Behavior of Boron in Molten Aluminum and its Grain Refinement Mechanism
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The mechanism of heterogeneous grain refining of aluminum by ultrafine elemental boron particles was investigated. In order to facilitate the observation of the boron-aluminum interface, a boron filament was introduced in a melt at 1013 K (740 °C) containing different levels of Ti. The Al/B interface was studied using transmission electron microscopy and different phases were identified using the electron diffraction method. The experimental results showed that boron is dissolved in pure aluminum while its dissolution is inhibited in presence of titanium solute. A thin layer of TiB2 formed at the surface of boron thickens with residence time in the melt. The mechanisms by which aluminum is crystallized on boron are discussed.
The addition of grain refiners into molten aluminum alloys is a common industrial practice to suppress the growth of columnar grains, to promote the formation of fine grains, and to improve castability and product quality. Grain refiners are usually added as Al-based master alloys that are rich in at least one of the following elements: titanium, boron and carbon. There are several types of grain refiners. Those produced by reaction of molten aluminum with halide salts (K2TiF6 and KBF4) and containing 5 pct Ti and 1 pct B, are the most widely used by industry. Their typical microstructure consists of TiB2 particles, TiAl3 needles and Ti solute in an aluminum matrix. Small additions of such master alloys in aluminum alloys (usually at 0.01 pct Ti and 0.002 pct B levels) result in a fine and equiaxed solidification microstructure. An appropriate temperature of addition practically lies between the liquids and 1073 K (800 °C).
A great deal of fundamental research has been done over the past 50 years in order to understand the possible mechanisms by which grain refiners yield a fine microstructure. The majority of fundamental research works have focused on the refining mechanisms of Al-Ti-B master alloys and several theories have been postulated in this matter which were summarized in very useful comprehensive reviews.[1, 2, 3, 4] According to these theories, the refinement effect could be ascribed to two phenomena: (i) the growth restriction factor and (ii) the inoculation and heterogeneous nucleation. The growth restriction is explained by the diffusion of solute atoms at the liquid/solid interface. In the inoculation theory, however, α-Al grains grow on the nucleation sites that are introduced in the liquid metal by the master alloy. Small particles of TiAl3, (Ti-Al)B2, and TiB2[5, 6, 7, 8, 9] have been proposed as potential nuclei contributing to the formation of α-Al and the efficiency and stability of these particles have been extensively studied. According to a general consensus, the heterogeneous nucleation of aluminum is attributed to the presence of TiB2 particles when an Al-Ti-B refiner is used. Some experimental evidence confirmed the nucleation potential of TiB2 particles, while it was also shown that TiB2 could be surrounded by a thin layer of TiAl3 phase which acts as the nucleation site.[10, 11, 12, 13, 14, 15]
Although the efficiency of grain refiners has been improved considerably over the past decades, there are still several issues which need to be addressed. One of these issues is the difficulty of controlling the size, morphology and agglomeration of TiB2 particles in Al-Ti-B master alloy. Individual particles of TiB2 have typically a size between 0.5 and 2 μm but some reach 10 μm. Agglomeration of particles increases with increasing residence time in the melt and worsens in the presence of halide salt traces[16,17] although recent studies have shown that Mg additions reduce the agglomeration and increase nucleation efficiency in Al-Si alloys. The coarse TiB2 particles and agglomerates are a serious concern and can cause surface defects, especially if the aluminum is to be rolled in thin foils. Settling of TiB2 particles is another issue. These particles tend to settle at the bottom of crucibles resulting in the formation of sludge and reducing the overall efficiency of the refiner. Grain refining of recycled aluminum is also a growing concern in cast houses. The level of Ti must be kept under the peritectic concentration in order to prevent the formation of brittle TiAl3 needles during solidification. Recycled metals contain a considerable amount of titanium. Addition of Ti-containing refiners in a recycled metal causes an increase of Ti level and limits the use of recycled metals in melt charge. Foundries keep this level typically under 1000 ppm. Titanium level is controlled by mixing recycled and pure aluminum in the melt charge.
In an attempt to overcome some of the abovementioned issues, Alamdari et al. have proposed a novel process for grain refinement of aluminum alloys by using fine elemental boron particles. These grain refiners are in the form of Al-matrix composites in which elemental boron particles are finely dispersed using high energy ball milling. The grain refining tests revealed that the addition of boron alone at the level of 0.002 pct does not provide any grain refining effect in pure aluminum. However they showed that when titanium is introduced prior to or along with boron this refiner is more efficient than the conventional Al-5Ti-1B refiners for commercially pure aluminum and some Al-Si based alloys such as A356 and A357. These observations confirm the fact that boron particles do not survive in pure aluminum above 1013 K (740 °C). However, in the presence of Ti, boron particles are not only protected in the melt but also contribute efficiently to heterogeneous nucleation of aluminum. Beside its great efficiency and good control of particle size refining, this grain refiner may also offer the possibility of treating recycled aluminum without any titanium addition. The mechanism by which elemental boron particles act as potential nuclei is however still unknown.
