Microstructure and mechanical properties of extruded TiB2/2024 aluminum matrix composites

TiB2/2024 aluminum matrix composites were prepared in situ from the Al–K2TiF6–KBF4 reaction system; then, we investigated the microstructure and mechanical properties of the composites in the as-cast and extruded states. X-ray diffraction (XRD) and scanning electronic microscope (SEM) analyses showed that TiB2 particles were successfully produced in the matrix by the in situ reactions. The optimal content of TiB2 particles in the composites was 3 wt%; moreover, the size of α-Al grains in the microstructure of the composites with 3 wt% content was the smallest among the composites reinforced with different content of TiB2 particles, and the TiB2 particles showed a uniform distribution. The tensile strength and elongation of the composites (246 MPa and 9.8%, respectively) were 21.8% and 18.1% higher compared with those of the alloy matrix. When the TiB2 particle content was 5 wt%, the cast composite exhibited the highest hardness of 113 HBW, which was 43.0% higher than that of the base alloy. Fracture analysis showed that the fracture mode changed from ductile to brittle as the mass fraction of TiB2 in the composite increased. The mechanical properties of the 3 wt% TiB2/2024 aluminum matrix composite were significantly improved after hot extrusion, with tensile strength and elongation values of 375 MPa and 19.7%, respectively, which were 52.4% and 101% higher than those of the as-cast material.


Introduction
Aluminum matrix composites are extensively applied in high-tech fields such as aerospace and automotive assembly because of their low density, high specific strength, good thermal conductivity, and easy designability [1][2][3]. 2024 aluminum alloy is a high-demand Al-Cu series alloy, noted for its excellent strength, plasticity and weldability, making it a suitable material for the fabrication of aerospace structural parts. Ceramic particles such as SiC, Al 2 O 3 , B 4 C, TiB 2, and TiC are commonly used as reinforcements in aluminum matrix composites [4][5][6][7][8]. Among them, TiB 2 particles attract great interest owing to their excellent thermal stability, high specific strength and stiffness, as well as inertness with respect to the aluminum matrix [9]. The addition methods of TiB 2 particles can be divided into two categories: in situ and ex situ methods [10]. Compared with those obtained by the ex situ method, TiB 2 particles prepared by the in situ approach exhibit better wettability with the base alloy, more uniform dispersion, and finer size (up to the micron level), which ensures the effective reinforcement of the base alloy [11,12].
Secondary processing of aluminum matrix composites through extrusion, forging, and rolling is a very effective way to improve the uniformity of the distribution of the reinforcing phase particles, eliminate casting defects, and improve the microstructure [13]. Soltani et al studied the effect of the varying extrusion ratio (7.4, 14.1 and 25) on the wear resistance of Al-15 wt% Mg 2 Si composites and found that, as the extrusion ratio increased, the Mg 2 Si particles in the composites became more uniformly distributed and more tightly bound to the matrix, with a gradual increase in the wear resistance of the composites [14]. Gajakosh  Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. reduction was achieved in four passes; after hot rolling, the reinforcing phase particles were distributed along the rolling direction and the microhardness, strength, and ductility of the composites were significantly increased [15]. Zeng et al investigated the effect of multidirectional forging with a 90°changing direction loading pass by pass was performed at 720 K and 12.5 mm s −1 on the structural organization and properties of Al6061/ZrB 2 composites; after multidirectional forging, the ZrB 2 reinforcing particles were better distributed in the composites and the matrix alloy grains were refined to smaller sizes. The tensile strength of the composites increased by approximately 22% [16].
Reports on the microstructure and strengthening effects of 2024 aluminum matrix composites with varying TiB 2 contents, as well as the organization and properties of the composites before and after hot extrusion, are relatively scarce. In this work, TiB 2 /2024 aluminum matrix composites were prepared by the in situ method and the optimum added amount of reinforcing particles was determined. Then, the optimized TiB 2 /2024 aluminum matrix composites were processed by hot extrusion, and the effects of the latter treatment on the microstructure and mechanical properties of the composites were investigated.

