Direct microscopy of alloy nucleation, solidification and ageing (coarsening) during stir casting

Journal of Crystal Growth 76 (1986) 151—169 North-Holland, Amsterdam

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DIRECT MICROSCOPY OF ALLOY NUCLEATION, SOLIDIFICATION AND AGEING (COARSENING) DURING STIR CASTING R.J. SMEULDERS

*

Research Group of Optics, Department of Applied Physics, Deift University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands

F.H. MISCHGOFSKY Laboratory of Inorganic and Physical Chemistry, Deift University of Technology and Del/I Soil Mechanics Laboratory, P.O. Box 69, 2600 AB Deift, The Netherlands

and Hi. FRANKENA Research Group of Optics, Department ofApplied Physics, Deift University of Technology, Lorentzweg 1, 2628 Cf Del/I, The Netherlands Received 14 April 1984; manuscript received in final form 17 April 1986

In order to explain the slurry thixotropy and spherical particle morphology observed during the stir casting of metal alloys, we performed real time microscopic observations during the stir casting process using as a model substance the transparent organic alloy neopentyl alcohol with water. An optical set-up was developed consisting of two pulsed lasers, an optically adopted model of a stir casting apparatus, a microscope system and three different recording system to take (simultaneously) holograms, microphotographs 1) small solidifying particles (10—10~sm). Our preliminary observations and video recordings of the fast moving (up to 10 m s indicate that several mechanisms occur (nearly) simultaneously. Initially these are: both primary and secondary nucleation, rapid growth of the first nuclei into equiaxed dendrites, fragmentation of dendrite arms, clustering and coarsening. The development of the distribution of particle size and dendrite tip diameter and of the distribution of particle flow velocity and direction is shown quantitatively. We found that: (i) The increase of the fraction solid was mainly due to the formation of new nuclei (even after partial remelting), instead of outgrowth of existing nuclei. (ii) Not the fragmentation or clustering mechanisms but coarsening of the (branches of) equiaxed dendrites leads to the typical spherical particle shapes after stir casting. (iii) The rate of coarsening increases with stirring rate. (iv) Complex flow patterns at higher stirring rates can be attributed to the presence of Taylor vortices. Thixotropy is discussed in terms of coarsening and (local) variations of process parameters. The observation techniques used to determine the development of size and shape distributions, interactions and flow patterns of alloy particles during solidification seem also suitable to study industrial bulk crystallization.

1. Introduction Stir casting is an innovative casting technique for metals. It means that the metals are stirred vigorously during nucleation and subsequent partial solidification before they are casted. It appeared as the topic in many recent studies [1—12, 14,17,18]. The morphology of the solidified par*

Present address: Koning en Hartman Elektrotechniek P.O. Box 125, 2600 AC Delft, The Netherlands.

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tides in metal alloys after stir casting is spherical, contrary to the dendritic morphology which is found after unidirectional solidification. But, in spite of much research [1—8],it is still unknown which mechanism is responsible. However, without this knowledge, process control, and so product quality are poor. The investigation of the morphology changes is very tedious and difficult with metal alloys, since the partially solidified slurry has to be quenched and samples have to be polished before they can

0022-0248/86/$03.50 C Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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be examined microscopically. Moreover, it is known that quenching has a large influence on the particle shapes and structures and is only possible during the later stages of stir casting. Another important property of the stirred solidifying alloys, which is not found during conventional casting, is the thixotropic behaviour of the slurry [1,3]. Direct microscopic observation of the solidification process during stir casting might reveal the cause of both thixotropy and morphology changes. This requires a transparent model alloy and an optical set-up by which the rapidly moving solidifying particles can be observed or recorded with very short exposure times. Van Dam and Mischgofsky [9,10,141 and the present authors [11,12] showed recently that succinodimtril and neopentyl alcohol (further denoted as NPA) alloyed with water are good transparent model substances for metal alloys to study the solidification during stir casting: both organic alloys exhibit a similar partide morphology [9,11,12] and rheological behaviour of the slurry [9,10,14] as the metal alloys do. We used the NPA/water alloy in a model device shaped after an actual stir-casting apparatus introducing optical facilities for observation purposes [11,12,17]. In this transparent model device the particle slurry is stirred by a rotating glass inner cylinder. Two pulse lasers are used to iiluminate the solidifying particles. We have taken holograms, photographs and videorecordings simultaneously, showing (magnified) images of the particles. This offers the possibility to combine the advantages of these three techniques: a large depth of image volume, a relatively high lateral resolution with a large field of view and a relatively large repetition rate (10 Hz) of the recordings. In the present paper we will show that the complex solidification process during stir casting can be observed in detail and that the different mechamsms such as nucleation, growth and ageing of the particles and the interactions of the particles (fragmentation and clustering) in the rapidly stirred slurry can be observed and studied separately. Similarly we will show how the cornplicated particle flow patterns can be determined from double exposure microphotographs and holograms. In order to quantify the influence of several mechanisms on the final alloy microstruc-

ture, we performed preliminary determinations of the development of the size distributions of partides and dendrite tip diameters with time.

