped to a more positive value at which the hydrogen atoms are re-oxidised, giving a transient anodic current corresponding to removal of the hydrogen atoms from the electrode. This paper gives the theory of the method. Fick's law for diffusion of hydrogen atoms in the metal is modified by allowing for trapping of some of the hydrogen. The trapping process is assumed to be of first order and irreversible, with no significant saturation of the traps. The ingress problem has been formulated and solved for two distinct cases:

• 1.(a) equilibrium ingress, giving a constant hydrogen atom concentration at the surface with the flux being controlled solely by diffusion and trapping in the metal; and
• 2.(b) constant ingress flux through the metal surface.

These cases have been solved to give explicit expressions, as functions of charging time, for the concentration profiles during the ingress and egress steps, the anodic current transients, and the charge densities corresponding to ingress and egress and trapped hydrogen.     

Chemically induced reduction: A viable process for synthesizing γ-TiAl based intermetallic matrix composite powders containing nanocrystalline TiC

A chemically induced reduction process has been developed for synthesizing intermetallic matrix composites (IMCs) consisting of titanium aluminide and titanium carbide. The process involves the reduction of metal chlorides (TiCl₄ and AlCl₃) with metallic lithium in polar organic solvents such as acetonitrile (MeCN) and tetrahydrofuran (THF) to form a colloidal precursor. The as-prepared precursors have been either directly heat treated in ultra-high-purity argon (UHP-Ar) or pretreated in hydrogen (H₂) followed by further heat treatment in UHP-Ar. The powders have been characterized primarily using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Results of the structural analyses conducted on the heat-treated precursors derived using MeCN as a solvent indicate the formation of either single-phase titanium carbide (TiC) or a composite mixture of γ-TiAl and nanocrystalline TiC, depending on the heat-treatment conditions. The formation of TiC is related to the strong interaction between TiCl₄ and the polar organic solvents, resulting in the formation of adducts which contain primary Ti-C linkages. Pretreatment of the precursors derived using MeCN as a solvent in H₂ promotes the removal of carbon and results in the formation of the composite mixture of γ-TiAl and TiC after subsequent Ar treatment at 1200 °C. At this stage, washing the pretreated powders in water helps to minimize and even eliminate any impurity phases to a large extent, leaving behind phase-pure composites containing γ-TiAl and TiC after the final Ar treatment. However, extended pretreatment in H₂ appears to be ineffective toward removal of additional carbon and leads to formation of hydride-phase impurities. On the other hand, the reductive reaction conducted using THF as a solvent results in minimizing the amount of carbon while inducing the formation of γ-TiAl during direct Ar treatment of the precursors. This is because of the weaker interaction between TiCl₄ and THF. Transmission electron microscopy was used to characterize the size distribution of the constituent phases. The analysis shows that the composite synthesized using these chemical approaches consist of discrete nanocrystalline TiC particles (<20 nm) that are uniformly distributed intermixed with submicron sized γ-TiAl (0.1 to 0.2 μm). Thus, the new chemical process proposed in this study demonstrates the potential for synthesizing in situ composites containing fine distribution of γ-TiAl and nanocrystalline TiC. Such composites could potentially exhibit unique mechanical properties and deformation behavior useful for high-temperature structural applications.     

Riction properties of composite materials of the system TiN - Ain. II. Effect of oxide films on friction and wear of a ceramic material — Steel system

Friction and wear are studied for materials of the system TiN — AlN preliminary oxidized at 800–1100°C. It was established that thin oxide films containing Al₂TiO₅ and α-Al₂O₃, that promote a decrease in frictional wear, form on the surface of composite materials of the system TiN — AlN. Our assumptions are confirmed that the improvement in tribological properties of TiN — AlN composites is caused by forming oxide screening layers that prevent direct contact between the ceramics and steel counter-body. At high rates (V=16 m/sec) and pressure (P=2.0 MPa) the oxide films form more rapidly. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(411), pp. 121–124, January–February, 2000.     

Microstructural design of cellular materials—I: honeycomb beams and plates

Performance indices for materials describe the mechanical efficiency of a component under a given mode of loading: the higher the performance index, the lower the mass of the component for a given mechanical requirement. Material selection charts offer a graphical means of comparing performance indices for a wide range of materials. The performance indices are first described. Micromechanical models for the behaviour of cellular materials are then used to suggest novel microstructural designs for cellular materials with improved performance. Model materials with two of the microstructures, honeycomb beams and plates, have been fabricated and tested. The results of the tests indicate that the new microstructures have higher values of some performance indices than those of the solids from which they are made.     

Characterisation of sintering of alumina matrix–stainless steel dispersion composite and interaction between chromium,carbon and alumina during powder metallurgy process

Abstract

This paper aims to study the sintering of 316L stainless steel and alumina composites. Compositions range from 0 to 100 vol.-% steel, and the experimental procedures involve density and microstructure analysis of the samples, as well as dilatometric measurements. In this study, it is shown that reducing atmosphere debinding can lead to carbon residues. These have a negative effect on alumina densification by delaying the sintering onset. For metal–ceramic composites, densification is modified by a complex interaction involving carbon (which lowers alumina density), chromium oxide (which is documented in literature to diminish alumina densification) and stainless steel phase. Chromium carbide formation is possible for some experimental conditions (1–30% stainless steel and hydrogenated argon debinding); this mechanism, locking both carbon and chromium outside alumina phase, leads to higher sintered densities.