Materials and methods
The base alloy used in the present experiments was 2024 aluminum alloy, and its composition is shown in table 1. The required amounts of KBF 4 and K 2 TiF 6 were calculated and weighed according to the Ti:B molar ratio of 1:2, followed by thorough mixing and drying at 300°C to remove water of crystallization. A 2024 aluminum ingot was melted in a graphite crucible heated to 850°C. The salt mixture was added to the aluminum solution in batches and stirred intermittently for 60 min using a mechanical stirrer to ensure the complete reaction between the salts and the aluminum solution. When the melt temperature was 730°C, nitrogen gas was passed through the melt for refining and degassing it, and finally poured into a preheated mold. The 2024 aluminum alloy and its composites were machined into extruded billets of Φ 46 mm × 32 mm. The billets were homogenized and annealed at 420°C/6 h + 510°C/24 h and subsequently extruded into bars by a hot extrusion process with extrusion temperature, strain rate, and extrusion ratio of 480°C, 0.001 s −1 , and 9:1, respectively.
Phase characterization and elemental analysis of the composites were performed by x-ray diffraction (XRD, PANalytical X-Pert PRO MPD) with Cu Kα radiation in the 2θ range of 5°-90°at a rate of 2°min −1 and energy dispersive spectroscopy (EDS). The microstructure and fracture morphology of the composites were inspected using a metallographic microscope (Zeiss Axio Lab A1) and a scanning electron microscope (ZEISS Sigma 300). The hardness testing was performed according to GB/T231.1-2018. The hardness of the composites was evaluated using a Brinell hardness tester (HBS-62.5) with a ball of 2.5 mm diameter at a load of 62.5kg for 30s, and the average hardness of each composite was obtained after five tests. The tensile testing was performed according to GB/T228.1-2021. The tensile properties of the composites were determined by an electronic universal material testing machine (SHIMADZU AG-Xplus 100KN) at the tensile speed of 1 mm/min. The specimen size is shown in figure 1. Three specimens for each composite were measured to reduce the experimental errors.

Results and discussion
3.1. Microstructure of as-cast composites Figure 2 shows the XRD pattern of the 5 wt% TiB 2 /2024 composite. The main phases present in the spectrum were α-Al, TiB 2 , and Al 2 Cu; no diffraction peak corresponding to the intermediate phase Al 3 Ti was observed, which indicates that the Al-K 2 TiF 6 -KBF 4 reaction system successfully produced the reinforcing TiB 2 phase. The basic reactions are shown below [17]: 3Al AlB 2KAlF 2 Al Ti TiB 4Al 3 Table 2 shows the thermodynamic parameters of reactions. The value of ΔG for each reaction equation was calculated with a reaction temperature of 850°C. The negative values show that a melt reaction will occur. Figure 3 shows the optical microscopy (OM) images of TiB 2 /2024 aluminum matrix composites with different TiB 2 mass fractions. Figure 3(A) shows the presence of α-Al grains with dendritic morphology and non-uniform size in the as-cast 2024 aluminum alloy. Figures 3(B)-(D) show that, as the TiB 2 content in the composites increased, the α-Al grain morphology mainly consisted of long strips and polygons, while the size of the α-Al grains in the alloy exhibited a decreasing and then increasing trend. Figure 4 shows the average size of α-Al grains in the 2024 aluminum alloy and its composites. Compared with the 2024 aluminum alloy matrix, the average grain size of the composite decreased by approximately 5.9% when the TiB 2 content in the composite was 1 wt%; the finest grains were found when the TiB 2 content was 3 wt%, with the corresponding average grain size decreasing by ∼15.2%; the α-Al grain size increased when the TiB 2 particle content was raised from 3 to 5 wt%. The scanning electron microscopy (SEM) images of the 2024 aluminum matrix composites reinforced with different mass fractions of TiB 2 particles are shown in figure 5. Figures 5(A), (B) show that, in the composites with TiB 2 particle contents of 1 and 3 wt%, the TiB 2 particles exhibited a relatively uniform distribution with clear boundaries and no large agglomerates. As shown in figure 5 (C), when the mass fraction reached 5 wt%, agglomeration of TiB 2 particles was observed in the composite material. The reinforcing phase particles were mainly polygonal and rectangular in shape, and their sizes ranged between 100 and 2000 nm; the most uniform distribution of the reinforcing phase particles was achieved for a TiB 2 content of 3 wt%.