2. Experimental set-up and procedures 2.1. Optical set-up In our microscopic set-up a ruby laser and a frequency doubled Nd3 ± : YAG laser are used as (coherent) light sources. A full description of the experimental set-up is given elsewhere [11,12,17]. In short, red and green laser beams are combined before entering the model device along the axis of rotation. Fig. 1 shows that the beam subsequently is reflected by a prism, passes an air gap and the rotating glass inner cylinder, traverses the gap containing the solidifying slurry and finally leaves the device through a glass window in a temperature controlled, static outer cylinder. A microscope objective is used to image the particles onto three different recording systems: an off-axis holographic system, a photo camera and a video camera. 2.2. Application of the optical set-up for crystal growth in a rapidly flowing substance Application of our microscopic set-up is limited by the properties of the lasers, the dimensions of the model device, the properties of the microscope objective and the recording techniques [11,12,17]. Table 1 shows some of the most important limitations of the set-up. The coherence of the laser light is responsible for the small diffraction patterns which are visible around particles that are not exactly in the focal plane of the microscope systern (cf. figs. 3, 5—7, 10 and 12a); on the other hand, due to the coherence of the light, holograms with a large depth of field can be recorded, and also gradients of the refractive index, caused by concentration gradients or temperature gradients around the growing crystals or within the gap in general, can be visualized using interferometry. A detailed discussion of the possibilities and advantages of holographic interference microscopy for the study of crystals and concentration gradi-

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ents during growth and dissolution or melting has been presented previously [15,16]. Here we only describe the application of the set-up to the study of nucleation, growth, ageing, fragmentation, clustering and displacements of particles in a rapidly stirred, cooled melt. 2.3. The model substance NFA alloyed with water The transparent organic substance neopentyl alcohol [(CH3) 3CCH2OH] was alloyed with a quantity of water and other impurities. Both these Table 1 Limitations of the optical set-up (see also refs. [11,12,15—17]) Technique Microphotographs and video recordings

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latter quantities were 0.7 ±0.2 wt% for the earlier experiments and 1.3 ±0.2 wt% for the later experiments, as determined from measurements with Differential Scanning Calorimetry by Van Dam and coworkers [19]. The non-aqueous impurities are according to the manufacturer always less than 0.4 wt%. Since this amount is much less than the quantitiy of water, we shall denote the alloy as NPA with water. For the liquidus temperature TL, we have taken the maximal temperature at which the first nucleated particles became visible at any of the experiments with the same alloy. TL was

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also determined by slowly rising the temperature until the last solid particle had disappeared. In both cases we found the same value for TL. We found TL 327.3 ±0.2 K for 0.7 wt% water and TL = 318.5 ±0.2 K for 1.3 wt% water. The temperature interval between TL and the temperature at which the substance is completely solidified at the solidus temperature (Ta) is in the order of 10 K. Presently, the phase diagram is being measured by Van Dam and coworkers [19]. The quantitative data in figs. 4, 8, 9the andsame 11 were determined from experiments with NPA/water content, since the model device was kept sealed all the time and TL did not vary. In spite of the fact that organic alloys are often used as models for metal alloys to study solidification [9—14,17],the mechamsms for crystal growth are not quite the same. For example, the heat transport from the growing crystals is a growth limiting factor for the organic alloys due to their limited heat conductivity, whereas it is of no importance during crystal growth in most metal alloys. But the morphology and mcrostructure of solidifying NPA alloys and of other organic substances of this typical class [9,131, and references given in ref. [13], show a large similarity with the morphology after casting of metal alloys. Further, Van Dam and Mischgofsky [10,14] showed that NPA/water alloys exhibit a rheological behaviour similar to that of stir cast metal alloys [1,3], i.e. a dense particle slurry behaves thixotropically (see also ref. [14]).

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The actual stir casting process was simulated by cooling the melt with a cooling rate e down to a temperature T0 below the liquidus temperature TL. An example is shown in fig. 2. The temperature T0 was chosen in between 0.5 ad 1.5 K below TL. We used cooling rates within the range of 0.001—0.03 K s~. During the whole experiment the stirring rate N was kept constant 1).in the range of 0.16—16.6 rotations per second (s The model substance was not pumped axially through the model device, i.e. we did batch-stircasting experiments. A cross section of the model device is shown in fig. 1. A more detailed description and a longitudinal cross section has been given previously [11]. In fig. 2a, the time tL is defined as the time which has elapsed since the temperature of the melt T passed the liquidus temperature T 1. Nucleation starts at a temperature TN below TL, i.e. at a time tL > 0.

