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.     

Dendritic growth during directional solidification of hypoeutectic Fe-C-Si alloys

The liquid decanting technique has been used to study the morphology of dendrites in directionally solidified Fe-3.08 pct C-2.01 pct Si alloy. The experimental results indicated that the morphology of primary dendrites in the Fe-C-Si system is very similar to those obtained in some transparent metal model systems and in some other metal systems. In order to study the morphological transition between cellular and dendritic growth, directionally solidified samples were quenched in cold water at various stages of solidification and the morphology was examined on the polished and etched surface. It has been found that when the growth velocity decreased from 326.6 to 0.8 μn/s, the average dendrite tip radius increased from 1.12 to 33.1 μm. At a growth velocity of about 0.65 μm/s, a transition from dendritic to cellular growth occurred. Models for dendritic growth proposed by various investigators have been briefly reviewed and compared with the present experimental results. Significant disagreements were found for some of the available theoretical models. Possible explanations have been given for these disagreements.     

Directional solidification of lead-copper immiscible alloys in a cyclic gravity environment

Hypermonotectic copper-lead alloys were directionally solidified at unit gravity on earth and also in the cyclic gravitational environment attainable during flight of NASA's KC-135 aircraft. In both cases macrosegregation developed that consisted of an initial lead-rich phase above which an aligned composite structure of apparent monotectic composition grew. Differences within these regions are examined, and the suitability of the KC-135 environment for directional solidification of monotectic alloys is discussed. 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 25–229, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Dendritic solidification of Cu-Ni alloys: Part I. Initial growth of dendrite structure

Small Cu-Ni melts were furnace cooled at different rates and with different "nucleation" temperatures. Thermal analysis showed that the thermal arrests associated with the formation of the dendrite skeletons showed a significant supercooling below the liquidus temperatures. This supercooling increased with increased rates of heat extraction,i.e. higher cooling rates and lower "nucleation" temperatures; and was associated with higher dendrite growth velocities. The solute content of the dendrites measured on samples quenched during the thermal arrest quantitatively supported these observations. The magnitude of the supercooling for a given rate of heat extraction varied directly with the freezing range of the alloy but remained finite (although small) for unalloyed copper. The approximate measurements of dendrite growth velocities for one alloy (40 wt pet Ni) at different supercoolings agreed well with the predictions of Trivedi’s theory of dendrite growth. E. A. FEEST, formerly Graduate Student, University of Sussex K. HOLM, formerly Visiting Research Student, University of Susses     

Natural convective effects in directional dendritic solidification of binary metallic alloys: Dendritic array primary spacing

This paper is concerned with the effect of natural convective patterns evidenced in a previous paper [Acta metall.37, 1143 (1989)], on the dendritic primary spacings. Primary spacings in normal gravity environment have been found to be much smaller than those measured in a microgravity environment, the latter being in good agreement with the diffusion controlled theoretical predictions. The scaling analysis of convective effects developed in the first paper allows us to propose a relationship, which gives the primary spacing as a function of the experimental parameters, in convective transport conditions of the solute in the liquid. This correlation is in good agreement with our experimental results, and those of the literature.     

Microstructural variations induced by gravity level during directional solidification of near-eutectic iron-carbon type alloys

The effects of gravity on the microstructure of directionally solidified near-eutectic cast irons are studied, using a Bridgman-type automatic directional solidification furnace aboard a NASA KC-135 aircraft which flies parabolic arcs and generates alternating periods of low-g (0.01 to 0.001 g, 30 seconds long) and high-g (1.8 g, 1.5 minutes long). Results show a refinement of the interlamellar spacing of the eutectic during low-g processing of metastable Fe-C eutectic alloys. Low-g processing of stable Fe-C-Si eutectic alloys (lamellar or spheroidal graphite) results in a coarsening of the eutectic grain structure. Secondary dendrite arm spacing of austenite increases in low-g and decreases in high-g. The effectiveness of low-gravity in the removal of buoyancy-driven graphite phase segregation is demonstrated.     

The effect of reduced gravity on solidification microstructures of NH4Cl-H2O alloys

The effect of gravity on the columnar-to-equiaxed microstructural transition was studied in small samples of NH₄Cl-H₂O. The behavior of the samples during laboratory (one gravity) experiments was contrasted with their behavior during a (low gravity) sounding rocket flight. In one gravity, the columnar zone accounted for 25 to 100 pct of the structure, depending on the superheat and orientation of the chill. Grain multiplica-tion occurred by showering and by convection induced dendrite arm remelting. Convec-tion was caused by both thermal gradients and solutal gradients. In low gravity, however, completely columnar structures were obtained; all grain multiplication mechanisms were entirely suppressed. Reduced gravity also modified the thermal conditions and caused the liquid to cool more slowly. This resulted in a steeper temperature gradient in the liquid ahead of the solidification interface. “Big bang” type nucleation occurred in two of the samples, distributing nuclei throughout the liquid. Despite this, an equiaxed zone did not form, indicating that the most significant effect of low gravity on this experiment was modification of the thermal conditions.     

Dendritic solidification of magnesium alloy AZ91

The magnesium alloy AZ91 has been studied under different solidification conditions, to establish the details of the as-cast structure. The electron backscattered pattern (EBSP) technique has been used to determine the crystallographic directions of the dendrite stem and the secondary arms. Under unidirectional solidification conditions, two different stem directions were found: at a low temperature gradient and high growth velocity, the dendrites grew in a «11-20» direction, while at higher temperature gradients and lower growth velocities, the growth direction was (2245). Dendrites with «11-20» stems have arms in six directions around the stem, while dendrites with «22-45» stems have three secondary arm directions. The crystallographic directions of the secondary arms are the same as the two stem directions.     

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 of Cu-Ni alloys: Part II. The influence of initial dendrite growth temperature on microsegregation

Microsegregation measurements and calculations were carried out for solidification of small melts of Cu-40 wt pct Ni under conditions in which both the cooling rate and the local rate of heat extraction during dendrite growth were varied. These demonstrated that the supercooling required to support dendrite growth significantly reduces the microsegregation in the completely frozen ingot. The importance of this for the different microsegregation in columnar and equiaxed regions of large ingots is briefly discussed. E. A. FEEST, formerly Graduate Student, University of Sussex, Brighton, Sussex, England     

Directional solidification of Cu- Pb and Bi- Ga monotectic alloys under normal gravity and during parabolic flight

Cu-Pb and Bi-Ga monotectic alloys of nominal hypermonotectic compositions were directionally solidified under various furnace translation rates, temperature gradients, and gravity levels. Gravity was varied by solidifying the alloys under ground conditions and in the furnace aboard NASA KC-135 aircraft, flying on parabolic trajectories. High translation rates, high gradients, high gravity levels, and higher density and lower thermal conductivity of the L₂ phase favored the formation of fiber composite structure, while the opposite conditions resulted in structures consisting of L₂ droplets in α matrix. A modified particle engulfment theory as originally enunciated by Ulhmannet al. is proposed to explain these observations. Formerly Graduate Student, Department of Metallurgical Engineering, The University of Alabama,     

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 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.