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.     

Nucleation and phase selection in undercooled Fe-Cr-Ni melts: Part II. Containerless solidification experiments

The solidification behavior of undercooled Fe-Cr-Ni melts of different compositions is investigated with respect to the competitive formation of δ-bcc (ferrite) and γ-fcc phase (austenite). Containerless solidification experiments, electromagnetic levitation melting and drop tube experiments of atomized particles, show that δ (bcc) solidification is preferred in the highly undercooled melt even at compositions where δ is metastable. Time-resolved detection of the recalescence events during crystallization at different undercooling levels enable the determination of a critical undercooling for the transition to metastable bcc phase solidifcation in equilibrium fcc-type alloys. Measurements of the growth velocities of stable and metastable phases, as functions of melt undercooling prior to solidification, reveal that phase selection is controlled by nucleation. Phase selection diagrams for solidification processes as functions of alloy composition and melt undercooling are derived from two types of experiments: X-ray phase analysis of quenched samples and in situ observations of the recalescence events of undercooled melts. The experimental results fit well with the theoretical predictions of the metastable phase diagram and the improved nucleation theory presented in an earlier article. In particular, the tendency of metastable δ phase formation in a wide composition range is confirmed.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

Rapidly solidified AlCu alloys—I. experimental determination of the microstructure selection map

Laser rapid solidification experiments have been performed on AlCu alloys of hypereutectic composition 36, 40 and 44 wt% Cu. By taking thin foils from the surface of the laser traces it has been possible to study the resulting growth morphologies using TEM; the microstructural orientation allowing the morphologies to be correlated with their local growth velocities. Microstructural orientation allowing velocity range 0.01–2.0 ms⁻¹ have been studied, and eutectic (both with and without oscillatory) instabilities), dendritic, cellular, banded and planar front growth have all been observed. By combining these results with earlier observations made on alloys with lower Cu concentrations, it has been possible to produce a microstructure selection map for AlCu alloys. The map correlates microstructure to growth velocity and composition in the ranges 0.01–2.0 ms⁻¹ and 0–44 wt% Cu, i.e. the major part of the binary AlAl₂Cu eutectic (0–54 wt% Cu). As well as providing an interesting overview of solidification structures, several features of this map can be used to gain information on the AlCu phase diagram.     

A solidification diagram for Ni-Cr-Mo-Gd alloys estimated by quantitative microstructural characterization and thermal analysis

A γ-Gd solidification diagram is proposed as an aid to understanding solidification behavior of Ni-Cr-Mo-Gd alloys. In this system, the Ni-Cr-Mo solid solution γ primary austenite phase is treated as the “solvent” and Gd is treated as the solute. The proposed diagram, which has features characteristic of a binary “eutectic” system, was constructed by combining differential thermal analysis and quantitative microstructural analysis data. As a result of the partially divorced solidification microstructure in the ingots studied, determination of the fraction eutectic, and hence the eutectic composition, requires the use of advanced image analysis techniques. The diagram displays a number of features that are very similar to the Ni-Gd binary system and can be used to assess the influence of the Gd concentration on solidification behavior.     

Solidification of binary hypoeutectic alloy matrix composite castings

We consider a binary hypoeutectic alloy casting which solidifies in dendritic form in an unreinforced engineering casting and seek to predict its microstructure in a metal matrix composite. We focus on the case where the reinforcement is fixed in space and fairly homogeneously distributed. We assume that the reinforcement does not catalyze heterogeneous nucleation of the solid. We show that the reinforcement can cause several microstructural transitions in the matrix alloy, depending on the matrix cooling rate, the width, A, of interstices left between reinforcing elements, and the initial velocityV of the solidification front. These transitions comprise the following: (1) coalescence of dendrite arms before solidification is complete, causing solidification to proceed in the later stages of solidification with a nondendritic primary phase mapping the geometry of interstices delineated by reinforcement elements; (2) sharp reduction or elimination of microsegregation in the matrix by diffusion in the primary solid matrix phase; and (3) a transition from dendrite to cell formation, these cells featuring significant undercoolings or a nearly plane front configuration when reinforcing elements are sufficiently fine. Quantitative criteria are derived for these transitions, based on previous work on composite solidification, observations from directional solidification experiments, and current solidification theory. Theory is compared with experimental data for aluminum-copper alloys reinforced with alumina fibers and for the dendrite to cell transition using data from directional succinonitrile-acetone solidification experiments. Theory and experiment show good agreement in both systems. This article is based on a presentation made at the “Analysis and Modeling of Solidification” symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2–6, 1994, under the auspices of the TMS Solidification Committee.     

