Cleavage Plane Orientation Research Articles

単結晶ＬｉＦの破壊じん性

The fracture toughness (KIc) of LiF single crystal was evaluated by the indentation microfracture technique (IMT) and the indentation strength technique (IST) at room temperature. The indentation crack on the (001) of ‹100› specimen resulted in the lowest KIc of about 0.5MNm-1.5 in both IMT and IST because the {110} is the preferred cleavage plane of this crystal. The Vickers hardness (HV) for the (001) was obtained to be about 0.9GPa. A fairly smooth planar was provided by the crack-propagation originated from the initial crack on the (001) of ‹110› specimen. However, cracks in the ‹110› (110) specimen became eventually angled at 45° to the original flaw. As the result, KIc≈0.7MNm-1.5, HV≈0.9GPa for the former, and KIc≈0.8MNm-1.5, HV≈1.2GPa for the latter. The indentation crack on both the (110) and (112) of ‹111› specimen didn't propagate along the extention of Vickers indentation diagonal and produced the rough surface after fracture. The KIc values for ‹111› (110) and ‹111› (112) were determined to be 1.5 and 1.8MNm-1.5, respectively, while HV≈1GPa. These results indicate that the KIc of single crystals is affected by the cleavability of crystals and the geometric configuration between the cleavage plane and the direction of crack propagation.

On the reconciliation of reduced cohesion and enhanced plasticity mechanisms for hydrogen embrittlement

The pernicious influence of hydrogen on the crack growth resistance of metals is well established. It is generally believed that the enrichment of hydrogen in the region of hydrostatic tension at the tip of a loaded crack results in embrittlement. As proposed by Westlake, the precipitation and subsequent cracking of brittle hydrides in the region of increased hydrogen content has been shown to be responsible for subcritical cracking in hydride forming metals. The mechanism for reduced resistance to crack propagation in non-hydride forming metals, principally iron and nickel, is much less clear. Hydrogen-assisted cracking in these materials usually occurs transgranularly in the absence of grain boundary segregation. Beachem made careful examinations of fracture surfaces from hydrogen embrittled steels and deduced that the macroscopic embrittlement was actually due to severely localized plasticity. Nakasato and Bernstein demonstrated that hydrogen charging of purified iron induced cleavage on (110) slip planes rather than the preferred cleavage plane, (100). Embrittlement of the iron by the addition of > 0.7% by weight of silicon was required to switch hydrogen induced cracking to the cube planes. Normally, one would not consider a loss of cohesion and an increase in crack tip plasticity to be compatible processes. Therefore,more » a debate has been taking place as to which of these mechanisms is actually responsible for observed macroscopic losses in ductility. The purpose of this note is to offer a model whereby it would be possible to explain this apparent anomaly by demonstrating that, for the special case of hydrogen-assisted slip band cracking, the two processes are complementary.« less

Early ontogeny of the annual fish genus Nothobranchius: Cleavage plane orientation and epiboly.

Living embryos of ten species of the teleost fish Nothobranchius were observed from fertilization through the ten-somite stage. Cellular behaviour during early development showed differences as well as similarities among species. In one species at the eight-cell stage the blastomeres are arranged in two rows of four cells, all in one plane. In three other species the blastomeres are arranged in two layers of four cells, each one above the other, and in the remaining six species the arrangement is intermediate between the one-layer and two-layer pattern. Hybridization experiments showed that the sequence of orientation of cleavage planes is not under the control of either the nucleus or of cytoplasmic factors from the sperm. At midcleavage stages, marginal cells of the blastoderm undergo fusion, thus giving rise to the external-Yolk Syncytial Layer. A second period of fusion of cells of the Enveloping Layer occurs just before epiboly starts and a third one after epiboly has concluded. Marginal cells of the Enveloping Layer do not possess cell protrusions such as blebs, lobopodia, or filopodia; therefore, spreading of the Enveloping Layer during epiboly by means of locomotion of marginal cells seems unlikely. Evidence is presented that the deep cells within the segmentation cavity remain separated from one another through contact inhibition during epiboly and that they use the Enveloping Layer as a substratum during their dispersion over the yolk. A description of normal stages applicable to all ten species of Nothobranchius is presented.

Cleavage of gallium phosphide

The cleavage of single-crystal GaP was studied by measuring the fracture toughness on the (100), (110), and (111) planes. It was observed that (110) is the preferred cleavage plane with a fracture toughness of only 0.65 ± 0.03 MPa m 1 2 and a fracture surface energy of 1.46 ± 0.07 J/m 2.

On the scribing and subsequent fracturing of silicon semiconductor wafers

The integrated circuits deposited on silicon wafers are often separated by scribing with a diamond tool followed by bending to produce fracture. Using a commercial scribing tool we find permanent deformation and three types of crack. The median crack which propagates downwards is the objective of the scribing process. Lateral cracks which form, apparently following plastic deformation, may lead to chipping on either side of the scribing tool. These cracks and also the chevron cracks which form on the surface are very similar to cracks observed in scratching glass. However, in silicon, because of its anisotropy, the chevron cracks may be a serious problem since they can guide the median crack out of the scribing direction onto a preferred cleavage plane. This aspect leads to a brief discussion of the crystallography of silicon and recommendations for scribing configurations which should minimize undesired fracture. Finally, it is shown that the established methods of linear elastic fracture mechanics may be used to predict the maximum radius of curvature required to fracture a wafer containing a prescribed series of median cracks.

