Precipitation in a Ni–Al–Mo Directionally Solidified Eutectic Composite
The most recently developed high temperature materials are known as in-situ composites or directionally solidified eutectics. Since the composites are formed during solidification, they have excellent morphological stabilities up to near their melting temperatures. The Ni-Al-Mo system has received the greatest attention for recent several years(1)~(3) since the alloy is expected to form a structure consisting of ductile α-Mo fibers aligned in aductile γ+γ' matrix†. However variations in microstructures and the resultant strength by thermal treatments have not been investigated in earlier papers(1)~(3). In a separate paper(4) the present authors have reported the detailed microstructural analysis of as-grown composite and the microstructural and strength variations of the composite due to quenching from high annealing temperatures. The present paper describes the microstructural changes occurring during ageing of a Ni-12.7at%Al-21.6at%Mo alloy composite after quenching from 1533K studied by transmission electron microscopy
Rotation of Mo-Fibers in Directionally Solidified Ni-Al-Mo Eutectic Alloys
The interface structure of α-Mo fib ers in directionally solidified Ni-Al-Mo eutectic alloys has been investigated by transmission electron microscopy and computer-aided graphical techniques. The Mo fibers have square or rectangular cross sections with well defined flat facets and fundamentally have the Bain orientation relationship with respect to the γ matrix lattice. The crystal lattices of the Mo fibers, however, are often rotated with respect to the matrix lattice about their longitudinal axes. Correspondingly, the facet planes deviate from the exact rational (1 1 0)γ′//(1 0 0)α relationship. A computer aided graphical plotting of the atom arrangement at the interfaces suggests that good atomic matching is kept even after the rotation of fibers. High resolution electron microscopy revealed the incorporation of single atomic structural ledges to keep the coherency of the lattices of the matrix and rotated fibers. The relation between the lattice rotation and the facet plane rotation is well explained by the moiré and O-lattice models.
Studies on root enamel (2). Enamel pearls. A review of their morphology, localization, nomenclature, occurrence, classification, histogenesis and incidence.
Enamel pearls are one of a number of different enamel structures that can be found on the roots of deciduous and permanent teeth. They have a distinct predilection for the furcation areas of molar teeth, particularly the maxillary third and second molars. They can consist primarily of enamel, but in most instances, a core of dentin is contained within them. On rare occasions, even pulpal tissues can be found. Enamel pearls usually occur singularly, but up to 4 enamel pearls have been observed on the same tooth. Depending on the study, enamel pearls on permanent molar teeth have an incidence rate of between 1.1%-9.7% with distinct differences among racial and national groups. The incidence of enamel pearls increases greatly in histological studies, suggesting that they are often obscured by a covering of cementum.