The purpose of this paper is to study the behavior of elemental boron in liquid aluminum in order to elucidate its influence on the grain refining mechanism of boron particles. The boron filament was introduced into liquid aluminum containing different concentrations of dissolved titanium and the microstructure and composition at the boron/aluminum interface were studied. The boron fiber facilitates the study of the boron-aluminum interface after immersion in liquid aluminum.
2 Experimental Details
Chemical Composition of the Liquid Aluminum Bath
The titanium level in the melt was adjusted by adding a master alloy containing 6 wt pct Ti. The master alloy had been previously prepared in the laboratory by dissolving 6 wt pct pure titanium powder in pure molten aluminum. The required mass of master alloy was added to molten aluminum at 1053 K (780 °C) prior to stirring and reducing the temperature to 1013 K (740 °C).
In a first series of experiments, boron filaments were immersed into pure aluminum and Al-Ti melts for different periods of time varying between 5 and 15 minutes. In order to provide Al-Ti melts, titanium was added to the melt 30 minutes before introducing the boron filament. The crucible was then taken out of the furnace and allowed to cool rapidly. Duration of solidification was about one minute. The solidified samples were then cut and polished and the cross section of the filament was characterized using optical and scanning electron microscopy in order to reveal the boron-aluminum interface.
In a second series of experiments, two specimens were immersed into the melt for 30 minutes and 6 hours respectively and then rapidly pulled out of the melt in order to allow rapid quenching directly from the melt temperature. This second series of test was performed to eliminate any cooling artifact at the interface and to reveal the interface state as it was in the melt. The specimens were thinned using focused ion beam (FIB) and their interface was examined using a transmission electron microscope (TEM).
3 Results and Discussion
3.1 Stability of Elemental Boron in Pure Al and Al-Ti Melts
3.2 Boron/Aluminum Interface
Titanium and boron concentrations overlap over a short distance of about 50 nm and could indicate the existence of an (Al,Ti)B2 phase. However, the presence of two distinct phases of TiB2 and AlB2 with a relatively sharp interface indicates that, at this temperature, these two phases do not form a continuous solid solution. Similar conclusions had been reported by other authors.[20,21] The Ti and boron overlap is therefore more probably an artifact due to the low spatial resolution of the EDX probe and the fact that the interface is barely parallel to the electron trajectory.
It is clear that Al diffuses through the TiB2 and oxide layers and thickens the AlB2 zone behind the initial interface. No evidence of Al gradient within TiB2 crystals was observed in our experiment. Since TiB2 and AlB2 are not soluble in each other, the diffusion of Al through TiB2 crystals is hardly conceivable. It is therefore logical to suppose that the diffusion of Al takes place through the TiB2 grain boundaries. It can also be seen that the thickness of AlB2 is not homogeneous over the examined areas. This is thus probably due to the TiB2 grain boundary defects in some areas favoring the diffusion of aluminum (interface on the right side of Figure 7). In the same way, boron can diffuse through AlB2 and at TiB2 grain boundaries to reach the solid/liquid interface and react with titanium solute to thicken the TiB2 layer. Actually the relative position of oxygen peak confirms this hypothesis since TiB2 crystals were detected only in front of this peak.
Although these experiments revealed the mechanisms by which boron is protected in aluminum melt containing titanium, they do not clarify the grain refining mechanism of elemental boron particles as reported earlier. The Al/TiB2 interface was examined using TEM to obtain more information about the solidification of aluminum over the TiB2 layer formed in situ. The area encircled in Figure 7 was further examined at higher magnification where a crystal of Al was found at the vicinity of a TiB2 crystal. The inter-planar spacing of TiB2 (001) and Al (111) planes were measured to be 0.262 and 0.230 nm, respectively. The angle between these two planes was measured to be 14.9 deg and the interface was found to be clear and free from any transitional phase. However no evidence of coherency between aluminum and TiB2 was observed. Examination of other TiB2/Al interfaces did not reveal any coherent interface.
3.3 Possible Refining Mechanism
As mentioned earlier, the objective of this work was to understand how elemental boron particles act as an efficient grain refiner for aluminum alloys as reported by Alamdari et al. At the light of this experiment, the refining mechanism of elemental boron seems to be close to that of Al-Ti-B master alloys and it can be attributed to the nucleation of Al directly on TiB2 particles.
Grain refining mechanism of Al-Ti-B master alloys has been ardently disputed in the literature for more than 20 years. The microstructure of these master alloys consists basically of two phases within an aluminum matrix; TiB2 and TiAl3. The refining mechanism is therefore attributed to the nucleation of aluminum on either one of these phases. Two major hypotheses have been put forward and supported by some experimental observations to explain the refining phenomenon. The first one considers that aluminum can directly nucleate on certain crystallographic planes of a TiB2 crystal while the second one only considers the nucleation of aluminum on TiAl3.
Mohanthy et al.[12,22] showed that when Ti is absent, synthetic TiB2 particles added to molten aluminum are pushed at the grain boundaries rather than being present within grains. Kaneko et al. also reported that TiB2 alone is not an efficient grain refiner.