3. Mechanisms that dominate the development of the particle size distribution Up to now only the structures of stir cast metals are known after quenching. These show spherical particles in contrast to unidirectional solidification, which gives dendritic particles [1—3,5,6,8].The particle size distribution depends

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on cooling rate and stirring rate [2,3,5,6]. Until now, there is no experimentally proved model to explain the presence of the spherical particles. Hypothetic mechanisms proposed by Vogel et al. [6] are (i) an increased rate of (primary) nucleation during stirring and (ii) dendrite fragmentation after the formation of high-angle grain boundaries in the dendrites followed by bending of the dentrite arms. Apart from microscopic observations on quenched structures one also can derive some information on the growth behaviour from viscosimetry. Spencer et al. [1] and Joly and Mehrabian [3] observed a thixotropic behaviour of the slurry at fractions of solid (f~)larger than 30%. They found that the viscosity depends on the stirring rate and time and on the history of the slurry. They explained the thixotropic behaviour by assuming a structural build-up and break-down of aggregates of primary particles during stirring (see also refs. [2,8]). Van Dam and Mischgofsky [9,10,14] carried out viscosimetric measurements on transparent organic model substances. They found a similar thixotropic behaviour for these model alloys. In our apparatus we can perform microscopic observations during real stir cast conditions (stirred batch) comparable to the experiments of Van Darn and Mischgofsky [9,10,14] and of the experiments with metal alloys [1—3,5,6,8]. Our aim for the present preliminary investigations was to find out if there were one or several mechanisms which govern the thixotropic behaviour and the development of the particle shape and size distributions, which determine the final product quality of the casted alloys. This knowledge may lead to a better control of the casting process, and so of the product quality. For crystal growers, particularly those in the field of industrial crystallization, it may be interesting to know that as far as we know this is the first time that nucleation, growth, clustering and fragmentation of any compound was observed in situ with microscopic detail at slurry speeds up to 10 m s’. So some of the outcomes of our preliminary observations might also be of interest for industrial bulk crystallization from stirred slurries, —

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3.1. Primary nucleation and growth At the onset of the solidification (to be referred to as primary nucleation) at the nucleation ternperature TN in all stir casting experiments with the NPA/water alloys, we observed equiaxed dendritic particles. Within a few seconds the particles grew out unto a certain size and then, surprisingly, growth seemed to stop. Contrary to the expectation, further cooling caused new nuclei instead of the further growth of the already existing nuclei. Every particle had six (mutually perpendicular) dendritic arms and each arm carried several secondary dendritic arms. During the experiments at relatively low stirring rates (about 0.16 to 1.6 s’), tertiary dendritic arms were also visible. In fig. 3 examples are shown of the equiaxed dendrites, nucleated during experiments with stirring rates between 0.16 and 16.6 s1 and cooling rates between 0.03—0.001 K s1. Note that figs. 3b and 3c already show some secondary nucleated particles (S) (see section 3.2) and some fragments (F) of broken particles (see section 3.4). Our observations of equiaxed dendrites at the onset of the solidification process are in agreement with the theoretical work of Vogel and Cantor [4]. They calculated that stirring decreases the particle interface stability and so promotes dendritic growth. The fact that equiaxed dendrites have never been observed after stir casting metal alloys [6] is probably due to their rapid coarsening into spherical particles, as well be discussed in section 5, whereas it is quite impossible to “freeze” the shape of particles at low fractions solid by any way of rapid quenching. Depending on various parameters, we observed the first solidified particles at different undercooling temperatures. In fig. 4 the temperature at which the first particles appeared is given as a function of the stirring and cooling rate for an NPA alloy with 1.3 wt% of water. From these and many other experiments it follows that the metastable, undercooled region is up to 1 K for N = 0.16 s~,but much smaller for N = 16.6 s1, which is to be expected. For low stirring rates we observed a few, relatively large particles within the field of view (fig. 3a). At higher stirring rates, much more but smaller particles are found. In the

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latter case the increased number of particles can be due to secondary nucleation (see section 3.2). The number of particles did not depend on the

undercooling temperature (see also the ranges of the nucleation temperatures TN in fig. 4), but depended only on the stirring rate (N). So mainly

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Fig. 3. Equiuxed dendritic particles s , s~’,0.02=0.002 K s ‘,K60s~, s after passing = 66 s. (b) 1, 30s after after primary passing nucleation. TN=rL=66(a)s. N(c)= 0.16 N=16.6 about 60s TN, after Lpassing TN, N=1.66 s~, =O.02 K s tL = 180 s. (b), (c) S = secondary nucleated particles, F = broken-off dendrite arms after fragmentation. NPA with 1.3 wt% water. Microphotographs taken with the YAG laser.

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secondary nucleation has a strong influence on particle size distributions during the onset of solidification in a stirred melt.

