Solidification Microsegregation Research Articles

Interplay between solidification microsegregation and complex precipitation in a γ/γ’ cobalt-based superalloy elaborated by directed energy deposition

This study examines the segregations and the precipitation appearing during the solidification path of a Co-Ni-Al-W-Ta-Ti-Cr γ/γ’ cobalt-based superalloy processed by directed energy deposition (DED). Observations reveal characteristic elements partitioning in the liquid during additive manufacturing. Due to this microsegregation, complex multiphase precipitation occurs and the various precipitates are identified and characterized in a cobalt-based superalloy fabricated by DED. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to investigate the spatial distribution and nature of the various phases detected in the as-fabricated microstructure. Energy dispersive X-ray spectrometry (EDS), wavelength dispersive X-ray spectrometry (WDS), and electron energy loss spectroscopy (EELS) are coupled with a fine analysis of the diffraction patterns to identify the different phases decorating the interdendritic regions. These characterizations allow the identification of different submicronic precipitates: Al 2 O 3 , (Ta,Ti)(N,C), HfO 2 , Cr 3 B 2 and (Ti,Zr,Hf) 2 SC. The solidification sequence is discussed at light of the experimental results. This work provides a first understanding of the interplay between solidification segregations and the formation of second phase precipitation in a cobalt-based superalloy processed by DED. • A cobalt-based superalloy was processed by Direct Energy Deposition. • Solidification segregations lead to the precipitation of interdendritic phases. • Automated crystallographic orientation mapping and chemical analyses were used. • Various interdendritic phases have been identified: oxides (Al 2 O 3 and HfO 2 ), carbonitrides (Ta, Ti)(N, C), sulphocarbides (Ti, Zr, Hf) 2 SC, and borides Cr 3 B 2 . • The solidification path can be established based on the experimental observations.

Alloy solidification: Assessment and improvement of an easy-to-apply model

• The Gong-Chen model requires no experimental parameters and is easy to apply. • It can be used as a generic solidification model up to the solid fraction (fs) of 0.9. • When fs > 0.9, its applicability depends on both solute diffusion and partition coefficient. • The Gong-Chen model requires f s Lever > 1, which has been examined and justified. • The Won-Thomas model provides the most practical predictions but requires experimental input. It has been a central task of solidification research to predict solute microsegregation. Apart from the Lever rule and the Scheil-Gulliver equation, which concern two extreme cases, a long list of microsegregation models has been proposed. However, the use of these models often requires essential experimental input information, e.g. , the secondary dendrite arm spacing ( λ ), cooling rate ( T ˙ ) or actual solidification range ( Δ T ). This requirement disables these models for alloy solidification with no measured values for λ, T ˙ and Δ T . Furthermore, not all of these required experimental data are easily obtainable. It is therefore highly desirable to have an easy-to-apply predictive model that is independent of experimental input, akin to the Lever rule or Scheil-Gulliver model. Gong, Chen, and co-workers have recently proposed such a model, referred to as the Gong-Chen model, by averaging the solid fractions ( f s ) of the Lever rule and Scheil-Gulliver model as the actual solid fraction. We provide a systematic assessment of this model versus established solidification microsegregation models and address a latent deficiency of the model, i.e., it allows the Lever rule solid fraction f s to be greater than one ( f s > 1). It is shown that the Gong-Chen model can serve as a generic model for alloy solidification until f s reaches about 0.9, beyond which ( f s > 0.9) its applicability is dictated by both the equilibrium solute partition coefficient ( k ) and the solute diffusion coefficient in the solid ( D s ), which has been tabulated in detail.

Migration of solidification grain boundaries and prediction

Solidification processing is essential to the manufacture of various metal products, including additive manufacturing. Solidification grain boundaries (SGBs) result from the solidification of the last liquid film between two abutting grains of different orientations. They can migrate, but unlike normal GB migration, SGB migration (SGBM) decouples SGBs from solidification microsegregation, further affecting material properties. Here, we first show the salient features of SGBM in magnesium-tin alloys solidified with cooling rates of 8−1690 °C/s. A theoretical model is then developed for SGBM in dilute binary alloys, focusing on the effect of solute type and content, and applied to 10 alloy systems with remarkable agreement. SGMB does not depend on cooling rate or time but relates to grain size. It tends to occur athermally. The findings of this study extend perspectives on solidification grain structure formation and control for improved performance (e.g. hot or liquation cracking during reheating, intergranular corrosion or fracture).

