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 highly undercooled Sn- Pb alloy droplets

Experimental work is described on undercooling and structure of tin-lead droplets emulsified in oil. The droplets, predominantly in the size range of 10 to 20 μm, were cooled at rates (just before nucleation) ranging from about 10^-1 K per second to 10⁶ K per second. The higher cooling rates were obtained by a newly developed technique of quenching the emulsified droplets in a cold liquid. Measured undercoolings (at the lower cooling rates) ranged up to about 100 K. Structures obtained depend strongly on undercooling, cooling rate before and after nucleation, and alloy composition. Droplets containing up to 5 wt pct Pb were apparently single phase when undercooled and rapidly quenched. Droplets in the composition range of about 25 wt pct to 90 wt pct Pb solidified dendritically, even at the most rapid quench rates employed, apparently because these alloys undercooled only slightly before nucleation of the primary phase. Formerly Graduate Research Assistant and Postdoctoral Associate in the Department of Materials Science and Engineering, Massachusetts Institute of Technology.     

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.     

Nonequilibrium Solidification Behavior of Co-Si Alloys Near the First Eutectic Point

Adopting a fluxing purification and cyclic superheating technique, Co-10 wt pct Si and Co-15 wt pct Si alloys had been undercooled to realize rapid solidification in this work. It was investigated that the solidification modes and microstructures of Co-Si alloys were deeply influenced by the undercooling of the melts. Both alloys solidified with a near-equilibrium mode in a low undercooling range; the peritectic reaction occurred between the primary phase and the remnant liquids, and it was followed by the eutectic reaction and eutectoid transformation. With the increase of undercooling, both alloys solidified with a nonequilibrium mode, and the peritectic reaction was restrained. As was analyzed, a metastable Co₃Si phase was found in Co-10 wt pct Si alloy when a critical undercooling was achieved.     

Steady-state electrotransport of carbon in nickel and in nickel-copper and iron-copper alloys

For comparison with previous data on iron-carbon alloys, steady-state electrotransport experiments were performed on three alloys: Nickel-0.15 wt pct carbon, nickel-20 copper-0.13 carbon, and iron-7.2 copper-0.10 carbon. The current density was 2.20 to 2.85 × 10⁷ AJm² and the temperature was 1300 K (and also 1200 K for the nickel-carbon alloy). The Z* values obtained were +5.7, +5.7, and +4.5 respectively, for the three alloys. Present theory appears to be unable to explain the relation of these data to similar data on iron-carbon alloys.     

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.     

Steady-state electrotransport of carbon in nickel and in nickel-copper and iron-copper alloys

For comparison with previous data on iron-carbon alloys, steady-state electrotransport experiments were performed on three alloys: Nickel-0.15 wt pct carbon, nickel-20 copper-0.13 carbon, and iron-7.2 copper-0.10 carbon. The current density was 2.20 to 2.85 × 10⁷ AJm² and the temperature was 1300 K (and also 1200 K for the nickel-carbon alloy). The Z* values obtained were +5.7, +5.7, and +4.5 respectively, for the three alloys. Present theory appears to be unable to explain the relation of these data to similar data on iron-carbon alloys. Formerly Postdoctoral Fellow, Department of Metallurgical and Materials Engineering, University of Florida, Gainesville, Florida     

Diffusion and electrotransport of carbon in γ-phase iron-nickel alloys

The diffusivity and electrotransport behavior of carbon in an iron-32.5 wt pct nickel alloy was investigated for the temperature range of 1220 to 1415 K for two carbon concentrations. The electrical resistivity was measured as a function of temperature between room temperature and 1625 K for carbon contents of 0.002, 0.10, and 0.21 wt pct. Values for the activation energy for diffusion of 32.0 ± 5.8 and 36.0 ± 5.2 kcal per mole were determined for alloys containing 0.10 and 0.21 wt pct carbon respectively, with slightly smaller values being found for the activation energy for electrotransport. The sign of the effective valence indicates that carbon migrates toward the cathode.     

