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.     

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.     

Alternative length scales for polycrystalline materials—II. cluster morphology

In a preceding paper we have proposed that a physically relevant length scale for polycrystalline materials is that associated with clusters of grains sharing special boundaries. Here we examine how that length scale changes with microstructure evolution under various external influences. We also discuss how the length scale may be reflected in constitutive relations for several examples of physical and mechanical properties of polycrystalline materials.     

Overview 12: Fundamentals of dendritic solidification—II development of sidebranch structure

Experiments have been carried out to study the development of the dendritic sidebranch structure in succinonitrile. Results obtained provide the first insights into such fundamental problems as the origin of sidebranch perturbations, the mechanism of sidebranch evolution, the influence of anisotropy in solid-liquid interfacial energy, the kinetics of isothermal coarsening, and, finally, the mechanism of dynamic sidebranch coarsening during solidification. All of the problems addressed have applications to the interpretation of microstructures arising from both conventional and rapid solidification processes.     

Modelling of texture evolution for materials of hexagonal symmetry—II. application to zirconium and titanium α or near α alloys

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.     

Cyclic deformation of ferritic steel—II. Stage II crack propagation

The crack propagation process is dominated by the formation of ductile fatigue striations and the classical “one load cycle per striation” concept is only valid in a narrow crack propagation rate interval. However, over 4 orders of magnitudes in crack propagation rates the striation rates is independent of the ΔK, which implies that the number of cycles necessary to form one striation probably is greater than one. The similarities between the fatigue damage processes in the cyclic plastic zone and in plastic strain controlled specimens are documented in detail. The micromechanisms of crack propagation is related to a local plastic collapse of the dislocation substructure at the crack tip. A model for fatigue crack propagation rate predictions has been developed. The model is a refinement of the existing accumulated damage, LCF-models and the most important parameters taken into account are: the constant striation spacing, a realistic dynamic cyclic yield stress, the Coffin-Manson constants, a threshold plastic strain and a constant which coincide with the average grain size.     

Pair potential model of intermetallic phases—II

Using data based on the properties of pure components and a model of intermetallic phases developed in a previous paper (I), lattice parameters have been predicted for cubic structures and heteroelectronic phases composed of mixed electronic configuration components. Contrary to the quantitative agreement found for the case of homoelectronic components (I), the predicted lattice parameters deviated from the observed, but systematically. These deviations correlated to component type, periodic table group number and principal quantum number and were independent of crystal stuctures. This latter result led to a prediction of the relative stability of the A15 and L1₂ structures. The test of this prediction showed agreement for 73 out of 80 phases. These results plus the agreement between prediction and experiment for the lattice parameters (found in I) and energies of formation of homolectronic phases having these structures (developed in the present paper) lead to the conclusion that the model correctly describes the effect of the electrochemical factor on the lattice parameter and energy of formation of intermetallic phases. Also, it is concluded that deviations between model predicted and experimental values of the lattice parameter and energy of formation correspond to real physical effects. Further, it is concluded that the model may be used to describe the relative stability of competing cubic crystal structures.     

A Study of Scrap Heating by Burners: Part II—Numerical Modeling

A computational fluid dynamics code was developed to model the heating of a bed of porous steel scrap by combustion gases from a burner. The code accounted for nonuniform void fraction in the bed; turbulent, non-Darcian flow, heat transfer from the gas to the scrap; and radiation. The measured bed porosity values were used in the code. Because steel scrap pieces are very irregular in shape and size, the effective particle diameter was fitted to measurements made in a 1-m³ capacity furnace, as reported in part I. It was found that the lower porosity of the scrap was the most beneficial in increasing the efficiency of heat transfer to the scrap bed because the interfacial area is larger. The effect of particle size was much smaller. It was found that the configurations that increased the residence time or path length of the gases increased the efficiency. The measured porosity of the bed approached unity at the walls, so this provided an easy path for the gas to short-circuit the bed, which limited the effectiveness of decreasing the porosity to increase heat-transfer efficiency. Similarly, simulations of nonuniform scrap distributions reduced efficiency of heat transfer due to short circuiting. The implications of the findings for industrial operations are discussed.     

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.     

On a stochastic theory of grain growth—II

The stochastic theory of grain growth proposed previously [C. S. Pande, Acta metall.35, 2671 (1987)] is developed further in this paper. This theory leads to a Fokker-Planck equation for the grain size distribution. Two different “deterministic” terms for this equation, one proposed by Hillert and other by Pande, are compared. It is found that contrary to expectations Hillert's determistic term even with the addition of a “diffusion” like term does not lead to a grain size distribution which could be in agreement with experiments. The deterministic term proposed by Pande, leads to a grain size distribution which is similar to lognormal distribution, and closer to experimental results.     

Interpretation of some metallic phase diagrams—II. Entropy of mixing

A new expression is obtained for the entropy of mixing of two or more metals by decoupling the volume-dependent variations and the pairwise correlations between ‘atoms’ in the Fisher expansion. The volume-dependent terms are the most important in aluminum-based alloys and depend very sensitively on the volume of mixing of these alloys, the volume expansion coefficient and isothermal compressibility of the pure metals involved.     

On constrained sintering—II. Comparison of constitutive models

Several authors have proposed constitutive equations to describe bodies that sinter under constraint. We provide a critical examination of these models and show that some of them imply a negative Poisson's ratio, in contradiction to experimental evidence.