Statistical Derivation of Basic Equations of Diffusional Kinetics in Alloys with Application to the Description of Diffusion of Carbon in Austenite

Basic equations of diffusional kinetics in alloys are statistically derived using the master equation approach. To describe diffusional transformations in substitution alloys, we derive the "quasi-equilibrium" kinetic equation which generalizes its earlier versions by taking into account possible "interaction renormalization" effects. For the interstitial alloys Me-X, we derive the explicit expression for the diffusivity D of an interstitial atom X which notably differs from those used in previous phenomenological treatments. This microscopic expression for D is applied to describe the diffusion of carbon in austenite basing on some simple models of carbon-carbon interaction. The results obtained enable us to make certain conclusions about the real form of these interactions, and about the scale of the "transition state entropy" for diffusion of carbon in austenite.Comment: 26 pages, 5 postscript figures, LaTe

Thermally Stable Nanocrystalline Steel

Two novel nanocrystalline steels were designed to withstand elevated temperatures without catastrophic microstructural changes. In the most successful alloy, a large quantity of nickel was added to stabilize austenite and allow a reduction in the carbon content. A 50 kg cast of the novel alloy was produced and used to verify the formation of nanocrystalline bainite. Synchrotron X-ray diffractometry using in situ heating showed that austenite was able to survive more than 1 hour at 773 K (500 ˚\u9aC) and subsequent cooling to ambient temperature. This is the first reported nanocrystalline steel with high-temperature capability.QC 20180604</p

Effect of niobium on the microstructure and mechanical properties of hot rolled microalloyed steels after recrystallization-controlled rolling

Microalloyed steels with increased strength and ductility are of considerable interest for use in the 'as-hotrolled' condition. However, there is a lack of information on their microstructural characteristics and mechanical properties. Seven different microalloyed steels with variable Nb and C content were evaluated in this work. First, characterization of the microstructure by optical and scanning and transmission electron microscopy was performed. Different microstructural constituents and grain size distributions were observed, and three different groups of precipitates were identified. For all steels, tensile tests were performed and ductile-to-brittle transition temperatures were determined. Finally, the complex interplay between microstructural features and mechanical properties was analyzed to determine structure-property relations for the steels under evaluation