Molybdenum accumulation at ferrite: Austenite interfaces during isothermal transformation of an Fe-0.24 pct C-0.93 pct Mo alloy

Abstract

A scanning transmission electron microscope (STEM) technique was used to measure Mo concentrations at ferrite:austenite (␣:␥) interfaces in an Fe-0.24 pct C-0.93 pct Mo alloy partially transformed at 650°C, 630°C, and 610°C. These concentrations were quite small at 650°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°C, a temperature at which transformation stasis (incomplete transformation) occurred. Concentrations at 610°C were intermediate between the values observed at 650°C and 630°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 ␣:␥ 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 2 C precipitation at ␣:␥ 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.

Coupled-solute drag effects on ferrite formation in Fe-C-X systems

“…In the companion article in this symposium, [8] an STEM technique [59,[114][115][116] was used to evaluate Mo enrichment at ␣:␥ boundaries in an Fe-0.24 pct C-0.93 pct Mo alloy as a function of isothermal reaction time at 650°C (just below the bay temperature), 630°C, and 610°C. The overall reaction kinetics at the two higher temperatures were previously investigated.…”

“…Root-mean-square diffusion calculations, made under the assumption that segregation occurred from ferrite, where diffusion is much more rapid than in austenite, [117] supported this deduction. Despite the ability of the STEM technique used to detect Ͻ2 pct of a monolayer of Mo enrichment, [6,8] a rough estimate of the number density of kinks on the risers of growth ledges indicated that this technique cannot reveal Mo accumulation at these sites. As noted in Section IV-A of the companion article, [8] it appears that these are the only viable sites for atomic attachment at partly coherent boundaries between crystals whose stacking sequence differs.…”

“…The second is the coupled-solute drag effect (C-SDE). [1][2][3][4][5][6][7][8] Although in Fe-C-X systems where the C-SDE is operative and the paraequilibrium Ae3 curve is shifted to higher temperatures and carbon concentrations, it will be seen that the C-SDE can be strong enough (at least in Fe-C-Mo alloys) so that when transformation takes place at intermediate reaction temperatures it can overpower the higher driving force made available for ferrite nucleation and growth by the Ae3 shift. [4] Problems with the definition of bainite [2] are circumvented by not discriminating between ferrite and bainite insofar as is practicable; only the ferritic component of pearlite is excluded from these considerations.…”

Comparative Effect of Mo and Cr on Microstructure and Mechanical Properties in NbV-Microalloyed Bainitic Steels

“…In high strength low alloyed steels Mo is well known to provide phase balance strengthening, via facilitating the bainite transformation [1][2][3][4][5], and solid solution strengthening [6][7][8][9]. It can decrease the rate of dynamic recrystallization of austenite [10][11][12], which may lead to grain refinement.…”

Scanning Transmission Electron Microscopy Study of Interfacial Segregation of Mn during the Formation of Partitioned Grain Boundary Ferrite in a Fe-C-Mn-Si Alloy

“…Interphase boundaries, in contrast, have received relatively little attention from experimentalists. [3][4][5][6][7][8] It is the purpose of this contribution to report results of studies of alloying element (Mn) distributions in the vicinity of (quenched) interfaces of grain boundary nucleated proeutectoid ferrite crystals, formed under substitutional solute partitioning conditions in a Fe-0.37C-3.0Mn-1.9Si steel. A lower carbon version of the same alloy was the subject of a previous study of ferrite growth and Mn accumulation at a/c interfaces, initially growing under nonpartitioning conditions.…”