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.     

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.     

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     

Constitution of the Lead-Tin-Strontium System up to 36 Atomic Percent Sr

The constitution of the Pb-Sn-Sr system from the Pb-Sn binary up to 36 at. pct Sr was determined by differential thermal analysis, metallography, microprobe analysis, and X-ray diffraction. Pb₃Sr forms a continuous series of solid solutions with Sn₃Sr, and is referred to here as the8 phase. Sn₄Sr was the only other intermetallic phase found and is designated here as γ. A eutectic-like trough is formed between (Pb) and δ. It originates at 1.0 at. pct Sr and 324.5 °C (the (Pb)/Pb₃Sr eutectic) and falls monotonically to ~75 at. pct Pb, 24.5 at. pct Sn, and 0.45 at. pct Sr at 283 °C. At 283 °C, a Class II, four-phase reaction occurs: L + δ→ (Pb) + γ. A eutectic-like trough between (Pb) and γ falls from the four-phase plane at 283 °C to the ternary eutectic at ~26 at. pct Pb, ~74 at. pct Sn and <0.3 at. pct Sr at 182 °C. The ternary eutectic reaction is L → (Pb) + (Sn) + γ.     

Microstructural Evolution During Laser Resolidification of Fe-25 Atom Percent Ge Alloy

The microstructural evolution of concentrated alloys is relatively less understood both in terms of experiments as well as theory. Laser resolidification represents a powerful technique to study the solidification behavior under controlled growth conditions. This technique has been utilized in the current study to probe experimentally microstructural selection during rapid solidification of concentrated Fe-25 atom pct Ge alloy. Under the equilibrium solidification condition, the alloy undergoes a peritectic reaction between ordered α ₂ (B2) and its liquid, leading to the formation of ordered hexagonal intermetallic phase ε (DO19). In general, the as-cast microstructure consists of ε phase and ε–β eutectic and α ₂ that forms as a result of an incomplete peritectic reaction. With increasing laser scanning velocity, the solidification front undergoes a number of morphological transitions leading to the selection of the microstructure corresponding to metastable α ₂/β eutectic to α ₂ dendrite + α ₂/β eutectic to α ₂ dendrite. The transition velocities as obtained from the experiments are well characterized. The microstructural selection is discussed using competitive growth kinetics. This article is based on a presentation made in the symposium entitled “Solidification Modeling and Microstructure Formation: In Honor of Prof. John Hunt,” which occurred March 13-15, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the TMS Materials Processing and Manufacturing Division, Solidification Committee.     

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.     

Determination of Solid Fraction from Cooling Curve

A new method to determine directly the solid fraction using the cooling curve was proposed for solidification of undercooled melts. Then, to construct three different baselines, a sudden function ξ _α (x) is introduced. In terms of the ξ _α (x) function, accordingly, the solid fractions during solidification of Ni-3.3 wt pct B, Al-7 wt pct Si, Al-14 wt pct Cu, and Fe-4.56 wt pct Ni alloys were predicted. The predictions of the primary, the regular lamellar eutectic, the anomalous eutectic, and the peritectic phases from cooling curves of the solidified samples coincide with the results of measurement or the available methods.     

The effect of temperature and microstructure on the creep damage found in low alloy ferritic steels

Constant strain rate tests at 10^-5 s^-1 have been carried out in the temperature range 723 to 973 K on two 1 1/2 pct Cr · 1/2 pct V ferritic steels, the first steel with a 20 pct bainite, 80 pct ferrite microstructure and the second with a fully ferritic structure. Measurements of the quantitative strain, ε_gb, due to grain boundary sliding (gbs), were made and in both steels the γ values (where γ = ε_gb/ε_T) increased with increasing temperature. In both structures, sliding was found to occur on all boundaries. A qualitative study of cavitation damage and final fracture mechanisms was also made. It is suggested that in the mixed structure, cavities are nucleated by gbs at carbides whereas in the fully ferritic structure, cavity nucleation is by the interaction of intragranular slip with a grain boundary. Optical observations showed that the large scale cavitation behavior was superficially very similar in both steels, but scanning electron microscope observations showed remarkable differences in the fine scale cavitation damage. The implications of these results are discussed in terms of the relationship between matrix deformation, grain boundary deformation and creep fracture. Formerly of the Department of Metallurgy, University of Manchester.     

