Fine particles of BaTiO3 and SrTiO3 have been synthesized by the spray pyrolysis technique, and the chemical homogeneity of their particles was analyzed by x-ray diffraction (XRD) and analytical electron microscopy (AEM). The stock solutions were prepared by dissolving Ti(OC3H7)4 and Ba(NO3)2 or Sr(NO3)2 in diluted nitric acid solution. They were atomized into a reaction chamber held at 1000 °C through a two-fluids atomizer with N2 gas. Mostly hollow spherical particles 3 μm in diameter were obtained, composed of very fine grains of 40 nm. As-prepared powders were crystallized to BaTiO3 or SrTiO3 with a small amount of by-product such as Ba2TiO4, Sr2TiO4, and TiO2. The AEM study revealed that the bulk composition of each particle was chemically homogeneous, but a local chemical composition segregation was observed within each particle. This chemical inhomogeneity was considered to be caused by the difference in the precipitating speed of each component from the precursor salts; that is, the precipitation of Ti4+ ion as TiO2 · xH2O was faster than those of Ba2+ and Sr2+ ions. To control this segregation, (a) replacing a part of the solvent of stock solutions with methanol or ethanol, (b) adding H2O2 to the solutions, and (c) increasing the concentration of the solution, are found to be effective. The reasons for these effects are discussed.

In this paper a model is presented for predicting the structural changes that occur in a metaphosphate glass during nitriding in anhydrous ammonia. Predicted values for the nitrogen content, the mole fraction of phosphate groups, the bridging to nonbridging oxygen ratio, and the ratio of the structural types for nitrogen are compared to the values obtained in earlier experimental work. A plausible correlation is found between the values predicted from this structural model and those observed experimentally. In addition, a maximum nitrogen content (N/P = 0.9) is predicted and the lower nitrogen content normally observed is explained.

In this paper, we address some fundamental questions regarding the response of a crack to externally generated dislocations. We note that since dislocations that formed at external sources in the material must be in the form of loops or dipoles, the theory must be couched in terms of crack shielding in a plastically polarizable medium. There are strong analogies to dielectric theory. We prove two general theorems: (1) Dipoles formed in the emission geometry relative to a crack tip always antishield the crack and (2) when dipoles are induced during uniform motion of a crack through a uniformly plastically polarizable material, then the net shielding is always positive. We illustrate these general theorems with a number of special cases for fixed and polarizable sources. Finally, we simulate the self consistent time dependent response of a crack to a polarizable source as the crack moves past it. The results show that the crack is initially antishielded, but that positive shielding always dominates during later stages of configuration evolution. The crack may be arrested by the source, or it may break away from it, depending upon the various parameters (source strength and geometry, dislocation mobility, Griffith condition for the crack, etc.). The results indicate that the time dependence of crack shielding in the presence of a nonuniform density of sources will be very important in practical cases of brittle transitions in materials.

We present a first principles model for abnormal grain growth in either two or three dimensions that has the virtue of simplicity on the one hand and of close agreement with recent computer simulations on the other. The model is based on Gladman's picture of one grain growing in a matrix of similar grains, retaining its shape as it grows. The grain must have a critical size advantage over its neighbors before the process is spontaneous. Once growth is under way the kinetics are identical to those in Hillert's classical model of grain growth. In the competition between abnormal growth of one grain and the normal growth in the matrix, the important parameters are the ratios γa/γm and μa/μm, where γ is the energy and μ is the mobility of a grain boundary and the subscripts a and m refer to abnormal and matrix, respectively. Values of these three factors: size advantage, energy advantage, and mobility advantage define the conditions under which abnormal grain growth may occur. The results agree closely with recent computer simulations and therefore permit interpolations to be made between computer answers. A hypothesis on the role of second phase particles as a trigger for abnormal grain growth is proposed.

Fe3Al-based iron aluminides have been of interest for many years because of their excellent oxidation and sulfidation resistance. However, limited room temperature ductility (<5%) and a sharp drop in strength above 600 °C have limited their consideration for use as structural materials. Recent improvements in tensile properties, especially improvements in ductility produced through control of composition and microstructure, and advances in the understanding of environmental embrittlement in intermetallics, including iron aluminides, have resulted in renewed interest in this system for structural applications. The purpose of this paper is to summarize recent developments concerning Fe3Al-based aluminides, including alloy development efforts and environmental embrittlement studies. This report will concentrate on literature published since about 1980, and will review studies of fabrication, mechanical properties, and corrosion resistance that have been conducted since that time.