Microstructure and Phase Evolution in Laser Cladding of Ni/Cu/Al Multilayer on Magnesium Substrates

A multilayer coating of Ni/Cu/Al was fabricated on magnesium substrates using laser cladding. The solidification behavior and the phase evolution of the compositionally graded coating were studied. The results of the X-ray diffraction (XRD) analysis together with the metallographic study showed that a series of phase evolutions had occurred along the gradient (Mg) → (Mg) + Al₁₂Mg₁₇ → (Mg) + Q + λ ₂ → λ ₁ → λ ₁ + γ ₁ → γ ₁ + (Cu) + λ ₁ → (Cu) + λ ₁ → (Cu) → (CuNi) → (Ni). The rapid solidification condition had suppressed the invariant reactions that existed in the ternary Mg-Al-Cu alloy system. As a result, many of the predicted Al-rich brittle intermetallic compounds, which are detrimental to the performance of the coating, were not produced. The solidification path during the laser cladding of the Al and Cu layers was determined and the various phases, as predicted by the corresponding phase diagram, agreed well with the experimental results. Finally, the primary arm spacing (PAS) and the solidification morphology of the dendrites in the Cu and Ni layers were analyzed in relation to the solidification conditions.     

Further discussions on the solute redistribution during dendritic solidification of binary alloys

After a review over former works about the solute redistribution during dendritic solidification, a new“local solute redistribution equation ”is deduced based on Flemings's model, where lim-ited diffusion in solid during solidification is carefully treated. Because a form parameter is also included, the equation can be used for the solidification processes with different shapes of den-drites. By solving the equation at the condition of directional solidification, more completef _s -C, functions for both needlelike and platelike dendritic solidifications with both linear and parabolic solidification rates are obtained. As examples, the volume fractions of nonequilibrium phase in Al-4.5 pct Cu alloy is evaluated with differentf _s -C _l functions. On the thinking that the dendrites in actual solidification process is usually between needlelike and platelike ones, the volume fraction of the nonequilibrium phase is suggested to be in the region between the one calculated by the model for platelike dendrites and that for needlelike dendrites. The relationship between the region and local solidification time is also presented by figures, which are compared with the data of former researchers.     

Effect of Shrinkage on Primary Dendrite Arm Spacing during Binary Al-Si Alloy Solidification

Upward and downward directional solidification of hypoeutectic Al-Si alloys were numerically simulated inside a cylindrical container. Undercooling of the liquidus temperature prior to the solidification event was introduced in the numerical model. The finite-volume method was used to solve the energy, concentration, momentum, and continuity equations. Temperature and liquid concentrations inside the mushy zone were coupled with local equilibrium assumptions. An energy equation was applied to determine the liquid fraction inside the mushy zone while considering the temperature undercooling at the solidifying dendrite/liquid interface. Momentum and continuity equations were coupled by the SIMPLE algorithm. Flow velocity distribution at various times, G, R, λ ₁, and solidification time at mushy zone/liquid interface during solidification were predicted. The effect of shrinkage during solidification on these solidification parameters was quantified. Transient temperature distribution, solidification time for the mushy zone/liquid interface, and λ ₁ were validated by laboratory experiments. It was found that better agreement could be achieved when the fluid flow due to solidification shrinkage was considered. Considering shrinkage in upward solidification was found to only have a marginal effect on solidification parameters, such as G, R, and λ ₁; whereas, in the downward solidification, fluid flow due to shrinkage had a significant effect on these solidification parameters. Considering shrinkage during downward solidification resulted in a smaller R, stronger fluid flow, and increased solidification time at the mushy zone/liquid interface. Further, the flow pattern was significantly altered when solidification shrinkage was considered in the simulation. The effect of shrinkage on G and λ ₁ strongly depended on the instantaneous location of the mushy zone/liquid interface in the computational domain. The numerical results could be validated by experimental data only when both the undercooling of the liquidus temperature prior to solidification and fluid flow in the liquid caused by the effect of shrinkage during solidification were included in the model.     