A comparative study of the microstructures observed in statically cast and continuously cast Bi-In-Sn ternary eutectic alloy

A ternary eutectic alloy with a composition of 57.2 pct Bi, 24.8 pct In, and 18 pct Sn was continuously cast into wire of 2 mm diameter with casting speeds of 14 and 79 mm min⁻¹ using the Ohno Continuous Casting (OCC) process. The microstructures obtained were compared with those of statically cast specimens. Extensive segregation of massive Bi blocks, Bi complex structures, and tinrich dendrites was found in specimens that were statically cast. Decomposition of γSn by a eutectoid reaction was confirmed based on microstructural evidence. Ternary eutectic alloy with a cooling rate of approximately 1 °C min⁻¹ formed a double binary eutectic. The double binary eutectic consisted of regions of BiIn and decomposed γSn in the form of a dendrite cell structure and regions of Bi and decomposed γSn in the form of a complex-regular cell. The Bi complex-regular cells, which are a ternary eutectic constituent, existed either along the boundaries of the BiIn-decomposed γSn dendrite cells or at the front of elongated dendrite cell structures. In the continuously cast wires, primary Sn dendrites coupled with a small Bi phase were uniformly distributed within the Bi-In alloy matrix. Neither massive Bi phase, Bi complex-regular cells, nor BiIn eutectic dendrite cells were observed, resulting in a more uniform microstructure in contrast to the heavily segregated structures of the statically cast specimens.     

Microstructure of AI2O3 fiber-reinforced superalloy (INCONEL 718) composites

Composites of INCONEL 718 alloy reinforced with either single-crystal (SAPHIKON) or polycrys-talline (Du Pont's FP) A1₂O₃ fiber were fabricated by pressure casting. Optical and transmission electron microscopy were used to characterize the microstructure of the composites and to determine the nature of the fiber/matrix reaction. The widely dispersed fibers in the SAPHIKON-fiber-reinforced composite had no influence on the solidification of the matrix. Six phases, γ-Ni₃Al, γ'-Ni₃Nb, δ-Ni₃Nb, TiC, NbC, and Laves, were present in the matrix of the composite. The last three phases were formed during solidification and the others precipitated during subsequent cooling. The high density of fibers in the FP-fiber-reinforced composite led to a more uniform microstructure within the matrix. Only three phases,γ″-Ni₃Nb, NbC, and Laves, were identified. Diffusion of Ti into the A1₂O₃ fiber resulted in preferential grain growth in the FP fiber in areas adjacent to the fiber/matrix interface. The fiber/matrix bond strength in shear in the SAPHIKON-fiber-reinforced composite was in excess of 150 MPa.     

A study of the hot-working behavior of SiC-Al alloy

The hot-working behavior of two metal matrix composites (7090 + 20 vol pct SiC whiskers and 6061 + 20 vol pct SiC whiskers) and their powder metallurgy matrix alloys (7090 and 6061) was studied by hot torsion testing. Flow stress (σ_o) and strain-to-failure (ε _f ) data were generated at deformation temperatures and strain rates corresponding to the potential range for commercially hot-working these alloys. Based on the hot torsion data, hot-working parameters were recommended where σ_o was low and ε _f was high. Strain rate sensitivities and activation energies of deformation were computed for the alloys. Formerly with Martin Marietta Laboratories, Baltimore, MD     

