Mechanical behavior of cast single phase alloys solidified from undercooled melts

Several ingots (0.0254 m in diam × 0.10 m long) of nickel-30 wt pct copper, nickel-10 wt pct cobalt and iron-25 wt pct nickel were solidified with various undercoolings up to about 200 K, prior to nucleation of the solid. The materials were mechanically tested in the ascast condition. In nickel-30 wt pct copper and iron-25 wt pct nickel alloys the 0.2 pct offset yield strength, ductility and fatigue strength increased with undercooling. A linear relationship was established between 0.2 pct offset yield strength and the square root of secondary dendrite arm spacing in dendritic alloys (undercooled less than 170 K) or that of grain diameter in nondendritic alloys (undercooled more than 170 K). In iron-25 wt pct nickel limited testing indicated improvements in Charpy V-notch impact strength and in fracture toughness with undercooling. No improvement of tensile properties with undercooling was observed in nickel-10 wt pct cobalt, an alloy which solidified normally with very low microsegregation.     

Heterogeneous nucleation of solidification of Si by solid AI in hypoeutectic Al-Si alloy

Melt-spun Al-3 wt pct Si with and without ternary additions of Na and Sr has been heat-treated above the Al-Si eutectic temperature in a differential scanning calorimeter to form a microstructure of Al-Si eutectic liquid droplets embedded in the α-Al matrix. During subsequent cooling in the calorimeter, the heterogeneous nucleation temperature for solidification of Si in contact with the surrounding Al matrix depends sensitively on the alloy purity, with a nucleation undercooling which increases with increasing alloy purity from 9 to 63 K below the Al-Si eutectic temperature. These results are consistent with Southin’s hypothesis that low levels of trace P impurities are effective in catalyzing Si nucleation in contact with the surrounding Al matrix. With a low Al purity alloy, 0.1 wt pct Na addition increases the Si nucleation undercooling from 9 to 50 K, 0.15 wt pct Sr addition does not affect the Si nucleation temperature, and 0.3 wt pct Sr addition decreases the Si nucleation undercooling from 9 to 3 to 4 K. The solidified microstructure of the liquid Al-Si eutectic droplets embedded in the Al matrix depends on the Si nucleation undercooling. With low Si nucleation undercooling, each Al-Si eutectic liquid droplet solidifies to form one faceted Si particle; however, with high Si nucleation undercooling, each Al-Si eutectic droplet solidifies to form a large number of nonfaceted Si particles embedded in Al. Formerly with the Oxford Centre for Advanced Materials and Composites, Department of Materials, Oxford University     

Dendritic Growth of Undercooled Nickel-Tin: Part III

Studies were made of structure and solute distribution in undercooled droplets of nickel-25 wt pct tin alloy and the eutectic nickel-32.5 wt pct tin alloy. Structures of levitation melted droplets of the Ni-25 wt pct Sn alloy showed a gradual and continuous transition from dendritic to fine-grained spherical with increasing initial undercooling up to about 180 K. Results suggest that all samples solidified dendritically and that the final structures obtained were largely the result of ripening. Experimental data on minimum solute composition in the samples produced are bounded by two calculated curves, both of which assume equilibrium at all liquid-solid interfaces during recalescence and subsequent cooling. One assumes complete diffusion in the solid during recalescence; the other assumes limited diffusion, but partial remelting to avoid superheating of the solid. Several observations support the view that the eutectic alloy solidifies dendritically, much as the hypoeutectic alloy does. Surface dendrites were seen in regions of surface shrinkage cavities and a coarse “dendritic” structure can be discerned on polished sections, which seems to correspond to the large surface “dendrites” seen by high-speed photographs of the hypoeutectic alloy. The structure of highly undercooled eutectic samples is composed fully of an anomalous eutectic. Samples solidified with intermediate amounts of undercooling possess some lamellar eutectic which, it is believed, solidified after recalescence was complete.     

