Seeding of single-crystal superalloys—Role of constitutional undercooling and primary dendrite orientation on stray-grain nucleation and growth

An experimental study, together with a two-dimensional numerical simulation of solute segregation, was conducted to investigate (1) the mechanism for stray-grain nucleation following seed melt-back and initial withdrawal and (2) the role of the primary dendrite disposition of the seed crystal in relation to the mold wall during growth. It is proposed that the factors contributing to stray-grain nucleation during initial withdrawal are (1) the magnitude of local, solute-adjusted undercooling and (2) the rapidly changing curvature of the solidification front close to the mold walls during the initial solidification transient. Based upon the calculated local undercooling and experimentally observed stray-grain morphologies, it was concluded that stray grains nucleate near the mold wall around the seed perimeter and behind the columnar dendrites that advance into the bulk liquid ahead of the melt-back zone. These grains then compete during growth with the dendrites originating from the seed. Therefore, the morphological constraints arising from the inclination of the primary dendrites from the seed crystal with respect to the mold wall (converging/diverging/axial 〈001〉) determines the probability of the stray-grain nuclei developing into equiaxed/columnar grains following competitive growth.     

Crystal-Growth Transition and Homogenous Nucleation Undercooling of Bismuth

The cross-sectional and surface morphologies of highly undercooled bismuth samples are investigated by optical microscopy and scanning electron microscopy. It is found that the grain morphology can be classified into three types. When the undercooling is less than 49 K (49 °C), flaky grains with pronounced edges and faces are arranged parallel to each other, showing the feature of lateral growth. When the undercooling is over 95 K (95 °C), refined equiaxial grains with several smooth bulges on the surface of each grain are randomly arranged, showing the feature of continuous growth. In the undercooling region from 49 K to 95 K (49 °C to 95 °C), the features of both lateral and continuous growth are observed. The microstructures within the sample grains obtained at different undercooling regions are dissimilar, but they all show features of anisotropic growth. Based on the critical growth-transition undercoolings, direct expressions that express the relationship between the solid-liquid interface energy and temperature are determined. Homogenous nucleation undercooling is also predicted according to the solid-liquid interface energy obtained from the critical growth-transition undercooling. The predicted results of homogenous nucleation undercooling for bismuth are in good agreement with the experimental results.     

The mechanism of formation of a fine duplex microstructure in Ti-48Al-2Mn-2Nb alloys

The mechanism of formation of the fine duplex microstructure resulting from the α → γ transformation in water-quenched Ti-48Al-2Mn-2Nb alloys was studied using transmission and analytical electron microscopy. As-cast Ti-48Al-2Mn-2Nb alloys were heat treated in the α phase field and water quenched to room temperature. The resulting microstructure (referred to as a fine duplex microstructure) consisted of equiaxed grains and abutting lath colonies. Both the colonies and the grains were composed of the γ phase, twinned γ laths, and α₂ laths. It was found that the transformation from α to γ in the fine duplex microstructure took place through long range diffusional processes, and compctitive growth between the equiaxed and lath morphology occurred. Nucleation of they phase from the α matrix can occur through nucleation on stacking faults, followed by growth through the sympathetic nucleation and growth of new γ laths on a substrate lath. The observed misorientations and the interfacial structures between the laths were found to be consistent with such a mechanism. Compctition between such nucleation and growth mechanisms for the equiaxed and lath morphologies of γ leads to the formation of lath colonies (of γ and α₂) interspersed with equiaxed grains in these alloys. Formerly Visiting Scientist, Metals and Ceramics Division, Oak Ridge National Laboratory This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Effect of lanthanum addition on microstructure and hardness of brass alloys produced by rheological squeeze casting

The effect of La addition (0–0.30 wt%) on the microstructure and hardness of rheological squeeze casting brass alloys was experimentally investigated. The rheological squeeze casting process is improved by controlling the wall surface crystals and melt flow rate to realise the preparation of semi-solid melt with flow, and a brass alloy workpiece with La is produced. The microstructure and properties of the brass alloy samples were investigated using metallography, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and hardness testing. The results indicate that the hardness of the rheological squeeze casting brass alloy is increased by 20.4% from 108 to 130 HBW with an increase in the La content from 0 to 0.30 wt%. The microstructural analysis results show that La significantly refines the primary α-phase grains, and the main mechanism is the constitutional undercooling and heterogeneous nucleation caused by the La enrichment in the front of the solid–liquid interface. The squeeze pressure promotes undercooling, which improves the nucleation rate and affects the solute diffusion and nucleus growth. The dual effects of these two aspects aggravate the grain refinement process, consequently increasing the number of grain boundaries and improving the hardness of the brass alloy.     

