The Influence of Fluid Flow on the Microstructure of Directionally Solidified AlSi-Base Alloys

To obtain a quantitative understanding of the effect of fluid flow on the microstructure of cast alloys, a technical Al-7 wt pct Si-0.6 wt pct Mg alloy (A357) has been directionally solidified with a medium temperature gradient under well-defined thermal and fluid-flow conditions. The solidification was studied in an aerogel-based furnace, which established flat isotherms and allowed the direct optical observation of the solidification process. A coil system around the sample induces a homogeneous rotating magnetic field (RMF) and, hence, a well-defined flow field close to the growing solid-liquid interface. The application of RMFs during directional solidification results in pronounced segregation effects: a change to pure eutectic solidification at the axis of the sample at high magnetic field strengths is observed. The investigations show that with increasing magnetic induction and, therefore, fluid flow, the primary dendrite spacing decreases, whereas the secondary dendrite arm spacing increases. An apparent flow effect on the eutectic spacing is observed. 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.     

Microstructures and properties of SnZn-xEr lead-free solders

The Sn9Zn eutectic alloy is the nontoxic lead-free solders alternative having a melting temperature which is closest to that of the eutectic SnPb alloy. In order to improve the properties of SnZn lead-free solders, 0-0.5 wt.% of rare earth Er was added to the base alloys, and the microstructures were studied. Results showed that the addition of rare earth Er could enhance the wettability of SnZn solders, with 0.08%Er addition, the spreading area gave an 19.1% increase. And based on the mechanical testing, it was found that the tensile force and shear force of SnZn-xEr solder joints could be improved significantly. Moreover, the oxidation resistance of SnZn0.08Er solder was better than that of SnZn solder. In addition, it was found that trace amounts of rare earth Er could refine the microstructures of SnZn solders, especially for Zn-rich phases, and excessive amount of rare earth Er led to a coarse microstructure.     

Effects of Indium Content on Microstructural,Mechanical Properties and Melting Temperature of SAC305 Solder Alloys

The present study investigated the effects of indium (In) addition on the microstructure, mechanical properties, and melting temperature of SAC305 solder alloys. The indium formed IMC phases of Ag₃(Sn,In) and Cu₆(Sn,In)₅ in the Sn-rich matrix that increased the ultimate tensile strength (UTS) and the hardness while the ductility (% EL) decreased for all In containing solder alloys. The UTS and hardness values increased from 29.21 to 33.84 MPa and from 13.91 to 17.33 HV. Principally, the In-containing solder alloys had higher UTS and hardness than the In-free solder alloy due to the strengthening effect of solid solution and secondary phase dispersion. The eutectic melting point decreased from 223.0°C for the SAC305 solder alloy to 219.5°C for the SAC305 alloy with 2.0 wt% In. The addition of In had little effect on the solidus temperatures. In contrast, the liquidus temperature decreased with increasing In content. The optimum concentration of 2.0 wt % In improved the microstructure, UTS, hardness, and eutectic temperature of the SAC305 solder alloys.     

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.     

Microstructural analysis of lead-free solder alloys

Among the many issues related to the performance of lead-free solder alloys, the dependence of their mechanical properties on the microstructure and the stability of the microstructure stability are some of the most important issues. A comprehensive understanding of the process-microstructure-property relationships is essential. Toward that goal, a microtextural analysis is performed using orientation imaging microscopy (OIM) for alloy Sn-3.8Ag-0.7Cu (wt pct) processed at four different temperatures. Sn-3.8Ag-0.7Cu is one of the most promising lead-free solder alloys that has shown superior mechanical properties to other candidate lead-free solder alloys. However, a comprehensive understanding of their microstructure and the dependence of microstructure on processing conditions are still lacking. In the present work, a detailed microstructure characterization with respect to phase compositions, grain size and size distributions, texture, and orientation relationships between various phases are performed. The measured microstructural features are correlated with the soldering temperatures.     

Crystallization of Amorphous Alloys

Crystallization of amorphous alloys is compared with conventional solidification of melts. Taking account of the temperature dependence of crystal nucleation and growth rates, the links between the two processes are explored. The fundamentals of nucleation and growth kinetics in amorphous alloys are reviewed. It is shown that the crystallization of amorphous alloys can be exploited (1) to obtain ultrafine grained microstructures with useful properties and (2) to elucidate nucleation mechanisms in conventional grain-refining practice. 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.     