Effect of dendritic arm spacing on mechanical properties and corrosion resistance of Al 9 Wt Pct Si and Zn 27 Wt Pct Al alloys

It has been reported that the mechanical properties and the corrosion resistance (CR) of metallic alloys depend strongly on the solidification microstructural arrangement. The correlation of corrosion behavior and mechanical properties with microstructure parameters can be very useful for planning solidification conditions in order to achieve a desired level of final properties. The aim of the present work is to investigate the influence of heat-transfer solidification variables on the microstructural array of both Al 9 wt pct Si and Zn 27 wt pct Al alloy castings and to develop correlations between the as-cast dendritic microstructure, CR, and tensile mechanical properties. Experimental results include transient metal/mold heat-transfer coefficient (h _i), secondary dendrite arm spacing (λ₂), corrosion potential (E _Corr), corrosion rate (i _Corr), polarization resistance (R ₁), capacitances values (Z _CPE), ultimate tensile strength (UTS, σ _u ), yield strength (YS, σ _y ), and elongation. It is shown that σ _U decreases with increasing λ₂ while the CR increases with increasing λ₂, for both alloys experimentally examined. A combined plot of CR and σ _U as a function of λ₂ is proposed as a way to determine an optimum range of secondary dendrite arm spacing that provides good balance between both properties.     

Solidification and phase equilibria in the Fe-C-Cr-NbC system

A solidification model is developed and experimentally checked for Fe-C-Cr-Nb alloys in the white cast irons range. It is based on a partial quaternary Fe-C-Cr-NbC phase diagram and predicts the possible solidification paths for the alloys containing γ, with (Fe,Cr)₇C₃ and NbC as the microconstituents at room temperature. The dendritic γ to massive (Fe,Cr)₇C₃ transition in experimental alloy microstructures with NbC contents up to 22 pet is explained by this model. Thermal analysis is also used to compare the solidification paths and model approach.     

A 500 °C isothermal section for the Al-Au-Cu system

The Al-Au-Cu system and its associated ternary alloys and intermetallic compounds is surprisingly poorly known, and the authors could find no phase diagram for it in the literature. This article addresses this omission by presenting an isothermal section at 500 °C, derived with the aid of X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), metallography, and hardness measurements. The samples studied had generally received an anneal of 2 hours at 500 °C, primarily in order to complete any transformations that occurred during solidification and cooling of the castings. The possibility of further changes on protracted annealing at 500 °C is not ruled out, and the diagram presented is, therefore, applicable only to material prepared by thermal processing of an industrial nature. The presence of a ternary β phase with a nominal stoichiometry of AlAu_2−x Cu_1−x (0≤x≤1) was confirmed, and its phase field at 500 °C was determined. A number of the binary intermetallic phases were found to exhibit some solid solubility of the ternary element. In particular, the γ-Al₄Cu₉ phase extends deep into the ternary and, in the vicinity of the commercially interesting 18-carat line, appears to exist in a ternary ordered form, designated here as γ ₂ .     