3.2. Secondary nucleation A short time after the first particles were ob served, much smaller new particles appeared. This will be referred to as secondary nucleation. During the experiments with stirring rates of N = 16.6 s1, secondary nucleation followed within a few seconds after primary nucleation, whereas the time interval could exceed 60 s at stirring rates of N = 0.16 s1. Most of the secondary particles also exhibit equiaxed dendritic shapes, although almost spherical particles and particles with the shape Of an octadedron have been observed, too. Their sizes are generally small. Fig. 5 shows an example of secondary nucleated particles. The primary nucleated particles are clearly visible in the upper half of fig. 5a. From these observations one can conclude that

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1L = 240 Fig. 5. (a) Secondary nucleation during an experiment with N = I 66 s and = 0.002 K s —‘ about 60 s after passing TN, s. (b) The small particles are caused by nucleation after remelting and subsequent temperature decrease (about 80 s before exposure. N 8.3 ~_1), tL> 10~s. Both microphotographs were taken with the YAG laser. NPA with 1.3 wt% water.

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3.3. Nucleation and growth after partial remelting After partially remelting the solidified particles during stirring at a slight temperature increase of about 1.6 K (to 0.1 K below TL) and a subsequent decrease of the temperature of about 0.4 K (i.e. after decreasing and subsequent increasing of the fraction of solid, f~)~ new fast-growing nuclei appeared. Simultaneously, small broken parts of par~ tially molten larger particles grew out also into the equiaxed dendritic shape. Fig. 5b shows these new particles as well as a larger partially molten partide. So fig. 5b shows that nucleation not only occurs during the primary cooling stages after passing the liquidus temperature TL, but that nucleation may occur also within the solidification range (between TL and the solidus temperature T~)each time a temperature drop is caused. It is striking that at temperature drops, the fraction of solid f~increases mainly by nucleation, but that growth of larger particles is not observed. Also Vogel et al. [6] concluded, from their experiments with Al—Cu alloys, that subsequent solidification appeared to occur by formation of more particles, rather than by the continued growth of the existing particles during stir casting. This is in agreement, too, with our observations on blocked growth during primary nucleation (section 3.1) and on secondary nucleation (section 3.2).

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increased to a value N = 50 s~,all large dendritic particles were broken into smaller pieces, as can be seen in fig. 6b. The particles also appear to be rounded off, which is due to a slight temperature increase, resulting from the increased stirring rate. In all our experiments the fragmentation occurs already at very low fractions of solid material, but, at the higher stirring rate (N = 16.6 s1), the fragmentation of the larger dendrite arms seemed to start after the occurrence of secondary nucleated particles. Vogel et al. [6] proposed, on the basis of observations on stir casted Al—Cu and on the basis of a theoretical model, that fragmentation of the dendrites is the main mechnism for getting spherical particles after stir casting. From figs. 6a and 6b and from our other observations, we determined that fragmentation of nearly all the dendntic particles occurs at very high stirring rates (N = 50 s 1)~But even then the particles are less spherical than the particles (or in the terminology of Swartzbeck and Kattamis [18], the so-called dendrite cells) after stir casting of metal alloys. Particularly at the lower stirring rates, the shape of large parts of the dendritic particles stays unchanged. It will be shown in section 5 that other mechanisms lead to the spherical shapes as found in metal experiments. So fragmentation decreases the average particle size, but does not govern the transition from dendritic to spherical particle shapes. —

3.4. Fragmentation It is generally assumed that the thixotropic increase and decrease of the viscosity as a function of stirring rate is caused by some sort of structure formation (e.g. clustering) and structure breakdown (e.g. fragmentation). In this and the next section we will describe our preliminary observations on fragmentation and clustering, Approximately 20 to 60 s after the primary nucleation, several pieces like broken-off dendrite arms became visible. Equiaxed dendritic particles with one missing primary dendritic arm were observed in the same period. This is interpreted as the start of fragmentation. In fig. 6a, some examples of broken parts of dendritic particles are shown after nucleation at a low stirring rate of N = 1.66 s~.When the stirring rate was suddenly

4. Clustering The building up of a highly viscous structure is only imaginable through the outgrowth of large dendritic structures or through the clustering of individual particles. In a stirred melt only the latter possibility seems realistic, which is confirmed by our observations. The main question then is, how are the clusters formed and how do they build up the strength in their “networks”. Almost simultaneously with the occurrence of the particle fragmentation, also clusters of several dendritic particles were observed. The cluster sizes were mostly in the order of that of the field of view of the microphotographs and video recordings (about ~ m [11]). Fig. 7 shows some

