Structure observations by high-resolution electron microscopy of Ni-B melt-spun alloys (B<30 at%)

The microstructure of a series of Ni-B alloys (15 at%3B phase (a=0.44 nm,b=0.52 nm,c=0.66 nm) is expected. High-resolution electron microscopy images of Ni-25 at% B rapidly quenched indicate that a two-phase structure is observed. A metastable phase is observed with a periodicity of 1.0 nm. HREM images and EELS analyses led us to propose the structure of this metastable phase. This phase (Ni₅B₂) is correlated with the monoclinic Hägg carbide Fe₅C₂. Depending on the velocity of the substrate and on the boron concentration, a variety of microstructures has been observed from a faulted to a semi-amorphous structure. The structures are related to the solidification behaviour and the heat flow in the undercooled melt. Particular attention was paid to the eutectic Ni-Ni₃B composition.     

Transparent monolithic zirconia gels: effects of acetylacetone content on gelation

Transparent yttria-stabilized zirconia monolithic gels were synthesized under controlled conditions for hydrolysis and chemical polymerization. The influence of acetylacetone on the previous chemical reactions was explored, and a plausible explanation was proposed concerning the outstanding role of this additive during the gelation process. Small-angle X-ray scattering was used to estimate the size of the polymeric species and to investigate the influence of acetylacetone on the structure of the gels prepared.     

Grain boundary resistivity in Y-TZP materials as a function of thermal history

Grain-boundary resistivity in yttria-containing tetragonal zirconia polycrystalline (Y-TZP) materials dominates the total resistivity. Impedance measurements combined with microstructural studies suggest that post-sintering heat treatments (in particular the cooling rate) influence the location of the grain-boundary phase which, in turn, has a significant effect on the grain-boundary resistivity. Higher cooling rates from the sintering temperature lead to reduction in the grain-boundary resistivity. In both alumina-containing and relatively pure tetragonal zirconia polycrystals, post-sintering heat treatments have a less conspicuous effect. The activation energy associated with the grain-boundary resistivity was independent of the post-sintering heat treatments but was 25 to 30 kJ mol^–1 higher than that for the oxygen-ion conduction within the grains at low temperatures.     

Damage analysis of a crack layer

Damage analysis of a crack layer in polystyrene is carried out by employing optical microscopy and principles of quantitative steteology. The results show that, within the quasistatic phase of crack layer propagation, the average crazing density, along the trailing edge of the active zone, is constant. This is consistent with a self-similarity hypothesis of damage evolution employed by the crack layer theory. The average crazing densities within the active zone and along its trailing edge are found to be practically equal. A layer of constant crazing density, adjacent to the crack planes, accompanies the crack during its quasi-static growth. This suggests that: (1) a certain level of crazing density should be reached, around the crack tip, prior to crack advance; (2) the specific energy, associated with this ‘core’ of damage, could be considered as a Griffith's type energy. The results are in favour of certain hypotheses adopted by the crack layer theory.     

Effects of manufacturing defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates

The effects of defects on the mechanical properties of carbon fibre reinforced polyethersulphone laminates have been measured. The defects studied were cut fibre plies, omission of polymer films and local delamination produced by the inclusion of foreign matter. Of these it was found that only cut plies had a significant detrimental effect on the strength of a laminate. For specimens with two cut plies, the failure stress, tensile, flexural and compressive in the remaining continuous plies was the same as in the defect-free material, provided that the cut plies were widely separated. However the failure stresses were 15–18% lower in the continuous plies in the specimens containing two cut plies which were more closely spaced and in specimens containing four cut plies.     

The formation of zinc ferrite

The formation of zinc ferrite from zinc oxide and iron oxide is evaluated with high temperature X-ray diffraction experiments. Despite the problem of accurately determining the temperature of the part of the sample actually investigated, a kinetic analysis of the isothermal formation is shown to be possible. From these data the energy and entropy of activation are derived. It is further shown that neither corrections for grain growth during formation nor the presence of impurities have any influence on these results. On the other hand, the size of the reactant iron oxide particles is found to have a strong influence on both the energy and entropy of activation.     

Thermomechanical behaviour of composites

This paper derives the governing equations for the thermomechanical behaviour of composites. When the basic equations for the thermoelastic behaviour of solids were first derived in the nineteenth century several approximations were made. The effect of these assumptions are discussed and illustrated by the results of a simple laboratory test. The implications of this work on the analysis of impact damaged laminates are then discussed.     

Theory of propagating deformation bands

A model for propagating deformation bands is developed, based on a mechanical equation of state and on evolution equations for the structure parameters. It is shown that, contrary to normal deformation, it is necessary to discriminate between global parameters which depend solely on time elapsed and local parameters which are functions of the strain history. Correspondingly, two sets of equations are presented, one describing the development of the global parameters in the course of time, the other giving the evolution of the local state parameters in the course of deformation, i.e. as a function of strain. Measurable quantities derived from the first set are the load serrations and the band velocity, while the second set yields the flow stress and the strain profile of the propagating band.