Materials Science Forum Vols. 539-543

Abstract: Non-toxic allergy free alloying elements are mostly selected for preparing metallic biomaterials. Currently, functionalities such as low modulus, shape memory, super elasticity, etc. are required for the metallic biomaterials, especially for β type titanium alloys. The harmonization of metallic, ceramic, and polymer biomaterials is needed for advanced biomaterials in the future. Titanium and its alloys are attracting considerable attention with regard to applications not only in the biomedical field, but also for dental and healthcare products. In dentistry, titanium and its alloys are applied to dental products such as crowns, inlays, bridges, etc., as well as dental implants. For fabricating dental products, the dental precision casting process is important. A new dental precision casting process using calcia is currently being developed. Noble alloys such as gold base or silver base alloys are widely applied for the precision casting of dental products. Allergy-free elements, particularly Pd-free low- noble dental alloys are required.

Abstract: The Representative Volume Element (so-called RVE) is the corner stone of continuum mechanics. In this paper we examine the scaling to RVE in linear elasticity, finite elasticity, elasto-plasticity, thermoelasticity, and permeability of random composite materials.

Abstract: Friction stir welding (FSW) has been in use for nearly fifteen years and a significant body of published research regarding process/property/structure relationships is now available; particularly with respect to FSW of aluminum alloys. In this paper, some pertinent literature will be reviewed and an attempt made to tie the numerous experimental observations together through some unifying concepts. Examination of relationships among control and response FSW process variables (respectively e.g. tool rotation rate and torque) and weld microstructure and properties can provide important insight regarding how weld properties develop and how best to approach process development for different alloy classes.

Abstract: The body-centered tetragonal (BCT) structure in quenched Fe-C steels is usually illustrated to show a linear change in the c and a axes with an increase in carbon content from 0 to 1.4%C. The work of Campbell and Fink, however, shows that this continuous linear relationship is not correct. Rather, it was shown that the body-centered-cubic (BCC) structure is the stable structure from 0 to 0.6 wt%C with the c/a ratio equal to unity. An abrupt change in the c/a ratio to 1.02 occurs at 0.6 wt%C. The BCT structure forms, and the c/a ratio increases with further increase in carbon content. An identical observation is noted in quenched Fe-N steels. This discontinuity is explained by a change in the transformation process. It is proposed that a two-step transformation process occurs in the low carbon region, with the FCC first transforming to HCP and then from HCP to BCC. In the high carbon region, the FCC structure transforms to the BCT structure. The results are explained with the Engel-Brewer theory of valence and crystal structure of the elements. An understanding of the strength of quenched iron-carbon steels plays a key role in the proposed explanation of the c/a anomaly based on interstitial solutes and precipitates.

Abstract: The quality of steel sheets is strongly affected by the surface defects that can be generated during hot rolling and are often related to scales removal operation. These defects are related to rather complex high temperature oxidation processes. In order to reduce an occurrence of the defects, it is necessary to understand better the formation of iron oxides during high temperature oxidation, the structure of the interfaces with the substrate and between different oxide phases. However, due to the lack of good experimental research tools details of iron oxide microstructures were not investigated. Conventional methods, such as backscattered electron images or fractography can only provide general characteristics of microstructures like grain morphology and grain size. In this paper the microstructure, phase distribution and texture in oxide formed during high temperature oxidation of iron and low carbon steels are investigated. The oxide microstructures are characterized by orientation imaging microscopy (OIM) on the cross-sectional area of the oxide layers. It is demonstrated that OIM using electron backscattered diffraction (EBSD) techniques, can be used to distinguish grains having different phase composition and orientation and can become invaluable tool for visualizing the oxide microstructure, texture and also can be used to study oxide defects. The three different iron oxides phases can be distinguished and the characteristics of oxides with different oxidation histories compared The characteristics of high temperature oxidation microstructure of iron are presented with description of iron oxide defects and cracking as well as the illustration of the interfacial microstructure between the layered iron oxides.

Abstract: Yield strength of highly dislocated metals is known to be directly proportional to the square root of dislocation density (ρ), so called Bailey-Hirsch relationship. In general, the microstructure of heavily cold worked iron is characterized by cellar tangled dislocations. On the other hand, the dislocation substructure of martensite is characterized by randomly distributed dislocations although it has almost same or higher dislocation density in comparison with heavily cold worked iron. In this paper, yielding behavior of ultra low carbon martensite (Fe-18%Ni alloy) was discussed in connection with microstructural change during cold working. Originally, the elastic proportional limit and 0.2% proof stress is low in as-quenched martensite in spite of its high dislocation density. Small amount of cold rolling results in the decrease of dislocation density from 6.8x1015/m-2 to 3.4x1015/m-2 but both the elastic proportional limit and 0.2% proof stress are markedly increased by contraries. 0.2% proof stress of cold-rolled martensite could be plotted on the extended line of the Bailey-Hirsch equation obtained in cold-rolled iron. It was also confirmed that small amount of cold rolling causes a clear microstructural change from randomly distributed dislocations to cellar tangled dislocations. Martensite contains two types of dislocations; statistically stored dislocation (SS-dislocation) and geometrically necessary dislocation (GN-dislocation). In the early deformation stage, SS-dislocations easily disappear through the dislocation interaction and movement to grain boundaries or surface. This process produces a plastic strain and lowers the elastic proportional limit and 0.2% proof stress in the ultra low carbon martensite.

Abstract: Numerous methods have recently emerged for fabricating cellular lattice structures with unit cells that can be repeated to create 3D space filling systems with very high interconnected pore fractions. These lattice structures possess exceptional mechanical strength resulting in highly efficient load supporting systems when configured as the cores of sandwich panels. These same structures also provide interesting possibilities for cross flow heat exchange. In this scenario, heat is transported from a locally heated facesheet through the lattice structure by conduction and is dissipated by a cross flow that propagates through the low flow resistant pore passages. The combination of efficient thermal conduction along the lattice trusses and low flow resistance through the pore channels results in highly efficient cross flow heat exchange. Recent research is investigating the use of hollow truss structures that enable their simultaneous use as heat pipes which significantly increases the efficiency of heat transport through the lattice and their mechanical strength. The relationships between heat transfer, frictional flow losses and topology of the lattice structure are discussed and opportunities for future developments identified.

Abstract: Significant technical challenges still remain today for the fuel cell in a number of areas including reliability, durability, cost, operational flexibility, technology simplification and integration, fundamental understanding and life cycle impact. New advanced materials and associated innovative engineering design will be required to close these technical gaps. This paper provides a perspective on fuel cell technology today, research and development directions, challenges going forward, and a future view of the fuel cell.