Influence of low-gravity solidification on heterogeneous nucleation in stable iron-carbon alloys

The processes of nucleation and growth of alloys during solidification are linked to the level of gravitational force. In a low-gravity environment, buoyancy-induced convection becomes negligible, resulting in lower convection as compared to normal or high gravity. In this paper, heterogeneous nucleation and grain multiplication during solidification of gray cast iron, and the effect of gravitational level on them, have been studied by means of directional solidification on ground and under low-gravity (low-g) and high-gravity (high-g) conditions obtained by aircraft parabolic flights. It has been assumed that the final number of eutectic grains results from the contribution of heterogeneous nucleation,N _h , heterogeneous nucleation induced by inoculation,N _i , and heterogeneous nucleation induced by convection,N _c . In turn,N _c has two components, a grain multiplication component,N _c ^m , and a kinetics of chemical reactions component,N _c ^k . In all cases, it was found that a higher number of grains are obtained when solidifying in highg as compared with lowg. This was attributed to higher convection in highg. It was demonstrated that grain multiplication due to convection can contribute 20 to 23 pct from the total number of grains resulting from heterogeneous nucleation of uninoculated samples. For the case of inoculated samples, it was shown that the contribution to the convection-induced nucleation of the kinetics of chemical reactions can be as high as 30 pct but can be zero at very low or very high grain numbers. A possible mechanism and an explanation have been given to those findings. The silicon distribution, graphite morphology, and the influence of soak time on experimental results have also been discussed.     

Phase nucleation and growth mechanisms during the solidification of Fe-B hypoeutectic alloys under melt-quenching conditions

The solidification laws of hypoeutectic alloys Fe-B during rapid cooling are studied. The solidification mechanisms of heterogeneous and homogeneous nucleation and the following crystal growth are considered depending on the cooling rate of a melt layer and the conditions before the solidification front. The nucleation conditions for different types of borides and α and γ-Fe are studied. The relation between the type and morphology of the forming crystalline phases and the temperature-concentration conditions of their nucleation and growth is shown.     

The directional solidification of Pb-Sn-Cd alloys

The growing interest in composite structures for new material applications makes it necessary to determine just how generally we can apply existing solidification theory to controlled three-phase ternary solidification. The Pb-Sn-Cd ternary eutectic system was used as a suitable model system to completely map the phase morphology as a function of G/R and compositions. By carefully controlling the freezing rate and the thermal gradient in the liquid ahead of the solid-liquid interface (in the range 400 to 500 C/cm) the following areas of interest were investigated: 1) the effect of growth velocity and composition on coupled structures, 2) ternary impurities and their effect on the minimum G/R for coupled growth in a binary system, 3) the effect of growth velocity and composition on the nonplanar interface structures, and 4) the adaptability of present theories (the constitutional supercooling criterion and Cline’s binary analysis) in predicting the region of coupled growth in a three-component eutectic system growing at steady-state. It was found that much of the one and two-phase directional solidification theory and terminology can be directly extended to a ternary eutectic system. This suggests a further extension to n-phase, m-component systems (m ≥ n) with at least a qualitative understanding of the solidification process. The Authors wish to acknowledge the support of the National Science Foundation which made this study possible.     

Nonequilibrium solidification of Fe-P alloys

The specific features of solidification of Fe-P alloys in the concentration range 5?C25 at % P have been studied by differential thermal analysis and X-ray diffraction. The equilibrium ??-Fe + Fe₃P eutectic is shown to form immediately from melt only for compositions with 17?C19 at % P when a melt is superheated below temperatures of 1130?C1160°C, at which the type of composition short-range ordering changes (Fe₃P ?? Fe₂P) under insignificant (??50°C) supercooling conditions. A nonequilibrium ??-Fe + Fe₂P eutectic forms during cooling of the alloy at a large (??200°C) supercooling.     

Eutectic solidification of aluminum-silicon alloys

A mechanism that describes nucleation and growth as well as morphology modification by chemical additives of the eutectic phases in aluminum-silicon hypoeutectic alloys is presented. The mechanism is supported with results of nonequilibrium thermal analyses, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), selected area electron diffraction, and elemental X-ray mapping, as well as results of high-temperature rheological measurements that are performed on alloy samples of precisely controlled chemistry.     

Dendritic solidification in binary alloys

Alloys generally solidify dendritically, and associated with that is the microsegregation of impurities. Pure metals also solidify in dendritic form as “thermal” dendrites, which actually segregate the system’s enthalpy. In this investigation, small additions of solute to succinonitrile have been studied and dendritic growth observed in a supercooled melt. This free dendritic growth-mode is similar to that experienced by equiaxed dendrites found in alloy castings. Observations of these free dendrites include measurement of velocity and tip radius of the dendrites at different supercoolings and solute concentrations.     

Densities of Pb-Sn alloys during solidification

Data for the densities and expansion coefficients of solid and liquid alloys of the Pb-Sn system are consolidated in this paper. More importantly, the data are analyzed with the purpose of expressing either the density of the solid or of the liquid as a function of its composition and temperature. In particular, the densities of the solid, Eqs. [15] and [16], and of the liquid, Eqs. [24] and [25], during dendritic solidification are derived. Finally, the solutal and thermal coefficients of volume expansion for the liquid are given as functions of temperature and composition (Figure 9).     

On solidification of hypereutectic Al-Si alloys

Precipitation of primary silicon was studied in Al-Si hypereutectic alloys with 15, 18, and 25 wt. % silicon content. The alloys were solidified with different cooling rates from different super heat temperatures. The liquidus and eutectic temperature were evaluated from the cooling curves. The liquidus temperature was found to decrease with cooling rate. The evaluation of microstructure showed that the fraction of primary silicon decreased with increasing the cooling rate and super heat temperature. Furthermore, the morphology of the primary silicon changed as an effect of cooling rate and super heat temperature.     

