Russell R. Mueller

Interlamellar failure at transcrystalline interfaces in glass/polypropylene composites

Abstract Transcrystalline microstructures are normally not observed at the interface between E-glass fibers and an isotactic polypropylene matrix, unless mechanical translation is applied to the fiber while it is in the supercooled polymer melt. We demonstrate here that transcrystallinity can form at the surface of E-glass fibers if appropriate nucleating agents are used to coat the fibers. These agents can nucleate either the α (monoclinic) or β (hexagonal) crystal forms of polypropylene. Single-fiber composite experiments were performed to assess the effect of transcrystallinity on matrix deformation. The preliminary results presented here reveal the occurrence of a previously unreported damage mechanism by which interlamellar fractures form preferentially at the interface well before any bulk matrix damage occurs. The density of this damage zone is higher in transcrystallinity of the β crystal form than of the α form, although it was found that in the α form the damage can propagate into the matrix. The occurrence of this damage mechanism suggests that toughness increases may potentially be obtained by careful design of the interfacial transcrystalline region in E-glass/polypropylene composites.

Morphology and Damage Mechanisms of the Transcrystalline Interphase in Polypropylene

Abstract By coating glass fibers with the appropriate nucleating agent, transcrystallinity can be generated in polypropylene/glass composities. Transcrystallinity can consist either of the alpha (monoclinic) or beta (hexagonal) crystal structure. Through the use of directional solidification, the transcrystalline morphology can be duplicated in polypropylene films on a level large enough for mechanical and morphological study. Permanganic etching and subsequent electron microscopy reveals that lamellar orientation in alpha transcrystallinity differs significantly from the beta form. Alpha transcrystallinity consists of lamellae which are edge-on relative to the polypropylene film thickness, while beta transcrystallinity consists of lamellae which are primarily flat-on. This difference in morphology results in significant variations in mechanical properties and damage mechanisms.

THE YOUNG'S MODULUS OF SILICA BEADS/EPOXY COMPOSITES : EXPERIMENTS AND SIMULATIONS

The effect of particulate volume fraction vp and diameter dp on the composite Young’s modulus Ec is studied both experimentally, using a silica bead/epoxy system, as well as with the help of computer simulations. The experimental and simulation results show that for a given particulate size, the overall Ec vs vp curve displays a concave upward shape and not a linear shape. This superlinear trend of the data implies that the average strain normalized to the applied strain λ=ep/ec transferred to the particulates increases with volume fraction. The above finding is explained in terms of a mean‐field picture, where a single particle interacts with an effective medium consisting of the remaining particles embedded in the matrix. As the modulus of the effective medium surrounding a reference particle increases with vp, the modulus mismatch between the reference particulate and the medium is consequently reduced. This leads to an overall increase in the normalized average strain λ transferred to each particulat...

Phase transformations at steel/IN626 clad interfaces

The microstructures of 4130 and 2.25Cr-1Mo steels clad to nickel base IN625 by welding and HIPing were examined by Analytical Electron Microscopy (AEM) and Secondary Ion Mass Spectroscopy (SIMS) to determine the interfacial microstructural characteristics which could affect their mechanical properties and corrosion resistance. The interface microstructures of the clads produced by the two methods were considerably different. The clad produced by welding was characterized by a low density of carbide precipitates confined to a very narrow region (∼1 μm) at the interface of ferrite and austenite. In addition, a thin region of untempered martensite was present at the interface which could affect its resistance to hydrogen embrittlement as well as other mechanical properties. The interface of the HIP clad composite contained several regions of distinct microstructural characteristics with widely varying densities of carbide precipitates. Relative to the clad produced by welding, extensive precipitation was observed both in the steel and in the IN625 at the interface, separated by a region free from precipitation. The extent of precipitation at the interface regions appears to be controlled essentially by the extent of carbon transport across the interface. The article describes the detailed analysis of the interface characteristics, and models are proposed to explain the microstructural evolution at the interface of the HIP and weld clad composites.

Microstructural characterization of the dispersed phases in Al-Ce-Fe system

Analytical electron microscopy studies were conducted on a rapidly solidified Al-8.8Fe-3.7Ce alloy and arc melted buttons of aluminum rich Al-Fe-Ce alloys to determine the characteristics of the metastable and equilibrium phases. The rapidly solidified alloy consisted of binary and ternary metastable phases in the as-extruded condition. The binary metastable phase was identified to be Al6Fe, while the ternary metastable phases were identified to be Al10Fe2Ce and Al20Fe5Ce. The Al20Fe5Ce was a decagonal quasicrystal while the Al10Fe2Ce phase was determined to have an orthorhombic crystal structure belonging to space group Cmmm, Cmm2, or C222. Microscopy studies of RS alloy and cast buttons annealed at 700 K established the equilibrium phases to be Al13Fe4, Al4Ce, and an Al13Fe3Ce ternary phase which was first identified in the present study. The crystal structure of the equilibrium ternary phase was determined to be orthorhombic with a Cmcm or Cmc2 space group. The details of X-ray microanalysis and convergent beam electron diffraction analysis are described.

Crystal structure of intermetallic phase in Fe--20Cr--4Al--0. 5Y alloy by convergent beam electron diffraction

The crystal structure and chemical composition of the intermetallic phase in a Fe--20%Cr--4%Al--0.5%Y (wt. %) alloy were investigated by electron microscopy. Convergent beam diffraction studies revealed that the intermetallic phase forms in three different crystal structures that could coexist in a single grain of the phase. The dominant crystal structure was shown to be hexagonal (a = 0.85, c = 0.84 nm) with a space group most likely to be P6/sub 3//mmc. Within the hexagonal phase, regions of a rhombohedral crystal structure (a = 0.85, c = 1.26 nm) were observed that had grown in without an apparent phase boundary separating the two crystal structures. The third crystal structure was determined to be monoclinic (a = 0.97, b = 0.85, c = 1.07 nm, and beta = 97.3/sup 0/) and formed by twinning on the )101-bar1) planes of the hexagonal phase. The chemical compositions of regions with different crystal structures were comparable and the stoichiometry of the intermetallic phase corresponds to (Fe,Cr)/sub 17/(Al,Y)/sub 2/. The relationship of the observed crystal structures to those previously reported is discussed.