Microstructure and mechanical behavior of spray-deposited High-Li Al-Li alloys

High-Li alloys, with the composition Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr, were synthesized using a spray deposition technique (wt. pct, X=0∼1.5). The microstructure of the spray-deposited Al-Li alloys consisted of equiaxed grains with an average grain size in the range from 20 to 50 μm. The grain-boundary phases were fine and discrete. The spray-deposited and thermomechanically processed materials were isothermally heat treated at 150 °C and 170 °C to investigate the age-hardening kinetics. It was noted that the spray-deposited Al-3.8Li-XCu-1.0Mg-0.4Ge-0.2Zr alloys exhibited relatively sluggish aging behavior. The peak-aged condition was achieved at 170 °C in the range from 20 to 90 hours. It was noted that Cu increases the hardness of alloys during aging. Moreover, the influence of Cu on age-hardening kinetics is marginal. The mechanical properties of the spray-deposited and extruded Al-Li alloys were studied in the underaged, peak-aged, and overaged conditions. For example, the peak-aged yield strength, tensile strength, and ductility of Al-3.8Li-1.0Cu-1.0Mg-0.4Ge-0.2Zr are 455 MPa, 601 MPa, and 3.1 pct, respectively. Moreover, an increase in the Cu content of the alloy led to improvements in strength, with only slight changes in ductility, for Cu contents up to 1.0 wt pct. Beyond this range, an increase in Cu content led to decreases in both strength and ductility.     

Tensile properties of directionally solidified Al-4 wt pct Cu alloys with columnar and equiaxed grains

The tensile properties of directionally solidified Al-4 wt pct Cu-0.15-0.2 wt pct Ti alloys with equiaxed grains were determined and compared with the properties of directionally solidified Al-4 wt pct Cu columnar structures. The tensile properties of the equiaxed structure were isotropic, but varied with the distance from the chill face. The mechanical properties of the equiaxed structure were generally between those of the longitudinal and transverse columnar structures. The 0.2 pct offset yield stress(σ _y, MPa) is represented as a function of the grain size,d (mm), the average concentration, C_o (wt pct), and the local concentration, C (wt pct), by σ_y = [(15.7 + 22.6 C_o) + (1.24 + 1.04 C_o)d ^-1/2] + [15.7 △C], where △C = C - C_o. The equiaxed structure exhibits inverse segregation similar to that in the columnar structure.     

Microstructure and mechanical properties of ductile Ni3AI-type compound in Fe-(Ni,Mn)-AI-C systems rapidly quenched from melts

By the rapid quenching technique, nonequilibrium Ni₃Al-type compounds with high strength and hardness as well as large elongation have been found in Fe-Ni-Al-C and Fe-Mn-Al-C systems. This formation region is limited to about 7 to 55 wt pct Ni, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Ni-Al-C and to about 7 to 65 wt pct Mn, 3 to 9 wt pct Al and 0.8 to 2.4 wt pct C for Fe-Mn-Al-C. The Ni₃Al-type compound has fine grains of about 1 to 10 μm in diam. Their Vickers hardness and yield strength increase with increase in the amounts of carbon, aluminum or nickel and the highest values attain about 665 DPN and 1690 MPa for Fe-Ni-Al-C and 600 DPN and 1740 MPa for Fe-Mn-Al-C. Elongation increases with decrease in carbon or aluminum and attains about 11 pct for Fe-20 wt pct Ni-6 wt pct Al-1.2 wt pct C and 28 pct for Fe-20 wt pct Mn-8 wt pct Al-1.6 wt pct C. The good strength and ductility of the Ni₃Al-type compounds remain unchanged on tempering for 1 h until heated to about 750 K. Further, it has been found that the addition of chromium, molybdenum or cobalt is effective for the improvement of mechanical properties and thermal stability of the compounds. Thus, the use of materials containing Ni₃Al-type compounds may be attractive for fine gage high-strength wire or plate applications. Formerly Graduate Student of Tohoku University.     

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     

Microstructure of Cu-Co alloys solidified at various supercoolings

The effects of supercooling on the microstructure of Cu-Co alloys containing 10 to 65 wt pct Co were investigated. Supercooling of the alloys below a characteristic temperature,t _SEP, resulted in a metastable phase separation into two liquids: one Co rich (L1) and the other Cu rich (L2). The microstructure of the phase-separated alloys consisted of spherulites of one phase embedded in a matrix of the other. The spherulites in alloys containing less than 40 wt pct Co were solidified from the L1 melt and from L2 in alloys containing more than 55 wt pct Co. Supercooling of copper alloys containing around 50 wt pct Co resulted in a duplex structure of fine and coarse dendrites. Microstructural evidence was presented for the formation of aε-Cu metastable phase in alloys containing less than 30 wt pct Co.     

Dendritic growth of undercooled nickel-tin: Part I

Solidification of undercooled Ni-25 wt pct Sn alloy was observed by high-speed cinematography and results compared with optical temperature measurements. Samples studied were rectangular in cross-section, and were encased in glass. Cinematographic measurements were carried out on samples undercooled from 68 to 146 K. These undercoolings compare with a temperature range of 199 K from the equilibrium liquidus to the extrapolated equilibrium solidus. At all undercoolings studied, the high-speed photography revealed that solidification during the period of recalescence took place with a dendrite-like front moving across the sample surface. Spacings of the apparent “dendrite” were on the order of millimeters. The growth front moved at measured velocities ranging from 0.07 meters per second at 68 K undercooling to 0.74 meters per second at 146 K undercooling. These velocities agree well with results of calculations according to the model for dendrite growth of Lipton, Kurz, and Trivedi. It is concluded that the coarse structure observed comprises an array of very much finer, solute-controlled dendrites.     