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smaller clusters. At higher stirring rates, the diameter of the clusters appeared to be smaller. The time interval between the onset of the nucleation and the appearance of clusters of particles depended only slightly on the cooling rate, but strongly on the stirring rate. In fig. 8 the temperature at which the first clusters appeared during the

cooling period is given as a function of the stirring rate N. The temperatures in fig. 8 are the average values from data of 12 experiments, with stirring rates of N = 0.16, 1.66 and 16.6 s1 and a cooling rate between 0.001—0.03 K s~ [11]. The variations between the plotted values are due to the variations in the cooling rate, but there was no

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Fig. 6. (a) Initial fragmentation after primary nucleation; N=l.66 s~, r = 0.02 K s1 about 45 s after passing TN. (b) Fragmentation and rounding off of dendrite arms after simultaneous increase of N from 1.66 to 50 s~ and of T from 317.7 to 318.3 K. Both microphotographs were taken with the YAG laser. NPA with 1.3 wt% water.

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1L = 306 s, To 317.4 K. (b) Fig. 7. Small clusters of dendritic particles. (a) N =1.66 s~, 0.02 K s_i, 300 s after passing TN, N = 16.6 s~, e = 0.002 K s~. 90 s after passing TN, tL = 210 s. NPA with 1.3 wt% water. Both microphotographs were taken with the YAG laser.

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ever, Apaydin et al.’s idea of “sintering” can be adapted to a concept of “welding together” of particles and clusters as suggested recently by Van

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Dam and Mischgofsky [14] from thixotropy measurements and by Mischgofsky [20] from microscopic observations (see also section 5). This con-

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significant relationship between these clustering temperatures and the cooling rate e. As said before, it is assumed from viscosity meaurements on metal [1,3] and organic [9] alloys that the slurry thixotropy is caused by aggregate formation and break-down in dependence on stirring rate [1,3,9]. It is also assumed that the particle aggregates contain entrapped melt, yielding an effective fraction of solid f~(i.e. solid clusters with enclosed melt) which is much larger than the actual fraction of solid Is (i.e. the solid particles alone) [3,9]. By increasing the stirring rate the clusters are broken, the enclosed liquid is released and I~approaches Is~i.e. the shear strength of the slurry decreases. This is now confirmed by our observation of smaller clusters at higher stirring rates, Another possible contribution to the variations in shear strength of slurry might be derived from special grain boundaries observed by Apaydin et al. [8]. They assume that these boundaries are formed by clustering of small spherical particles followed by sintering. Our observations, however, indicate that the “spherical” particles on both sides of these so-called low energy grain boundaries are parts of a single equiaxed dendritic particle after ageing (i.e. coarsening and coaliscing), as will be discussed in section 5. So these spherical partides are equal to the “dendrite cells” [18] found in other metal experiments and also equal to the dendrite tips observed in our experiments. How-

dept includes local temperature variations which nearly always will occur within the slurry. During cooling minimal temperature variations will exist between the thermostated outer well and the rotating inner wall of the slurry in the gap. During nucleation and growth, latent heat is released locally. Moreover, during decrease or increase of the stirring rate the heat caused by the friction and .

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colhsion processes within the slurry and with the walls of the stir-cast apparatus vary temporarily and locally. The net result of such local temperature variations on contact points between particles or clusters during continuous cooling and/or decrease of stirring rate can cause a coalescence or “welding” together of the particles by slight outgrowth at the contact points. Similarly, increase of temperature and/or stirring rate can cause the melting of point contacts between particles and clusters long before melting of the bulk of the particles becomes visible, since at the (e.g. dendritic) point contacts, the radius of curvature is minimal and so the surface energy is maximal with respect to the larger crystal parts. These effects are very close to the coarsening effects discussed in section 5. The effect of time on the yield strength of the slurry at constant temperature and stirring rate will be discussed in section 5.

5. Shape development during ageing of the solidified particles 5.1. Coarsening As pointed out previously, immediately after nucleation most particles exhibit an equiaxed dendritic structure, which is modified somewhat by fragmentation. However, when the final temperature T0 and so the corresponding fraction solid is attained, it is observed that the particles start to round off, which we shall denote by “ageing of the

