Influence of a High Static Magnetic Field on the Morphology of Primary Al3Fe Phase in Al-3%Fe Alloy

It had been done the experiments of the solidification on Al-Fe alloy under a high static magnetic field (10T). The effect of high magnetic field on the morphology of primary Al3Fe phase in Al-3%Fe alloy solidification structure has been investigated by analyzing the microstructures. The experimental results shew that the variation of the morphology of Al3Fe phase was obvious under a high static magnetic field, and them changed to particle-likes and short needles from needle-likes, and they were arranged in chains along the direction of magnetic field to form oriented layered structure. The critical nucleation work reduced and the nucleation rate increased under the applied field, and the magnetic interaction caused by the field can suppress the growth of needle-like Al3Fe phase, both of them resulted in the particle-likes and short needles grains of primary Al3Fe phase to nucleate and grow preferentially. Under the action of magnetic moment and the magnetic interaction force a high static magnetic field, the grains of Al3Fe rotated and then polymerized, and finally formed chain arrangements and layer structures.

Solidified Structure Refinement of H13 Tool Steel under a Multi-Rotational Speed Super Gravity Field

In this paper, the effects of a super-gravity field with multi-rotational speeds on the grain refinement and tensile properties of as-cast H13 steel were investigated systematically. The experimental results showed that compared to the single-rotational speed (conventional) super-gravity field, the as-cast grains of H13 steel can be significantly refined in a multi-rotational speed (speed increased in stages) super-gravity field. In the conventional super-gravity field, with the decrease in rotational radius, the secondary dendrite arm spacing (SDAS) and the prior austenite grain size (PAGS) increase, and the maximum values of SDAS and PAGS are 90 and 55 µm, respectively, while in multi-speed super-gravity fields, at the range of increasing rotational speeds, SDAS and PAGS decrease as the rotational radius decreases. In the three-rotational speed super-gravity field, the maximum values of SDAS and PAGS are 80 µm and 50 µm. In the five-rotational speed super-gravity field, the maximum values of SDAS and PAGS are reduced to 58 µm and 34 µm. Accordingly, both the tensile strength and the plasticity are enhanced when increasing the number of rotational speeds in the super-gravity field, especially for the inner position of the super-gravity sample. The ultimate tensile strengths at outer, middle, and inner positions of H13 steel solidified in the conventional super-gravity field are 1445 MPa, 1378 MPa, and 1023 MPa, corresponding to elongations of 2%, 1.5%, and 0.5%, respectively, while in the five-rotational speed super-gravity field, they are 1408, 1443, and 1453 MPa, corresponding to elongations of 1.8%, 3.9%, and 2.2%, respectively. The mechanism for the grain refinement is that multi-speed super-gravity can reduce the critical nucleation work of austenite and the tangential force produced by increasing the rotational speed break dendrites at the solidification front, refining the solidified structure.

Refinement on the Solidification Structure of H13 tool steel under Multi-Rotational Speeds Super-Gravity Field

In this paper, the effect of multi-rotational speeds super-gravity field on the grain refinement and tensile properties of as-cast H13 steel were investigated systematically. The experimental results revealed that the as-cast grains of H13 steel can be significantly refined in multi-rotational speeds supergravity field. In conventional supergravity field, with the decrease of rotational radius, the secondary dendrite average spacing (SDAS) and the austenite grain average size (AGAS) increase, and the maximum values of SDAS and AGAS are 90 µm and 55 µm, respectively. while in multi-speeds supergravity fields, at the range of increasing rotational speeds, SDAS and AGAS decrease as the rotational radius decreases. In three-rotational speeds supergravity field, the maximum values of SDAS and AGAS are 80 µm and 50 µm. In five-rotational speeds supergravity field, the maximum values of SDAS and AGAS are reduced to 58 µm and 34 µm. Accordingly, both the tensile strength and the plasticity are enhanced with the increasing the number of rotational speeds in supergravity field, especially for the inner position of supergravity sample. The ultimate tensile strengths at outer, middle, and inner positions of H13 steel solidified in conventional supergravity field are 1445 MPa, 1378 MPa, and 1023 MPa, corresponding elongations of 2%, 1.5%, and 0.5%, while in the five-rotational speeds supergravity field, they are 1408 MPa, 1443 MPa, and 1453 MPa, corresponding elongations of 1.8%, 3.9%, and 2.2%. The mechanism for the grain refinement is that multi-speeds super-gravity can reduce the critical nucleation work of austenite, and the tangential force produced by changing the rotational speeds breaks dendrites at the solidification front, refining solidification structure.

On the thermodynamic and kinetic aspects of immersion ice nucleation

Heterogeneous ice nucleation initiated by particles immersed within droplets is likely the main pathway of ice formation in the atmosphere. Theoretical models commonly used to describe this process assume that it mimics ice formation from the vapor, neglecting interactions unique to the liquid phase. This work introduces a new approach that accounts for such interactions by linking the ability of particles to promote ice formation to the modification of the properties of water near the particle–liquid interface. It is shown that the same mechanism that lowers the thermodynamic barrier for ice nucleation also tends to decrease the mobility of water molecules, hence the ice–liquid interfacial flux. Heterogeneous ice nucleation in the liquid phase is thus determined by the competition between thermodynamic and kinetic constraints to the formation and propagation of ice. At the limit, ice nucleation may be mediated by kinetic factors instead of the nucleation work. This new ice nucleation regime is termed spinodal ice nucleation. The comparison of predicted nucleation rates against published data suggests that some materials of atmospheric relevance may nucleate ice in this regime.

On the Thermodynamic and Dynamic Aspects of Immersion Ice Nucleation

Heterogeneous ice nucleation initiated by particles immersed within droplets is likely the main pathway of ice formation in the atmosphere. Theoretical models commonly used to describe this process assume that it mimics ice formation from the vapor, neglecting interactions unique to the liquid phase. This work introduces a new approach that accounts for such interactions by linking the ability of particles to promote ice formation to the modification of the properties of water near the particle-liquid interface. It is shown that the same mechanism that lowers the thermodynamic barrier for ice nucleation also tends to decrease the mobility of water molecules, hence the ice-liquid interfacial flux. Heterogeneous ice nucleation in the liquid phase is thus determined by the competition between thermodynamic and kinetic constraints to the formation and propagation of ice. At the limit, ice nucleation may be mediated by the dynamics of vicinal water instead of the nucleation work. This new ice nucleation regime is termed spinodal ice nucleation. Comparison of predicted nucleation rates against published data suggests that some materials of atmospheric relevance may nucleate ice in this regime.

Analysis of the effect of water activity on ice formation using a new thermodynamic framework

In this work a new thermodynamic framework is developed and used to investigate the effect of water activity on the formation of ice within supercooled droplets. The new framework is based on a novel concept where the interface is assumed to be made of liquid molecules "trapped" by the solid matrix. Using this concept new expressions are developed for the critical ice germ size and the nucleation work, with explicit dependencies on temperature and water activity. However unlike previous approaches, the new model does not depend on the interfacial tension between liquid and ice. Comparison against experimental results shows that the new theory is able to reproduce the observed effect of water activity on nucleation rate and freezing temperature. It allows for the first time a phenomenological derivation of the constant shift in water activity between melting and nucleation. The new framework offers a consistent thermodynamic view of ice nucleation, simple enough to be applied in atmospheric models of cloud formation.