Dendritic Growth Front Research Articles

Phase-Field Simulation for Non-isothermal Solidification of Al-Cu Binary Alloy

Based on the principle of dilute solution approximation, a phase field model for dendritic growth during the non-isothermal solidification process was proposed by coupling the phase field, concentration field and temperature field, and was adopted to investigate the dendritic growth of Al-Cu binary alloy during the non-isothermal solidification process. Also, the simulations of free dendritic growth in an undercooled melt of Al-4.5%Cu binary alloy with different perturbation intensity and anisotropy were carried out by the present phase field model. The results show that during the non-isothermal solidification process of Al-4.5%Cu binary alloy, the dendrite grow into undercooled melt with the solute rejection and latent heat release at the front of solid/liquid interface, and the solute enriches at the dendrite root and high temperature appears at the dendritic growth front. With the increase of perturbation intensity, the dendritic growth becomes more developed and more branches appear. Moreover, the anisotropy coefficient also has a great effect on the dendritic growth, and the growth speed of dendrite increases with the increase of anisotropy coefficient.

The effect of welding conditions on stray grain formation in single crystal welds – theoretical analysis

Stray grain formation during solidification of nickel-based single crystal superalloy welds leads to a degradation of mechanical properties and cracking. Based on the mechanism of constitutional supercooling ahead of the advancing dendritic growth front, the effect of welding conditions (weld speed and power) on the tendency to form stray grains during solidification was evaluated. A simple 3D thermal model was combined with a geometric model to determine the extent of stray grain formation as a function of welding conditions and position in the weld pool, taking into account the influence of dendrite growth orientation. A parameter describing the degree of stray grain formation, averaged over the entire solidification front, was calculated. Processing maps that show the severity of stray grain formation as a function of weld speed, power and orientation were developed. It was found that low power and high weld speed were optimal for minimizing the stray grain formation potential. The effect of crystallographic orientation of the weld on the overall tendency to form stray grains was minimal, although local variations based on orientation were quantified. The theoretical analysis was compared to earlier experimental work on laser welded nickel-based single crystals and all of the experimental observations could be reproduced in the calculations.

Nickel induced lateral crystallization behavior of amorphous silicon films

Nickel induced lateral crystallization of amorphous silicon with and without electric field has been studied. Dendritic silicon growth behavior is observed, with crystallites of a few hundred nanometers in width and up to a few microns in length. The behavior can be understood from the preferential epitaxial growth of silicon from the (1 1 1) facets of the NiSi 2 precipitate, which forms during the early stage of the annealing process. The dendritic growth fronts are different with and without electric field in the nickel induced lateral crystallization process. Electric field is found to be beneficial in increasing the lateral crystallization rate and improving the film crystallinity. Joule heating plays an important role as well to enhance the lateral crystallization.

Directional solidification of white cast iron

Several studies of the ledeburite eutectic (Fe-Fe{sub 3}C), in pure Fe-C alloys have shown that it has a lamellar morphology under plane front growth conditions. The structure of ledeburite in white cast irons, Fe-C-is, consists of a rod morphology. It is generally not possible to produce plane front growth of Fe-C-Si eutectic alloys in the Fe-Fe{sub 3}C form, because at the slow growth rates required for plane front growth, the Fe{sub 3}C phase is replaced by graphite. By using small additions of Te, the growth of graphite was suppressed, and the plane front growth of the ledeburite eutectic in Fe-C-Si alloys was carried out with Si levels up to 1 wt pct. It was found that the growth morphology became a faceted rod morphology at 1 wt pct Si, but in contrast to the usual rod morphology of white cast irons, the rod phase was Fe{sub 3}C rather than iron. It was shown that the usual rod morphology only forms at the sides of the two-phase cellular or dendritic growth fronts in Fe-C-Si alloys. Possible reasons for the inability of plane front directional solidification to produce the usual rod morphology in Fe-C-Si alloys are discussed. Also, data are presented onmore » the spacing of the lamellar eutectic in pure Fe-C ledeburite, which indicates that this system does not follow the usual {lambda}{sup 2}V = constant relation of regular eutectics.« less

Some Consequences of Thermosolutal Convection: The Grain Structure of Castings

The essential principles of thermosolutal convection are outlined, and how convection provides a transport mechanism between the mushy region of a casting and the open bulk liquid is illustrated. The convective flow patterns which develop assist in heat exchange and macroscopic solute segregation during solidification; they also provide a mechanism for the transport of dendritic fragments from the mushy region into the bulk liquid. Surviving fragments become nuclei for equiaxed grains and so lead to blocking of the parental columnar, dendritic growth front from which they originated. The physical steps in such a sequence are considered and some experimental data are provided to support the argument.

