Molybdenum Accumulation at Ferrite: Austenite Interfaces during Isothermal Transformation of an Fe-0.24 Pct C-0.93 Pct Mo Alloy

摘要

A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (alpha:gamma) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650 deg C, 630 deg C, and 610 deg C. These concentrations were quite small at 650 deg C, which is just below the bay temperature of the time-temperature-transformation (TTT) curve for the initiation of ferrite formation. There were larger concentrations at 630 deg C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610 deg C were intermediate between the values observed at 650 deg C and 630 deg C. The average accumulation at the latter temperatures increased appreciably as a function of transformation time. After each heat treatment, there was considerable variation in Mo accumulation from one alpha:gamma interface to another and, to a lesser extent, from one region to another along the same interface. These higher Mo concentrations were deduced to have developed largely through volume diffusion of Mo, mainly through ferrite, to interfaces whose ledgewise growth had been interrupted by growth stasis. (Mo_2C precipitation at alpha:gamma boundaries occurred only at the end of growth stasis.) It appears that only a very small amount of Mo segregation is needed, probably at specific interfacial sites, in order to produce growth cessation. Growth kinetics anomalies of this kind continue to provide the best evidence available for the operation of a coupled-solute drag effect.