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particles” or coarsening. After a period of ageing, the particles can even exhibit a spherical shape (fig. 10), like it is found for metal alloys. In our experiments, about ~ s after the onset of primary nucleation, the transformation of the shape of the dendritic particles into a shape resembling bunches of grapes was completed. At the same time, all differently shaped particles (particle pieces, clusters of particles, and secondary nucleated partides) also exhibited large spherical shapes. The rounding off of the dendrite arms starts already a few seconds after nucleation, as can be observed in fig. 3a. This rounding off of the (primary) dendritic arms can be explained by the mechanism of coarsening (both dendrite coarsening [10—14] and particle coarsening or Ostwald ripening [10—12,14,20]),i.e. surface areas with a small radius of curvature (and therefore a high surface energy) tend to lower their surface energy by increasing their radius of curvature. A similar behaviour was observed for the secondary and tertiary dendrite arms of which some became sperical, while other arms disappeared. If both primary and secondary dendrite arms were present, no differences between the size distribution of their tip diameters (d) were found (cf. also section 7). Metallurgists [1—3,5,6,8] find only spherical particles after quenching of their stir-cast metal alloys, whereas we observed that most particles nucleate as equiaxed dendrites (section 3.1). However, it is quite impossible to quench a metal alloy with a low fraction of solid without completely destroying the shape of the primary nucleated particles. The fact that no equiaxed dendrites are found afer quenching of metal alloys with a higher fraction of solid, therefore suggests that coarsening of metal dendrites happens at a much higher rate than in our organic alloys, Another striking phenomenon, not noted by the metallurgists, is that we, for some equiaxed dendrites, could directly observe that some of the dendrite tips grew out into large spheres. During this process these equiaxed dendritic particles (or at least relatively large parts of them) stayed unchanged. This shows that an initial nucleus with its six primary dendritic branches, each of which develops into one or several spheres, ends up as a

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sphere-like particle consisting of six or more smaller spheres (looking like a bunch of grapes, cf. fig. 10). If one quenches a stirred batch with these “bunches of grapes” and afterwards studies cross sections of the quenched batch under the microscope, one easily interprets the “grapes” as “primary” particles and the “bunches of grapes” as “clusters” of individual particles. Apaydin et al. [8] called these bunches “primary particles” and attributed their origin to a collision mechanism of previously separated small particles (the “grapes” or dendrite tips as we called them) followed by sintering or a growth twinning mechanism. The terminology in other stir-cast literature [1—3,5,6] can also give a misleading interpretation of the particle formation. We suggest to call the spherical tips of the dendrites, which become the “grapes” in the later stage of ageing, the “dendrite tips” or the “dendrite cells” (the latter suggested by Swartzbeck and Kattamis [18]. So our conclusion is that the “bunches of grapes” are no clusters, but coarsened equiaxed dendrites. Clustering occurs when several equiaxed dendrites get connected before, during or after coarsening, finally yielding structures of clustered “bunches of grapes”. Our next step will be to study the clustering and fragmentation behaviour of large amounts of “bunches of grapes” into thixotropic microstructures by both simultaneous viscosimetry and (video-)microcsopy at high recording rates and high particle densities (or .f~).

5.2. The effect ol coarsening on the shear strength From the foregoing it will be clear that coarsening will exert a strong influence on the thixotropic behaviour of the slurry. Both, the building up and the break down of particle network, and the entrapment of melt within clusters (as discussed in section 4), will depend on the shape of the partides, i.e. their dendritic or spherical shapes. Under constant conditions of temperature and stirring rate, coarsening will lead to a gradual decrease of shear strength as discussed by Van Dam and Mischgofsky [10,14]. Therefore the rate of the coarsening process is important for the interpretation of the thixotropic slurry behaviour. In section

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7, some preliminary quantitive results on the rate of the dendrite tip coarsening will be presented.

6. Development of the particle size distribution In order to quantify the influence of each of the mechanisms (primary and secondary nucleation, growth, fragmentation and coarsening) on the final particle size distribution, it is necessary to statistically evaluate the recorded particle sizes in various solidification stages. Since the particle shapes range from dendritic to spherical, we first have to define a good measure for the particle size. The measurements were performed on video images (10 recordings per second) and microphotographs [ll]. They gave sufficient numbers of particles for a statistical analysis since subsequent pictures show different sets of particles at the used stirring rates and field of view. As an example, we give the size distribution just after primary nucleation (section 6.1) and give an impression of the difficulties which will be encountered at later solidification stages (section 6.2). 6.1. Size of the dendritic particles just after primary

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Fig. 9a defines the longest diameter D and the tip diameter d of a dendritic particle. After some time of ageing (coarsening) the particles may assume a shape like a bunch of grapes (see section 5). In this case the diameter of the grapes (or the spherical “dendrite cells” as they are called in ref. 1181) is defined to be d and the diameter of the whole bunch is D, as logically follows from our discussion in secton 5.1. As an example, the distribution of the longest diameter D of the 44 particles in the microphotograph of fig. 3c is given in fig. 9b. It shows a smooth spread of the different D values due to the occurrence of secondary nucleated particles and broken off particle fragments as visible in fig. 3c. The average value (0) of the longest diameter D over 12 experiments, as measured in the period between primary nucleation and clustering, is given in fig. 9c. In this period (typically in between 20 and 60 s), the average particle size becomes smaller

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Fig. 9. (a) Definition of the largest diameter D and the tip diameter d of an equiaxed dendrited particle. (b) The distribution of the diameter D of 44 particles at lL = 60 s, measured from the microphotograph of fig. 3c. (c) The average value (D) of the diameter D of all particles observed in the time interval (about 20 to 60 s) between the onset of nucleation and the onset of clustering before T, is reached, as a function of the stirring rate N and the cooling rate c (NPA with 1.3 wt% water).