Characterization of Hypereutectic Al-Si Powders Solidified under Far-From Equilibrium Conditions

The rapid solidification microstructure of gas-atomized Al-Si powders of 15, 18, 25, and 50 wt pct Si were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In order of increasing particle size, the powders exhibited microcellular Al, cellular/dendritic Al, eutectic Al, and primary Si growth morphologies. Interface velocity and undercooling were estimated from measured eutectic spacing based on the Trivedi–Magnin–Kurz (TMK) model, permitting a direct comparison with theoretical predictions of solidification morphology. Based on our observations, additional conditions for high-undercooling morphological transitions are proposed as an extension of coupled-zone predictions. 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.     

建筑用钎焊接头组织与力学性能的研究

采用OM、XRD、SEM和拉力试验机,研究了钎焊工艺参数对SnAg0.5CuZn0.1Ni/Cu无铅微焊点界面金属间化合物(IMC)和力学性能的影响。结果表明:添加0.1%Ni(质量分数)能显著细化SnAg0.5CuZn钎料合金的初生β-Sn相和共晶组织;钎焊温度为270℃、钎焊时间为240 s时,钎焊接头的剪切强度达到最大值47 MPa。     

Microstructure and mechanical behavior of Cr-Cr2Hfin situ intermetallic composites

A detailed investigation of the effects of microstructural changes on the mechanical behavior of twoin situ intermetallic composites with Cr and Cr₂Hf phases in the Cr-Hf system was performed. The nominal compositions (at. pct) of the alloys were Cr-5.6Hf (hypoeutectic) and Cr-13Hf (eutectic). The study included evaluations of strength, ductility, and fracture toughness as a function of temperature and creep behavior. Two microstructures in each alloy were obtained by heat treatments at 1250 ‡C (fine microstructure) and 1500 ‡C (coarse microstructure). A decrease in elastic strength (stress at the onset of inelastic response in the load-deflection curve) with the coarsening of the microstructures was noted for both alloys below 1000 ‡C. The Cr-13Hf alloy retained strength to a higher test temperature, relative to Cr-5.6Hf alloy, under both microstructural conditions. The alloys showed no evidence of ductility at room temperature. However, in the coarse microstructure of the Cr-5.6Hf alloy, the primary Cr exhibited ductility at and above 200 ‡C; ductility in primary Cr could be seen only at and above 1000 ‡C for the fine microstructure. In other words, the temperature at which ductility was first observed decreased from about 1000 ‡C to about 200 ‡C due to high-temperature heat treatment in this alloy. Both microstructures of Cr-5.6Hf alloy showed a significant increase in fracture toughness with increasing test temperature. However, the increases in fracture toughness with temperature for the Cr-13Hf alloy microstructures were relatively small. Both alloys showed about four orders of magnitude reduction in steady-state creep rates relative to pure Cr at 1200 ‡C. The results are analyzed in the light of deformation characteristics and fracture micromechanisms. The effects of microstructural factors, such as the size and continuity of phases, solubility levels of Hf as well as interstitial elements in Cr, on the observed mechanical behavior are discussed. Formerly Research Scientist, Materials and Processes, UES, Inc.     

Measurement of segregation and distribution coefficients in MAR-M200 and hafnium-modified MAR-M200 superalloys

Crystals of the MAR-M200* and the Hf-modified MAR-M200 superalloys were grown by plane front solidification. The solidification microstructures of both alloys contain the proeutectic γ phase, the γ + γ’ eutectic, and a 3-phase eutectic. The chemical compositions of these alloys were measured along the growth direction with the use of an electron microprobe. In the MAR-M200 alloy, W preferentially segregates to the proeutectic while Ti, Cr, and Al preferentially segregate to the eutectic. In the Hf-modified MAR-M200 alloy, W, Cr, and Al preferentially segregate to the proeutectic while Ti and Hf preferentially segregate to the eutectic. The segregation trends observed in the plane front solidified alloys are generally in agreement with the results reported in the literature for the dendritically solidified alloys. The distribution coefficients determined from the composition data range from 1.1 to 1.3 for W, 0.5 to 0.7 for Ti, 0.85 to 0.95 for Al, and 0.9 to 0.95 for Cr in the MAR-M200 alloy, and from 1.1 to 3.7 for W, 0.8 to 1.2 for Ti, 0.95 to 1.2 for Al, and 0.17 to 0.29 for Hf in the Hf-modified MAR-M200 alloy.     

