Solidification of undercooled Sn-Sb peritectic alloys: Part II. Heterogeneous nucleation

The undercooling behavior of fine droplet samples of Sn-rich, Sn-Sb alloys was investigated using differential thermal analysis (DTA). Undercooling levels measured during cooling from the liquid state follow the trend of the intermetallic phase liquidus, suggesting that solidification of all droplet samples (even those which solidify to yield a supersaturatedβ-tin product) was probably initiated with formation of primary intermetallic phase. Heterogeneous nucleation thermal cycling treatments were then used to measure the relative catalytic potency of primary intermetallic phases in this system for nucleation ofβ-tin during cooling. Crystallization reactions below the equilibrium peritectic temperature of 250 °C, at 187 °C and 230 °C, have been interpreted as corresponding to nucleation ofβ on Sn₃Sb₂ and SnSb substrates, respectively. The behavior observed in the Sn-Sb system can be generalized to guide the interpretation of heterogeneous catalysis and the analysis of solidification pathways in other peritectic alloy systems. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

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.     

Microanalysis of precipitate free zones (PFZ) in Al-Zn-Mg and Cu-Ni-Nb alloys

In order to better understand the formation of Precipitate Free Zones (PFZ), microanalysis was conducted on heat treated Al-2.2 at. pct Zn-4.7 at. pct Mg and Cu-30 at. pct Ni-0.9 at. pct Nb alloys. In both the alloys, no appreciable solute depletion at the grain boundaries was observed in the as-quenched condition. After aging, marked solute depletion was observed in the PFZ of both the alloys. In the Al-Zn-Mg alloy, the PFZ were supersaturated with respect toη andT phases up to 4 h of aging at 473 K. In the Cu-Ni-Nb alloy, the PFZ were supersaturated only with respect to theβ phase but not the metastable γ″ phase. Based on the results, the factors affecting the formation and growth of PFZ are discussed.     

The microstructure of rapidly solidified Ti-Fe melt-spun ribbons

Ti-Fe binary alloys were rapidly solidified by the melt-spinning technique, and four compositions were examined: Ti-5 wt pct Fe, which is the critical composition for theβ to ω athermal transformation; Ti-10 wt pct Fe, which represents a hypoeutectoid composition; the eutectoid composition Ti-15 wt pct Fe; and Ti-20 wt pct Fe, as an example of a hypereutectoid alloy. The Ti-5 wt pct Fe rapidly solidified ribbons are composed of two different structures. The first consists of α′-martensite plates inβ matrix and the second, athermal ω particles inβ matrix. The Ti-10, 15, and 20 wt pct Fe alloys are also composed of two structures. These areβ grains and isothermal-like ω particles inβ matrix. A solidification model is suggested which explains the existence of two different microstructures at the same composition and the for-mation of two kinds of ω particles.     

The influence of alloy composition on precipitates of the Al-Mg-Si system

To study how changes in solute elements affect precipitation, six Al-Mg-Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.     

Dendrite coherency during equiaxed solidification in binary aluminum alloys

Dendrite coherency, or dendrite impingement, is important to the formation of the solidification structure and castability of alloys. Dendrite coherency in the systems Al-xMn, Al-xCu, Al-xFe, and Al-xSi(x = 0 to 5 wt pct) has been studied by continuous torque measurement in solidifying samples. The fraction solid at the dendrite coherency point, fs*, varies with the alloy system and the solute concentration in the alloy, from 18 to 56 pct for the present alloys investigated. An increase in solute concentration decreases the coherency fraction solid,fs*. An alloy system with a large slope of the liquidus line has a high coherency fraction solid. A theoretical approach has been developed to account for the effects of the alloy system and solute concentration on the dendrite coherency in the alloy. The grain sizes of the alloys were evaluated using the parameters at coherency point.     

Microstructure and phase relations for Ti-Mo-Al alloys

The influence of aluminum additions to a Ti-7 at. pet Mo alloy on the phase equilibria was investigated. The microstructures of the alloys, Ti-7 pct Mo-7 pct Al and Ti-7 pct Mo-16 pct Al, were determined by light and electron microscopy. It was found that with increasing aluminum concentration the formation of the metastable w phase was suppressed. In the Ti-7 pct Mo-16 pct Al alloy the β phase decomposed upon quenching by precipitating coherent, ordered particles having a B2 type of crystal structure (β₂). At low temperatures the equilibrium phases for this alloy were β + α+ β ₂, whereas at high temperature (850° to 950°C) the Ti₃Al phase was in two-phase equilibrium with the β phase. The four-phase equilibrium which exists at a temperature of about 550°C involves the reaction β + Ti₃Al ⇌ α + β₂. G. LUETJERING, formerly Staff Member Materials Research Center, Allied Chemical Corp., Morristown, N. J.,     

The compctition between martensite and omega in quenched Ti-Nb alloys

A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system. Formerly a Graduate Student in the Materials Science Program at the University of Wisconsin-Madison.     