Interplay between Solidification Microsegregation and Complex Precipitation in a Γ/Γ’ Cobalt-Based Superalloy Elaborated by Directed Energy Deposition

Interplay between Solidification Microsegregation and Complex Precipitation in a Γ/Γ’ Cobalt-Based Superalloy Elaborated by Directed Energy Deposition

Non-Equilibrium Solidification Behavior With Solute Trapping Associated With Powder Characteristics During Electron Beam Additive Manufacturing

For components built by powder bed fusion with electron beam (PBF-EB), the resulting microstructure arising from non-equilibrium solidification–microsegregation and the formation of interdendritic phases significantly affects the mechanical properties and hot cracking resistance. Notably, the powder characteristics influence heat absorption and conduction, thereby altering the molten pool behavior and solidification parameters. However, the effect of powder feedstock on non-equilibrium solidification during PBF has not been widely investigated. In this study, a CoCrMo alloy was built using powders prepared by gas-atomization (GA) and plasma rotating electrode process (PREP). Under the given operating conditions, the samples built with the two powders were experimentally characterized and their compression properties were compared. By performing multi-scale numerical simulations, powder melting and solidification were visualized and analyzed to elucidate the mechanism through which the powder characteristics influence the non-equilibrium solidification behavior during PBF-EB. The study revealed that upon appropriated size control, compared to the GA powder, the PREP powder had a smaller specific surface area and higher sphericity; thus, the generated powder layer exhibited higher heat absorption and dissipation rates. Therefore, a high solidification rate is facilitated, thereby suppressing microsegregation. The findings contribute to PBF knowledge related to feedstock, thus proving to be an essential reference for selecting and optimizing metallic powders applicable to additive manufacturing.

On the use of metastable interface equilibrium assumptions on prediction of solidification micro-segregation in laser powder bed fusion

ABSTRACTMany models have been developed to explore solidification segregation and dendrite structure in additively manufactured parts. However, these models tend to be computationally expensive and consider only a limited number of alloying elements, compromising their practical application. In this work, a methodology to extend the Scheil model, based on interface metastable equilibrium assumptions, is established to predict the spatial compositional maps due to micro-segregation for a laser-powder bed fusion (L-PBF) build of alloy 718. The compositional maps are contrasted against experimental data measured in a unit dendrite cell by transmission electron microcopy. The validity of Scheil's implicit assumptions under the rapid solidification conditions in L-PBF is further discussed. The extended Scheil model is shown to be computationally efficient and readily applicable to multi-component systems.

Micro-segregation and precipitates in as-solidified Al-Sc-Zr-(Mg)-(Si)-(Cu) alloys

In this work, we investigate the as-cast microstructure of three model alloys, an Al-Sc-Zr, an Al-Mg-Si-Sc-Zr and an Al-Mg-Si-Cu-Sc-Zr alloy. Transmission electron microscopy and atom probe tomography are used to investigate the precipitates present in the microstructure after casting. We find that higher Si content results in the formation of dispersoids upon casting. The as-cast dispersoids are found to be rich in Si with Si substituting for ~25% of the Al in the L12 structure. Variations in the local alloy composition and dispersoids size is explained by the presence of solidification micro-segregation which is fitted using a one-dimensional Scheil model. This segregation is found to be responsible for the formation of MgSi rod-like precipitates in the inter-dendritic regions. These precipitates are found to nucleate on the as-cast dispersoids. A closer look at the solid solution reveals that most of the Sc and Zr is supersaturated after casting even in the presence of Mg, Si and Cu. We discuss the impact of these results on the design of a suitable heat treatment for 6xxx-series alloys with Sc.