The effect of heat treatment variables on the phase transformations at 1420°c in ti-48a1 and Ti-48al-2Mn-2Nb alloys

The effect of heat treatment variables such as initial microstructure, isothermal reaction time and cooling rate on the phase transformations occurring at 1420°C in Ti-48A1 and Ti-48Al-2Mn-2Nb alloys was studied. The main effect of the initial microstructure, which comprised either lamellae of α₂and γ or equiaxed γ grains, was to alter the kinetics, through a change in the chemical driving force of the phase transformations. Therefore, the equiaxed γ grains transformed to α much faster than the lamellar structure and in the initially lamellar structure, growth of α resulted in the delineation of the initial dendritic structure formed during solidification. The effect of the rate of cooling from the heat treatment temperature on the final morphology of these alloys was drastic and resulted in a change in morphology from lamellar grains obtained on furnace cooling to a feathery and mottled morphology obtained on water quenching. TEM analysis of water quenched Ti-48Al-2Mn-2Nb revealed complex morphologies including a structure which consisted of equiaxed γ grains and residual α₂and abutting colonies of γ and α₂. Based on the TEM results, the early stages of formation of γ from α were studied and mechanisms of nucleation and growth discussed. The relative importance and the coexistence of massive and martensitic transformation products is also discussed.     

Growth of interdendritic eutectic in directionally solidified Al-Si alloys

Interdendritic eutectic microstructures in Al-Si (6 to 12.6 wt pct Si) alloys have been investigated as a function of growth velocity and temperature gradient. The interface morphology, as well as the behavior of the eutectic spacing and undercooling, suggest that the resultant microstructure is governed by two different growth processes. That is, at low growth rates, steady-state columnar eutectic growth is found and obeys the relationship, λ²V = constant, where λ is the eutectic spacing andV is the growth rate. At higher growth rates, the nucleation of equiaxed eutectic grains occurs in the interdendritic liquid. The experimental findings are interpreted in the light of recently developed models for the columnar to equiaxed transition and for irregular eutectic growth.     

Effect of Cooling Rate on Microstructure Evolution and Plastic Flow of Ti-6Al-4V

The effect of cooling rate following subtransus solution treatment on microstructure evolution and plastic flow of Ti-6Al-4V was established. For this purpose, three types of tests were performed using samples of Ti-6Al-4V with an initial structure of equiaxed α in a matrix of β and a cooling rate of 11, 42, or 180 K/min: (i) Static cooling following solution treatment without or with a prestrain, (ii) constant-strain-rate hot compression during concurrent cooling, and (iii) static cooling to a specified temperature followed by constant-strain-rate isothermal hot compression. The volume fraction of equiaxed α developed during cooling was strongly dependent on cooling rate, but pre- or concurrent deformation resulted in relatively-small changes in this quantity. In addition, the cooling rate through its effect on the growth kinetics of equiaxed α had a noticeable effect on plastic-flow behavior under both isothermal and non-isothermal conditions. In both instances, the retention of the high-temperature microstructure (characterized by a low fraction of equiaxed α) during rapid cooling gave rise to lower flow stresses than samples with equilibrium equiaxed phase fractions. By contrast, when secondary α was formed during cooling, higher flow stresses were generated due to a Hall-Petch-like effect. The results were interpreted using models for the diffusional growth of equiaxed α, the onset of nucleation of secondary α, and predictions of the plastic-flow response of equiaxed two-phase microstructures based on a self-consistent approach. Unlike previous findings which indicated a large increase in the rate of dissolution of equiaxed α due to concurrent deformation/pipe diffusion during heating transients, the present work did not reveal a corresponding enhancement of growth during cooling.

     

快速凝固奥氏体不锈钢薄带的显微结构特征

熔液自旋含钛奥氏体不锈钢316L薄带具有典型的快速凝固结构和均匀的化学成分。显微结构由细小柱晶和等轴晶构成。测得晶胞间距约0.2μm。算得其平均冷却速度为106℃/S。结构中有一无特征区域,说明凝固时的过冷度较高,算得其形核过冷度约200K。     