The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys

The microstructure and tensile behavior of two Al-7 pct Si-Mg casting alloys, with magnesium contents of 0.4 and 0.7 pct, have been studied. Different microstructures were produced by varying the solidification rate and by modification with strontium. An extraction technique was used to determine the maximum size of the eutectic silicon flakes and particles. The eutectic Si particles in the unmodified alloys and, to a lesser extent, in the Sr-modified alloys are larger in the alloys with higher Mg content. Large Fe-rich π-phase (Al₉FeMg₃Si₅) particles are formed in the 0.7 pct Mg alloys together with some smaller β-phase (Al₅FeSi) plates; in contrast, only β-phase plates are observed in the 0.4 pct Mg alloys. The yield stress increases with the Mg content, although, at 0.7 pct Mg, it is less than expected, possibly because some of the Mg is lost to π-phase intermetallics. The tensile ductility is less in the higher Mg alloys, especially in the Sr-modified alloys, compared with the lower Mg alloys. The loss of ductility of the unmodified alloy seems to be caused by the larger Si particles, while the presence of large π-phase intermetallic particles accounts for the loss in ductility of the Sr-modified alloy.     

Analysis of the composition of α plates isothermally formed in titanium binary alloys

The solute distribution within and near the proeutectoid α plates which were isothermally formed from the β solid solution in Ti-V, Cr, and Fe alloys was measured in a scanning transmission electron microscope (STEM) equipped with an energy dispersive X-ray (EDX) analyzer. In the Ti-4.9 at. pct Cr and 5.1 at. pct Fe alloys in which all measurements were made above the calculated T_o temperature (the maximum temperature at which the diffusionless β to α transformation can occur), the solute concentration in the α plates was close to final equilibrium as long as the analyzed plates had grown to the sizes comparable to the STEM spatial resolution (∼50 nm). In the Ti-5.4 at. pct V and 2.6 at. pct Fe alloys, the solute concentration in the α plates was already close to equilibrium at very short reaction times at temperatures more than 100 °C below T_o. The ledgewise diffusion growth mechanism can roughly account for the thickening behavior of these plates, consistent with the results of the composition analysis. In the Cr alloy, a microstructure resembling the lower bainite in steels, i.e., nonlamellar distribution of TiCr₂ particles in thick α plates, was formed by coalescence of solute-depleted thin α plates which were formed in aggregates (in sheaves) at an earlier stage of growth. This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

Examination of solidification pathways and the liquidus surface in the Nb-Ti-Al system

The solidification pathways, subsequent solid-state transformations, and the liquidus surface in the Nb-Ti-Al system have been examined as part of a larger investigation of phase equilibria in Nb-Ti-Al intermetallic alloys. Fifteen alloys ranging in composition from 15 to 40 at. pct Al, with Nb to Ti ratios of 4:1, 2:1, 1.5:1, 1:1, and 1:1.5, were prepared by arc melting and the as-cast microstructures were characterized by optical microscopy (OM), microhardness, X-ray diffraction (XRD), differential thermal analysis (DTA), backscattered electron imaging (BSEI), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The results indicate that the range of primary β solidification is much wider than that indicated in previously reported liquidus surfaces, both experimental and calculated. Differential thermal analysis has identified the existence of a β to σ+γ transformation in three alloys where it was previously thought not to exist; confirmation was provided by high-temperature vacuum heat treatments in the single-phase β region followed by rapid quenching. The location of the boundary between the β, σ, and δ primary solidification fields has been redefined. A massive β → δ transformation, which was observed in the cast microstructure of a Nb-25Ti-25Al alloy, was repeatable through cooling following homogenization. A β → δ+σ eutectoid-like transformation in the 25 at. pct Al alloys, was detected by DTA and evaluated through microstructural analysis of heat-treated samples. Trends in the β phase with variations in composition were established for both lattice parameters and microhardness. As a result of this wider extent of the primary β solidification field, a greater possibility exists for microstructural control through thermal processing for alloys consisting of either σ+γ, β+σ, or β+δ phases. An erratum to this article is available at .     

