Effect of Cross-linking Density on Creep and Recovery Behavior in Epoxy-Based Shape Memory Polymers (SMEPs) for Structural Applications

Epoxy-based shape memory polymers (SMEPs) are gaining importance in the area of aerospace structures due to their high strength and stiffness which is a primary requirement for an SMEP in structural applications. The understanding of viscoelastic behavior of SMEPs is very essential to assess their shape memory effect. In the present work, three types of SMEPs with varying cross-linking densities were developed by curing an aromatic epoxy resin with aliphatic amines. Glass transition temperature (T _g) was measured for these SMEPs using advanced rheometric expansion system, and from the T _g measurements, a range of temperatures from glassy to rubbery regimes were chosen. At selected temperatures, creep-recovery tests were performed in order to evaluate the viscoelastic behavior of SMEPs and also to investigate the effect of temperature on creep-recovery. Further, a three-parameter viscoelastic model (Zener) was used to fit the data obtained from experiments. Model parameters like moduli of the springs and viscosity of the dashpot were evaluated by curve fitting. Results revealed that Zener model was well suited to describe the viscoelastic behavior of SMEPs as a function of test temperatures.     

Diffusion studies in the β (B2), β′ (bcc), and γ (fcc) Fe-Ni-Al alloys at 1000 °C

Diffusion studies were carried out in the Fe-Ni-Al system at 1000 °C with solid-solid diffusion couples assembled with β (B₂), β′ (bcc), and γ (fcc) single-phase alloys for the development of diffusion structures, diffusion paths, and for the determination of interdiffusion and intrinsic diffusion coefficients. The diffusion structures were examined by optical and scanning electron microscopy, and the concentration profiles were determined by electron microprobe analysis. Diffusion couples included several series of β vs γ and β′ vs γ diffusion couples characterized by a common terminal alloy bonded to several terminal alloys with varying compositions. The development of planar and nonplanar interfaces, as well as two-phase layers, as observed in various couples, were related to the diffusion paths. The interdiffusion fluxes of individual components were calculated directly from the experimental concentration profiles, and the diffusional interactions among components were examined in the light of zero-flux planes (ZFPs) and flux reversals, which were identified in several couples. Ternary interdiffusion coefficients ( (i, j = Al, Ni)), with Fe considered as the dependent concentration variable, were evaluated at composition points of the intersection of diffusion paths of single-phase couples and of multiphase couples that developed planar interfaces. The interdiffusion coefficients were the largest in magnitude for the β′ alloys, especially near the β/β′ miscibility gap, and decreased for the β and γ alloys. In the β and γ phases, the main interdiffusion coefficient for Al was larger than those for Ni and Fe. Also, Fe interdiffused faster than Ni in the Fe-rich β and β′ phases. The cross-interdiffusion coefficients ( and ) were negative in all three phases. In general, the coefficients were larger in magnitude than the coefficients; however, the magnitude of was greater than that of near the β/(β + γ) phase boundary on the ternary isotherm. In the β phase, the magnitude of (i, j=Al, Ni) coefficients increased over 1 to 2 orders of magnitude with a decrease in the Al concentration and increase in the Fe/Ni concentration ratio. Interdiffusion coefficients, extrapolated from the ternary coefficients for binary alloys, were consistent with those in literature. Intrinsic diffusion coefficients were also determined at selected compositions. In addition, tracer diffusion coefficients were estimated for the binary Fe-Al and Ni-Al alloys at selected compositions, from an extrapolation of ternary interdiffusion coefficients.     

The influence of the die and floating plug geometry on the drawing process of tubing

Nowadays, the development of tube drawing process with a floating plug technique has moved our limit up to the production of thick-walled tubes; production of which still belongs to marginal techniques which are different from drawing of standard tubes. The submitted paper is basically a survey of research in fundamental characteristics of tube drawing with a floating plug with the aim to summarize some important results from the theory, which have not yet been published in any English publication. The purpose of this survey is to fill the gap in English literature to promote transfer and exploitation of knowledge. Research in fundamental characteristics was further developed and conditions under which it is possible to draw thick-walled tubes are defined. Confirmed possibility of thick-walled tubes drawing with support provided by a floating plug having negative overlap is the result of theoretical study and practical verification, which enables further moving of drawing process limit.     

Interdiffusion in the Ni-Cr-Co-Mo system at 1300 °C

Interdiffusion was investigated with solid-solid diffusion couples in theα (fcc) region of the quaternary Ni-Cr-Co-Mo system at 1300 °C for the determination of diffusion paths and diffusional interactions among the components. The concentration profiles for a given couple exhibited a common cross-over composition, Y_c, which reflected the relative depths of diffusion in the terminal alloys. Interdiffusion fluxes were calculated directly from the concentration profiles, and the quaternary interdiffusion coefficients were calculated at selected compositions. Ni and Co exhibited uphill diffusion against their individual concentration gradients in a direction opposite to the interdiffusion of Cr. Quaternary diffusion paths were presented as a set of partial diffusion paths on the basis of relative concentration variables.     