The alstruc microstructure solidification model for industrial aluminum alloys

The model predicts the solidification path in the aluminum corner of the AlCuFeMgMnSi phase diagram, with compensation for solid-state diffusion and particle growth undercoolings. Input is the composition and the rate of cooling. Output is the temperature vs fraction solid; the solid-state concentration profiles; the type, volume fraction, and size of the intermetallic particles; and also the temperature-dependent thermal conductivity, density, specific heat, and heat of fusion for use in thermal models.     

Density and solidification shrinkage of hypoeutectic aluminum-silicon alloys

The density of liquid and solid hypoeutectic aluminum-silicon alloys has been measured with high accuracy in the temperature range 400 °C to 800 °C by using the indirect Archimedian method. A eutectic mixture of KCl and LiCl salts was used as reference liquid. This method allowed measurements of density for both liquid and solid in the same experiment and thus reduced the systematic error in estimating the solidification shrinkage. The results show that the density of liquid aluminum-silicon alloy increases with increasing silicon content, while silicon reduces the density in the solid state. Silicon content reduces the solidification shrinkage from 6.6 pct for pure aluminum to 4.4 pct for Al-11.6 pct Si.     

The solidification characteristics of laser surface-remelted Fe-12Cr-nC alloys

Laser surface remelting experiments have been carried out in Fe-12Cr-nC alloys in order to characterize the effect of growth velocity on the microstructural scale and the mode of solidification. Two types of growth morphologies have been observed: cellular/dendritic at a low carbon content and dendritic at a high carbon content. The existing band of primary spacings for a given solidification condition was characterized and confirmed. A recent analytical expression by Hunt and Lu for cellular and dendritic structures was compared to the experimentally measured primary arm spacings, and a good agreement was obtained. Further, the relationship between the primary spacing and the dendrite-tip radius was also determined and compared to the experimental results using a simple geometrical consideration between the dendrite-tip radius and the primary spacing. Finally, a theoretical microstructural map was constructed and compared to the experimental results. A good agreement between the observed and the calculated phases was obtained.     

The breakdown of single-crystal solidification in high refractory nickel-base alloys

The breakdown of single-crystal solidification has been studied over a wide range of solidification conditions in ten superalloys with large variations in Re, Ta, and W content. Over the range of experimental conditions investigated, grain defect formation was sensitive to local thermaland solutal conditions. For a fixed alloy composition and withdrawal rate, the transition from single-crystal to equiaxed solidification did not occur abruptly. Instead, as thermal gradients were decreased in a series of experiments, isolated, highly misoriented columnar grains with the same composition as that of the base alloy developed in the presence of positive (stabilizing) thermal gradients with increasing frequency until the advance of the single-crystal front was completely blocked. The onset of columnar grain formation occurred when the primary dendrite arm spacing exceeded a critical value, corresponding to a morphological transition in the dendritic array. The onset of “freckling” was observed at the same primary dendrite arm spacing where misoriented columnar grains began to appear. In experiments with varying levels of refractory alloy content, there was also a strong correlation between the onset of grain formation and freckle formation. These observations strongly suggest that in high refractory content superalloys, the breakdown of single-crystal solidification and the formation of misoriented grains as well as freckle-type defects are sensitively dependent on thermosolutal convection processes.     

Contraction of aluminum alloys during and after solidification

A technique for measuring the linear contraction during and after solidification of aluminum alloys was improved and used for examination of binary and commercial alloys. The effect of experimental parameters, e.g., the length of the mold and the melt level, on the contraction was studied. The correlation between the compositional dependences of the linear contraction in the solidification range and the hot tearing susceptibility was shown for binary Al-Cu and Al-Mg alloys and used for the estimation of hot tearing susceptibility of 6XXX series alloys with copper. The linear thermal contraction coefficients for binary and commercial alloys showed complex behavior at subsolidus temperatures. The technique allows estimation of the contraction coefficient of commercial alloys in a wide range of temperatures and could be helpful for computer simulations of geometrical distortions during directchill (DC) casting.     

Dendritic solidification of alloys in low gravity

Gravity-driven convective flow influences dendrite morphology, interdendritic fluid flow, dendrite interface morphology, casting macrosegregation, formation of channel type casting defects, and casting grain structure. Dendritic solidification experiments during multiple parabolic aircraft maneuvers for iron-carbon type alloys and superalloys show increased dendritic spacing in low-gravity periods. Larger dendrite spacings for low-gravity solidification have also been reported for sounding rocket and space laboratory experiments for metal-model and binary alloys. Convection decreases local solidification time and increases the rate of interdendritic solute removal. The elimination of convection in low gravity is thus expected to increase dendritic spacing. Convection's effect on dendritic arm coarsening is expected to be dependent on the coarsening mechanism. Decreased coarsening in low gravity found for Al-Cu is indicative of coarsening predominately by arm coalescence. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Macrosegregation during directional solidification of Pb-Sb alloys

Macrosegregation of Sb was investigated during directional solidification of binary Pb-Sb alloys containing 2.2 and 5.8 wt% Sb over growth rates varying from 0.8 to 30 μm s^?1. The cellular to dendritic transition was observed at a growth rate of 3.0 μm s^?1 in Pb-2.2 Sb alloy in contrast to a growth rate of 1.5 μm s^?1 in Pb-5.8 Sb alloy. The chemical analysis data revealed considerable macrosegregation of Sb along the longitudinal section of alloys. The degree of macrosegregation increased with a decrease in the growth rate. This behavior is discussed in light of thermo-solutal convection in the mushy zone as well as that in the melt ahead of the solid-liquid interface.