1 Introduction

It is well known that aluminum matrix composites (AMCs) are considerably prominent due to their potential to offer desirable properties, including low density, high specific strength, high specific stiffness, excellent wear resistance, and controllable expansion coefficient. These advantages make AMCs receive great attention for numerous applications in aerospace, automobile, and military industries [1, 2]. In recent decades, particles reinforced aluminum matrix composites (PRAMCs) have become more and more significant in the research field of AMCs [3, 4]. Currently, there are several methods for fabricating PRAMCs, which can be classified into three categories: solid-phase process, liquid-phase process, and semi-solid fabrication process. Among them, liquid processes, such as stir casting and compocasting, have several advantages including high production rate, low cost, and the feasibility of producing complex parts [5]. But it is extremely challenging for stir casting to disperse submicron- and nano-sized particles uniformly in the melt due to their large surface-to-volume ratio and poor wettability.

Ultrasonic vibration is applied extensively in the treatment of metallic melt, such as degassing, refinement, and purifying [6–11]. In addition, ultrasonic vibration is used in the fabrication of particles reinforced metal matrix composites, for it can improve the wettability between reinforcements and matrix. However, the ordinary ultrasonic equipment mainly consists of the piezoelectric ceramic transducer, whose energy output is limited. Thus, the ultrasonic treatment does not perform as well as expected. There is a patented process termed flux-assisted synthesis (FAS) (also known as mixed-salt reaction or reaction cast) which was developed by the London Scandinavian Metallurgical Company (LSM) to produce in situ aluminum matrix composites [12]. Although some works [13–17] investigated the reactions between molten aluminum and Ti- and B-bearing potassium fluoride salts, K2TiF6 and KBF4, few studies were reported on the production of in situ TiB2/Al composites using K2TiF6 and KBF4 under high-intensity ultrasound produced by magnetostrictive transducer.

In this work, the conventional K2TiF6 and KBF4 were used as the reaction system. The objective of this paper is to study the morphologies and distribution of in situ TiB2 particles and TiAl3 phase in aluminum matrix via ultrasonic treatment produced by magnetostrictive transducer. Their impacts on mechanical properties of this versatile composite were also discussed.

2 Experimental

2.1 Materials and apparatus

In this research, raw materials used were refined aluminum ingot for re-melting (99.99 % purity) as the composite matrix, and two types of commercial inorganic salt, namely potassium hexafluorotitanate (K2TiF6, 99 % purity) and potassium tetrafluoroborate (KBF4, 98 % purity), were used to produce TiB2 reinforcement. In casting process, the atmospheric control electrical resistance furnace with the ultimate temperature of 1,200 °C was used. The ultrasonic processing system mainly consisted of ultrasonic probe made of TC4 titanium alloy and magnetostrictive transducer (4 kW power, 20 kHz frequency) made of Fe–Ga alloy.

2.2 Material preparation

The mixed salts of K2TiF6 and KBF4 with the stoichiometric atomic ratio in accordance with Ti/2B were dried at 200 °C to remove moisture and reduce the temperature difference subsequently. When the pure aluminum ingot was superheated to the given temperature (850 °C) and held for 10 min, the desired amount of the mixture of fluoride salts was added into the melt in batches and the liquid matrix was stirred for 10 min by the graphite agitator to facilitate the melting of the salts mixture. In order to protect materials against oxidation and other undesirable reactions during casting process, pure argon gas had to be charged in an appropriate amount into the furnace. After adequately stirring the melt, the melt was reheated to 800–820 °C and held for 5 min. Then, the ultrasonic probe was dipped into the melt from the upper surface and sonicated for 15 min to make the generated TiB2 particles agglomeration scatter, the particles distribute uniformly in the melt and the composite melt degas as well.

Specimens with different TiB2 contents of 0 wt%, 2.5 wt%, 5.0 wt%, and 7.5 wt% were melted in graphite crucible; after cooling down to 720–740 °C and holding for 10 min, the melt was poured into a preheated Reynolds standard golf tee mold [6] made of copper. In order to compare the properties and structures of the samples, the composite melts with the same components were prepared without ultrasonic treatment but in other identical conditions (including casting temperature, furnace, crucible types).