Solidification of undercooled Fe-Cr-Ni alloys: Part I. Thermal behavior

Solidification of undercooled Fe-Cr-Ni alloys was studied by high-speed pyrometry during and after recalescence of levitated, gas-cooled droplets. Alloys were of 70 wt pct Fe, with Cr varying from 15 to 19.7 wt pct, balance was Ni. Undercoolings were up to about 300 K. Alloys of Cr content less than that of the eutectic (18.1 wt pct) have face-centered cubic (fee) (austenite) as their equilibrium primary phase, and alloys of higher Cr content have body-centered cubic (bcc) (ferrite) as their equilibrium primary phase. However, except at low undercoolings in the hypoeutectic alloys, all samples solidified with bcc as the primary phase; the bcc then transformed to fcc during initial recalescence for the lower Cr contents or during subsequent cooling for the higher Cr contents. The bcc-to-fcc transformation, whether in the semisolid or solid state, was detected by a second recalescence. In the hypoeutectic alloys, the growth of primary metastable bcc apparently results from preferred nucleation of bcc. The subsequent nucleation of fcc may occur at bcc/bcc grain boundaries. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Numerical analysis of the rapid solidification of gas-atomized Al-8 Wt Pct Fe droplets

A numerical analysis of the microstructural evolution of microcellular and cellular α-Al phase in gas-atomized Al-8 wt pct Fe droplets was represented. The two-dimensional (2-D) non-Newtonian heat transfer and the dendritic growth theory in the undercooled melt were combined, assuming a point nucleation on the droplet surface and the macroscopically smooth solid-liquid interface enveloping the cell tips. It reproduced the main characteristic features of the reported microstructures quite well and predicted a considerable volume fraction of thermal dendritic growth region in a droplet smaller than 10 μm if an initial undercooling was larger than 100 K. The volume fractions of the microcellular region, g_A, and the sum of the microcellular and cellular region, g_α, were predicted as functions of the heat-transfer coefficient, h, and the initial undercooling, ΔT. It was shown that g_A and g_α, in the typical atomization processes with h = 0.1 to 1.0 W/C.M²K, are dominated by ΔT and h, respectively, but for h larger than 4.0 W/C.M² K, a fully microcellular structure can be obtained irrespective of the initial undercooling.     

Microstructural characteristics of Ni-Sb eutectic alloys under substantial undercooling and containerless solidification conditions

Both Ni-36 wt pct Sb and Ni-52.8 wt pct Sb eutectic alloys were highly undercooled and rapidly solidified with the glass-fluxing method and drop-tube technique. Bulk samples of Ni-36 pct Sb and Ni-52.8 pct Sb eutectic alloys were undercooled by up to 225 K (0.16 T _E ) and 218 K (0.16 T _E ), respectively, with the glass-fluxing method. A transition from lamellar eutectic to anomalous eutectic was revealed beyond a critical undercooling ΔT ₁*, which was complete at an undercooling of ΔT ₂*. For Ni-36 pct Sb, ΔT ₁*≈60 K and ΔT ₂*≈218 K; for Ni-52.8 pct Sb, ΔT ₁*≈40 K and ΔT ₂*≈139 K. Under a drop-tube containerless solidification condition, the eutectic microstructures of these two eutectic alloys also exhibit such a “lamellar eutectic-anomalous eutectic” morphology transition. Meanwhile, a kind of spherical anomalous eutectic grain was found in a Ni-36 pct Sb eutectic alloy processed by the drop-tube technique, which was ascribed to the good spatial symmetry of the temperature field and concentration field caused by a reduced gravity condition during free fall. During the rapid solidification of a Ni-52.8 pct Sb eutectic alloy, surface nucleation dominates the nucleation event, even when the undercooling is relatively large. Theoretical calculations on the basis of the current eutectic growth and dendritic growth models reveal that γ-Ni₅Sb₂ dendritic growth displaces eutectic growth at large undercoolings in these two eutectic alloys. The tendency of independent nucleation of the two eutectic phases and their cooperative dendrite growth are responsible for the lamellar eutectic-anomalous eutectic microstructural transition.     