Easton and StJohn showed that TiB2 are effective nucleants while the excess Ti solute restricts grain growth providing constitutional undercooling. Powerful refining effect of a master alloy, where both TiB2 and Ti are present, led to development of the duplex nucleation theory. Mohanty and Gruzleski detected a thin Ti-rich layer around the synthetic TiB2 particles using electron probe micro analysis technique. The surrounding layer had a chemical composition close to TiAl3. A crystallographic approach has also been used to explain the grain refining mechanisms. Detection of an epitaxial layer, with lattice spacing consistent with TiAl3, on TiB2 particles led Schumacher et al. to conclude that such a layer could be responsible for heterogeneous nucleation of aluminum. The refining effect was thus attributed to the formation of a TiAl3 layer on TiB2 particles.
Low lattice disregistry between aluminum and TiAl3 compared to that between aluminum and TiB2 suggests a superior nucleation potential of the former but it does not exclude the refining potential of the latter. The interface between Al and TiB2 was investigated by several authors but a consensus has not been yet reached regarding the existence of an orientation relationship between these two phases. An orientation relationship between (111) Al and (0001) TiB2 planes was found in a melt-spun Al-5Ti-1B master alloy. Mitra et al. and Kostka et al. reported semi-coherent interface between TiB2 and Al where no reaction layer or segregation were found. Although these data show the good potential of certain planes of TiB2 for nucleation of aluminum, the experimental conditions prevailing in these works are far from those of grain refining process. It is therefore difficult to extrapolate directly these observations to explain the grain refining process in aluminum alloys.
The interface between Al and TiB2 crystals examined in the present work simulates very closely the real grain refining conditions of boron particles reported earlier. A clean interface without any consistent crystallographic orientation between TiB2 and Al matrix was revealed. This observation suggests that the presence of a TiAl3 intermediate layer is not essential for nucleation; hence Al nucleates directly from the TiB2 crystals formed in situ on boron particles in the melt. Consequently, in grain refining experiments with elemental boron particles, it is speculated that each boron particle acts as a substrate for nucleation of several TiB2 crystallites as shown in Figures 5 and 7. These crystallites with different crystallographic orientations increase the probability of exposing a suitable TiB2 facet, having minimum interface energy with α-Al, and result in higher refining efficiency of a single boron particle. It is probably the reason why elemental boron shows a higher efficiency than the conventional grain refiner with equivalent Ti and B addition levels. Another important role of TiB2 crystallites is to protect boron particles and prevent their dissolution in aluminum melt.
Boron-based refiner may offer other considerable advantages compared to the conventional ones. For grain refining of recycled aluminum already containing Ti, no further addition of Ti is required. In addition, no potassium fluoride salts are required in the manufacturing process, overcoming the agglomeration of TiB2 particles associated with these salts. The boron particle size can be controlled during manufacturing eliminating the problems encountered with large TiB2 particles. A TiB2-shielded boron particle has a lower density than a bulk TiB2 particle and settle less rapidly in liquid aluminum than bulky TiB2 does.
In this paper, the grain refining mechanism of boron particles was studied by observing the behavior of boron filaments immersed in Al-Ti melts in different conditions. Boron filaments were immersed into liquid aluminum having different concentrations of titanium (between 0 and 500 ppm) and the microstructure and composition at the B/Al interface were investigated. Boron filament was used to facilitate the observation of the Al-B interface.
The results showed that boron dissolves rapidly in pure aluminum. However, a concentration of about 100 ppm of Ti inhibits its dissolution. When boron comes in contact with titanium containing melts a thin layer of TiB2 crystallites is formed at the surface of boron and protects it from dissolution. The boron surface is therefore covered with very fine TiB2 crystals having different crystallographic orientations. The TiB2 layer thickens as the boron residential time in the melt increases. Aluminum on the other hand, diffuses through the TiB2 layer and reacts with boron to form AlB2 behind the TiB2 layer.
During solidification, aluminum crystallizes on this in situ formed TiB2 layer. The interface between TiB2 and aluminum was found to be clean and free from TiAl3 or any transitional phase. A disregistry of 15 deg was revealed between (111) Al plane and (100) TiB2 plane. These observations suggest that the grain refining effect of boron is most likely due to the presence of this TiB2 layer.
When a boron-based grain refiner is added to an aluminum melt, boron particles are released into the melt and a large number of TiB2 crystallites with different crystallographic orientations are formed in situ on the surface of these boron particles. These diverse orientations increase the probability of exposing a suitable TiB2 plane, (0001) planes for instance, having minimum interface energy with α-Al and result in nucleation of aluminum. This may explain why elemental boron show a higher efficiency than the conventional grain refiner with equivalent Ti and B addition levels as reported in the literature.
The financial support of this project was provided by the “Natural Sciences and Engineering Research Council of Canada (NSERC)” and “Fonds de Recherche du Québec - Nature et Technologies (FRQ-NT)” via the Aluminum Research Centre—REGAL. The authors wish to acknowledge the contribution of Specialty Materials Inc., for providing the boron filaments, of Mr. H. Heidari for his contribution in some experiments and of Dr. Z. Zhang, Université du Québec à Chicoutimi, for TEM analysis.
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