2.3 Microstructural and properties characterization

Optical microstructures of the resulting composites were examined under optical microscope (OM, Olympus BX60M) after being etched by Keller’s reagent (1.5 ml HCl, 2.5 ml HNO3, 1 ml HF, and 95 ml H2O). The morphology, size, and dispersion of in situ TiB2 particles in the matrix were observed by scanning electron microscope (SEM, LEO-1450) and field emission scanning electron microscope (FESEM, SUPRA™ 55) equipped with energy-dispersive spectroscopy (EDS) detector. X-ray diffraction (XRD) patterns were obtained on Rigaku DMAX-RB X-ray diffractometer, using monochromatic Cu Kα radiation with the wavelength of 0.154 nm, at 40 kV and 150 mA.

According to China’s National Standard (GB/T 228-2002), tensile tests were carried out at room temperature with the tensile rate of 0.5 mm·min−1 on a CMT-4105 testing machine. The tensile properties reported in this paper were the average values of three tensile samples.

3 Results and discussion

3.1 Grain microstructure

Figure 1 shows the grain size variation of as-cast 5.0 wt% TiB2/Al composites without and with ultrasonic treatment, respectively. The metallographic microstructure of as-cast TiB2/Al composites containing different TiB2 contents with ultrasonic treatment is shown in Fig. 2. The in situ TiB2 content level is 5.0 wt% and 7.5 wt%, respectively. It can be observed clearly that the primary α-Al phase is effectively refined due to high-intensity ultrasound. With the ultrasonic vibration, the composite presents a fine uniform microstructure, which is composed of fine grains with average size of 50 μm. Most of the TiB2 particles are found to be distributed along the grain boundary regions, and very minimal agglomerations of the particles are observed in grains. The results indicate that the average grain size of as-cast 7.5 wt% TiB2/Al composite is reduced to 25 μm or so, half of the size of 5.0 wt% TiB2/Al composite. It can be attributed to coupled effects of the TiB2 contents in the aluminum matrix and the ultrasonic vibration.

Fig. 1
figure 1

OM images of as-cast 5.0 wt% TiB2/Al composites prepared a without and b with ultrasonic treatment

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Fig. 2
figure 2

OM images of as-cast composites with different TiB2 contents prepared by ultrasonic method: a 5.0 wt% TiB2 and b 7.5 wt% TiB2

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3.2 Distribution and morphology of TiAl3 and TiB2

Microstructures of as-cast TiB2/Al composites with different TiB2 contents with and without ultrasonic treatment applied in the melting process are shown in Fig. 3. Unlike the coarse acicular or plate-like TiAl3 particles which were reported by many researchers [6, 13], the TiAl3 phase produced in the aluminum matrix in this experiment is in the shape of irregular gullies as shown in Fig. 3a, b, d. Meanwhile, TiAl3 is found in two different morphologies as shown in Fig. 4. Blocky and rounded particles are generated with the same time as TiB2. In addition, the second morphology is elongated. TiB2 particles form inside the banded structure of TiAl3 when the amount of the reactants (K2TiF6 and KBF4) increases.

Fig. 3
figure 3

SEM images of as-cast composites with different TiB2 contents: a 2.5 wt% TiB2, b 5.0 wt% TiB2, c 7.5 wt% TiB2 with ultrasonic treatment, and d 5.0 % TiB2 without ultrasonic treatment

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Fig. 4
figure 4

High-magnification SEM image of dotted circle region in Fig. 3a

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The results show that with the increase in TiB2 content, the TiAl3 particles gradually disappear under the ultrasound field. Emamy et al. [14] stated that the formation of TiB2 particles was expected during the following reaction:

$$ {\text{TiAl}}_{3} + 2{\text{B}} \to {\text{TiB}}_{2} + 3{\text{Al}} $$
(1)
$$ \Delta G^{0} = - 43.4 \times 10^{3} + 15.8\,T $$
(2)

where ΔG 0 is the Gibbs’ free energy and T is the reaction temperature. When TiB2 particles account for 7.5 wt%, the conversion efficiency of TiAl3 into TiB2 almost reaches up to 100 % from the comparison among Fig. 3a–c. This means that at the lower amount level of the reactants, K2TiF6 and KBF4, the reaction to form TiB2 is less complete and the intermediate TiAl3 exists in a larger quantity than required to form TiB2.