The above results show that the particles of the reinforcing TiB 2 phase were mainly distributed on grain boundaries in the composites, and played a role in pinning grain boundary, hindering the growth of α-Al, and when the TiB 2 content is appropriate, it can be utilized as the nucleus of heterogeneous shapes thus leading to finer grains. However, the excessive content of TiB 2 in the composites leads to more severe agglomeration of the reinforcing phase particles at the grain boundaries due to surface tension, which reduces the heterogeneous nucleation rate while significantly reducing the number of TiB 2 particles originally distributed in these regions, which in turn weakened the pinning effect of particles on the grain boundary migration and impaired the overall performance of the alloy. Figure 6 shows the tensile properties of 2024 aluminum matrix composites reinforced with different mass fractions of TiB 2 . Upon the addition of TiB 2 particles, the tensile and yield strengths of the composites were significantly improved. When the content of TiB 2 was 5 wt%, the tensile strength and yield strength of the  composites reached the highest values of 254 and 185 MPa, respectively, which were 25.7% and 29.4% higher compared with those of the alloy matrix. When the content of TiB 2 in the composites was increased from 0 to 1 wt%, the elongation changed from 8.3% to 10.9%, corresponding to an increase of approximately 31.3%; moreover, as the TiB 2 mass fraction increased to 3 and 5 wt%, the elongation of the composites decreased to 9.8% and 6.1%, respectively. The comparison of the tensile strength and elongation values shows that the composite exhibited the best overall mechanical properties when the TiB 2 content was 3 wt%, with tensile strength and elongation of 246 MPa and 9.8%, respectively, which were 21.8% and 18.1% higher compared to those of the matrix. Figure 7 shows the Brinell hardness of 2024 aluminum matrix composites reinforced with different mass fractions of TiB 2 . The Brinell hardness of the composites gradually increased with increasing TiB 2 mass fraction. In particular, the Brinell hardness of the 2024 matrix alloy was 79 HBW, and reached 113 HBW (43.0% increase) for a TiB 2 mass fraction of 5 wt%. The strengthening effect of the TiB 2 reinforcing particles on the alloy is mainly due to the synergy of fine grain and Orowan strengthening mechanisms [18]. The TiB 2 particles are uniformly distributed in the grain boundaries of the composite, and will hinder the grain growth during solidification. The fine TiB 2 particles can also play the role of heterogeneous nucleation sites and increase the number of grain particles, thus achieving grain refinement. According to the Hall-Petch relationship, grain refinement increases the yield strength of the material [19]. Grain refinement strengthening is generally evaluated by the well-known Hall-Patch strengthening equation [20]:

Mechanical properties of as-cast composites
where the constant k is related to the material equal to 0.12 MPa·m 1/2 , d and d 0 are the mean grain sizes of the composite and the Al2024 alloy, respectively. According to the Orowan strengthening theory, the movement of internal dislocations in the composite is hindered by the uniformly distributed TiB 2 particles, and the strength of the composite increases [21]. Based on the size distribution, most TiB 2 particulates are submicron-sized and effective in activating the Orowan strengthening. Therefore, the Orowan-Ashby equation is employed to calculate the strengthening effects as follows [22]:  When an optimum amount of TiB 2 particles is added in the composite, they will be uniformly distributed at the grain boundaries and hinder the grain growth. Moreover, due to the small size of TiB 2 particles and the narrow gaps between them, their hindering effect on the dislocation motion is more significant, which results in a significant improvement in the overall mechanical properties of the composite. A lower content of TiB 2 particles in the composite results in a weaker grain refinement effect. At the same time, the larger spacing between TiB 2 particles limits their hindering effect on the dislocation motion. When the TiB 2 content in the composites is too high, the TiB 2 particles will agglomerate at the grain boundaries, resulting in an uneven distribution of the reinforcing phase in these regions, which makes it difficult to effectively hinder the grain growth; hence, the grain size in the composite will increase again. In addition, as the agglomeration becomes more serious, the agglomerated particles tend to become sources of internal cracks during the tensile process. Figure 8 shows OM and SEM images of the 3 wt% TiB 2 /2024 aluminum matrix composite after extrusion. Figures 8(A), (B) show that the grains formed in the hot-extruded structure were elongated along the extrusion direction(ED), while the α-Al phase in the composite was mainly fibrous at this time. The reinforcing phase inside the composite was distributed in strips along the extrusion direction. The results of the quantitative EDS analysis of the two regions A and B in figure 8(B) are shown in table 3. Previous studies [23,24] pointed out that TiB 2 particles in extruded composites were mainly distributed in two regions; one region exhibited a strip-like structure consisting of S-and θ-phases together; the TiB 2 particles in this region were larger and could be easily detected as agglomerates. In the other region, the TiB 2 particles were uniformly distributed in the gaps of the strip-like structures, and were usually difficult to detect due to their smaller size and more diffuse distribution. Table 4 shows the mechanical properties of the 2024 aluminum alloy and its composites before and after extrusion. The tensile strength, elongation, and hardness of the extruded alloy (294 MPa, 28.8%, and 88 HBW, respectively) were 45.5%, 247%, and 11.4% higher than those of the as-cast alloy. The tensile strength, elongation, and hardness of the extruded 3 wt% TiB 2 /2024 aluminum matrix composite were 375 MPa, 19.7%, and 118 HBW, respectively; these values were 52.4%, 101%, and 13.5% higher than those of the as-cast 3 wt% TiB 2 /2024 composite.

