World Journal Of Engineering Nanoscaled Grain Growth F. Liu, M.M. Gong State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, P.R. China Email: liufeng@nwpu.edu.cn; Tel : 0086-29-88460374 Owing to the purpose for industrial of grain growth, the steady solute distribution application, to suppress grain growth of can hardly exist. Then, aimed to operate the nanocrystalline materials and to hold the nonsteady solute distribution, with the excellent properties resulted from nano-scale assumption that the drag exerted by the appear to be extremely important [1, 2]. As a segregation is proportional to the difference promising method to improve stability and to between solute concentration in bulk and GBs, inhibit grain growth, solute addition has been a grain size dependent drag was presented. demonstrated in various nanocrystalline Noting that, as shown by J.W. Cahn, the drag materials, ascribed to the effect from the force is a complex function of both the grain segregation of solute in grain boundaries size and the GB velocity, at the limit of low (GBs). The role played by GB segregation on growth velocity, the drag force was found to grain growth of nanocrystalline materials has be proportional to the product of grain size been modeled: and GB velocity. Furthermore, it could be I. Thermodynamically, as a mainly part of deduced that the drag force dependent on driving force for grain growth, the grain grain size and on growth velocity can only boundary energy is regarded as const in retard but not stop GB migration; see Fig. 2. common. However, under the condition of As mentioned above, the reduction of the solute segregation in GBs, on the basis of the GB energy and the generation of drag force Gibbs adsorption theorem the GB energy is both affect the grain growth of nanocrystalline thought to be reduced with segregation materials by the thermodynamic and kinetic promoting (see Fig. 1) and demonstrated in ways separately. Thereafter, combining each Fe-P alloys. Furthermore, a systemic factor individually with the parabolic equation metastable state where the GB energy reduces can provide an insight for grain growth of to zero with solute segregating has been nanocrystalline materials. However, the whole proposed. That is to say, GB segregation description of growth kinetics requires reduces the driving force to retard grain incorporating these two factors into the growth; and when the GB energy becomes parabolic equation simultaneously. Applying zero, the grain growth stops. these three revised grain growth formulas II. Kinetically, considering the difference of which are subjected to the thermodynamic solute concentration between GBs and the factor, the kinetic factor and the coupling of bulk, a drag force on GB migration has been these two factors to the related experimental assumed to explain the retardation of grain data, it shows that the reduction of the GB growth from the reduction of GB migration. A energy, i.e. thermodynamic factor plays the const drag term was firstly proposed to depend predominated effect on the process of grain on the final grain size. Nevertheless, only in growth for nanocrystalline materials the situation where a steady-state solute meanwhile the solute drag, i.e. kinetic factor distribution in GBs has been established, the only slows down this process; see Fig. 3. const drag is adaptive. In fact, before the stop 705 World Journal Of Engineering lines are calculated using these three revised grain growth formulas which are subjected to the kinetic factor, the thermodynamic factor and the coupling of these two factors [5]. Reference [1] H. Gleiter, Prog. Mater. Sci. 33, 233 (1989) [2] K. Lu, Mat. Sci. Eng. R, 16, 1611 (1996) [3] R. Kirchheim, Acta Mater. 55, 5129 (2007) [4] Natter, H., Löffler, M. S., Krill, C. E., Hempelmann, R., Scripta Mater. 44, 2321 (2001) [5] Chen, Z., Liu, F., Wang, H. F., Yang, W., Yang, G. C., Zhou, Y. H., Acta Mater. 57, 1466 (2009) Fig. 1 Schematic illustration of the GB energy, σb with increasing chemical potential, μB according to the Gibbs adsorption isotherm [3]. Fig. 2 Comparison of Burke’ model, Michels’ model, and Rabkin’s model. Assuming an initial grain size D0 equaling to 0.05Dmax, the scaled grain size D/Dmax is plotted as function of the scaled annealing time τ=(k/Dmax2)t. Fig. 3 Evolutions of average grain size of the oxygen-doped nickel samples with different oxygen contents at 673 K [4] with annealing time. Meanwhile, the dotted, dashed and solid 706