R.J. Smeulders et at

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Direct microscopy of alloy during stir casting

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Fig. 1U. Microphotograph of particles with the shape of a “hunch of grapes” (N taken with the YAG laser).

as time lapses due to the origin of the secondary nucleated particles and the beginning of fragmentation. Growth of individual particles was not of much influence on
165

1. i~ = 450 s, NPA with 0.7 wt% water,

8.3 s~

smaller than 50 ~smwere impossible, since noise in the image, as well as small dust particles and air bubbles, deteriorated in some cases the image quality, and so made particle recognition and measurement difficult. In the time interval before clusters appear, the average diameter
166

R.J. Smeulders eta!.

/ Direct microscopy 0/alloy during stir casting

will give a third peak at higher values of D. Unfortunately not all the videorecordings and microphotographs could be used for the determination of D values. At the onset of nuclealion on some of the images no particles at all are visible, or only parts of them, whereas in the later stages of the solidification process, so many partides are visible that it is difficult to discriminate the sizes of individual particles. During these preliminary experiments, it was difficult to measure the sizes of clustes, because they were of the same order of magnitude as our field of view, as pointed out in section 5.1. Although we did measure several size distributions, as presented recently [11], it was not the issue of this preliminary work to obtain a statistically significant development of the particle size distribution after the onset of solidification. For such a purpose, a large number of experiments is necessary.

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the dendrite arm the dendrite tip diameter 7. Development of

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We measured the diameters of the dendrite tips (d), as defined in fig. 9a, as a function of the time lapse ~L after the temperature TL is passed. In fig. 11 the maximal values of d (dmas), as observed within the field of view, are plotted as a function of tL for three different stirring rates: N = 0.16, 1. The cooling rate e at the start 1.66 and 16.6 s of each experiment was between 0.001 and 0.03 K s — We did not find a significant influence of the cooling rate e on the development of the maximum tip diameter dm~at constant stirring rates. The influence of the stirring rate on the coarsening, however, seemed to be significant. Assuming the relationship dm~= /3t~,we found for N = 0.16 s1: a = 0.24 ±0.11, /3 = 17 ±3 ~m s_a, for N = 1.66 s~1a = 0.36 ±0.10, /3 = 11 ±1 ~tm s_a, and

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Fig. 11. ~The 0.001< <003 maximal K 5 values ad 316.8 d~
Ri. Smeulders et a!. / Direct microscopy of alloy duringstir casting

for N =

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tween primary and secondary dendrite arm tips in

From fig. 11 one can see that, for instance, dm~ 150 .tm is attained for tL = 600 s at N = 16.6 s~, but about 6000 s later for N = 0.16 s~. This conforms the prediction of Vogel et al. [6] for metal alloys of a rapid increase of coarsening with increasing stirring rate. During the coarsening of the secondary dendrite arms, in experiments with N = 0.16 and 1.66 s we observed that the secondary dendrite arm spacing is of the same order of magnitude as the tip diameter. This has also been found for metal alloys [5]. Therefore we did not discriminate be-

the measurements for fig. 11. We restricted the measurements to the tip diameters, since in experiments with the highest stirring rate (N = 16.6 s_i) no secondary dendrite arms were visible.

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Flow patterns

With the aid of double pulses from the ruby laser, double exposure microphotographs and holograms have been made of the fast moving par-

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Fig. 12. (a) Part of a double exposure microphotograph, of equiaxed dendritic particles shortly after nucleation, taken with the ruby laser (NPA with 0.7 wt% water, N = 8 s~, e = 0.04 K s~, tL = 10 s). The time interval between exposures 4t = 100 ~ss.Insert: Definition of particle displacement v~~4 t and angle ~, z direction of axis of rotation, x = tangential direction, (b) The vectorial displacement field (v~~4t) of the microphotograph of (a). (c) Distribution of the particle velocity component ~ in the x—z plane. (d) Distribution of the particle direction component tan ~ in the x z plane.