The formation of lamellar-eutectic grains in thin samples

We present an experimental study of the formation of lamellar-eutectic grains in directional solidification of thin hypereutectic samples of the transparent nonfaceted alloy CBr₄-C₂Cl₆. We start solidification from a partly stabilized solid residue. This solid is in a single phase (β phase) along the solid-liquid interface. The successive stages of the transient leading to the final lamellar structure are (1) the solute redistribution transient of the β-liquid front; (2) the appearance, without nucleation, of seeds of the other solid phase (α phase) onto the front; (3) the growth of the α phase along the β-liquid front (primary invasion); (4) the secondary invasion of the newly formed α-liquid front by the β phase; and (5) the oscillatory instability, called periodic lamellar branching, occurring during the secondary invasion. We study stages (2) through (5) in detail. Stages (2) through (4) are similar to those leading to banded microstructures in peritectics. Stage (5) is specifically responsible for the onset of two-phase growth and the formation of eutectic grains.     

The Role of Si and Cu Alloying Elements on the Dendritic Growth and Microhardness in Horizontally Solidified Binary and Multicomponent Aluminum-Based Alloys

Horizontal directional solidification (HDS) experiments were carried out with Al-3wtpctCu, Al-3wtpctSi, and Al- 3wtpctCu-5.5wtpctSi alloys in order to analyze the interrelation between the secondary dendrite arm spacing (λ ₂) and microhardness (HV). A water-cooled horizontal directional solidification device was applied. Microstructural characterization has been carried out using traditional techniques of metallography, optical, and SEM microscopy. The ThermoCalc software was used to generate the phase equilibrium diagrams as a function of Cu and Si for the analyzed alloys. The effects of Si and Cu elements on the λ ₂ and HV evolution of the hypoeutectic binary Al-Cu and Al-Si alloys have been analyzed as well as the addition of Si in the formation of ternary Al-Cu-Si alloy. The secondary dendrite arm spacing was correlated with local solidification thermal parameters such as growth rate (V _L), cooling rate (T _R), and local solidification time (t _SL). This has allowed to observe that power experimental functions given by λ ₂ = Constant (V _L)^−2/3, λ ₂ = Constant (T _R)^−1/3 and λ ₂ = Constant (t _SL)^1/3 may represent growth laws of λ ₂ with corresponding thermal parameters for investigated alloys. Hall–Petch equations have also been used to characterize the dependence of HV with λ ₂. A comparative analysis is performed between λ ₂ experimental values obtained in this study for Al-3wtpctCu-5.5wtpctSi alloy and the only theoretical model from the literature that has been proposed to predict the λ ₂ growth in multicomponent alloys. Comparisons with literature results for upward directional solidification were also performed.

     

Phase selection in undercooled ti3sn melts

The solidification of undercooled Ti₃Sn melts was investigated using electromagnetic levitation and electrohydrodynamic atomization experiments followed by extensive microstructural char- acterization. The study was motivated by several reports on the kinetic preference for the body- centered cubic (bcc) phase over more closely packed disordered and ordered structures during competitive crystallization from undercooled melts. At low undercoolings, Ti₃Sn melts yield the equilibrium ordered hexagonal DO₁₉ structure, which is retained without change upon cool- ing. Undercoolings between ~100 and ~300 K yield primary dendrites with hexagonal sym- metry but a final microstructure which is clearly martensitic in origin. Two previously unknown metastable forms of Ti₃Sn were identified: an ordered base-centered orthorhombic derived from the α martensite and an ordered monoclinic phase related to the face-centered orthorhombic martensite observed in the Ti-V system. Both phases are believed to evolve from the solid state transformation of a high temperature β phase, but the dendritic structure clearly indicates the formation of a hexagonal phase different from DO₁₉,i.e., α. The latter forms in preference to β, which has a larger driving force in at least part of the undercooling regime studied. It is proposed that the primary α transforms to β as a consequence of recalescence, which subse- quently transforms martensitically and orders to yield the observed metastable forms of Ti₃Sn.     

