Microstructural design of cellular materials—II: Microsandwich foams

The linear elastic behaviour of foams is primarily controlled by bending of cell walls. The mechanical performance of foams can be improved by increasing the bending stiffness of the cell walls. Recently, our group has developedmicrosandwich foams in which the cell walls are microsandwich elements. The improved bending stiffness of the walls resulting from the sandwich microstructure increases their performance indices. Here, we describe a finite element analysis of the Young's moduli and Poisson's ratio of microsandwich foams. We then use the analysis to perform a parametric study of the way in which the performance of the microsandwich foams depends on microstructural parameters. Finally we compare the results of the analysis with data for three microsandwich foams. The analysis describes the data well.     

Thin film forced velocity cells and cellular dendrites—I. Experiments

A new, compreheensive set of experimental systematics for the alloys succinonitrile-salol (S-Sa), succinonitrile-acetone (S-A), and pivalic acid-ethanol(P-E) is summarized. The contribution deals synthetically with transients involving the Mullins and Serkerka instability together with Ostwald ripening, pattern selection mechanisms, cell root instabilities, steady state wavelength and amplitude systematics spanning near threshold cellular and dendritic forms, anisotropic effects, facetting and hysteresis and systematics of the cell to dendrite transition. The observations reveal that there are two main stable or nearly stable branches of non-planar cellular solutions, both significantly involving solid state diffusion and excluding single-valued local equilibrium spatial structures. The normal single-valued,non-local equilibrium branch which we designated asdynamical has a scale decreasing with increasing velocity. The other triple-valued, near-marginal branch which we designate asquasi-equilibrium has a scale increasing with increasing velocity.     

Microstructural evolution in ultra-low-carbon steel weldments—Part I: Controlled thermal cycling and continuous cooling transformation diagram of the weld metal

The transformation behavior and microstructural evolution of the as-deposited weld metal from an ultra-low-carbon (ULC) weldment were characterized by dilatometry, optical microscopy, transmission electron microscopy, and microhardness measurements. These results were used to construct a continuous cooling transformation (CCT) diagram for this weld metal. The major microconstituents observed in this ULC weldment were (in order of decreasing cooling rate) coarse autotempered martensite, fine lath martensite, lath ferrite, and degenerate lath ferrite. No polygonal ferrite was observed. These results were also used to develop criteria to differentiate between the two predominant microstructures in these ULC steels, lath martensite, and lath ferrite, which can look quite similar but have very different properties.     

The transformation hardening of steel surfaces by laser beams—I. Hypo-eutectoid steels

Surfaces can be treated with a scanning laser beam to modify their properties. The beam subjects the near-surface to a thermal cycle with an amplitude and shape which depends on the process variables: the beam size, energy, scan rate and on whether the laser is pulse or continuous, and the chemistry and metallurgy of the steel. Approximate solutions to the equations of heat flow are combined with kinetic models to predict the near-surface structure and hardness of plain carbon steels after laser treatment. The results are assembled into diagrams which show immediately the combination of process variables for a desired surface structure and the associated hardness profile.     

Electron microscopy study of Ni/Ni3Al diffusion-couple interface—I. Microstructural observation and microchemical analysis

The Ni/Ni₃Al diffusion-couple interface has been examined using the techniques of analytical electron microscopy and high-resolution electron microscopy. It is shown that the Ni-rich solid solution phase (γ phase) is transformed from the Ni₃Al-based intermetallic phase (γ′ phase) and that it extends into the γ′ phase during diffusion annealing. The orientations of the γ and γ′ phases remain the same. Coherency exists across the γ/γ′ phase boundary. The position of the boundary is not well defined but it is in an equilibrium condition in terms of composition.     