X-Ray Videomicroscopy Studies of Eutectic Al-Si Solidification in Al-Si-Cu

Al-Si eutectic growth has been studied in-situ for the first time using X-ray video microscopy during directional solidification (DS) in unmodified and Sr-modified Al-Si-Cu alloys. In the unmodified alloys, Si is found to grow predominantly with needle-like tip morphologies, leading a highly irregular progressing eutectic interface with subsequent nucleation and growth of Al from the Si surfaces. In the Sr-modified alloys, the eutectic reaction is strongly suppressed, occurring with low nucleation frequency at undercoolings in the range 10 K to 18 K. In order to transport Cu rejected at the eutectic front back into the melt, the modified eutectic colonies attain meso-scale interface perturbations that eventually evolve into equiaxed composite-structure cells. The eutectic front also attains short-range microscale interface perturbations consistent with the characteristics of a fibrous Si growth. Evidence was found in support of Si nucleation occurring on potent particles suspended in the melt. Yet, both with Sr-modified and unmodified alloys, Si precipitation alone was not sufficient to facilitate the eutectic reaction, which apparently required additional undercooling for Al to form at the Si-particle interfaces.     

Undercooling and solidification of Al-50 at. pct Si Alloy by electromagnetic levitation

Electromagnetic levitation is applied to achieve containerless solidification of 10-mm-diameter droplets of Al-50 at. pct Si. A maximum undercooling of 320 K is obtained. Phase morphologies on the droplet surfaces and on the deeply etched sections of the samples solidified at different undercoolings are examined by scanning electron microscopy. The primary silicon shows well-developed faceted dendrites at a small undercooling, but a fine granular form at a large undercooling. Stratified deposits of aluminum are found within the primary silicon plates, arising from solute pileup during growth. The microstructural refinement at a large undercooling has its origins in solute restriction of crystal growth and in fragmentation of the primary silicon dendrites. The form of the Al-Si eutectic is also found to be changed into an anomalous form at a large undercooling.     

The production of ultrafine ferrite in low-carbon steel by strain-induced transformation

An investigation into the production of ultrafine (1 μm) equiaxed ferrite (UFF) grains in low-carbon steel was made using laboratory rolling, compression dilatometry, and hot torsion techniques. It was found that the hot rolling of thin strip, with a combination of high shear strain and high undercooling, provided the conditions most suitable for the formation of this type of microstructure. Although high strains could be applied in compression and torsion experiments, large volume fractions of UFF were not observed in those samples, possibly due to the lower level of undercooling achieved. It is thought that ferrite refinement was due to a strain-induced transformation process, and that ferrite grains nucleated on parallel and linear deformation bands that traversed austenite grains. These bands formed during the deformation process, and the undercooling provided by the contact between the strip and the work rolls was sufficient to drive the transformation to homogeneous UFF grains.     

Friction Stir Processing of Investment-Cast Ti-6Al-4V: Microstructure and Properties

Investment-cast titanium components are becoming increasingly common in the aerospace industry due to the ability to produce large, complex, one-piece components that were previously fabricated by mechanically fastening multiple pieces together. The fabricated components are labor-intensive and the fastener holes are stress concentrators and prime sites for fatigue crack initiation. The castings are typically hot-isostatically-pressed (HIP) to close internal porosity, but have a coarse, fully lamellar structure that has low resistance to fatigue crack initiation. The as-cast + HIP material exhibited 1- to 1.5-mm prior β grains containing a fully lamellar α + β microstructure consistent with slow cooling from above the β transus. Friction stir processing (FSP) was used to locally modify the microstructure on the surface of an investment-cast Ti-6Al-4V plate. Friction stir processing converted the as-cast microstructure to fine (1- to 2-μm) equiaxed α grains. Using micropillars created with a dual-beam focused ion beam device, it was found that the fine-grained equiaxed structure has about a 12 pct higher compressive yield stress. In wrought products, higher strength conditions are more resistant to fatigue crack initiation, while the coarse lamellar microstructure in the base material has better fatigue crack growth resistance. In combination, these two microstructures can increase the fatigue life of titanium alloy castings by increasing the number of cycles prior to crack initiation while retaining the same low-crack growth rates of the colony microstructure in the remainder of the component. In the current study, high-cycle fatigue testing of investment-cast Ti-6Al-4V was performed on four-point bend specimens. Early results show that FSP can increase fatigue strength dramatically. This article is based on a presentation given in the symposium entitled “Materials Behavior: Far from Equilibrium” as part of the Golden Jubilee Celebration of Bhabha Atomic Research Centre, which occurred December 15–16, 2006 in Mumbai, India.     

