Bulk nanostructured alloy formation with controllable grain size

Acta mater. 48 (2000) 2117±2121 www.elsevier.com/locate/actamat

BULK NANOSTRUCTURED ALLOY FORMATION WITH CONTROLLABLE GRAIN SIZE W. H. GUO and H. W. KUI{ Department of Physics, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (Received 16 February 1999; accepted 30 January 2000) AbstractÐIt was demonstrated that bulk nanostructured materials can be prepared by rapidly solidifying a eutectic alloy melt. Prior to solidi®cation, the undercooled melt has to undergo liquid phase spinodal decomposition. Although the average grain size of an as-prepared bulk nanostructured alloy can be as small as 3±6 nm, there is actually little control over its grain size. It turned out that by introducing intermediate isothermal annealing to the rapid solidi®cation process, bulk nanostructured alloy ingots of diameter > 1 cm can be synthesized with controllable grain size. 7 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Mesostructure; Alloys; Spinodal decomposition; Scanning/transmission electron microscopy (STEM); Fluxing technique

1. INTRODUCTION

By de®nition, when the diameter r of the constituent grains of a material falls in the range of 1 nmRrR100 nm, it is called a nanostructured material. It possesses some novel physical properties that its coarse-grained counterpart lacks. For instance, the yield strength of a nanostructured metal is enhanced, though less ductile [1±3]. Also, ceramics made up of nanometer size grains exhibit some ductility [3±6]. The notion of nanostructured materials was ®rst put forward by Gleiter [7] and Turnbull [8]. It was later put into practice by Gleiter and co-workers [9]. The synthesis method that they introduced involves the preparation of nanometer size powders by an evaporation technique and sintering of these as-prepared nanometer powders under pressure (sometimes also at elevated temperatures). There are other modi®ed techniques that mainly focus on how to prepare the nanometer powders. The typical dimensions of an as-prepared specimen, in cylindrical shape, are diameter 11 cm and height 11 mm: The main drawback in these methods is contamination. Since impurities including trapped gas are unavoidable, sintering is either slowed down or even prohibited. Consequently, there are microvoids left in an as-prepared nanostructured specimen, serving to reduce the mechanical strength of the sys-

{ To whom all correspondence should be addressed.

tem [2]. In addition, the grain size distribution is quite wide, e.g. in a nanocrystalline Cu specimen, the size at half-width of the size distribution spans from 15 to 70 nm [2]. Nanostructured materials can also be synthesized by annealing glassy Fe74.5Nb3Si13.5B9 ribbons [10]. Uniformity in grain size can be substantially improved by adding Cu [10, 11]. More recently, Johnson and co-workers [12] found that nanocrystals emerge in an amorphous matrix when an original homogeneous amorphous metal is annealed at a temperature near its glass transition temperature Tg. They suggested that there is a chemical decomposition in the amorphous alloy during annealing. The nanocrystals are the outcome of preferential crystallization of one of the decomposed phases. The advantage of this technique is the removal of microvoids. Recently, Guo et al. [13] demonstrated that bulk, microvoid free nanostructured materials of relatively uniform grain size can be prepared by rapid solidi®cation. This technique is applicable to systems that would undergo liquid state phase separation. Eutectic alloys belong to this category. By de®nition, the same kind of atoms in a eutectic alloy prefer to stay together, i.e. the attraction between the same type of atoms is larger than that between di€erent ones. When a eutectic alloy becomes a melt at high temperatures, the entropic term in the Gibbs free energy forces all the constituent atoms to mix well with each other. On the other hand, at lower temperatures, if the melt can sustain

