The solidification microstructures of Pb-45% Sn hypoeutectic and Pb-85%Sn hypereutectic alloys were studied in rotating magnetic field (RMF). A transition of primary phase from dendrite to spherical growth was caused by the RMF, which simultaneously fractured and fined grains. The fracture and refinement increased first and then decreased with the increase in RMF intensity. When magnetic intensity exceeded a critical value, the size of the primary phase became bigger on the contrary. Therefore, there existed an optimum value of magnetic intensity in fracture and refinement of grain. Moreover, the rotating frequency determined the skin depth of magnetic field, and further affected the homogenization of temperature and solute. The rotating frequency and magnetic intensity were key factors affect- ing refinement and uniformity of solidification microstructures. The introduction of RMF not only changed the solidification thermodynamics, but also led to a reduction in Gibbs free energy associated with the formation of critical crystal nucleus and atom diffusion activation energy, thus enhancing the rate of nucleation.
The influence of the rotating magnetic field (RMF) on the solidification proc- ess of Pb-Sn binary alloys is studied by comparing the solidification microstructures under the common condition and RMF condition. It is found that the RMF can completely elimi- nate the gravity induced macrosegregation, and result in dendrite fragmentation and promote solute diffusion velocity. These differences are regarded as the effect of compli- cated melt flow caused by RMF. Moreover, when the content of the primary phase is small, many spherical microstructures form under the RMF condition. The analyses indicate that these special microstructures are likely the conjunction action of melt flow and concentra- tion and temperature field uniformity caused by RMF.
MENG Xiaohua CHEN Changle HONG Zhenyu WANG Jianyuan
Liquid-solid interface morphological evolution of SCN-2wt%H2O alloy was investigated in the presence of shear flow. It has been found that shear flow strikingly enhances the stability of planar interface at lower pulling rates. In the intermediate range of pulling rate, the interface experiences periodical oscillating "planar-cellular" and "cellular-dendritic" growth patterns. The formation of oscillating growth modes can be explained by two reasons: (i) The stabilizing effect induced by shear flow competes with the growing destabilizing effect by the increase of pulling rate, which provides the driving force for morphological evolution; and (ii) the establishment of stable solute field at the liquid-solid interface needs a period of time, leading to the result that the interface evolution can not stay stable at the equilibrium state. However, as the puling rate becomes higher, it plays more and more important roles in pattern selection and dendritic growth forms.
The directional solidification process of SCN-3wt%Salol transparent alloy is investigated in the presence of the shear flow at the liquid-solid interface.It is found that the shear flow induces a stabilizing effect on planar interface.At higher pulling rates,oscillation of the growth pattern together with fluctuation of the growth velocity takes place.With the increase of the pulling rate,the interface growth pattern transits from"planar-cellular"oscillation to"cellular-dendritic"oscillation,and the periodicity increases.The modification of the growth pattern is due to the effect of the shear flow on solute distribution,and the time and history dependent character of interface morphology evolution also plays an important role in the formation of the oscillating growth pattern.
An experimental apparatus consisting of a crystal growth room and a crystal growth observation system was developed for the study of the effect of the gravity convection perpendicular to the growth direction on the growth process by use of model alloy succinonitrile (SCN)-5wt%ethanol. It was found that the convection improves the stability of the interface and causes the downstream alternation of the cell growth direction because of the dual effect of the Stokes force and the gravity. The second dendrite arm facing the flow comes into being earlier than that at an- other side when the interface transforms cell to dendrite. Then the dendrite at the side facing the flow comes into being earlier. The second dendrite arm facing the flow grows faster and is more developed than that at another side. In addition, the primary dendrite arm spacing increases and the dendrite tip radius decreases un- der the gravity convection.
Numerical simulation based on phase field method is performed to describe solidification process of pure material in a free or forced flow. The evolution of the interface is showed, and the effects of mesh grid and flow velocity on succinonitrite shape are studied. These results indicate that crystal grows into an equiaxial dendrite in a free flow and into an asymmetrical dendritic in a forced flow. With increasing flow velocity, the upstream dendritic arm tip grows faster and the downstream arm grows slower. However, the evolution of the perpendicular tip has no significant change. In addition, mesh grid has no influence on dendritic growth shape when mesh grid is above 300×300.
This paper reports that the rapid solidification of mixed Li 2 B 4 O 7 and KNbO 3 melted in a Pt loop heater has been performed experimentally by the method of quenching, and various morphologies of KNbO 3 crystals have been observed in different regions of the quenched melt-solution. Dendrites were formed in the central region where mass transfer is performed by diffusion, whereas polygonal crystals with smooth surface grew in the marginal region where convection dominates mass transport. Based on measurement of KNbO 3 concentration along crystal interface by electronic probe analysis, it finds the variety of crystal morphologies, which is the result of different solute distributions: in the central region the inhomogeneity of solute concentration is much sharper and morphological instability is easier to take place; nevertheless in the marginal region the concentration homogeneity has been greatly enhanced by convection which prevents the occurrence of morphological instability. Additional solute distribution in the melt along the primary dendrite trunk axis as well as that in mushy zones has also been determined. Results show that the solute concentration in the liquid increases linearly with distance from the trunk tip and more solutes were found to be concentrated in mushy zones. The closer the mushy zone is to trunk tip, the lower the solute concentration will be there.
Magnetocaloric and colossal magnetoresistance effects of the layered perovskite La 1.4 Sr 1.6 Mn 2 O 7 compound have been studied.A broad peak of magnetic entropy change (-△S M) is found above the Curie temperature (T C =120 K),which can be associated with the existence of two-dimensional short range ferromagnetic order.Additionally,the curvilinear shape of-△S M for layered perovskite is quite different from that of the Ln 1-x A x MnO 3 probably arising from magnetocrystalline anisotropy.At the same time,a wide peak of colossal magnetoresitance effect near T C is found in the layered provskite La 1.4 Sr 1.6 Mn 2 O 7.