Mechanical properties of extruded composites
There are two main reasons for the substantial improvement in the properties of the extruded alloy: on the one hand, hot extrusion eliminates internal casting defects in the material; on the other hand, during the extrusion process, the grains and agglomerated TiB 2 particles break under the action of three-way compressive stress, which further promotes grain refinement and facilitates the uniform distribution of the reinforcing phase particles, the presence of some high-angle grain boundaries endows the alloy with a greater capacity to regulate the plastic deformation of grain boundaries, thereby augmenting its plasticity by virtue of its capacity to absorb more energy during the fracture process; on a macroscopic scale, this leads to a substantial improvement in the mechanical properties [25]. However there are some reasons that can reduce the performance of the composite, such as the salt film coating the generated TiB 2 particles reduces the strengthening impact of TiB 2 and processing casting defects are diverse and can range from porosity, segregation, hot tears, and the generation of hydrides and oxides [26,27]. Figure 9 shows the fracture morphology of the as-cast 2024 aluminum alloy and its composites. As shown in figure 9(A), the fracture morphology was mainly dominated by dimples and tear ridges. The dimples were deeper, more numerous, and unevenly sized. The number of tear ridges was relatively small and their size was also relatively small. The overall fracture characteristics were typical of ductile fractures, and their macroscopic appearance was characterized by good plasticity and high elongation. Figure 9(B) reveals that when the TiB 2 content was 1 wt%, the size of the tear ridges and dimples in the fracture increased, but the size and distribution of the dimples was relatively uniform, and still reflected ductile fracture characteristics. Figure 9 (C) shows that increasing the content of reinforcing phase particles in the alloy to 3 wt% resulted in the gradual appearance of internal cleavage planes, with both brittle and ductile fracture characteristics existing at the same time, and the elongation of the composite decreased compared with that measured for a content of 1 wt%. As shown in figure 9(D), when the TiB 2 content reached 5 wt%, a large cleavage plane appeared in the fracture, showing typical brittle fracture characteristics, which were macroscopically reflected in the poor plasticity and low   elongation of the composite. The presence of TiB 2 clusters in the composite disrupts the continuity of the matrix alloy, making it susceptible to cracking at the interface with the reinforcing phase during tensile deformation; then, the cracks gradually spread and eventually lead to fracture as the tensile deformation continues [28]. Figure 10 shows the fracture morphology of the extruded 2024 aluminum alloy and its composites. As shown in figure 10(A), the dimples in the fracture of the extruded 2024 aluminum alloy adopted a fine honeycomb structure and extended deeper; the size of the tear ridges in the fracture was also smaller, which is typical of ductile fractures. The comparison with figures 10(A) and 10(B) shows that, upon addition of TiB 2 particles in the alloy, the size of the dimples in the fracture increased significantly and the their depth decreased; moreover, small bright-white TiB 2 particles were present at the bottom of the dimples, and the tear ridges in the fracture of the composite were coarser. In short, the TiB 2 particles generated by the in situ reaction had good wettability with the matrix alloy and exerted a certain pinning effect on the alloy grains; on a macroscopic scale, this translated into an increased tensile strength and a decreased plasticity.