Solidification and Room Temperature Microstructure of a Fully Pearlitic Compacted Graphite Cast Iron

Compacted graphite cast irons are rapidly developing for they have better mechanical properties than lamellar graphite cast irons and present less porosity than spheroidal graphite cast irons. For many applications, an as-cast fully pearlitic matrix would be desired which can hardly be achieved when graphite is compacted. Addition of manganese, copper and tin are thus made as these elements are known to be pearlite promoters. However, their amount should be limited so as to avoid detrimental effects amongst which are heterogeneities in the matrix properties which impede easy machining. In the present work, a compacted graphite cast iron containing 0.3 wt% Mn, 0.8 wt% Cu and 0.1 wt% Sn was cast in sand mould and in standard thermal analysis cup. The cup sample showed a nearly fully pearlitic matrix and was selected for further study. The characterization consisted of measuring and correlating the distributions of pearlite interlamellar spacings and microhardness values. An attempt was made to look for the effect of solidification microsegregation on microhardness which did not reveal any trend.

Solidification Microsegregation and Hot Ductility of Fe-Mn-C-Al-xNb TWIP Steels

The influence of Nb addition on casting microstructures and high-temperature mechanical properties of Fe-Mn-C-Al-xNb TWIP steels was analyzed by phase-field modeling and experiments. Phase-field simulations showed that Mn, Nb, and C are enriched in inter-dendritic regions while Al is enriched in dendritic regions during solidification process of the investigated TWIP steels. Both phase-field simulations and microstructural characterization show that NbC precipitates are preferentially present near inter-dendritic boundaries. Nb addition slightly reduces hot ductility of the investigated steel at 1173 K (900 °C) while the Nb-added TWIP steels show better hot-ductility than the reference steel for deformation temperatures above 1373 K (1100 °C). NbC precipitates and inter-dendritic distances appear to be the most important variables that affect hot-ductility behavior of the investigated TWIP steels.

Study of the Eutectoid Transformation in Nodular Cast Irons in Relation to Solidification Microsegregation

Eutectoid transformation in cast irons may proceed in the stable or the metastable systems giving ferrite and graphite for the former and pearlite for the latter. The present work demonstrates that composition profiles across ferrite/pearlite boundaries are smooth and similar to those issued from the solidification step. No trace of long-range diffusion of substitutional solutes due to austenite decomposition could be observed. In turn, this ascertains that both stable and metastable transformations proceed with the product matrix—either ferrite opearlite—inheriting the parent austenite content in substitutional solutes. This result sustains a physical model for eutectoid transformation based on the so-called local para-equilibrium which is commonly used for describing solid-state transformation in steels.

Microstructure Evolution in the Mushy Zone of a β‐Solidifying TiAl Alloy under Different Cooling Processes

A novel method termed mushy zone cooling control is applied to tailor the microstructure of a β‐solidifying titanium aluminide alloy in this study. Ti‐44.5Al–4Nb–2Cr–0.1B (at%) alloy is subjected to temperature‐controlled cooling through the mushy zone by different processes. The solidified microstructure consists of α2/γ lamellar colony, B2 phase, and massive γ. It is found that both the fast cooling and the thermal cycling can play a role in refining α2/γ lamellar colonies and in reducing B2 phase, which is brittle at room temperature. α2/γ colonies with an average size of 28 µm are obtained after 10 thermal cycles. The results also show that solidification microsegregation can be largely eliminated by fast cooling. However, α2/γ colonies will continuously coarsen during such an isothermal holding process.

Different crystal growth and microsegregation mechanisms based on interface evolution in 0Cr18Ni9 stainless steel ingot

Through the microstructure characterization, the different microsegregation and crystal growth phenomena were revealed in a 0Cr18Ni9 stainless steel ingot. Because the solute redistribution is slight in the step growth of δ ferrite-austenite transition, the different distribution of residual δ ferrite, which are depended on the solute segregation, should be traced back to the microsegregation in solidification. And the residual δ ferrite traced back to microsegregation in solidification further implies the complicate crystal growth behaviors. Under different solidification conditions, the shape and forward direction of crystal interface are different. Especially, the discrete crystal growth can happen in a scale smaller than dendrite grains and dendrite stems. The underlying mechanism for discrete crystal growth is explained based on interface evolution. Finally, the effect of different crystal growth behaviors on microsegregation is analyzed.