Modeling of the Coupling of Microstructure and Macrosegregation in a Direct Chill Cast Al-Cu Billet

The macroscopic multiphase flow and the growth of the solidification microstructures in the mushy zone of a direct chill (DC) casting are closely coupled. These couplings are the key to the understanding of the formation of the macrosegregation and of the non-uniform microstructure of the casting. In the present paper we use a multiphase and multiscale model to provide a fully coupled picture of the links between macrosegregation and microstructure in a DC cast billet. The model describes nucleation from inoculant particles and growth of dendritic and globular equiaxed crystal grains, fully coupled with macroscopic transport phenomena: fluid flow induced by natural convection and solidification shrinkage, heat, mass, and solute mass transport, motion of free-floating equiaxed grains, and of grain refiner particles. We compare our simulations to experiments on grain-refined and non-grain-refined industrial size billets from literature. We show that a transition between dendritic and globular grain morphology triggered by the grain refinement is the key to the explanation of the differences between the macrosegregation patterns in the two billets. We further show that the grain size and morphology are strongly affected by the macroscopic transport of free-floating equiaxed grains and of grain refiner particles.

     

Direct observation of the crystal-growth transition in undercooled silicon

Using an electromagnetic levitation facility with a laser heating unit, silicon droplets were highly undercooled in the containerless state. The crystal morphologies on the surface of the undercooled droplets during the solidification process and after solidification were recorded live by using a high-speed camera and were observed by scanning electron microscopy. The growth behavior of silicon was found to vary not only with the nucleation undercooling, but also with the time after nucleation. In the earlier stage of solidification, the silicon grew in lateral, intermediary, and continuous modes at low, medium, and high undercoolings, respectively. In the later stage of solidification, the growth of highly undercooled silicon can transform to the lateral mode from the nonlateral one. The transition time of the sample with 320 K of undercooling was about 535 ms after recalescence, which was much later than the time where recalescence was completed.     

高性能取向硅钢的工业化生产研究

采用光学显微镜、X射线衍射仪等分析了宁波钢铁有限公司生产的取向硅钢不同工序下的组织及织构演变规律.结果 表明:铸坯经过热轧后,沿着厚度方向组织不均匀;一次冷轧并经脱碳退火后,组织由条状纤维状变成等轴状的初次再结晶晶粒,初次再结晶平均晶粒尺寸为18.17 μm,织构主要以α织构和γ织构为主;在二次冷轧后,晶粒再次被压缩,转变为纤维状,织构主要为γ织构;经过高温退火后,发生二次再结晶,晶粒异常长大,晶粒尺寸达到厘米级,织构成分为单一且锋锐的Goss织构.     

Friction Stir Welding (FSW) of Aged CuCrZr Alloy Plates

Friction Stir Welding (FSW) of Cu-0.80Cr-0.10Zr (in wt pct) alloy under aged condition was performed to study the effects of process parameters on microstructure and properties of the joint. FSW was performed over a wide range of process parameters, like tool-rotation speed (from 800 to 1200 rpm) and tool-travel speed (from 40 to 100 mm/min), and the resulting thermal cycles were recorded on both sides (advancing and retreating) of the joint. The joints were characterized for their microstructure and tensile properties. The welding process resulted in a sound and defect-free weld joint, over the entire range of the process parameters used in this study. Microstructure of the stir zone showed fine and equiaxed grains, the scale of which varied with FSW process parameters. Grain size in the stir zone showed direct correlation with tool rotation and inverse correlation with tool-travel speed. Tensile strength of the weld joints was ranging from 225 to 260 MPa, which is substantially lower than that of the parent metal under aged condition (~ 400 MPa), but superior to that of the parent material under annealed condition (~ 220 MPa). Lower strength of the FSW joint than that of the parent material under aged condition can be attributed to dissolution of the precipitates in the stir zone and TMAZ. These results are presented and discussed in this paper.

     

On the columnar to equiaxed transition in small ingots

The columnar to equiaxed transition (CET) in small ingots of, aluminum alloys was found to occur more easily for alloys with a larger value of the constitutional supercooling parameter (−mC _o (1-k)/k). The CET was found to be completely suppressed by increases in the mold temperature by preheating before casting. These results are discussed in terms of the model proposed by Burden and Hunt that the CET occurs by the effect of the thermal gradient, arising from the slow, solidification of equiaxed dendrites, which increases the undercooling of the columnar dendrites. The application of the model due to Burden and Hunt is shown to require, the use of the ‘big bang’ model for equiaxed nucleation on pouring. A higher density of the nuclei, that grow into equiaxed grains, formed by pouring with lower superheat and into a cold mold, gives a higher thermal gradient immediately in front of the growing columnar grains. Other evidence in favor of the model is briefly discussed.