Upon non-equilibrium solidifications, dendrite growth, generally as precursor of as-solidified structures,has severe effects on subsequent phase transformations. Considering synergy of thermodynamics and kinetics controlling interface migration and following conservation of heat flux in solid temperature field, a more flexible modeling for the dendrite growth is herein developed for multi-component alloys,where, two inherent problems, i.e. correlation between thermodynamics and kinetics(i.e. the thermokinetic correlation), and theoretical connection between dendrite growth model and practical processing,have been successfully solved. Accordingly, both the thermodynamic driving force G and the effective kinetic energy barrier Qeffhave been found to control quantitatively the dendrite growth(i.e. especially the growth velocity, V), as reflected by the thermo-kinetic trade-off. Compared with previous models, it is the thermo-kinetic correlation that guarantees quantitative connection between the practical processing parameters and the current theoretical framework, as well as more reasonable description for kinetic behaviors involved. Applied to the vertical twin-roll casting(VTC), the present model, realizes a good prediction for kissing points, which influences significantly alloy design and processing optimization.This work deduces quantitatively the thermo-kinetic correlation controlling the dendrite growth, and by proposing the parameter-triplets(i.e. G-Qeff-V), further opens a new beginning for connecting solidification theories with industrial applications, such as the VTC.
Yubing ZhangJinglian DuKang WangHuiyuan WangShu LiFeng Liu
Second-phase particle pinning has been well known as a mechanism impeding grain boundary(GB)migration, and thus, is documented as an efficient approach for stabilizing nanocrystalline(NC) materials at elevated temperatures. The pinning force exerted by interaction between small dispersed particles and GBs strongly depends on size and volume fraction of the particles. Since metallic oxides, e.g. Al_2O_3,exhibit great structural stability and high resistance against coarsening at high temperatures, they are expected as effective stabilizers for NC materials. In this work, NC composites consisting of NC Fe and Al_2O_3 nanoparticles with different amounts and sizes were prepared by high energy ball milling and annealed at various temperatures(T_(ann)) for different time periods(t_(ann)). Microstructures of the ball milled and annealed samples were examined by X-ray diffraction and transmission electron microscopy. The results show that the addition of Al_2O_3 nanoparticles not only enhances the thermal stability of NC Fe grains but also reduces their coarsening rate at elevated temperatures, and reducing the particle size and/or increasing its amount enhance the stabilizing effect of the Al_2O_3 particles on the NC Fe grains.
Grain boundary(GB) segregation in nanocrystalline alloys can cause reduction of GB energy, which leads to thermodynamic stabilization of nanostructures. This effect has been modelled intensively. However,the previous modelling works were limited to substitutional alloy systems. In this work, thermodynamics of nanocrystalline binary interstitial alloy systems was modelled based on a two-sublattice model proposed by Hillert [M. Hillert, et al. Acta Chem. Scand., 24(1970) 3618] and an atomic configuration for nanocrystalline systems proposed by Trelewicz and Schuh [J.R. Trelewicz, et al. Physical Review B, 79(2009) 094112]. The modelling calculations agree with the reported experimental data, indicating that the current thermodynamic model is capable of accounting for the alloying effect in the nanocrystalline binary interstitial alloys.
Guibin ShanYuzeng ChenMingming GongHao DongFeng Liu
