Influence of Silicon and Aluminum Additions on the Oxidation Resistance of a Lean-Chromium Stainless Steel

The effect of Si and Al additions on the oxidation of austenitic stainless steels with a baseline composition of Fe–16Cr–16Ni–2Mn–1Mo (wt.%) has been studied. The combined Si and Al content of the alloys did not exceed 5 wt.%. Cyclic-oxidation tests were carried out in air at 700 and 800°C for a duration of 1000 hr. For comparison, conventional 18Cr–8Ni type-304 stainless steel specimens were also tested. The results showed that at 700°C, alloys containing Al and Si, and alloys with only Si additions showed weight gains about one half that of the conventional type-304 alloy. At 800°C, alloys that contained both Al and Si additions showed weight gains approximately two times greater than the type-304 alloy. However, alloys containing only Si additions showed weight gains four times less than the 304 stainless. Further, alloys with only Si additions preoxidized at 800°C, showed zero weight gain in subsequent testing for 1000 hr at 700°C. Clearly, the oxide-scale formation and rate-controlling mechanisms in the alloys with combined Si and Al additions at 800°C were different than the alloys with Si only. ESCA, SEM, and a bromide-etching technique were used to analyze the chemistry of the oxide films and the oxide–base-metal interface, in order to study the different oxide film-formation mechanisms in these alloys.     

Effect of manganese addition on reactive evaporation of chromium in Ni-Cr alloys

Chromium is used as an alloy addition in stainless steels and nickel-chromium alloys to form protective chromium oxide scales. Chromium oxide undergoes reactive evaporation in high-temperature exposures in the presence of oxygen and/or water vapor. Deposition of gaseous chromium species onto solid oxide fuel-cell electrodes can reduce the efficiency of the fuel cell. Manganese additions to the alloy can reduce the activity of chromium in the oxide, either from solid solution replacement of chromium with manganese (at low levels of manganese) or from the formation of manganese-chromium spinels (at high levels of manganese). This reduction in chromium activity leads to a predicted reduction in chromium evaporation factors as much as 35 at 800 °C and 55 at 700 °C. Quantifying the effects of manganese additions on chromium evaporation should aid alloy development of metallic interconnects and balance-of-plant alloys. This paper was presented at the ASM Materials Solutions Conference & Show held October 18–21, 2004 in Columbus, OH.     

Microstructural and failure characteristics of metal-lntermetallic layered sheet composites

A processing technique for the fabrication of layered metal-intermetallic composites is presented, in which a self-propagating, high-temperature synthesis reaction (SHS) was initiated at the interface between dissimilar elemental metal foils. The resultant composite microstructure consisted of a fully dense, well-bonded metal-intermetallic layered composite. In this United States Bureau of Mines study, metal (Fe, Ni, or Ti) foils were reacted with Al foils to produce metal-metal aluminide layered composites. Tensile tests conducted at room temperature revealed that composites could be designed to behave in a high-strength and high-toughness manner by altering the thicknesses of the starting elemental foils. Failure characteristics revealed that the processes that govern ductilevs brittle behavior of the composites occur early in the fracture.     

Empirical colorant mixture models

The color behavior of unconventional colorant systems and certain other mixtures cannot be described adequately by available theory. In such cases alternative approaches to formulation and shading need to be developed. In this article we investigate empirical models using Scheffé polynomials to describe color response surfaces for three- and four-component olorant mixtures. We describe experimental designs that allow the efficient estimation of the coefficients of high-order polynomial models over the full mixture-design space and that also allow estimation of lower-order models over subspace mixture regions. The experimental designs are applied to real colorant systems, and the accuracy of color response prediction from linear, quadratic, cubic, and quartic polynomial models is compared to that of theoretical models. The color response surfaces are visualized by preparing contour plots that depict color variation over a compositional region. These maps allow one to observe the relationship between color and composition, to assess the color gamut available with a given colorant set, and to estimate the formula or adjustments required to match a given color position. The effective use of model predictions to perform a sensitivity analysis on the compositional variables is also demonstrated in the context of manufacturing process control.     

Structure and properties of aligned short fibre-reinforced intermetallic matrix composites

Powder injection moulding techniques were utilized to align short fibres (Al₂O₃ and SiC) in a variety of intermetallic matrices (NiAl, MoSi₂ and TaTiAl₂). The alignment was accomplished by extruding a mixture of powders and short fibres with a polymer-based binder through a constricting nozzle. The binder was removed and the powder and fibres were consolidated, producing an aligned short fibrous composite. The effects of powder morphology, fibre volume fraction and fibre diameter on the alignment were demonstrated. Small diameter powders were required to ensure alignment of an appreciable loading of fibres in a powder matrix. Tensile and hardness tests were used to evaluate the effectiveness of the aligned short fibres to strengthen and toughen the matrices. The mechanical behaviour of these aligned short fibrous composites were found to be comparable to similar aligned continuous fibrous composites produced by conventional techniques.