Effects of Undercooling and Cooling Rate on Peritectic Phase Crystallization Within Ni-Zr Alloy Melt

The liquid Ni-16.75 at. pct Zr peritectic alloy was substantially undercooled and containerlessly solidified by an electromagnetic levitator and a drop tube. The dependence of the peritectic solidification mode on undercooling was established based on the results of the solidified microstructures, crystal growth velocity, as well as X-ray diffraction patterns. Below a critical undercooling of 124 K, the primary Ni₇Zr₂ phase preferentially nucleates and grows from the undercooled liquid, which is followed by a peritectic reaction of Ni₇Zr₂+L → Ni₅Zr. The corresponding microstructure is composed of the Ni₇Zr₂ dendrites, peritectic Ni₅Zr phase, and inter-dendritic eutectic. Nevertheless, once the liquid undercooling exceeds the critical undercooling, the peritectic Ni₅Zr phase directly precipitates from this undercooled liquid. However, a negligible amount of residual Ni₇Zr₂ phase still appears in the microstructure, indicating that nucleation and growth of the Ni₇Zr₂ phase are not completely suppressed. The micromechanical property of the peritectic Ni₅Zr phase in terms of the Vickers microhardness is enhanced, which is ascribed to the transition of the peritectic solidification mode. To suppress the formation of the primary phase completely, this alloy was also containerlessly solidified in free fall experiments. Typical peritectic solidified microstructure forms in large droplets, while only the peritectic Ni₅Zr phase appears in smaller droplets, which gives an indication that the peritectic Ni₅Zr phase directly precipitates from the undercooled liquid by completely suppressing the growth of the primary Ni₇Zr₂ phase and the peritectic reaction due to the combined effects of the large undercooling and high cooling rate.     

The impact of cooling rates on the microstructure of Al-U alloys

The impact of cooling rates on the microstructure of Al-U alloys was studied by optical, scanning electron, and transmission electron microscopy. A variety of solidification techniques were employed to obtain cooling rates ranging between 3 × 10⁻² and 10⁶ K/s. High-purity uranium (99.9 pct) and high-purity aluminum (99.99 pct), or “commercially pure” type Al-1050 aluminum alloys were used to prepare Al-U alloys with U concentration ranging between 3 and 22 wt pct. The U concentration at which a coupled eutectic growth was observed depends on the cooling rates imposed during solidification and ranged from 13.8 wt pct for the slower cooling rates to more than 22 wt pct for the fastest cooling rates. The eutectic morphology and its distribution depends on the type of aluminum used in preparing the alloys and on the cooling rates during solidification. The eutectic in alloys prepared from pure aluminum was evenly distributed, while for those prepared from Al-1050, the eutectic was unevenly distributed, with eutectic colonies of up to 3 mm in diameter. Two lamellar eutectic structures were observed in alloys prepared from pure aluminum containing more than 18 wt pct U, which solidified by cooling rates of about 10 K/s. One structure consisted of the stable eutectic between UAl₄ and Al lamella. The other structure consisted of a metastable eutectic between UAl₃ and Al lamella. At least three different eutectic morphologies were observed in alloys prepared from Al-1050.     

Dendritic growth of undercooled nickel-tin: Part II

High-speed optical temperature measurements were made of the solidification behavior of levitated metal samples within a transparent glass medium. Two undercooled Ni-Sn alloys were examined, one a hypoeutectic alloy and the other of eutectic composition. Recalescence times for the 9 mm diameter samples studied decreased with increasing undercooling from the order of 1.0 second at 50 K under-cooling to less than 10⁻³ second for undercoolings greater than 200 K. Both alloys recalesced smoothly to a maximum recalescence temperature at which the solid was at or near its equilibrium composition and equilibrium weight fraction. For the samples of hypoeutectic alloy that recalesced above the eutectic temperature, a second nucleation event occurred on cooling to the eutectic temperature. For samples which recalesced only to the eutectic temperature, no subsequent nucleation event was observed on cooling. It is inferred in this latter case that both the α and β phases were present at the end of recalescence. The thermal data obtained suggest a solidification model involving (1) dendrites of very fine structure growing into the melt at temperatures near the bulk undercooling temperature, (2) thickening of dendrite arms with rapid recalescence, and (3) continued, much slower recalescence accompanying dendrite ripening.     