Synthesis of γ-TiAl by Reactive Spark Plasma Sintering of Cryomilled Ti and Al Powder Blend,Part I: Influence of Processing and Microstructural Evolution

To provide insight into the influence of an electric field on the kinetics of diffusion, fully lamellar γ-TiAl was processed by a rapid, two-stage, solid-state reactive sintering via spark plasma sintering (SPS) of a cryomilled Ti, Al powder blend. Cryomilling was implemented in the current study to attain a nanostructured grain size in the Ti and Al powder blend, and thereby provide insight into the influence of grain size on the underlying diffusion kinetics. Following a two-step process involving SPS at 873 K (600 °C) for 15 minutes and 1523 K (1250 °C) for 30 minutes, a fully lamellar TiAl alloy, with submicron lamellar spacing, was successfully obtained. Microstructural refinement in the Ti and Al powders during cryomilling led to an increase in solid-state diffusion, and the underlying mechanisms are discussed in detail.     

Modelling of texture evolution for materials of hexagonal symmetry—I. Application to zinc alloys

This work describes the texture and microstructure evolution during cold rolling of zinc alloys. The choice of the Taylor model and its applicability to the ZnCuTi alloy are being discussed. The use of this kind of model in the case of ZnCuTi alloys permits to predict, with a good agreement, the texture evolution by cold rolling up to 80% reduction. The conditions of occurrence of various texture components are discussed. The authors compare their results with those published in the literature and compare also their results with those obtained according to different models (Sachs, Taylor..). They show, in particular, that the hypotheses of the model have to be in accordance with the evolution of the microstructure on the one hand and that the critical stresses of the deformation systems have to be well known on the other hand.     

Interfacial structure and crystallographic studies of transformations in β′ and β CuZn alloys—I. Isothermal formation of α1 plates from β′

A TEM study was made of the structure of the broad faces and edges of α₁ plates formed in β′ CuZn alloys. Thickening kinetics of α₁ plates were also measured. α₁ plates were found to be fully coherent and also free of internal structure during the earliest observed stage of their growth. A single array of misfit dislocations bounding stacking faults, and also growth ledges, develop during the second growth stage and a second array of misfit dislocations appears during the third stage of growth. The inter-dislocation spacings in the two arrays are roughly 2 and 12 nm, respectively. Both interphase boundary structure and thickening kinetics are shown to be inconsistent with various shear mechanisms of growth but to fit comfortably into the framework provided by diffusion-controlled ledge-wise thickening. A ledge structure was also observed on α₁ plate edges, in agreement with deductions made by Simonen and Trivedi from their measurements of the lengthening kinetics of these plates.     

Cyclic grain boundary migration during high temperature fatigue—I. Microstructural observations

Experiments were conducted on high purity lead at room temperature using reverse bending and torsion fatigue at low cyclic frequencies (⩽ 1.50 Hz). Metallographie observations after testing show that there is a one-to-one correspondence between the markings from grain boundary migration and the number and pattern of cyclic loading, and this correspondence is maintained up to s>100 cycles. Grain boundary sliding occurs in each cycle in addition to the migration, and this leads to the development of broad triple point folds. If the strain amplitude is maintained constant, it is shown that the average distance migrated in each cycle increases as the imposed frequency is decreased. The distance migrated is often exceptionally large in the first cycle of testing, and there is often a similar large initial displacement if the test is interrupted for periods of time from 1 to 24 h and then continued. For large grain sizes ( ≳ 2000 μm), the migration markings may lead to a zig-zag pattern where the individual segments lie fairly close to 45° to the stress axis. A model is described which accounts for the one-to-one correspondence and which is consistent with a fine structure observed within the migration markings.     

Alternative length scales for polycrystalline materials—I. Microstructure evolution

It is a well-documented experimental observation that properties of grain boundaries depend on the atomic structure of the boundary. Yet constitutive relations for properties of polycrystalline materials containing a variety of grain boundaries currently do not take account of this boundary-to-boundary variability. Instead, a single-length scale—the average grain diameter—is utilized with the underlying assumption that all grain boundaries are the same. In this (I) and the following paper (II), we extend the accepted viewpoint to encompass a binary classification of grain boundaries based on their misorientation angle. The resultant new length scale is that associated with clusters of grains linked by grain boundaries sharing misorientations in the same category. This first paper focuses on how a model polycrystal is generated, the energetics of the model, and its evolution under various external influences.     

