Papers by Keyword: Solute Drag

Abstract: The controversy about the start up of the abnormal growth as heterogeneously engendered in the microstructure or produced by peculiar continuous incubation process has still not been solved. In this work the statistical theory of grain growth has been applied to treat the case of grain growth in the presence of an homogeneously unstable Zener drag. By the simulations presented it will be shown as abnormal grain growth is a result out of a continuous and homogeneous process without requiring any heterogeneity in the microstructure presumed to give local advantages to some grains. The mechanism by which during an incubation period the preconditions for the unstable growth are built up in the microstructure is clarified and discussed. Moreover the peculiar shape of Grain Size Distribution (GSD) approaching the “structural instability” will be also analytically defined and compared with experimental results obtained in a grain oriented Silicon Iron just before the onset of abnormal grain growth.

Abstract: A way to overcome the low deformability of magnesium alloys at room temperature is toincrease the temperature of forming operations. The stress exponent n, which is known to be a keyparameter in the control of plastic stability, generally decreases when temperature increases.Nevertheless, low n-values are not enough to ensure large capacity of deformation since fracturecan also result from strain induced cavitation. In the present investigation, both the mechanisms ofhigh temperature deformation and damage were studied in selected Mg alloys. Since damage datacan also give information on the deformation mechanisms, the strain induce cavitation behaviourwas mainly studied thanks to X-ray micro tomography which provides 3D information like thecavity shapes or the variation with strain of the number of cavities. Moreover, additionally toconventional post mortem analyses, it was attempted to perform the 3D damage characterisation inin situ conditions, namely directly during high temperature deformation tests.

Abstract: The deformability of wrought magnesium alloys at room temperature is limited and a way to overcome this limit is to carry out forming operations in warm or hot conditions. In the case of fine grained alloys, superplastic properties can be generally achieved but in this regime, the Mg alloys are sensitive to strain induced cavitation. However, large grained alloys can also exhibit quite large deformabilities when they are deformed at high temperature. This can be due to the fact that on one hand, the Mg alloys may quite easily dynamically recrystallize and on the other hand, that dislocation movements may be controlled by a solute drag effect leading to significant strain rate sensitivity parameters. These various mechanisms of deformation will depend on the composition, the mean grain size and the conditions of deformation (i.e. temperature and strain rate). In this work, the high temperature deformation mechanisms as well as the associated damage mechanisms of two wrought magnesium alloys are discussed.

Abstract: A simple analytical model is proposed for estimating grain boundary mobility during dynamic recrystallization in metallic alloys. The combined effects of solutes (solute drag) and second phase particles (Zener pinning) on mobility are considered. The approach is based on (and is consistent with) a recently published mesoscale model of discontinuous dynamic recrystallization. The dependence of grain boundary mobility on solute concentration and particle size is summarized in the form of two-dimensional maps.

Abstract: Abnormal grain growth (AGG) proceeds in case that normal grain growth is inhibited. It has long been known that the inhibition involves finely dispersed particles and/or the development of specific textures. There is another strong obstacle against the grain boundary (GB) motion; the solute atoms can reduce their energy by moving from the bulk into a GB. Resultant interaction between the solute atoms and a GB makes the GB motion more difficult. However the role of the GB segregation effect on AGG has not been clarified. In this study we simulate the 2D and 3D grain growth accompanying boundary segregation of solute atoms by using a phase-field model. It is shown that the segregation plays an important role on the occurrence of AGG. The boundary-segregation-induced AGG can take place when the average driving force of grain growth approaches a critical condition for pinning-depinning transition in solute-drag atmosphere.

Abstract: The current study aims to improve our fundamental understanding of solute segregation and solute drag on migrating grain boundaries (GB) in three dimensions. Computer simulation combines finite difference and finite element methods. An exemplary case study is reported, in which a spherical grain is embedded inside a cubic grain and shrinks as a result of motion by curvature, as a preliminary to modeling grain growth in single phase materials. The results agree qualitatively with literature studies in 1-D.

Abstract: The statistical model of grain growth is able to predict the effect of Zener drag on the grain size distribution evolution and on grain growth kinetics [1, 2]. This paper, in the same framework, will treat the case of atoms drag on grain boundary movement. The mechanism by which atoms drag operates is significantly different by that of Zener. The corresponding peculiar features will result in a specific grain size distribution evolution with considerable change of grain growth kinetics and distribution shape from that of normal grain growth case as a function of the intensity of the pinning conditions.

Abstract: Control of grain size during recrystallization of aluminum alloys is critical when tailoring material properties for structural applications. Most commonly the grain size is controlled by adding alloying elements which form second phases during homogenization heat treatments small enough to impose a Zener drag on the grain boundary mobility. These phases are known as dispersoids and are in the 10 to 200 nm in diameter range. In Al-Zn alloys, zirconium has been successfully used in controlling the degree of recrystallization after solution heat treatments. It is commonly understood that the Al3Zr dispersoids of about 20 nm in diameter present in the microstructure are the key features affecting grain boundary mobility. With the success of controlling recrystallization in Al- Zn alloys, zirconium has been added to other alloy systems, such as Al-Cu-Mn, and a similar retarding effect in recrystallization kinetics has been observed as seen in the Al-Zn systems. However, in Al-Cu-Mn alloys, zirconium bearing dispersoids are not observable in the microstructure. Consequently, additional microstructural effects such as solute drag need to be considered to explain the experimental observations. In this paper, the role of zirconium additions in aluminum alloys will be summarized.

Abstract: A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures by either a solute drag or by a precipitation pinning (when bonded to C or N) effect. Both mechanisms result in improved mechanical properties due to grain size refinement of the transformed ferrite. In this study, 3 Nb-microalloyed model alloys were designed to investigate the solute drag and the precipitation effect separately. The first alloy, containing a stoechiometric ratio of Nb and C, was designed to study the retarding effect of NbC on the recrystallization behavior. A second alloy, containing Nb and only few ppm C, was casted in order to study the effect of Nb in solid solution. The two alloys were compared with a C-Mn reference alloy. The recrystallization behavior of the three alloys were compared by multi-hit torsion tests and double hit compression tests. The Nb-C and the Nb-very low C showed small differences in recrystallization behavior. These results show that Nb delays the recrystallization by a solute drag effect or by the formation of a very small amount of precipitates.

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.