A systematic study of the sodium and calcium salts of an ethylene–methacrylic acid copolymer is reported. Fourier-transform infrared spectroscopy (in the midinfrared region) is applied to the characterization of stru...
详细信息
A systematic study of the sodium and calcium salts of an ethylene–methacrylic acid copolymer is reported. Fourier-transform infrared spectroscopy (in the midinfrared region) is applied to the characterization of structural changes as a function of temperature and time of annealing. In the spectra of calcium ionomers, bands associated with carboxylate dimers are identified and assignments of specific spectral features to multiplets and clusters are discussed. The spectroscopic changes observed in the spectra of sodium ionomers differ somewhat from their calcium counterparts in that a single infrared band attributed to isolated carboxylate groups is observed. Assignments of specific bands to multiplets and clusters can, however, be made in a manner consistent with the interpretation of the spectra of calcium ionomers.
In this study molecular alignment in copolymers of polyethylene terephthalate (PET) and p-hydroxybenzoic acid (PHB) was examined by means of differential radial distribution function (DRDF) analysis of wide angle X-ra...
详细信息
In this study molecular alignment in copolymers of polyethylene terephthalate (PET) and p-hydroxybenzoic acid (PHB) was examined by means of differential radial distribution function (DRDF) analysis of wide angle X-ray scattering data. The study was carried out using two representative copolymers with the PHB content of 30 and 60 mol%. The DRDF curves for the amorphous and the partially crystalline samples of these copolymers showed periodic intermolecular peaks up to various radial distances. The appearance of these peaks at periodicity suggested the existence of more-or-less parallel chain segments in the copolymers. The extent of the lateral molecular organization was for the amorphous copolymers containing 30 and 60 mol% PHB, respectively. A substantial structural difference was therefore shown for the copolymers in this composition range. The DRDF curve for the amorphous copolymer with 30 mol% PHB was found to be very similar to that for the previously reported glassy PET with 0 mol% PHB, indicating that the two materials had almost the same intermolecular structure. The structural information revealed by these DRDF results was in agreement with the various property changes caused by varying PHB contents of the copolymers.
Molecular dynamics simulations of the crack-tip f.c.c. --> b.c.c. martensitic transformation in Fe-20Ni and Fe-40Ni (at.%) were carried out. Embedded atom method potential functions were used to represent the inter...
详细信息
Molecular dynamics simulations of the crack-tip f.c.c. --> b.c.c. martensitic transformation in Fe-20Ni and Fe-40Ni (at.%) were carried out. Embedded atom method potential functions were used to represent the interaction between the atoms in both crystal structures. The results show that in both alloys the crack-tip stresses are relaxed owing to formation of the b.c.c. phase which is twinned to help accommodate the transformation strain associated with the f.c.c. --> b.c.c. transformation. As a result of the transformation, the crack tip becomes blunted which, in turn, enhances the material toughness. To quantify the effect of the crack-tip f.c.c. --> b.c.c. transformation on material toughness, components of Eshelby's F conservation integral were evaluated in the two alloys. It was found that the F-1 component of the integral which represents the force acting to propagate the crack tip has become negative owing to the transformation. As a result, crack propagation has ceased. Comparison of the two alloys showed that in Fe-40Ni dislocation emission in addition to the transformation can take place at the crack tip, causing additional crack blunting.
The heteroepitaxy in DyMnO3/Er1Ba2Cu3O7-delta bilayer thin films on LaAlO3 (100) substrates was characterized by four-circle X-ray diffractometry. The Er1Ba2CU3O7-delta thin films on LaAlO3 (100) substrates were prepa...
详细信息
The heteroepitaxy in DyMnO3/Er1Ba2Cu3O7-delta bilayer thin films on LaAlO3 (100) substrates was characterized by four-circle X-ray diffractometry. The Er1Ba2CU3O7-delta thin films on LaAlO3 (100) substrates were prepared by molecular-beam deposition (MBD) and post-growth annealing in wet and dry O2 at 880-degrees-C, whereas the DyMnO3 thin films OD the Er1Ba2Cu3O7-delta/LaAlO3 (100) heterostructure were deposited by MBD and post-growth annealing in dry O2 at 750-degrees-C. The conventional X-ray diffraction (XRD) patterns as well as pole figures (phi-scans) for specific (hkl) reflections were acquired. The Er1Ba2Cu3O7-delta thin film in the DyMnO3/Er1Ba2Cu3O7-delta/LaAlO3 (100) heterostructure showed [001] oriented epitaxial growth, as expected. The DyMnO3 thin film on the Er1Ba2Cu3O7-delta epilayer in the heterostructure grew with (110) epitaxy in its metastable orthorhombic phase (lattice constants: a0 = 5.272 angstrom, b0 = 5.795 angstrom and c0 = 7.38 angstrom). The heteroepitaxial relationships at the orthorhombic-DyMnO3 (110)/Er1Ba2Cu3O7-delta (001) interface was determined as the following: DyMnO3 (110) parallel-to Er1Ba2Cu3O7-delta (001), DyMnO3 [110BAR] parallel-to Er1Ba2Cu3O7-delta [100] or Er1Ba2Cu3O7-delta [010], and DyMnO3 [001] parallel-to Er1Ba2Cu3O7-delta [010] or Er1Ba2Cu3O7-delta [100].
The dry sliding wear performance of discontinuously reinforced aluminum (DRA) as investigated by numerous Studies during the last two decades is reviewed and discussed. In general DRAs feature substantially better wea...
