arXiv subjects

Qiaodan Hu

Publications and source records attributed to Qiaodan Hu.

3 recordsLinked to original sources

arXiv2022

The role of solute concentration in interface instability during alloy solidification: A viewpoint from the free energy

Solidification structures are determined by the interaction between the interfacial processes and transport processes of heat and solute. In this paper, we investigate planar instability in directional solidification. Firstly, the interfacial evolution at the initial growth stage is simulated, indicating the planar instability is represented by the transition from the planar to the cellular. Secondly, to represent the history-dependence of solidification, constant thermal gradient G and varying pulling speed VP are used in the simulations. The results indicate the cooling rate R ( = G*VP) dominates the overall propagation speed of the interface, to maintain the local thermodynamic equilibrium. The solute segregation determines the stability of the interface, by changing the excess free energy at the interface and corresponding interface energy. Finally, the simulations of the grains with different preferred crystallographic orientations are performed, indicating the surface energy and its anisotropy do not affect the solute diffusion and planar growth. The results also verify the conclusion that solute segregation influences the interface energy and results in interface instability. On the other hand, for the planar-cellular transition, the minimum surface stiffness rule is more suitable than the maximum surface energy rule. The influence of the solute concentration on the excess free energy and interface energy can be applied to other solidification patterns induced by the interface instability, which will be studied in the future.

cond-mat.mtrl-sci↗

arXiv2022

The effect of the interface energy on pattern selection in alloy solidification: A phase-field study

A thorough understanding of pattern selection is necessary for the control of solidification structures, which are dissipative structures created by irreversible processes. In this paper, we simulate solidification evolution with different Preferred Crystallographic Orientations (PCOs) through the Phase-Field model. Then we study the effect of solute segregation on the interface energy, as well as the influence of the interface energy on the pattern selection. At the initial stage, the solute segregation influences the interface energy, determining the instability of the planar interface. During the detailed evolution of the Planar-Cellular-Transition (PCT), the surface stiffness dominates this stage. At the PCT stage, high degree of solute segregation refers to the low interface energy, resulting in the appearing of the sidebranches behind the tip of the primary dendrites. At the steady-state stage, the overall propagation velocities of the interfaces are the same, while the tip velocities are different in the simulations with different PCOs. The different tip velocities give rise to the different morphological evolution of the interfaces. The viewpoints of the whole dissipative system and the local free energy are discussed, respectively. This paper demonstrates the effect of solute segregation on the interface energy, as well as the influence of the interface energy on the pattern selection.

cond-mat.mtrl-sci↗

arXiv2022

Microstructure evolution under the space-time variational solidification conditions in a melt pool: A multi-scale simulation study

The properties of welded components are dominated by the microstructure evolution in the pool, where the solidification conditions are space-time variational. To represent the variational solidification conditions in the pool, the multi-scale simulation is carried out in this paper, combining the microscopic Phase-Field (PF) equations with the macroscopic thermal processes. Firstly, two different models, the GR model and TF model, are employed to simulate the single crystal solidification at a local region of the pool. Results suggest the TF model is more suitable to reflect the variational conditions than the GR model. Secondly, the single-crystal solidification and poly-crystal solidification at the whole region of the pool are performed through the TF model. The results demonstrate the space-time variabilities of the solidification conditions across the melt pool. Meanwhile, the variational conditions affect the microstructure evolution significantly, including the onset of initial instability at the epitaxial growth stage and the directional evolutions of the converging grain boundaries (GBs) and diverging GBs at the competitive growth stage. Moreover, the formation of axial grain structures is observed, which can be regarded as the competition between the grains along the axial direction and radial direction. This study indicates the necessity of considering the variational conditions in a pool. Meanwhile, the PF model can simulate microstructure evolution under the variational conditions accurately, which has a great potential for investigating solidification dynamics in the melt pool.

cond-mat.mtrl-sci↗