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     

Solidification of the Cu-35wt pct Fe Alloys with Liquid Separation

The microstructures of the Cu-35wt pct Fe alloys were investigated by melt-fluxing in combination with cyclic superheating and melt-spinning technique, respectively. Using the melt-fluxing with cyclic superheating technique, it was found that a complicated sub-microstructure formed in the separated minor phase, when the undercooling was 120 K (120 °C). The processes of the phase transformation from a liquid state to room temperature for undercooled Cu-35wt pct Fe alloys were discussed, in order to understand the solidification with metastable liquid separation. By means of melt-spinning technique, it was indicated that the microstructure of solidification for Cu-35wt pct Fe alloys could be refined due to the high cooling rate.     

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     

Magnetic properties of maraging steels in relation to nickel concentration

Magnetic properties of maraging steels have been investigated as a function of nickel concentration. The alloys nickel content varied from 12 to 24 wt pct, while other alloying constituents were kept at a level maintained in the 18Ni-2400 MPA-grade maraging steel. The magnetic properties were determined following aging for 1 hour in the temperature range of 450 °C to 750 °C. In every alloy investigated, the coercive field increased with aging temper-ature, reaching a maximum around 670 °C ± 30 °C. The saturation magnetization values were lowest around temperatures where maximum coercive field was observed. The coercive field increased from ∼55 to ∼ 175 Oe (∼4380 to ∼ 13,900 amp/meter) and the corresponding sat-uration magnetization decreased from ∼18,500 to ∼ 4000 G (∼1.85 to ∼0.4 T) in the alloys containing 12 and 24 wt pct Ni, respectively. The reverted austenite increased from 25 vol pct at 12 wt pct Ni to 100 vol pct at 24 wt pct Ni. The hardness and Charpy impact strength of the alloys have also been determined. An attempt has been made to correlate magnetic properties with different phase transformations occurring in maraging steels.     

Solidification of undercooled Fe-Cr-Ni alloys: Part II. Microstructural evolution

Results are reported on microstructures of Fe-Cr-Ni alloys, solidified over a range of undercoolings and quenched during or after recalescence. Alloys studied contained 70 wt pct Fe and with Cr varying from approximately 15 to 20 wt pct. The three lower Cr alloys were hypoeutectic (with fee as primary phase in equilibrium solidification); the two higher Cr alloys were hypereutectic (with bcc as primary phase in equilibrium solidification). Results obtained are in agreement with predictions based on thermal analyses previously presented; they confirm and extend the understanding gained in that work. The primary phase to solidify in the hypoeutectic alloys is bec when undercooling is greater than an amount which decreases with increasing Cr content. At the lower Cr contents, the stable fcc phase then forms by solid-state transformation of the metastable phase and its subsequent engulfment by additional fcc. At the higher Cr content, transformation is by a peritectic-like reaction in the semisolid state, except near the surface at higher undercoolings where the transformation is massive. In the hypereutectic alloys, primary solidification at all undercoolings is the stable bcc phase. Partial transformation to fcc occurs in the semisolid or solid state, depending on composition and undercooling. Formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

A novel experiment for the study of substrate-induced nucleation in metallic alloys: Application to Zn-Al

In order to investigate the nucleation and growth mechanisms of Zn-0.2 wt pct Al grains in thin coatings deposited on steel sheets by hot dipping, a new directional solidification experiment has been specifically designed with a steel substrate immersed into the melt. The competition (CTG) between regular columnar (C) grains growing parallel to the thermal gradient and transverse grains (TGs) nucleated on the sheet surface and growing transversely has been observed in longitudinal sections of Zn-0.2 wt pct Al (-Sb) alloys. Without Sb, the CTG began at midheight of the casting, whereas with Sb, no CTG was found. A simple CTG criterion similar to that of Hunt for the columnar-to-equiaxed Transition (CET) was derived. The calculated extension of the TGs was successfully compared with that measured on micrographs reconstructed from electron backscattered diffraction (EBSD) measurements. This indicated that the critical nucleation undercooling for the formation of Zn-0.2 wt pct Al grains on the steel sheet is about 0.7 K and that Sb poisons these available nucleation sites.