Convective flow effects on diffusion layers during NH4Cl− H2O dendritic solidification

Diffusion layer widths ahead of NH 4Cl-71.5wt%H 2O dendritic growth fronts are compared for microgravity and ground based experiments. It is argued from the data that both micro (dendritic stalk boundary layer) and macro (bulk liquid) convection significantly influence the size of the ground-based layer, enhancing the dendrite's lateral growth but also limiting the tip growth. The use of the microgravity environment illustrates the significant aspects of convection free measurements in solidifying systems.

Anisotropy in twinned terfenol-D crystals

The highly magnetostrictive cubic compound Terfenol-D (Tb0.3Dy0.7Fe2) solidifies via a {211} dendritic growth front when grown by a free-standing zone technique. The resulting material is usually composed of dendritic plates often containing crystallographic twins, the predominant plate and twin plane being the (11̄1) orthogonal to the (2̄1̄1) growth plane. Results of room temperature magnetic torque analysis from (011) disk specimens, having differing twin densities, are presented which yield both the magnetic anisotropy constants, K1=−1.6×105 J/m3 and K2=−0.16×105 J/m3, and the relative parent/twin volume. Magnetic susceptibility data both parallel and transverse to the applied field are presented which, in conjunction with the anisotropy results, emphasize the importance of twin density on magnetoelastic response for typical application geometries.

Channel formation in Pb-Sn, Pb-Sb, and Pb-Sn-Sb alloy ingots and comparison with the system NH4CI-H2O

The formation of segregation channels during the unidirectional solidification of base chilled ingots has been studied as a function of composition in binary Pb-Sn and Pb-Sb and ternary Pb-Sn-Sb alloys. The patterns of channel distribution were characterized in the binary and ternary systems and are described as functions of temperature gradients, growth rates, dendrite spacings, and interdendritic permeabilities. Channels appear to nucleate at random across a dendritic front and subsequently to interact as they propagate, decreasing in density across the front. Assuming that the interdendritic spacing is the characteristic distance for a liquid perturbation, yields critical effective Rayleigh numbers which lie within a factor of x40 for both metallic and aqueous systems. This correlation is close, considering the sensitivity to any assumed dimension and the range of material properties involved, and is taken to support a model for channel nucleation occurring close to the dendritic growth front.

The growth of single crystal Terfenol-D crystals

Rods of the highly magnetostrictive material, Terfenol-D, Tb0.3Dy0.7Fe2, have been prepared by both Bridgman and float zone techniques. It is found that the dendritic growth front consists of parallel sheets of dendrites growing with a primary direction of and sheet planes of {111}. Unseeded rods show a strong preference for alignment of the grains. Seeding experiments have been successful for orientations but not , and the cause of this difference is discussed. The single crystal rods are seen to contain parallel {111} twin boundaries throughout their volume, growing near the central plane of the dendrite sheets. These results are analogous to those of Ge and Si. The role of the twin planes on domain wall motion and methods of eliminating the twins are discussed.

The mechanisms of formation and prevention of channel segregation during alloy solidification

Conditions for the formation of macroscopic segregation channels have been examined in the ammonium chloride-water and lead-tin systems, using base chilled molds. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (≺5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 deg to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. Artificially created channels or those momentarily blocked fail to continue and are overgrown, but channels can be initiated by drawing liquid upward from close to the growth front in fine capillaries. Examination of these effects leads to the conclusion that channels originate at the growth front, rather than within the dendritic array, and that their formation is necessarily preceded by a liquid perturbation from the less dense boundary layer into the supernatant, quiescent bulk liquid. Intermittent ‘solute fingers’ are then fed by dendritic entrainment to produce stable convective plumes and concomitant channels. It is considered that the effects of mold precession are primarily caused by translation of bulk liquid across the dendritic growth front, shearing off convective perturbations from the boundary layer before they have time to develop. The nature of the liquid movements is discussed and shown to be a function of the mold dimensions. The inclination of the gravitational vector within the solid-liquid, dendritic array is considered to be of secondary importance to the formation or prevention of channels.

Solute distributions at non-planar, solid-liquid growth fronts: III. With convective mixing in the liquid

The distribution of solute at cellular and dendritic growth fronts has been studied in aluminium—copper alloys under conditions of natural convective mixing in the liquid. The information presented concerns (a) the macroscopic segregation at a constant freezing rate, (b) observations and calculations of the thickness of the unstirred boundary layer, and (c) interface morphology and microsegregation with conditions of liquid stirring.