Phase-Field Study of the Cellular Bifurcation in Dilute Binary Alloys

Phase-field simulations in both two and three dimensions are used to investigate the microstructures that form closely above the threshold of the Mullins Sekerka instability in the directional solidi fication of dilute binary alloys. It is found that in this regime of shallow cells the simulation results strongly depend on the thickness of the diffuse interfaces even for model parameters that yield quantitative results for deep cells. For the material parameters of a dilute Sn-Bi alloy, the bifurcation is found to be supercritical, whereas weakly nonlinear amplitude expansions predict a subcritical bifurcation. Furthermore, an oscillatory instability of the cell grooves is found, which leads to the pinch-off of liquid inclusions even for relatively shallow cells. Finally, in three dimensions, three different morphologies are found, in agreement with experiments and previous numerical studies: regular hexagons, elongated cells (“stripes”), and inverted hexagons (“node” or “pox” structure, a hexagonal array of local depressions of the solidification front). Nodes and stripes are stable steadystate solutions only very close to the bifurcation. 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.     

Microstructural development in undercooled lead- tin eutectic alloys

A model was developed to predict micro structural development in lead—61.9 wt pct Sn (eutectic) alloys which were undercooled 5 to 25 K below their equilibrium freezing temperature prior to being preferentially nucleated. While the initial solidification velocity rapidly increases with increasing undercooling, the model predicts it to quickly decrease, prior to 10 pct solid formation, after which growth continues near the equilibrium temperature. Experimentally, and in accordance with the prediction, the eutectic emanated from the nucleation site with an initially fine spacing that increased with distance. However, in contrast to the model, the eutectic grew outward in a spokelike manner with each arm surrounded by a globular structure, this being attributed to the difficulty of lateral nucleation. Microstructural uniformity was further compromised by equiaxed eutectic grains which grew ahead of the advancing interface in the now only slightly undercooled liquid. Consequently, while containerless techniques may ensure sample purity and permit processing of high-temperature materials, development of a continuously fine and uniformly aligned microstructure cannot be assumed. This article is based on a presentation made in the symposium entitled “Microgravity Solidification, Theory and Experimental Results” as a part of the 1993 TMS Fall meeting, October 17-21, 1993, Pittsburgh, PA, under the auspices of the TMS Solidification Committee.     

Design and development of hot corrosion-resistant

A systematic study of the effects of refractory metals Ti, Ta, and Nb on the microstructures and properties was conducted with a hot corrosion-resistant alloy system Ni-16Cr-9Al-4Co-2W-lMo-(0~4)Ti-(0~4)Ta-(0~4)Nb (in atomic percent) which was selected based on thed-electrons alloy design theory and some basic considerations in alloying features of single-crystal nickel-base superalloys. The contour lines of solidification reaction temperatures and eutectic (γ + γ′) volume fraction in the Ti-Ta-Nb compositional triangle were determined by differential thermal analysis (DTA) and imaging analyzer. Compared with the reference alloy IN738LC, in most of the compositional ranges studied, the designed alloys show very low amounts of eutectic (γ + γ′) (⪯0.4 vol pct), narrow solidification ranges (⪯65 °C), and wide “heat-treatment windows” (>100 °C). This indicates that the alloys should have the promising microstructural stability, single-crystal castability, and be easier for complete solution treatment. In a wide compositional range, the designed alloys showed good hot corrosion resistance (weight loss less than 20 mg/cm² after 24 hours kept in molten salt at 900 °C). By summarizing the results, the promising alloy compositional ranges of the alloys with balanced properties were determined for the final step of the alloy design,i.e., to grow single crystal and characterize mechanical properties of the alloys selected from the previously mentioned regions. Formerly with the Institute of Metal Research, Academia Sinica, Shenyang 110015, China     

Influence of Solidification Thermal Parameters on the Columnar-to-Equiaxed Transition of Aluminum-Zinc and Zinc-Aluminum Alloys