O35] Modeling of dendritic microstructure and microsegregation in solidification of Al-rich multi-component alloys

O35] Modeling of dendritic microstructure and microsegregation in solidification of Al-rich multi-component alloys

Simulation of the Dendrite Morphology and Microsegregation in Solidification of Al–Cu–Mg Aluminum Alloys

Since most typical alloys in industrial applications are multicomponent with three or more components, and various CA models proposed in the past mainly focus on the binary alloys, a two-dimensional modified cellular automaton model allowing for the quantitatively predicting dendrite growth of multicomponent alloys in the low Peclet number regime is presented. The elimination of the mesh-induced anisotropy is achieved by adopting a modified virtual front tracking method. A new efficient method based on the lever rule is applied to calculate the solid fraction increment of the interfacial cells. The thermodynamic data such as liquidus temperature, the partition coefficients, and the slope of liquidus surface, needed for determining the dynamics of dendrite growth, are obtained by coupling with PanEngine. This model is applied to simulate the dendrite morphology and microsegregation of Al–Cu–Mg ternary alloy both for single and multi-dendrites growth. The simulated results demonstrate that the difference of the concentration distribution profiles ahead of the dendrite tip for each alloying element mainly results from the different partition coefficients and solute diffusion coefficients. Comparison with the prediction of analytical model is carried out and it reveals the correctness of the model. Consequently, the difference in interdendritic microsegregation behavior of different components is analyzed.

Directional Solidification Microsegregation and Concentration Analysis with Different Supersaturation

In order to obtain the directional microstructure of different supersaturation and growing velocity, three simulations is calculated with different initial temperature. When the initial temperature is 1576K, and the supersaturation and growing velocity are smaller. The average space length of columnar crystals is bigger, and microsegregation is smaller. when the initial temperature is 1574K, the supersaturation and growing velocity increase. The average space length of columnar crystals decreases, and microsegregation increase; when the initial temperature falls to 1566K, the supersaturation and growing velocity reach 1.176 and 3.238 cm per second. The microsegregation decreases rapidly. The average space length of columnar crystals is the smallest. The solute curve of columnar crystal is like a “U” figure. When the growing velocity is bigger than VA, the phenomenon of solute trapping takes place.

Modeling of Microstructure and Microsegregation in Solidification of Multi-Component Alloys

Driven by industrial demand, extensive efforts have been made to investigate microstructure evolution and microsegregation development during solidification of multicomponent alloys. This paper briefly reviews the recent progress in modeling of microstructures and microsegregation in solidification of multicomponent alloys using various models including micromodel, phase field, front tracking, and cellular automaton approaches. A two-dimensional modified cellular automaton (MCA) model coupled with phase diagram software PanEngine is presented for the prediction of microstructures and microsegregation in the solidification of ternary alloys. The model adopts MCA technique to simulate dendritic growth. The thermodynamic data needed for determining the dynamics of dendritic growth are calculated with PanEngine. After validating the model by comparing the simulated values with the prediction of the Scheil model for solute profiles in the primary dendrites as a function of solid fraction, the model was applied to simulate the microstructure and microsegregation in the solidification of Al-rich ternary alloys. The simulation results demonstrate the capabilities of the present model not only to simulate realistic dendrite morphologies, but also to predict quantitatively the microsegregation profiles in the solidification of multi-component alloys.

Computer-aided design of transformation toughened blast resistant naval hull steels: Part I

A systematic approach to computer-aided materials design has formulated a new class of ultratough, weldable secondary hardened plate steels combining new levels of strength and toughness while meeting processability requirements. A theoretical design concept integrated the mechanism of precipitated nickel-stabilized dispersed austenite for transformation toughening in an alloy strengthened by combined precipitation of M2C carbides and BCC copper both at an optimal ∼3 nm particle size for efficient strengthening. This concept was adapted to plate steel design by employing a mixed bainitic/martensitic matrix microstructure produced by air-cooling after solution-treatment and constraining the composition to low carbon content for weldability. With optimized levels of copper and M2C carbide formers based on a quantitative strength model, a required alloy nickel content of 6.5 wt% was predicted for optimal austenite stability for transformation toughening at the desired strength level of 160 ksi (1,100 MPa) yield strength. A relatively high Cu level of 3.65 wt% was employed to allow a carbon limit of 0.05 wt% for good weldability, without causing excessive solidification microsegregation.

