Boundary Phenomenon Research Articles

Some crystal boundary phenomena in aluminum bicrystals

The authors have found that in the melt growth of aluminum bicrystals under certain conditions, the crystal boundary rotates about the direction of heat flow during progressive solidification, even though the crystal boundary area increases. This crystal boundary phenomenon may not be analogous to that found by Chalmers in tin and lead. It is suggested that the former occurs in the bicrystals the boundaries of which have twist components, and in which a cellular substructure grows appreciably so that there will be a variation in the thermal conductivity for different directions. In the high purity aluminum bicrystals this boundary phenomenon does not occur.

亜鉛・アルミニウム共析合金の高温引張りにおけるひげ結晶の生長

A typical superplastic alloy, fine-grained Zn-22 wt%Al eutectoid, was strained at elevated temperatures in a 500 kV electron microscope in order to understand the mechanism of apparently “superplastic” elongation of zinc-rich grains.Superplasticity is generally thought as a grain boundary phenomenon and not as the elongation of individual grains. Thus, the unusual elongation of zinc-rich grains was a puzzling feature of the deformation in favor of the grain deformation as the cause of superplasticity. The “in situ” observation proved that the elongated grain in fact grew at the grain boundary under the influence of the applied tensile stress. The growth rate is determined by the diffusion of zinc through the grain boundary. The apparently superplastic elongation was a diffusional creep that prevails when bulk super-plasticity is prohibited due to the thin foil geometry.The phenomenon is not only important the for elucidation of the mechanism but also is interesting as a process to grow whiskers.

A Consolidated Concept of Joint Lubrication

Animal joints contain two systems which require lubrication: a soft-tissue system, involving the sliding of synovial membrane on itself or on other tissues, and a cartilage-on-cartilage system. The resistance to motion caused by soft-tissue stretching and friction is far greater than that due to cartilage rubbing on cartilage; in fact, soft-tissue resistance may be primarily responsible for clinical joint stiffness. Lubrication of the softtissue system depends on molecules of hyaluronate in the synovial fluid which stick to the synovium in a layer and keep moving surfaces apart. Cartilage-on-cartilage lubrication, by contrast, is independent of hyaluronic acid. A hyaluronate-free glycoprotein fraction which has been isolated from synovial fluid confers on a buffer solution a lubricating advantage equal to that of whole synovial fluid. Removal of this fraction from synovial fluid deprives the fluid of any lubricating advantage over buffer. The action of this glycoprotein on cartilage is a boundary phenomenon similar to that of hyaluronate on synovium. In addition to this boundary effect, cartilage surfaces are kept apart by a fluid squeeze film made up of joint fluid and interstitial fluid which weeps from the articular cartilage itself. The squeeze-film effect is probably potentiated by the undulations of the surface and the elasticity of the cartilage, which may lower frictional resistance by elastohydrodynamic effects.

Role of Intermolecular Attractions in Liquids and Gases

Certain conventional elementary ideas relating to the role of intermolecular attractions in liquids and gases are examined critically, and an effort is made to bring out a rationally defensible picture of the effects of these forces, without postulating the presence of any permanent, net force fields. A clear statement of the origin of surface tension is presented. The modification of pressure found in a real gas due to intermolecular attractions is shown not to be a boundary phenomenon. The need for a dynamic mechanism to account for this modification is pointed out.

Fatigue deformation in the interior of aged ternary aluminium-magnesium-zinc alloys

Abstract Metallographic observations have been made of deformation, associated with the fatigue process in the interior of several aged ternary aluminiumzinc-magnesium alloys. It has been found that the deformation occurring in these relatively complex alloys differs from that observed in pure metals. With alloys which had a high zinc content the fatigue process was essentially a grain boundary phenomenon and cracking was intercrystalline. As the magnesium: zinc ratio was increased deformation occurrcd within the grains but was concentrated in certain slip bands along which cracking could occur. Cavities have been observed in these slip hands and appeared to precede the formation of an actual crack. Zones which were depleted of precipitate were found adjacent to slip bands in heavily deformed regions and these zones are considered to form by a process of re-solution of the precipitate. As the magnesium: zinc ratio was increased still further the fatigue process was accompanied by little visible evidence o...