Understanding the interaction between the parameters involved in the columnar-to-equiaxed transition (CET) has gained considerable attention over the last two decades in the study of the structure of ingot castings. The present investigation was undertaken to investigate experimentally the directional solidification of Al-Zn and Zn-Al (ZA) alloys under different conditions of superheat and heat-transfer efficiencies at the metal/mold interface. The CET is observed; grain sizes are measured and the observations are related to the solidification thermal parameters: cooling rates, growth rates, thermal gradients, and recalescence determined from the temperature vs time curves. The temperature gradient in the melt, measured during the transition, is between –0.338 °C/mm and 0.167 °C/mm. In addition, there is an increase in the velocity of the liquidus front faster than the solidus front, which increases the size of the mushy zone. The size of the equiaxed grains increases with distance from the transition, an observation that was independent of alloy composition. The observations indicate that the transition is the result of a competition between coarse columnar dendrites and finer equiaxed dendrites. The results are compared with those previously obtained in lead-tin alloys. 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.     

Computational Modeling of Microstructure Evolution in Solidification of Aluminum Alloys

In this article, a front tracking (FT) model and a modified cellular automaton (MCA) model are presented and their capabilities in modeling the microstructure evolution during solidification of aluminum alloys are demonstrated. The FT model is first validated by comparison with the predictions of the Lipton–Glicksman–Kurz (LGK) model. Calculations of the steady-state dendritic tip growth velocity and equilibrium liquid composition as a function of melt undercooling for an Al-4 wt pct Cu alloy exhibit good agreement between the FT simulations and the LGK predictions. The FT model is also used to simulate the secondary dendrite arm spacing as a function of local solidification time. The simulated results agree well with the experimental data. The MCA model is applied to simulate dendritic and nondendritic microstructure evolution in semisolid processing of an Al-Si alloy. The effect of fluid flow on dendritic growth is also examined. The solute profiles in equiaxed dendritic solidification of a ternary aluminum alloy are simulated as a function of cooling rate and compared with the prediction of the Scheil model. The MCA model is extended to the multiphase system for the simulation of eutectic solidification. A particular emphasis is made on the quantitative aspects of simulations. This article is based on a presentation made in the symposium ”Simulation of Aluminum Shape Casting Processing: From Design to Mechanical Properties,” which occurred March 12–16, 2006, during the TMS Spring Meeting in San Antonio, Texas, under the auspices of the Computational Materials Science and Engineering Committee, the Process Modeling, Analysis and Control Committee, the Solidification Committee, the Mechanical Behavior of Materials Committee, and the Light Metal Division/Aluminum Committee.     

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.     

Natural Convection and Columnar-to-Equiaxed Transition Prediction in a Front-Tracking Model of Alloy Solidification

A meso-scale front-tracking model (FTM) of nonequilibrium binary alloy dendritic solidification has been extended to incorporate Kurz, Giovanola, and Trivedi (KGT) dendrite kinetics and a Scheil solidification path. Model validation via comparison with thermocouple measurements from a solidification experiment, in which natural convection is limited by design, is presented. Via solution of the flow field due to natural thermal buoyancy, it is shown that resultant liquid-phase convection creates conditions in which equiaxed solidification is favored. Comparison with simulations in which casting solidification is diffusion controlled show that natural convection has greatest effect at intermediate times, but that at early and late stages of columnar solidification, the differences are relatively small. It is, however, during the time of greatest divergence between the simulations that the authors’ predictive index for equiaxed zone formation is enhanced most by convection. Finally, the columnar-to-equiaxed transition is directly simulated, in directional solidification controlled by diffusion. 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.     

合金元素对Sn-9Zn基无铅钎料润湿性和组织的影响

研究了分别添加混合轻稀土、磷和铋对Sn-9Zn共晶合金在铜基上的润湿性能及对钎料内部显微结构和钎料/铜界面的影响。研究结果表明:Bi、RE和P都能有效地改善Sn-9Zn基合金钎料在铜基上的润湿性;且添加RE和P对钎料/铜界面无明显的影响,也不改变Sn-9Zn钎料内部的扫把状共晶结构;而添加Bi促进了Sn-9Zn合金中锌的富集析出。