INCONEL 718: A solidification diagram

As part of a program studying weldability of Ni-base superalloys, results of an integrated analytical approach are used to generate a constitution diagram for INCONEL 718^* in the temperature range associated with solidification. Differential thermal analysis of wrought material and optical and scanning electron microscopy, electron probe microanalysis, and analytical electron microscopy of gas tungsten arc welds are used in conjunction with solidification theory to generate data points for this diagram. The important features of the diagram are an austenite (γ)/Laves phase eutectic which occurs at ≈19.1 wt pct Nb between austenite containing ≈9.3 wt pct Nb and a Laves phase which contains ≈22.4 wt pct Nb. The distribution coefficient for Nb was found to be ≈0.5. The solidification sequence of INCONEL 718 was found to be (1) proeutectic γ, followed by (2) a γ/NbC eutectic at ≈1250°C, followed by (3) continued γ solidification, followed by (4) a γ/Laves phase eutectic at ≈1200°C. An estimate of the volume fraction eutectic is made using the Scheil solidification model, and the fraction of each phase in the eutectic is calculatedvia the lever rule. These are compared with experimentally determined values and found to be in good agreement.     

Microstructural study of the interface in laser-clad Ni-Al bronze on Al alloy AA333 and its relation to cracking

The interface toughness between a laser clad and the substrate determines whether the cladding is useful for engineering application. The objective of this investigation is to correlate the interface properties of laser-clad Ni-AI bronze on Al alloy AA333 with the microstructure and crystal structure of the interface. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX) are used to examine the interface. In a good clad track, the interface is an irregular curved zone with a varying width (occasionally keyholing structure) from 30 to 150 μm. A compositional transition from the Cu-rich clad (83 wt pct Cu) to the Al-rich substrate (3.2 wt pct Cu) occurs across this interface. Three phases in the interface are identified in TEM: Al solid solution, θ phase, and γ₁ phase, as described in the Cu-Al binary phase diagram. In a good clad track, the θ and γ₁ phases are distributed in the Al solid solution. In a clad track with cracks, the interface structure spreads to a much larger scale from 300 μm to the whole clad region. Large areas of θ and γ₁ phases are observed. The mechanism of cracking at the interface is related to the formation of a twophase region of θ and γ₁ phases. To understand the microstructure, a nonequilibrium quasibinary Cu-Al phase diagram is proposed and compared with the equilibrium binary Cu-Al phase diagram. It is found that the occurrence of many phases such as η₁η₂, ζ₁, ζ₂, ε₁, ε₂, γ₀, β₀, and β, as described in the equilibrium binary Cu-Al phase diagram, is suppressed by either the cladding process or by the alloying elements. The three identified phases (Al solid solution, θ phase, and γ₁, phase) showed significant extension of solubility. Formerly Visiting Research Associate, Department of Mechanical and Industrial Engineering, Center for Laser Aided Material Processing, University of Illinois at Urbana-Champaign, Urbana, IL 61801     

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.     

Solidification kinetics and metastable phase formation in binary Ti-Al

Near-equiatomic alloys of Ti-Al were solidified at various bulk undercoolings using electromagnetic levitation. Detailed thermal histories were acquired during experiments using optical pyrometry with sampling rates as fast as 500 KHz. Solidification and other high-temperature transformation pathways were deduced from the thermal data and microstructural analysis. Re- calescence rise times were used to determine semiquantitative primary solidification kinetics for the different phases. Primary β solidification was observed at compositions well into the equi- librium α regime; this is presented as part of a near-equiatomic nucleation domain diagram mat shows the primary solidification phase (β, α, ordered γ, or disordered γ) that results for each combination of nucleation temperature and composition. Solidification kinetics are faster for primary β (V_max ≈ 15 to 18 m s^-1) than they are for primary α (V_max ≈ 10 to 12 m s^-1). For undercoolings less than about 150 K, the primary solidification kinetics are about an order of magnitude slower for γ than for α. However, at an undercooling of about 150 K, the solidi- fication kinetics for γ increase discontinuously. This discontinuity is associated with a change in the primary solidification phase from ordered γ (V_max ≈ 0.5 m s^-1) to disordered γ (V_max ≈ 10 m s^-1). formerly Doctoral Student, Vanderbilt formerly Doctoral Student, Vanderbilt formerly Doctoral Student, Vanderbilt