168

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Direct microscopy of alloy during stir casting

tides. Fig. 12 gives an example of the velocity distribution, measured from a single double exposure microphotograph (fig. 12a). In the insert of fig. 12a we also define the displacement ~ of a particle and the angle 4 of the direction of the displacement. Because the imaged volume is only a small segment of the total cylindrical shaped gap containing the slurry, we used simple cartesian coordinates to describe the displacements. Here, x

ions. Equiaxed dendrites, however, have never been observed after quenching of real stir cast metal alloys [1—3,5,6,8].This may be explained by the fact that the metal dendrites rapidly transform into spherical particles due to coarsening (section 5). We also observed nucleation after partial remelting and subsequent cooling of the slurry. This means that within the solidification range T5 <

is the direction of the average (tangential) flow, while z is the vertical axis (parallel to the direction of the axis of rotation) in the focal plane of the imaged volume, Fig. 12a shows a part of the double exposure microphotograph used for the picture of the partide desplacement vectors of fig. 12b. The particle velocity distribution ~ (fig. 12c) and the particle velocity orientation distribution in the x—z plane (fig. 12d) are derived from fig. 12b. From figs. 12b and 12d, it can be seen that the particles have a velocity component in the z direction. Moreover, in the reconstructed double exposure holograms we also observed particle displacements in the y direction (perpendicular to the x—z plane). Since we have observed microscopic bands of Taylor vortices [12,17], it might be concluded that fig. 12b shows about one Taylor vortex (cf. also refs. [12,17]).


9. Conclusions The contribution of two pulse lasers and three recording techniques in a microscopic set-up with a model device of a stir cast apparatus, adjusted for direct optical observation, has proved to be a good observation system to study mechanisms which dominate the slurry rheology and the development of the particle shape and size distribution during stir casting of low fractions of solid material (about 1 to 10 vol%) up to high stirring rates (50 s_i), in detail over long periods (10~s). We did preliminary observations on the primary and secondary nucleation and the subsequent growth of particles as a function of the stirring rate and the temperature. Immediately after nucleation most particles were equiaxed dendrites, which is in accordance with theoretical predict-

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Direct microscopy of alloy during stir casting

larger tip diameters were found for higher stirring rates. The influence of small local variations in ternperature and of variations in stirring rate on the building up and break-down of cluster, and so on the thixotropic shear strength variations, is discussed in terms of coalescense or welding together (or reversily, the melting off) of point contacts between particles. Coarsening will lower the shear strength with time. With the help of double exposure holograms and microphotographs it appeared also possible to map and to evaluate quantitatively the value and direction of the velocity of the particles in a plane through the slurry. We found that the particles not only moved in the stirring direction. The observed particle displacements in the axial and the radial direction can be attributed to the occurrence of Taylor vortices for higher stirring rates.

Acknowledgements These investigations in the program of the Foundation for Fundamental Research on Matter (FOM) have been supported (in part) by the Foundation for Technical Research (STW) the future Technical Science Branch/Division of the Netherlands Organization for Advancement of Pure Research (ZWO). We gratefully acknowledge J.C. van Dam for valuable discussions.

References [1] D.B. Spencer, R. Mehrabian and M.C. Flemings, Met. Trans. 3 (1972) 1925.

169

[2] M.C. Flemings, R.G. Rick and K.P. Young, Mater. Sci. Eng. 25 (1976) 103. [31R.A. Joly and R. Mehrabian, J. Mater. Sci. 11(1976)139. [4] A. Vogel and B. Cantor, J. Crystal Growth 37 (1977) 309. [5] S.D.E. Ramati, D.G. Backman, Y.V. Marty, G.J. Abbaschian and R. Mehrabian, in: Proc. Workshop on Rheocasting (Columbus, OH, 1977) [MCIC-78-35 (1977)

131.

[61 A. Vogel, R.D. Doherty and B. Cantor, The Metals Society, ~ndon (1977) 518; (1979) 479. [71R. Trivedi, J. Crystal Growth 48 (1980) 93.

[81N.

Apaydin, K.V. Prabhakar and R.D. Doherty, Mater. Sri. Eng. 46 (1980) 145. [91(1982) J.C. van Dam and F.H. Mischgofsky, J. Mater. Sci. 17 989. [101J.C. van Dam and F.H. Mischgofsky, Rheol. Acta, 21 (1982) 445. [11] R.J. Smeulders, SPIE 369 (1982) [12] R.J. SPIESmeulders, 370 (1982)

F.H. Mischgofsky and H.J. Frankena, 482. F.H. Mischgofsky and HJ. Frankena, 74.

[13] S.C. Huang and M.E. Glicksmann, Acta Met. 29 (1981) 717. [14] J.C. van Dam and F.H. Mischgofsky, presented at 7th Intern. Conf. on Crystal Growth, ICCG-7, Stuttgart, 1983.

[151F.H. Mischgofsky,

J. Crystal Growth 43 (1978) 549. [16] F.H. Mischgofsky, SPIE 236 (1981) 86. [171 RJ. Smeulders, F.H. Mischgofsky and H.J. Frankena, in: Proc. Intern. Conf. on Optical Techniques in Process Control, The Hague, 1983 [BHRA Fluid Engineering (1983) 2651. [181G.W. and T.Z. Kattamis, J. Mater. Sci. 9 (1974) Swartzbeck 635. [191J.C. van Dam, private communication, 1983. [20] F.H. Mischgofsky, J. Crystal Growth 65 (1983) 500.