Papers by Keyword: Solute Drag

Abstract: The comparative effectiveness of solute Nb and NbC particles at impeding grain boundary motion is treated theoretically. It is shown that, for a steel containing ~0.05 at% Nb (~0.1 wt%Nb), under typical recrystallization conditions, solute Nb is more effective in the ferrite, whereas in the austenite, depending on the exact recrystallization temperature, either solute Nb or NbC precipitates may be more effective.

Abstract: A nominally pure Al slab was thermo-mechanically treated to result in a near random texture of 90 m grain size. Subsequent cold rolling with intermediate anneals at 230, 275, and 300°C reduced the Fe solute to near equilibrium compositions below 0.5 ppm atomic. The final cold rolled sheet continuously recrystallized; grain growth of this structure is reported. A grain-growth kinetics mapping was generated, correlating the parameters of Fe-in-Al solubility limit, Fe diffusivities in the grain boundaries and the Al lattice and the activation energies for migration rates.

Abstract: This paper described new characterization methods and data to quantify the influence of solute atoms on grain boundary and sub-grain boundary mobilities in Al-Mn alloys with a view to their integration into recovery and recrystallization modelling. Detailed SEM measurements of grain boundary mobilities during recrystallization have been made by in-situ annealing experiments on cold deformed Al – 0.1 and 0.3wt.% Mn binary alloys. Stored energies are estimated from the sub-grain sizes and misorientations and the boundary velocities directly measured in the temperature range 200-450°C. It is shown that in many cases good agreement with the Cahn, Lücke, Stüwe model for solute drag is obtained, e.g. the activation energies are intermediate between those of boundary and volume solute diffusion. Some particular cases of rapid growth occur in Al-0.1%Mn indicating boundary breakaway from solute clouds. A complementary study of sub-grain boundary mobilities has started on the same alloys; in this case the average mobilities are estimated from FEG-SEM growth data for the average sub-grain size for temperatures in the range 150-300°C. The results are compared with some previous data on Al-Si and show similar rates.

Abstract: In this paper we discuss the principles of a combined approach to solve the problem of solute drag as it occurs in microstructure evolution processes such as grain growth, recrystallization and phase transformation. A recently developed irregular grid cellular automaton is used to simulate normal grain growth, in which the energy of the grain boundaries is the driving force. A new, discrete diffusion model is used to simulate solute segregation to the grain boundaries. The local concentration of the solute is then taken into account in the calculation of the local grain boundary mobility and/or grain boundary energy, thereby constituting a drag force. The relation between solute concentration and grain boundary mobility/energy is derived from molecular dynamics simulations.

Abstract: Impurity segregation at grain boundaries in polycrystalline alloys is known to have a tremendous impact on the material properties such as grain boundary mobility, cohesion... But direct measurement of grain boundary chemistry is quite complex and there are few results concerning polycrystals. In this paper we present an indirect method to measure segregationmisorientation dependence on polycrystalline Ni-S alloys using 3D reconstruction of etch grooves. Samples of Ni-S alloy (1 ppm at) have been cold rolled at respectively 0.3 and 0.9 Von Mises equivalent strain and then annealed at 455°C to promote recrystallization. Then they have been etched near the transpassive potential to form etch grooves, whose geometry depends on the sulfur segregation level. It is found that the sulfur concentration at grain boundary decreases significantly when the driving force for grain boundary migration (i.e. the initial strain) increases, as predicted by solute drag theory.

Abstract: Atomic-scale interface segregation behaviour of Nb and Mo during different stages of recrystallisation of a-Fe has been investigated using a three-dimensional atom probe (3DAP). Experimental procedures to analyse a specific region of interest in the specimens and to determine an orientation relationship between analysed contiguous grains are briefly described, and then analytical artefacts which may affect the measured solute distribution are discussed. Atom probe analysis reveals that Gibbs free energy of segregation of Nb is larger than that of Mo in a-Fe, implying that a stronger solute Nb-interface interaction can be a reason for the larger retardation effect of recrystallisation by Nb addition. The comparison of measured solute profiles at migrating recrystallisation interfaces with calculated solute profiles show that Cahn’s solute drag model gives a reasonable fit to solute profiles for migrating interfaces.

Abstract: The migration rates (V) of “random” high angle grain boundaries (HAGB) during annealing of a cold deformed (e=1.3) high purity Al-0.1wt% Mn alloy were determined using a combination of in-situ annealing and EBSD in the SEM at temperatures between 200 and 330°C. The SEM heating stage used for these experiments is described and results on the local recrystallization kinetics of the Al-Mn alloy are presented. For this, the local stored energies (P) were determined by subgrain size and misorientation analyses to give the boundary mobilities (M) through the standard V = M.P equation. The solute drag “force” was analysed with the atomistic model of Lücke and Stüwe (1971) for a “loaded” boundary and used to estimate the diffusion rates in the above temperature range. The activation energies for boundary migration were found to be consistent with those of solute atoms moving behind the grain boundary, i.e. intermediate between the values for bulk and boundary diffusion of Mn.

Abstract: The recrystallisation kinetics experimentally measured on a supersaturated AA3103 after hot rolling is analysed using a numerical model. It is shown that temperature and nucleation inhibition due to precipitation on the subgrain structure are the two most important parameters in controlling the recrystallisation kinetics. On the opposite, particle pinning of recrystallised grain boundaries is negligible while pinning of subgrain boundaries during recovery and solute drag are relevant but second order effects.

Abstract: The historical development of the two approaches to the interaction between solute atoms and a migrating interface, based on dissipation of Gibbs energy and on solute drag, are reviewed and compared. In the way the solute drag was formulated long ago for recrystallization and grain growth, it does not apply to phase transformations. With a new solute drag equation, which was recently proposed, it turns out that the two approaches are completely equivalent for phase transformations as well as grain boundary migration.