The blastomere pattern in echinoderms: cleavages one to four

The results of a longitudinal study of the blastomere pattern in six embryos during the first four cleavages are reported. At each cleavage stage optical sections through an embryo, taken at vertical intervals of 5 or 10 micron, were recorded on 35 mm film: digitization of the blastomere contours and computer analysis allow calculation of the center, radius, surface area and volume of each blastomere. The subjective impression of exquisite regularity seen in normal echinoderm blastulae acquires a quantitative dimension from the present study. For example, the individual angles formed by the various quartets of blastomeres depart from right angles by at most a few degrees. The egg volume was found to be conserved up to the fourth cleavage. At the 16-cell stage, unlike the earlier stages, the blastomere positions cannot be ascribed solely to the position and orientation of the respective cleavage planes. Finally, a few features of a formal model of these early cleavages are sketched.

Laser-induced fracture in silicon

Utilizing the scanning electron microscope, the laser-impact zone on irradiated P-type silicon wafers has been characterized by a central region of compressive stress which decreases to zero and becomes tensile in character with increasing radius from the impact centre, with symmetrical fracturing occurring around the impact centre at a point of maximum tensile stress. The effect of surface flaws on the fracture of brittle Si wafers and their interaction with the impact zone were observed to be small for scratch widths of 60 μm, and lengths of 1 cm. Direct observations of advancing crack tips were made in the transmission electron microscope, and shown to have associated arrays of partial and total dislocations. The presence of numerous precipitates in the P-type Si wafers possessing long-range strain fields which overlapped along the {111} cleavage planes was observed to have a direct influence on the fracture process. Fracture was observed to occur in wafers containing cleavage-oriented scratches or scratches deviating by ∼ 5° from a cleavage-plane orientation, but little influence was noted for randomly oriented scratches or those which deviated from a cleavage-plane orientation by ∼ 10° or greater.

What Is the Role of Spores in Fern Taxonomy?

Ferns are a group of plants that illustrate many changes in taxonomic concepts which are useful in establishing systems of classification and in the delimiting of genera and species. Linnaean genera were distinguished on such gross morphological features as the shape, size, and placement of the sori. Later, stelar anatomy became an important criterion. Then such items as venation, origin of the sorus, indusium, development of the sporangium, scales, hairs, glands on the indusium, chromosome number, and, to a lesser extent, spore morphology were used to separate species. This is not a definitive paper on the use of spore morphology in fern taxonomy but rather an assessment of the role that may be attained by this much neglected field. concept of the structure and the terminology applied to different portions of the spore is quite varied. Douglas H. Campbell (1905) described spore structure as follows: The young spores are thin-walled, but later the wall becomes thicker and shows a division into two parts, an inner layer, which generally shows a cellulose reaction and is called the endospore (intine), and an outer more cuticularized coat, the exospore (exine). In addition, a third outer coat (perenium, epispore) is very generally present. Structurally, the spore consists of a protoplast, the living portion encased in a thin layer, the intine, which is surrounded by the exine. This in turn is generally enclosed by one or more layers which have been termed perispore, epispore, perine, or sclerine by various authors. There is no unanimity of usage among those interested in spore morphology. It was recognized early that differences in the number and orientation of cleavage planes gave rise to two types of spores from the spore mother cell, namely, the tetrahedral and bilateral spores. These were so named because, in the case of the former, three planes or faces were common to the proximal side of the spores tetrahedral or bilateral, and Coniogramme with spores 6

Degree of lateral order in various rayons as deduced from x‐ray measurements

AbstractX‐ray measurements on 16 different specimens of regenerated cellulose reveals that there exists a pronounced negative correlation between the integrated intensity of the 101 interference and its halfwidth. The 101 plane is a preferred cleavage plane and represents the lamellar plane of the ribbon‐shaped crystallites. Variations in the intensity of the other interferences are much less or nonexistent. These results show that the degree and the perfection of lateral order according to the 101 and 002 planes is little at variance, but that order according to the 101 plane is extremely variable in such a way that if the average thickness and (or) perfection of the lamellae increases, the total number of well‐ordered 101 plane contacts diminishes. As a possible explanation of this phenomenon it is suggested that various degrees of recrystallization with reference to this particular plane (i.e., various degrees of lateral order) may be associated with the various rayon specimens. This, however, does not necessarily imply differences in the total amount of ordered substance. This explanation is corroborated by the fact that cellulose acetate fibers which were previously allowed to “recrystallize” by heating in methanol, after saponification yield a rayon with a particularly sharp, but also particularly weak, 101 interference, without exhibiting a higher total percentage of ordered substance than the saponification products of acetate treated in this manner.

The Effect of Tension on the Electrical Resistance of Single Bismuth Crystals

The compensated potentiometer method of measuring minute variation in small resistances which was developed by P. W. Bridgman has been used in the study of the effect of tension, applied parallel to the direction of current flow, on the resistance of single bismuth crystals. The tension coefficient of resistance at 30\ifmmode^\circ\else\textdegree\fi{}C has been found to depend on the orientations both of the principal (111) and secondary ($11\overline{1}$) cleavage planes with respect to the tension. For the limiting case of the principal cleavage plane perpendicular to the tension, the coefficient is independent of the orientation of the secondary cleavage planes; in that of the principal cleavage plane parallel to the tension, the coefficient varies very little and is very probably independent of the secondary orientation. In the case where the normal to the principal cleavage plane makes an angle of about 60\ifmmode^\circ\else\textdegree\fi{} with the tension, the variation of the coefficient with the orientation of the secondary cleavage plane seems to be a maximum. This variation involves a change in sign as well as in magnitude, so that for certain orientations the coefficient becomes positive instead of remaining negative. The coefficient shows trigonal symmetry, as it must do if it is to be consistent with the known corporeal trigonal symmetry of the bismuth crystal. The paper presents only these empirical results.