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in the liquid phase, H now dominates and liquid phase separation is expected to take place. We have indeed demonstrated that liquid phase separation does occur in undercooled molten Pd±Si [14, 15] and undercooled molten Pd40.5Ni40.5P19 [16, 17]. The term ``undercooled'' means that the temperature T of a melt is below its liquidus Tl. The extent in temperature below Tl is called undercooling DT de®ned as DT ˆ Tl ÿ T: There are two transformation mechanisms in phase separation, namely, nucleation and growth, and spinodal decomposition. The characteristic morphologies in the latter are intertwining networks of wavelength l. According to Cahn [18], l of the networks depends on DT [more precisely, lAf…Ts ÿ T †=Tc g ÿ1=2 where Tc is the critical temperature of the miscibility gap and Ts is the onset temperature of the chemical spinodal]. Yuen and co-workers [16, 17] demonstrated that for undercooled molten Pd40.5Ni40.5P19, at DT ˆ 100 K, l ˆ 350 nm, and at DT ˆ 150 K, l ˆ 150 nm: Therefore, l exhibits a sharp drop with DT. It is plausible that if l decreases further and falls in the nanometer size regime, surface tension may induce the breakup of the liquid networks leading to the formation of nanometer size liquid droplets, which then solidify to nanostructured solid materials by rapid solidi®cation. Lee and Kui [14] found that molten Pd82Si18 behaves similarly in the undercooling regime. In order to prepare liquid networks of even smaller l, Guo et al. [13] employed the water-quenching method and bulk nanostructured Pd82Si18 alloys with an average grain size of 6 nm were synthesized. The as-prepared specimens are microvoid free and of narrow grain size distribution. This novel synthesis method by rapid solidi®cation still su€ers from two drawbacks. First, it is important to be able to prepare nanostructured alloys of various grain sizes. Rapid quenching however does not leave us much room for this practice. Second, the diameter of an as-prepared nanostructured Pd82Si18 alloy, in spherical shape, seldom exceeds 3 mm. Both problems are resolved as follows. The merit of the rapid solidi®cation technique developed by Guo et al. [13] is the achievement of a very large undercooling prior to solidi®cation so that an original homogeneous melt can be split into multiple liquids. Furthermore, at a very large DT, liquid networks of extremely short l, e.g. 6 nm [13, 18], would break up readily into nanometer size undercooled liquid droplets during the quenching period. Finally, the morphologies of the tiny liquid droplets are frozen by the subsequent solidi®cation to become a nanostructured material. It is apparent that if intermediate thermal annealing is incorporated into the rapid solidi®cation process so that liquid networks of longer wavelength have more

time to break up, then nanostructured materials of controllable grain size can be synthesized. The second goal is to prepare nanostructured alloys of larger physical size. According to Cahn's analysis [18], spinodal decomposition occurs homogeneously throughout an entire specimen, irrespective of its size. The liquid networks then break up into tiny liquid droplets of more or less uniform grain size after thermal annealing. However, as a crystal±liquid interface advances forward during crystallization, the heat generated due to crystallization is mostly dumped into the undercooled liquid ahead of the crystal front, raising its temperature. Consequently, the diameter of the liquid droplets ahead of the crystal front increases and the asobtained undercooled specimen has a wide grain size distribution [17, 19]. The diculty can be removed by undercooling a melt to a very large DT so that the solid±liquid interface advances slowly. Equivalently, heat (due to crystallization) is generated at a slower rate and can be dissipated away readily so as to keep the entire specimen more or less of uniform temperature or ®nally a nanostructured alloy of uniform grain size. 2. EXPERIMENTAL

Pd82Si18 ingots were prepared from elemental Pd (99.99% pure) and Si (99.999% pure) granules. After weighing in the right proportion, they were put in a clean fused silica tube. Alloying was carried out in a RF induction furnace under Ar atmosphere. The liquid phase spinodal decomposition for molten Pd82Si18 takes place at very large DT. Experience indicates that a melt can be undercooled substantially below its Tl by a ¯uxing technique. For instance, Kui et al. [20] demonstrated that molten Pd40Ni40P20 can be undercooled to its glass state bypassing crystallization with a cooling rate of 0.75 K/min. Also, molten Ge [21] can be undercooled to DT ˆ 342 K: In this work, the ¯uxing technique was again used and the ¯uxing agent was anhydrous boron oxide B2O3. Cooling cycles and isothermal annealing were performed in a computer-controlled furnace. In the experiment, a Pd82Si18 ingot (diameter 01 cm† and anhydrous B2O3 were put in a clean fused silica tube of vacuum 010 ÿ3 Torr: The whole system was then heated up by a torch to 01350 K: After melting, the molten Pd82Si18 ingot should be completely immersed in the molten anhydrous B2O3. Prolonged heating was applied for 4 h to facilitate the removal of impurities from the molten specimen. After the high temperature heat treatment, the whole system was transferred to the furnace sitting on a thermocouple as shown in Fig. 1. The thermocouple served to record the temperature of the molten specimen. The furnace was initially set at a temperature of 1173 K, just above the Tl of

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by conventional metallography methods, scanning (SEM) and transmission electron microscopy (TEM). Compositions were analyzed by EDX installed in both SEM and TEM. 3. RESULTS AND DISCUSSIONS

Fig. 1. Schematic diagram of the experimental setup.

Pd82Si18 …ˆ 1133 K). We waited for 20 min for the establishment of thermal equilibrium between the furnace and the system. The temperature of the furnace was next lowered down at a rate of 10 K/min to a temperature T which is below the onset temperature of the liquid phase spinodal decomposition reaction for molten Pd82Si18 …ˆ 863 K). As soon as T was reached, it was changed to an isothermal condition. This condition continued until the undercooled molten specimen crystallized. Immediately after the crystallization event, the whole system was removed from the furnace and allowed to cool down in air to room temperature. Microstructures of the ascrystallized/undercooled specimens were examined

Fig. 2. A SEM micrograph displaying the morphologies of an undercooled specimen that had undergone liquid phase spinodal decomposition. There are two subnetworks that are traced out by solid lines.