Modeling of irregular eutectic microstructures in solidification of Al-Si alloys

A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally.     

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct). Formerly Research Assistant, Department of Materials Science, University of Southern California     

Steady-state cellular solidification of Al-Cu Reinforced with alumina fibers

The steady-state directional solidification of aluminum-4.5 wt pct copper and aluminum-1.0 wt pct copper alloys reinforced with parallel,continuous, closely spaced alumina fibers is investigated under growth conditions that produce a plane front or cells in corresponding unreinforced alloys. Specimens were designed to have a central reinforced region surrounded by unreinforced metal of the composite matrix composition. Each was produced by pressure infiltration, subsequently remelted, directionally solidified, and quenched to reveal the liquid/solid metal interface. Both unreinforced and composite sections were characterized to determine solidification front morphology and degree of microsegregation. In the unreinforced portion of the samples, the transition from plane-front to cellular solidification was observed to correspond to a coefficient of diffusion of copper in liquid aluminum of 5 − 10⁻⁹ m² − s⁻¹, in agreement with published values. Cell lengths, analyzed using a finite-difference model of microsegregation, are in agreement with the Bower-Brody-Flemings (BBF) model for cell tip undercooling. In the composite portion of the samples, the alloys solidify free of lateral microsegregation for all solidification conditions investigated, in agreement with theory. The shape of the liquid/solid metal interface near the fibers indicates a much lower fiber/liquid metal interfacial energy than fiber/solid metal interfacial energy. In the composite, plane front solidification is therefore not observed even when plane front solidification obtains in the unreinforced alloy. It is shown that geometrical constraint imposed on deep cells by the fibers causes significant increases in cell tip undercoolings, in agreement with current analyses of deep cell solidification.     

Predicted volume change behavior accompanying the solidification of binary alloys

For the volume changes accompanying solidification, distinctions are made between the volume changeβ _Mfor the whole freezing process, the volume changeβ _mfor liquid entrapped within the freezing zone, and the localized volume changeβ _Taccompanying the liquid-solid phase transformation at a given temperature. The first volume change is important in mold design, while the latter two are important factors in the formation of casting defects such as shrinkage pores, solidification cracks, and inverse segregation. Values ofβ _M, β_M, andβ _mare deduced for equilibrium conditions in the representative alloy systems Al-Cu, Bi-Sb, Fe-C and Pb-Sn. While the volume changeβ _Mmay vary only moderately with alloy composition,β _mis a strong function of composition and of the temperature of enclosure. The isothermal volume change,β _T, equal to the relative density difference between solid and liquid, varies during the freezing process and is strongly dependent upon composition. Isothermal volume changes and hence density differences as large as 20 pct are deduced for some Bi-Sb and Pb-Sn alloys.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part II. A phase-diagram approach

The mechanism by which iron causes casting defects in the AA309 (Al-5 pct Si-1.2 pct Cu-0.5 pct Mg) may be related to the solidification sequence of the alloy. Superimposing calculated segregation lines on the liquidus projection of the ternary Al-Si-Fe phase diagram suggests that porosity is minimized at a critical iron content when solidification proceeds directly from the primary field to the ternary Al-Si-βAl₅FeSi eutectic point. Solidification via the binary Al-βAl₅FeSi eutectic is detrimental to casting integrity. This hypothesis was tested by comparing the critical iron content observed in the standard AA309 alloy to that of a high-silicon (10 pct Si) variant of this alloy.     

The influence of alloy composition on precipitates of the Al−Mg−Si system

To study how changes in solute elements affect precipitation, six Al−Mg−Si alloys aged at 175 °C were investigated by transmission electron microscopy (TEM). In alloys with 1.3 at. pct solute, when the Si/Mg ratio exceeds 5/6, a sharp hardness peak appears after 3 hours that correlates with a high density of fine Guinier-Preston (GP) zones. A second, broader peak correlates with β″ precipitates and U phases. With high Si/Mg ratios, GP zones survive for long aging times. The β″-Mg₅Si₆ phase becomes very stable in the alloy with its Si/Mg ratio closest to 6/5. Deviation from this ratio increases fractions of β′, U-phases and disordered precipitates. In Mg-rich alloys less GP zones form and the first peak is suppressed. A coarse precipitate microstructure of β″ and β′ develops, the volume fraction being much higher than in Si-rich alloys. The Mg-rich alloys overage faster. Reducing the content of solutes causes alloys with high Si/Mg ratios to have a more Mg-rich behavior.