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.     

Undercooling-Induced macrosegregation in directional solidification

The accepted primary mechanism for causing macrosegregation in directional solidification (DS) is thermal and solutal convection in the liquid. This article demonstrates the effects of under-cooling and nucleation on macrosegregation and shows that undercooling, in some cases, can be the cause of end-to-end macrosegregation. Alloy ingots of Pb-Sn were directionally solidified upward and downward, with and without undercooling. A thermal gradient of about 5.1 K/cm and a cooling rate of 7.7 K/h were used. Crucibles of borosilicate glass, stainless steel with Cu bottoms, and fused silica were used. High undercoolings were achieved in the glass crucibles, and very low undercoolings were achieved in the steel/Cu crucible. During under-cooling, large, coarse Pb dendrites were found to be present. Large amounts of macrosegregation developed in the undercooled eutectic and hypoeutectic alloys. This segre-gation was found to be due to the nucleation and growth of primary Pb-rich dendrites, continued coarsening of Pb dendrites during undercooling of the interdendritic liquid, Sn enrichment of the liquid, and dendritic fragmentation and settling during and after recalescence. Eutectic ingots that solidified with no undercooling had no macrosegregation, because both Pb and Sn phases were effectively nucleated at the start of solidification, thus initiating the growth of solid of eutectic composition. It is thus shown that undercooling and single-phase nucleation can cause significant macrosegregation by increasing the amount of solute rejected into the liquid and by the movement of unattached dendrites and dendrite fragments, and that macrosegregation in excess of what would be expected due to diffusion transport is not necessarily caused by convection in the liquid.     

The reaction behavior of Fe-C-S droplets in CaO-SiO2-MgO-FeO slags

The rate of reduction of FeO in the slag by carbon in iron droplets (2.9 wt pct C, 0.01 wt pct S) was studied for CaO-SiO₂-MgO slags containing between 3 and 35 wt pct FeO and temperatures ranging from 1643 to 1763 K. The effects of Fe₂O₃ additions to the slag and sulfur variations in the metal on the reaction rate were also studied. It was found that the behavior of the metal droplets in the slag, as observed by X-ray fluoroscopy, changed significantly with FeO content in the slag. Below 10 wt pct FeO, the droplet remained intact while reacting with the slag; however, above this FeO concentration, the droplet became emulsified within the slag. The large increase in surface area of the metal droplet due to emulsification caused the rate of reaction to be one to two orders of magnitude faster than for droplets that did not become emulsified. It was suggested that when the droplet is emulsified, the surface area and reaction kinetics are greatly increased, and the rate becomes controlled by mass transfer of FeO as Fe²⁺ and O²⁻ ions in the slag to the emulsified droplet. At low FeO contents for which the droplet does not emulsify, the rate is controlled by dissociation of CO₂ on the metal. It was also found that a critical temperature exists for a given FeO content at which point the rate of CO evolution increases dramatically. Additions of Fe₂O₃ to the slag and sulfur to the metal caused significant changes to the rate of reaction possibly by affecting the emulsification behavior of the droplet.     

Microstructures of undercooled Germanium droplets

Small liquid Ge droplets (0.3–0.5 mm diameter) have been undercooled 150–415 ± 20°C below T_m in B₂O₃ flux before solidifying to the diamond cubic phase. A correlation was found between initial undercooling and final grain size. Droplets undercooled <300°C exhibited a coarse grain structure. At greater undercoolings, the grain size became progressively finer. This correlation may be subsidiary to the dependence of grain size on interfacial undercooling. Ge droplets lightly doped with Sn solidified dendritically for undercoolings greater than 250°C. Twinned dendrites have been observed at small undercoolings (~ 10°C) in other experiments. It appears that larger interfacial undercoolings are necessary to grow the twin-free dendrites which we have observed. The correlation between grain size and the presence of dendrites suggests that the grain refinement observed in Ge samples undercooled > 300°C stems from dendritic break-up during solidification.     