Since the morphologies of intertwining liquid subnetworks change with annealing time, in the following only the microstructures of those undercooled specimens with annealing periods that are shorter than 12 min, but longer than 9 min are displayed and compared. The microstructure of an undercooled specimen with DT ˆ 280 K is shown in Fig. 2 (SEM micrograph). It depicts a network (made up of two subnetworks marked, respectively, by H ' and L ') of l12:5 mm: To avoid confusion, lines are drawn to identify the boundaries that separate the two subnetworks. There are two phases in the subnetwork marked by H ', which are Pd3Si and Pd9Si2. Again, there are two phases in the subnetwork marked by L', which are Pd and Pd9Si2. The crystallization in both subnetworks can be described as eutectic. It is apparent that the wavelength of the networks is still too long to break up for an annealing period of 9± 12 min. The microstructure of an undercooled specimen with DT ˆ 320 K is shown in Fig. 3 (SEM micrograph). Similar to the one shown in Fig. 2, it composes of two broken subnetworks, but of smaller l …1120 nm). The crystallization process has also changed. There is only one phase inside each subnetwork, Pd9Si2 in one and Pd3Si in the other. Besides the two broken subnetworks, particles of Pd87.5Si12.5 solid solution are found quite frequently right next to the broken subnetworks. Since connectivity of the two subnetworks has reduced substantially compared with that shown in Fig. 2, it is

Fig. 3. A SEM micrograph depicting the spinodal network of an undercooled specimen with DT ˆ 320 K: The liquid subnetworks have partially broken up.

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Fig. 4. A TEM micrograph illustrating the microstructures of an undercooled specimen with DT ˆ 350 K: The constituent grains of the undercooled specimen are more or less granular.

apparent that an annealing period of 9±12 min is already e€ective in breaking up the liquid subnetworks for an undercooled specimen with DT ˆ 320 K: The microstructure of an undercooled specimen with DT ˆ 350 K is shown in Fig. 4 (TEM micrograph). The microstructure is now more or less in granular form. It has a characteristic dimension of 060 nm: In other words, with an annealing period of 9±12 min, breakup of liquid networks of l160 nm becomes possible. There are three phases present in the undercooled specimen as marked in the ®gure. The average grain size of the constituent grains or l of the networks of the undercooled specimens

Fig. 5. A plot showing grain size vs DT. For DT < 320 K, the liquid subnetworks are connected. For DT > 320 K, the liquid subnetworks begin to break up.

Fig. 6. A TEM micrograph showing the microstructures of an undercooled specimen with DT ˆ 350 K: The annealing period for this undercooled specimen is 21 min, longer than that shown in Fig. 4 by about 10 min.

with an annealing period of 9±12 min is plotted in Fig. 5. It illustrates a rapid drop in the average grain size at smaller DT, but slows down at large DT. At DT ˆ 360 K, l150 nm: An attempt is made to divide the plot into two regions. On the left, the prescribed annealing time (longer than 9 min but shorter than 12 min) is apparently not long enough for the breaking up of the liquid subnetworks. However, partial if not complete breakup of the

Fig. 7. A TEM dark ®eld image showing the Pd3Si nanocrystals in a nanostructured alloy.

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In conclusion, nanostructured alloy ingots of diameter 01 cm have been successfully synthesized. Moreover, by introducing intermediate annealing, the average grain size of these nanostructures can be controlled. AcknowledgementsÐWe thank Hong Kong Research Grants Council for ®nancial support. REFERENCES

Fig. 8. An X-ray di€raction pattern taken from the undercooled specimen shown in Fig. 7. The di€raction peaks are broad due to the small grain size.

liquid subnetworks has occurred for those undercooled specimens lying on the right of the plot. In Fig. 4, it is apparent that the liquid networks have not broken up completely. In order to demonstrate that the length of annealing period is essential in preparing nanostructured alloy of random/ equiaxed grains, the microstructure of another undercooled specimen also of DT ˆ 350 K but with an annealing period of 21 min is shown in Fig. 6. It consists of more or less very ®ne equiaxed grains. Due to a longer annealing period, the average grain size is 170 nm, slightly larger than that shown in Fig. 4. A dark ®eld image showing the Pd3Si grains is shown in Fig. 7. They are equiaxed and of narrow grain size distribution. The undercooled specimen shown in Fig. 6 was also studied by the X-ray method. The di€raction pattern is shown in Fig. 8. Broadening of di€raction peaks is clearly visible. According to Ref. [22], line broadening due to small grain size in K space is given by DK ˆ 0:9…2p=r† where r is the average grain size as de®ned earlier. Line broadening due to strain [23] was found to be negligible. The line broadening shown in Fig. 8 was therefore attributed to small grain size and it corresponds to an average grain size of 170 nm, consistent with the measurement obtained by the TEM method.

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