A new analysis for the determination of ternary interdiffusion coefficients from a single diffusion couple

Concentration profiles that develop in a ternary diffusion couple during an isothermal anneal can be analyzed directly for average ternary interdiffusion coefficients. A new analysis is presented for the determination of average values for the main and cross-interdiffusion coefficients over selected regions in the diffusion zone from an integration of interdiffusion fluxes, which are calculated directly from experimental concentration profiles. The analysis is applied to selected isothermal diffusion couples investigated with α (fcc) Cu-Ni-Zn alloys at 775 °C, β (bcc) Fe-Ni-Al alloys at 1000 °C, and γ (fcc) Ni-Cr-Al alloys at 1100 °C. Average ternary interdiffusion coefficients treated as constants are calculated over composition ranges on either side of the Matano plane and examined for the diffusional interactions among the diffusing components. The ternary interdiffusion coefficients determined from the new analysis are observed to be consistent with those determined by the Boltzmann-Matano analysis at selected compositions in the diffusion zone. The ternary interdiffusion coefficients are also employed in analytical solutions based on error functions for the generation of concentration profiles for the selected diffusion couples. The generated profiles are a good representation of the experimental profiles including those exhibiting uphill diffusion or zero-flux plane (ZFP) development for the individual components. Uncertainties in the values of the interdiffusion coefficients calculated on the basis of the new analysis are found to be minimal.     

A model for predicting the transmission rate of malaria from serological data

A model is developed to estimate the duration for which malaria antibody levels in the blood remain high in a closed population. This estimate can be used to calculate the transmission rate within a region, in conjunction with the serological information contained in the population. The model is used on data obtained from a study of malaria in the Philippines and shows excellent agreement. It is subsequently utilised for predictions and seems to be an appropriate vehicle for this purpose.     

Diffusion in β2 Fe-Ni-Al alloys

Diffusion in the β(bcc) phase field of the Fe-Ni-Al system was investigated at 1004°C with solid-solid diffusion couples assembled with β₂ alloys of selected composition. Experimental diffusion paths were determined for all couples and interdiffusion coefficients calculated at composition points corresponding to intersections of diffusion paths and maxima and minima of concentration profiles. The dependence of interdiffusion coefficients on composition was most clearly presented in terms of the parameter Fe/(Fe + Ni). The diffusive interactions between aluminum and nickel as represented by the cross coefficients were either positive or negative depending on the ternary composition. The Fe/(Fe + Ni) ratio appeared to be a significant parameter since iron and nickel atoms behave differently in affecting the degree of ordering in nonstoichiometric (Fe, Ni)Al alloys with less than 50 at. pct aluminum.     

Multiphase diffusion in ternary Cu-Zn-Ni alloys

Isothermal, multiphase diffusion was studied in the Cu-Zn-Ni system at 775°C in order to investigate the development of time-dependent diffusion structures. Solid-solid couples set up with disks of a 46.5 Zn-31.5 Cu-22 Ni wt pct ternary alloy (bcc) sandwiched between disks of either pure nickel or copper (fcc) were diffusion-annealed for times of 2 hr to 2 days and studied metallographically and by electron microprobe analysis. The ternary alloy/nickel couples developed structures that exhibited complex, morphological evolution with time; single phase bands and highly orientation-dependent nonplanar morphologies and nonisolated precipitates observed initially in the diffusion zone gradually transformed to two-phase regions with isolated precipitates. Only minor structural changes in the diffusion zone were displayed by the ternary alloy/copper couples.     

Zero-flux planes and flux reversals in Cu−Ni−Zn diffusion couples

Concentration profiles of isothermal diffusion couples in binary as well as multicomponent systems can be analyzed directly for interdiffusion fluxes without the need for a prior knowledge of interdiffusion coefficients. Such an analysis is presented and applied for the calculation of interdiffusion fluxes of each component at various sections of several diffusion couples in the Cu−Ni−Zn system investigated at 775°C. A major outcome of these calculations is the identification of “zero-flux planes” for the individual components within the diffusion zones of ternary couples. At a zero-flux plane the interdiffusion flux of a component goes to zero and on either side of the plane occurs a change or reversal in the direction of the interdiffusion flux of the component. The formation as well as the number of zero-flux planes of the components is dictated by the terminal alloys of the diffusion comple. The compositions of zero-flux planes for Ni and Cu identified in several Cu−Ni−Zn couples are found to correspond to composition points of intersection of diffusion paths and isoactivity lines drawn through the terminal alloys of the couples on a ternary isotherm. C. W. KIM, formerly a graduate student at Purdue University