In order to identify the TiB2 crystals, FESEM, EDS, and XRD were employed in the study. Figure 5 shows the morphologies of the crystallites in the matrix. It can be seen that the size is 100–500 nm and the shape is regular polygon with clear profile, such as hexagonal platelet. It can be seen from the EDS analysis result that these particles contain both titanium and boron, which are pushed into the inter-dendritic region by the α-Al dendrites.

Fig. 5
figure 5

FESEM image of TiB2 particles from dotted circle region in Fig. 3c

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XRD analysis of the particles, which were extracted by a special extraction method using solution of 10 % HCl, indicates that the particles have the same crystal structure as TiB2 as shown in Fig. 6. So it is reasonable to confirm that these particles are TiB2 crystals. The isolated TiB2 particles do not cling to each other although they are still in the form of loose agglomeration. Therefore, both the mean size and size distribution of TiB2 particles in the aluminum matrix are significantly modified by applying ultrasonic vibration produced by the magnetostrictive transducer in the manufacture process.

Fig. 6
figure 6

XRD pattern of extracted TiB2 particles

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3.3 Tensile properties

Figure 7 shows the ultimate tensile strength (UTS), yield strength (YS), and elongation of as-cast pure aluminum and TiB2/Al composites under the ultrasound field. It can be clearly seen that the UTS and YS of the composites are simultaneously enhanced compared with those of as-cast pure aluminum with the same treatment. Furthermore, with 7.5 wt% TiB2, the UTS and YS of the composite are enhanced by 63 % and 75 %, respectively, compared with the aluminum matrix. To our knowledge, these results would be attributed to the coupled effects of the increase in grain boundary region due to the grain refinement and the obstruction of dislocation movement by TiB2 particles. The gliding dislocations have to overcome the barriers of TiB2 particles either by Orowan mechanism or by the processes of cross-slip or climb [18].

Fig. 7
figure 7

Tensile properties of as-cast pure aluminum and TiB2/Al composites with ultrasonic treatment

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However, compared with the ultrasonically cast pure aluminum, there is a gradually decreasing tendency of the elongation (a measure of ductility) of the composites with TiB2 content increasing. The elongation of the ultrasonically cast pure aluminum is about 47 % and, as anticipated, reduces to 13 % in the case of 7.5 wt% TiB2/Al composite. The results demonstrate that the dispersion of submicron-sized TiB2 in pure aluminum results in a significant increase in the tensile strength with the huge sacrifice of ductility. This may be the result of greater agglomeration of TiB2 particles and higher degree of micro-porosity present in the composite with higher TiB2 content. Besides, the increased TiB2 content would decrease the effective slip distance of dislocations during the deformation, which would lead to the decrease in the elongation [19].

For the subsequent studies, it is expected that if the optimal content of TiB2 particles is better understood and the process parameters are optimized, the dispersion and mechanical properties of the composites will be further improved.

4 Conclusion

In this study, the effects of high-intensity ultrasonic treatment on microstructural features and tensile properties of in situ TiB2/Al composites were investigated. The results show that the improvement in grain refining performance benefits from coupled effects of the increasing stoichiometric mass fraction of TiB2 particles and the ultrasonic vibration produced by the magnetostrictive transducer in the melt. Ultrasound applied in the process of manufacturing the TiB2/Al composites changes the morphology of TiAl3 phase into irregular gullies shape, accelerates the dissolution of coarse TiAl3 phase, and makes the distribution of TiB2 particles more uniform in the melt. It is demonstrated that the formation of TiB2 particles occurs via the transformation from TiAl3, and at the optimal amount of the reactants, K2TiF6 and KBF4, the conversion efficiency of TiAl3 into TiB2 almost reaches up to 100 %. Compared with the aluminum matrix, the UTS and YS of the TiB2/Al composites can be enhanced by 63 % and 75 %, respectively. The improvement is due to the uniform distribution of reinforcement and grain refinement of aluminum matrix.