Kinetics of growth of the Ti-Co

The growth behavior of the disordered and ordered intermetallic compounds in the Ti-Co system from undercooled melts during rapid solidification has been studied in detail. It was found that cooling rates of the order of 10¹⁰ to 10¹² K s^-1 were insufficient to suppress the nucleation and growth of the intermetallic compounds. The compound isomorphous to Ti₂Co does not exhibit faceting, indicating a nonsingular crystal/liquid interface morphology at high growth velocities. While a TiCo-like compound was found to have a ordered bcc structure, the formation of even the disordered TiCo₃ was completely suppressed by the high isothermal velocities, leading to glass formation. The growth velocity of compounds formed in the composition range 23.3 to 59.7 at. pct Co was found to be diffusion limited with nucleation times smaller than the total melt lifetimė in the process. The overall glass formation range was found to be very small, although the equilibrium phase diagram shows the presence of a variety of intermetallic compounds.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part I. Theoretical analysis of nucleation behavior

The selection of the primary solidifying phase in undercooled stainless steel melts is theoretically analyzed in terms of nucleation theory. Nucleation phenomena are considered using different models for the solid-liquid interface energy. The classical nucleation theory for sharp interfaces and an improved modification, the diffuse interface theory, are applied. The influence of deviations of the nucleus composition from the overall alloy composition is also revealed. A preferred nucleation of the metastable bcc phase in fcc equilibrium solidification-type alloys is predicted. The critical undercooling of metastable crystallization as a function of alloy composition is calculated for an isoplethal section at 69 at. pct Fe of Fe₆₉Cr_31-x Ni _x alloys. The results are summarized in a phase selection diagram predicting the primary solidification mode as a function of undercooling and melt composition.     

Liquid separation effects in Fe−Cu alloys solidified under different cooling rates

The effect of cooling rates on the microstructure of Fe−Cu alloys was investigated. A variety of solidification techniques was employed, in order to obtain a wide range of cooling rates. At high cooling rates (about 10⁴ K/sec), and in the composition range 30 to 80 wt pct Cu, the microstructures showed clear evidence of metastable liquid separation. This indicates a melt supercooling of about 50 to 100 K. Liquid separation coupled with high interfacial velocities resulted in solute trapping, and in a spherical morphology for one of the solids. At cooling rates lower than 10⁴ K/sec no liquid separation was observed, and the alloys solidified in a conventional manner,i.e., with a polycrystalline or a dendritic microstructure, depending on the Cu content. The type of the γ-Fe to α-Fe solid state transformation, taking place during cooling after solidification, depends on the cooling rates as well as on the Cu content in the γ-Fe phase. At medium cooling rates the transformation is martensitic, while at low or high cooling rates a polycrystalline transformed structure is obtained. A. MUNITZ, formerly Visiting Research Associate at the University of Florida at Gainesville, FL 32611     

Microstructural development in undercooled lead- tin eutectic alloys

A model was developed to predict micro structural development in lead—61.9 wt pct Sn (eutectic) alloys which were undercooled 5 to 25 K below their equilibrium freezing temperature prior to being preferentially nucleated. While the initial solidification velocity rapidly increases with increasing undercooling, the model predicts it to quickly decrease, prior to 10 pct solid formation, after which growth continues near the equilibrium temperature. Experimentally, and in accordance with the prediction, the eutectic emanated from the nucleation site with an initially fine spacing that increased with distance. However, in contrast to the model, the eutectic grew outward in a spokelike manner with each arm surrounded by a globular structure, this being attributed to the difficulty of lateral nucleation. Microstructural uniformity was further compromised by equiaxed eutectic grains which grew ahead of the advancing interface in the now only slightly undercooled liquid. Consequently, while containerless techniques may ensure sample purity and permit processing of high-temperature materials, development of a continuously fine and uniformly aligned microstructure cannot be assumed. This article is based on a presentation made in the symposium entitled “Microgravity Solidification, Theory and Experimental Results” as a part of the 1993 TMS Fall meeting, October 17-21, 1993, Pittsburgh, PA, under the auspices of the TMS Solidification Committee.     