Analyses of diffusion-related phenomena in steel process

In steel production processes, there are various diffusion-related phenomena, and these are controlled to obtain the required steel quality. In the refining process, the slag-metal reaction that is controlled by the diffusion of various reactants and products in boundary layers in both the slag and metal side along the slag-metal interface is analyzed by a coupled reaction model. Solidification microsegregation is controlled by the solute diffusion in the liquid and solid phases, and various analytical microsegregation models have been proposed. The tertiary precipitation of nonmetallic inclusions is also affected by solute diffusion in the solid phase and is analyzed by a coupled precipitation model, the modified solidification segregation model. The formation of a carbon (C)-rich band structure along the centerline of steel plates and weld cracking are also prevented by regulating the diffusion of C by accelerated cooling and by encouraging hydrogen to diffuse out by preheating the weld part, respectively. A key element of intragranular ferrite precipitation for ferrite grain refinement is the formation of manganese (Mn)-depleted zone by slow Mn diffusion around a precipitate.

Development and use of layered ferrous microstructure

The method by which bloomery iron was agglomerated into usable artefacts by forge welding, piling, and reforging gave a layered microstructure. This was not only associated with the directionality of residual slag inclusions, but also related to variation in solute concentration, which became directional. This variation could originate in the bloom, or it could be developed by the partitioning of solutes such as phosphorus between coexisting high temperature phases. The oxidation enrichment of solutes such as Ni and As in individual surfaces, before forging together, could also lead to a layered microstructure being produced. On a larger scale, the deliberate use of differently sourced irons to make a composite product could also generate a sandwich structure.Although many early iron artefacts contain substantial amounts of phosphorus overall, the layered microstructures that phosphorus segregation generates may have offset the embrittling effect that could otherwise be expected in the presence of some carbon. The purposeful selection of high phosphorus iron (identified from effect and source) for the backs of knives, etc., could have been intended to keep a stable, high carbon content in the steel edge to which the iron was welded. The final distribution of carbon in such a forged, banded microstructure is responsible for the pattern welding or damascening of sword blades where controlled forging, folding, and twisting is carried out, the variation in hardness and etching characteristics enabling a desired pattern to be revealed at the final surface. Banded or layered microstructures also occur in wrought steels produced from cast ingots, which have variation in concentration of solutes (e.g. phosphorus) originating from solidification microsegregation. This segregation was also significant in the development of dual phase steels, of importance for the production of car bodies, where the use of an intercritical quench results in banded martensite-ferrite microstructures, and subsequent low temperature tempering precipitates excess carbon from the ferrite. Such a sheet structure work hardens more rapidly than normal sheet of the same or somewhat greater carbon content, which is appropriate for the small amount of forming usually required.Attempts have also been made to produce a tough, more stable, layered microstructure synthetically by roll bonding layers of aluminium stabilised ferrite with high carbon steel. Although wires of martensite cored ferrite made in a similar manner gave promising results of substantially higher plastic strain to fracture, attempts to swage skeins of such wire to rod were unsuccessful. Layered austenite-martensite structures can also be produced by the choice of suitable compositions. In the historical Malayan kris dagger, layers of a high nickel content iron were developed which stabilised the austenite phase, as in recent work on a composite based on nickel coated components for rapier blades and for improved resistance to stress corrosion cracking in wider use.

Gas Atomization Processing of LaNi5-x.Mm, Modified with Silicon and Tin

ABSTRACTA high pressure gas atomization (HPGA) process has been employed to study microsegregation in LaNi4.75 Sn0.25 and LaNi4.6Si0.4 alloy powders to serve as a basis for further investigations of low cost production of multicomponent alloys in combination with mishmetal (Mm). This investigation is an attempt to produce high quality powders for battery cathode fabrication that can be used in an as-atomized condition without prolong annealing, hydridingdehydriding, and grinding. Argon atomizing gas was used in the HPGA process of the LaNi4.75Sn0.25 and LaNi4.6Si0.4 alloys to investigate rapid solidification effects on microsegregation. Short annealing treatments of 5 minutes at 900°C were able to homogenize both alloy compositions, eliminating solidification micro-segregation along the grain boundaries phases. The rapid homogenization can be attributed primarily to the refined cell structures in gas-atomized particles that provide short diffusion pathways for the dissolving elements.