详细信息
The dry sliding wear performance of discontinuously reinforced aluminum (DRA) as investigated by numerous Studies during the last two decades is reviewed and discussed. In general DRAs feature substantially better wear resistance than their respective unreinforced matrix. However, under particular conditions and wear mechanisms, the wear performance of the overall DRA-metal couple is similar to or lower than its respective unreinforced alloy-metal couple. Furthermore conflicting results regarding the effect of the different tribological parameters on the wear performance of DRAs are noted among the reviewed studies, thereby increasing the difficulty in achieving the optimization of DRA-metal couples. It is shown that parameter interactions exist and are responsible for these conflicting results. The development of a general framework identifying groups of interactive parameters and assessing wear material performance through investigating these groups of parameter is therefore presented. It is suggested that material loss can be rationalized from the understanding and description of the three wear precursors which delineate these groups of parameters, the wear precursors being surface, subsurface and third body behavior. It is proposed that optimization of a tribosystem can be done through optimizing the three behaviors independently, thus allowing consideration of a reduced number of parameters while providing a higher degree of practical extrapolation.
An experimental method was devised to increase the "packing density" of random spherical powder packings by agitation with an applied compressive force. Maximum random packing densities of both monosize and ...
详细信息
An experimental method was devised to increase the "packing density" of random spherical powder packings by agitation with an applied compressive force. Maximum random packing densities of both monosize and bimodal-size powder packings of various size ratios and volume fractions are compared to the as-received gas-atomized SS316L powder. Since the presence of a compressive load will tend to restrict the opening of interparticle passages through the powder pack, uniform mixing of the powders beforehand is important for achieving optimum packing. At the same time, however, macroscopic size segregation of mixer-mill-premixed powder packings was found not to be significant using this technique. Densities of optimized bimodal-size powder packings far exceed those of non-optimized packings and the as-received (continuously distributed) powder. This enhancement is also retained during subsequent hot isostatic pressing.
The F.C.C. --> B.C.C. martensitic transformation in Fe-Ni alloys containing between 20 and 40 at.% Ni was studied using molecular dynamics simulations. The interatomic interactions were represented using the embedd...
详细信息
The F.C.C. --> B.C.C. martensitic transformation in Fe-Ni alloys containing between 20 and 40 at.% Ni was studied using molecular dynamics simulations. The interatomic interactions were represented using the embedded atom method pseudo-monoatomic potentials, which were shown to represent correctly the relative stability of the F.C.C. and B.C.C. structures as a function of the nickel content. The simulation results dearly demonstrate the important role that crystal defects, such as the free surface or grain boundary, play in the formation of martensite. This is consistent with the experimental observations, which suggest that the martensitic nucleation is of a heterogeneous nature. The simulation results further indicate the need for lattice invariant deformation, such as twinning and slip, for the continued growth of martensite.
The embedded-atom method type interatomic potentials for h.c.p. titanium and b.c.c, vanadium, originally proposed by Johnson and coworkers, were modified in order to be able to model both the b.c.c. and the h.c.p. str...
详细信息
The embedded-atom method type interatomic potentials for h.c.p. titanium and b.c.c, vanadium, originally proposed by Johnson and coworkers, were modified in order to be able to model both the b.c.c. and the h.c.p. structures and the phase transformation between the two in titanium-base Ti-V alloys. The reliability of the modified potentials was tested with respect to lattice dynamics (the phonon dispersion curves and the phonon density of states) in the two structures and with respect to the relative thermodynamic stability of the two phases as a function of temperature and the alloy composition. The model predictions are found to be in reasonable agreement with their experimental counterparts. In particular, the predicted effect of vanadium on two phonon modes, a T-1 N-point 1/2[110] phonon and T-1 [112] phonon, which play the key roles in the b.c.c. --> h.c.p. transformation, was found to be consistent with the experiment.
Dynamic material modeling (DMM) has been applied to the analysis of the mechanical behavior of a (Widmanstatten + primary alpha2 + beta) Ti-25Al-11Nb alloy to establish the flow stress-strain rate-temperature-strain c...
详细信息
Dynamic material modeling (DMM) has been applied to the analysis of the mechanical behavior of a (Widmanstatten + primary alpha2 + beta) Ti-25Al-11Nb alloy to establish the flow stress-strain rate-temperature-strain criteria for stable flow. Unstable and stable flow regimes predicted by DMM were correlated to microstructural observations. Unstable flow zones were related to transformation of orthorhombic O --> alpha2, kinking of the alpha2 lamellae, shear band formation and coarsening of the dynamically recrystallized gram structure;stable flow regimes were shown to be associated with dissolution of the Widmanstatten alpha2, coarsening of primary alpha2 and dynamic spheroidization of the lamellar alpha2 microconstituents.
The influence of reinforcement size and matrix heat treatment on the wear mechanisms and wear rates during dry sliding of 2009 Al-20 vol.% SiCp composite have been examined. Friction and wear measurements were correla...
详细信息
The influence of reinforcement size and matrix heat treatment on the wear mechanisms and wear rates during dry sliding of 2009 Al-20 vol.% SiCp composite have been examined. Friction and wear measurements were correlated with surface and subsurface deformation mechanisms utilizing scanning electron micrography and EDX analysis. Friction analysis showed that increasing particulate size from 4 to 29 mu m increased the friction coefficient. The predominant friction mechanism at particulate sizes below 13 mu m involved adhesion and microplowing, these being augmented by hard third body SIC abrasion with increasing particulate size. Wear rate also increased with increasing particulate size and matrix hardness. Adhesion and micro-cutting were the predominant wear mechanisms for smaller reinforcements, the higher wear rates observed in the larger particulate reinforced composite tribosystem being associated with increased particulate cracking-induced subsurface delamination.
暂无评论