Solidification of undercooled Sn-Sb peritectic alloys: Part I. Microstructural evolution

The droplet emulsion technique, which involves dispersal of a bulk liquid alloy into a collection of fine droplets (5 to 30μm), was applied to Sn-Sb alloys to yield high levels of controlled undercooling. The maximum undercooling levels achieved varied from 179 °C for pure Sn to 113 °C for a Sn-16 at. pct Sb alloy. Analysis of hypoperitectic alloy samples (alloys with an Sb content less than that of the liquid at the peritectic temperature) indicates that solute trapping occurs during solidification at the levels of undercooling and cooling rate investigated, yielding nearly homogeneousβ-tin solid solutions with compositions approaching those of the bulk alloys. With increasing undercooling and/or cooling rate, hyperperitectic alloys exhibit a transition from a highly segregated structure consisting of faceted primary intermetallic phase and cellularβ to a structure consisting primarily of a supersaturated tin-rich solid solution. Lattice constant measurements confirm that virtually complete supersaturation ofβ-tin was achieved in emulsion samples cooled at 200 °C s^s−1 for compositions up to approximately 20 at. pct Sb. The development and characteristics of subsequent solid-state precipitation were used to guide the interpretation of the often complex solidification reaction sequences in the hyperperitectic alloys. The formation of supersaturatedβ-tin solid solutions in the undercooled samples is related to the appropriate metastable phase equilibria and the development of solute trapping. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

Quantification of microsegregation during rapid solidification of Al-Cu powders

A new technique is introduced to quantify microsegregation during rapid solidification. The quantification involves calculation of the average solute solubility in the primary phase during solidification of an Al-Cu binary alloy. The calculation is based on using volume percent eutectic and weight percent of second phase (in the eutectic), which were obtained experimentally. Neutron diffraction experiments and stereology calculation on scanning electron microscope images were done on impulse atomized Al-Cu alloys of three compositions (nominal), 5 wt pct Cu, 10 wt pct Cu, and 17 wt pct Cu, atomized under N₂ and He gas. Neutron diffraction experiments yielded weight percent CuAl₂ data and stereology yielded volume percent eutectic data. These two data were first used to determine the weight percent eutectic. Using the weight percent eutectic and weight percent CuAl₂ in mass and volume balance equations, the average solute solubility in the primary phase could be calculated. The experimental results of the amount of eutectic, tomography results from previous work, and results from the calculations suggest that the atomized droplets are in metastable state during the nucleation undercooling of the primary phase, and the effect of metastability propagates through to the eutectic formation stage. The metastable effect is more pronounced in alloys with higher solute composition.     

Effect of the primary phase on grain coarsening in undercooled Fe-Co alloys

Fe-Co alloy melts with Co contents of 10, 30, and 60 at. pct were undercooled to investigate the dependence of the primary phase on grain coarsening. A pronounced characteristic is that the metastable fcc phase in the Fe-10 at. pct Co alloy and the metastable bcc phase in the Fe-30 at. pct Co alloy will primarily nucleate when undercoolings of the melts are larger than the critical undercoolings for the formation of metastable phases in both alloys. No metastable bcc phase can be observed in the Fe-60 at. pct Co alloy, even when solidified at the maximum undercooling of ΔT = 312 K. Microstructural investigation shows that the grain size in Fe-10 and Fe-30 at. pct Co alloys increases with undercoolings when the undercoolings of the melts exceed the critical undercoolings. The grain size of the Fe-60 at. pct Co alloy solidified in the undercooling range of 30 to 312 K, in which no metastable phase can be produced, is much finer than those of the Fe-10 and Fe-30 at. pct Co alloys after the formation of metastable phases. The model for breakage of the primary metastable dendrite at the solid-liquid interface during recalescence and remelting of dendrite cores is suggested on the basis of microstructures observed in the Fe-10 and Fe-30 at. pct Co alloys. The grain coarsening after the formation of metastable phases is analyzed, indicating that the different crystal structures present after the crystallization of the primary phase may play a significant role in determining the final grain size in the undercooled Fe-Co melts.