Chemistry and distribution of phases produced by solid state sic/nicrai reaction

Aspects of the solid state reaction between SiC and a model superalloy consisting of Ni-20 at. pct Cr-10 at. pct Al at 1150 °C are studied in detail using analytical electron microscopy. The metal reaction zone formed on the metal side of the original metal/ceramic interface is found to consist of a complex mixture of four phases: γ′-Ni₃Al, β-NiAl, α-Cr, and Ξ, a ternary Ni-Si-Al phase. The chemistry of each phase is determined using X-ray spectroscopy in the analytical electron microscope. The effect of silicon on the γ —γ′ microstructure of the model superalloy is described, and is shown to alter the chemistry, morphology, and size of the γ′ phase. The γ-γ′ lattice parameter mismatch is calculated from the chemistry data, and changes in this mismatch caused by silicon correlate well with γ′ morphology changes. Finally, phase equilibria concepts are used to explain the presence and distribution of phases in the metal reaction zone. Department of Materials Science and Engineering, and during the course of this work was employed at the General Electric Corporate Research and Development Center as part of the Cornell University Engineering Cooperative Program.     

Microstructure, mechanical properties, and fracture mechanism of As-cast (Ti0.5Cu0.25Ni0.15Sn0.05Zr0.05)100−x Mo x composites

Copper mold cast cylinders of (Ti_0.5Cu_0.25Ni_0.15Sn_0.05Zr_0.05)_100−x Mo _x composites are prepared. Addition of Mo in the bulk glass-forming alloy induces the formation of a dendrite/matrix composite. For 3-mm-diameter cylinders, the matrix exhibits a homogenous ultrafine microstructure for Mo content of 2.5 at. pct, and a fine eutectic microstructure for 5 at. pct Mo. For 5-mm-diameter cylinders, the matrix exhibits a dendritic microstructure for 2.5 at. pct Mo, and exhibits a coarser eutectic microstructure for 5 at. pct Mo. Despite the formation of a dendrite/nanostructured matrix composite in the cylinders, the quenched surface layer with a nanoscale grain size dominates the deformation and fracture of the 3-mm-diameter cylinders. More than 56 vol pct quenched layer leads to a distensile fracture mode and the samples exhibit high fracture strength and high Young’s modulus but low ductility. For 5-mm-diameter cylinders, the composite microstructure becomes dominant due to its more than 64 vol pct volume fraction leading to a cone-shaped fracture surface. The samples exhibit lower yield strength and lower Young’s modulus but better ductility compared to the 3-mm-diameter cylinders. The mechanical behavior of the Mo-bearing composites strongly depends on the microstructural homogeneity and casting defects formed upon solidification.     

Martensitic transformations induced by plastic deformation in the Fe-Ni-Cr-C system

Martensitic transformations induced by plastic deformation are studied comparatively in various alloys of three types: Fe-30 pct Ni, Fe-20 pct Ni-7 pct Cr, and Fe-16 pet Cr-13 pct Ni, with carbon content up to 0.3 pct. For all these alloys the tensile properties vary rapidly with temperature, but there are large differences in the value of the temperature rangeM _s toM _d, which strongly increases with substitution of chromium for nickel or with carbon addition. Using the node method, it is found that the intrinsic stacking fault energy in the austenite drastically increases with temperature in all the chromium-bearing alloys investigated. This variation is consistent with the observed influence of temperature on the appearance of twinning or ε martensite during plastic deformation. Very different α’ martensite morphologies can result from spontaneous and plastic deformation induced transformations, especially in Fe-20 pct Ni-7 pct Cr-type alloys where platelike and lath martensites are respectively observed. As in the case of ε martensite, the nucleation process is analyzed as a deformation mode of the material, using a dislocation model. It is then possible to account for the morphology of plastic deformation induced α’ martensite in both Fe-20 pct Ni-7 pct Cr and Fe-16 pct Cr-13 pct Ni types alloys and for the largeM _s toM _d range in these alloys. This paper is based upon a thesis submitted by F. LECROISEY in partial fulfillment of the degree of Doctor of Philosophy at the University of Nancy.     