Optimization of parameters for cerium(Ⅲ) biosorption onto biowaste materials of animal and plant origin using 5-level Box-Behnken design: Equilibrium,kinetic, thermodynamic and regeneration studies

Response surface methodology(RSM) employing 5-level Box-Behnken design was used to optimize the biosorption of cerium(Ⅲ) onto biowaste materials of animal and plant origin viz. prawn carapace(PC) and corn style(CS). Various process parameters viz. pH(A: 3.0–9.0), biomass dosage(B: 0.05–0.35 g/L), initial metal concentration(C: 50–350 mg/L), contact time(D: 2–6 h) and temperature(E: 20–60 °C) were chosen for optimization. A log transformation was suggested by the Box-Cox plot in the present case. A low p-value of 0.0001 validated the significance of the model. Maximum Ce(Ⅲ) uptake of 218.3 mg/g for PC and 180.2 mg/g for CS was noted under optimum conditions. Among the equilibrium isotherms, Freundlich model was found to be the best fitted one suggesting a heterogeneous mode of biosorption on PC whereas Langmuir model showed the best fit suggesting homogeneous mode of cerium biosorption on CS. This was further confirmed by scanning electron microscopy(SEM). Kinetic studies showed better applicability of pseudo-first order model suggesting physisorption as phenomena underlying the process. Film-diffusion was suggested by the non-linearity of the Boyd plot. Thermodynamic studies showed that the process was endothermic and spontaneous. FTIR analysis confirmed a major involvement of the participation of amide, amines, ketones and primary alcohol groups during Ce(Ⅲ) biosorption. EDAX analysis confirmed the major participation of carbon group during Ce(Ⅲ) biosorption. This was the first report on parameter optimization of Ce(Ⅲ) biosorption onto biowaste materials using 5-level Box-Behnken experimental design which might be helpful for the recovery of Ce(Ⅲ) from aqueous environment.     

Monte-carlo simulations of interstitial diffusion and trapping—I. One type of traps and dislocations

The long-range tracer diffusion coefficient of interstitials their jump rate and their distribution among different sites in one-, two- and three-dimensional lattices have been determined by Monte-Carlo calculations. In one part of this study traps or antitraps were introduced with equilibrium site energies being lower or higher, respectively, and energies of the surrounding saddle points being higher or lower when compared with the normal sites. Already a small decrease of the saddle point energies around the trap sites, reduces the diffusion coefficient remarkably. For an overall constant saddle point energy the results could be compared with various trapping models, but for lower or higher energy barriers around the trap sites new analytical solutions for the site occupancy and the diffusivity had to be derived which are in agreement with the computer calculations. According to the theoretical results the free energy of a site is also determined by its accessibility where lower saddle points decrease the free energy and vice versa. In a second part of this study a site energy distribution was constructed which corresponds to the elastic interaction energy between an interstitial solute and an edge dislocation. Again the segregation of solute atoms in trap sites of the dislocation is strongly dependant on the distribution of saddle point energies and remarkable deviations from the treatment of Cottrell and Bilby occur if the saddle point energy is lowered in the same way as the energy of the equilibrium points.     

Dendritic solidification—I. Analysis of current theories and models

Recent advances in the theory of dendritic growth have been reviewed. These theories have attempted to obtain mathematically rigorous solutions to dendritic growth under steady state conditions, incorporating in them, with various degrees of success, essentially all the physical principles governing dendritic growth. Traditionally, these theories have relied on an “ad hoc” hypothesis requiring that the dendrite grow at the maximum possible growth rate. More recently, this hypothesis has been abandoned in favor of criteria which determine the conditions under which a dendrite tip will advance in a stable fashion into the liquid ahead of it. Simpler approaches to the problem are available, particularly for dendritic growth under an imposed thermal gradient. However, all the models relying on the simpler approach have been shown to be theoretically inconsistent and do not satisfactorily predict the limiting undercoolings at a dendrite tip for both very small and very large growth rates. The essential elements of any satisfactory quantitative model for dendritic growth have been summarized and a new model proposed. This new model will be shown to be theoretically consistent, at both the very low and very large growth rates, in the second part of this paper.