High-temperature strength and ductility increases of Ti-Mo-Al alloys by step aging

Step-aging programs, based on principles of particle-dislocation interactions, were developed systematically to obtain increases in the high-temperature strength and ductility properties of Ti-7 at. pct Mo-Al alloys. A triple-step aging program applied to Ti-7 Mo-16 Al produced a yield stress σ_0.2 = 1,500 MN/m², elongation to fracture ε _F = 4 pct at room temperature, and σ_0.2 = 900 MN/m², ε _F = 12 pct at 600°C. A two-step aging program resulted in σ_0.2 = 1,350 MN/m², ε _F = 5 pct at room temperature; σ_0.2 = 800 MN/m², ε _F = 20 pct at 600°C. Formerly Assistant Research Professor, Materials Research Laboratory, Rutgers University     

Mechanisms of Hf dopant incorporation during the early stage of chemical vapor deposition aluminide coating growth under continuous doping conditions

A laboratory-scale chemical vapor deposition (CVD) reactor was used to perform “continuous” Hf doping experiments while the surface of a single-crystal Ni alloy was being aluminized to form an aluminide (β-NiAl) coating matrix for 45 minutes at 1150 °C. The continuous doping procedure, in which HfCl₄ and AlCl₃ were simultaneously introduced with H₂, required a high HfCl₄/AlCl₃ ratio (>∼0.6) to cause the precipitation of Hf-rich particles (∼0.1 μm) at grain boundaries of the coating layer, with the overall Hf concentration of ∼0.05 to 0.25 wt pct measured in the coating layer by glow-discharge mass spectroscopy (GDMS). Below this ratio, Hf did not incorporate as a dopant into the growing coating layer from the gas phase, as the coating matrix appeared to be “saturated” with other refractory elements partitioned from the alloy substrate. In comparison, the Hf concentration in the aluminide coating layer formed on pure Ni was in the range of ∼0.1 wt pct, which was close to the solubility of Hf estimated for bulk NiAl. Interestingly, the segregation of Hf and the formation of a thin γ′-Ni₃Al layer (∼0.5 μm) at the coating surface were consistently observed for both the alloy and pure-Ni substrates. The formation of the thin γ′-Ni₃Al layer was attributed to an increase in the elastic strain of the β-NiAl phase, associated with the segregation of Hf as well as other refractory alloying elements at the coating surface. This phenomenon also implied that the coating layer was actually growing at the interface between the γ′-Ni₃Al layer and the β-NiAl coating matrix, not at the gas/coating interface, during the early stage of the coating growth.     

A study of the hot-working behavior of SiC−Al alloy composites and their matrix alloys by hot torsion testing

The hot-working behavior of two metal matrix composites (7090+20 vol pct SiC whiskers and 6061+20 vol pct SiC whiskers) and their powder metallurgy matrix alloys (7090 and 6061) was studied by hot torsion testing. Flow stress (σ ₀ ) and strain-to-failure (ε _f ) data were generated at deformation temperatures and strain rates corresponding to the potential range for commercially hot-working these alloys. Based on the hot torsion data, hot-working parameters were recommended where (σ ₀ ) was low and (ε _f ) was high. Strain rate sensitivities and activation energies of deformation were computed for the alloys. M. LIEBSON, formerly with Martin Marietta Laboratories. Baltimore, MD     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part III. A microstructural model

Iron has been shown to have a significant effect on the formation of porosity and shrinkage defects in Al-Si-Cu-based foundry alloys. This is not simply a direct consequence of the physical presence of the β-Al₅FeSi platelets in the microstructure, but is also due to the effect that these platelets have on the nucleation and growth of eutectic silicon. The alloy-dependent critical iron content determines when the β phase first solidifies and, hence, when it can participate in the silicon nucleation event. At critical iron contents, the β phase solidifies as the initial component of the ternary eutectic. However, at supercritical iron contents, the β phase is already well developed when ternary eutectic solidification begins, while, at subcritical iron contents, the β phase forms as a component of the ternary eutectic only after the binary Al-Si eutectic is well established. Each of these paths of microstructural evolution leads to different variations in microstructural permeability and, hence, interdendritic feedability and porosity formation. The actual porosity-forming response to these alloy-induced microstructural changes is influenced by the solidification conditions in the casting.