A novel Al matrix composite reinforced by nano-AlNp network

In pursuit of lightweighting of automobiles and low emission of transportation, the efforts to develop high-strength, heat-resistant and fatigue-resistant Al alloys and/or composites have been ongoing. Here we report a novel Al matrix composite with ultrahigh strength reinforced by a three dimensional network of nano-AlN particles for the first time. The in-situ synthesized AlN particles are connected by twinning bonding chains and built up a three dimensional network strengthening Al matrix enormously like the skeleton to human body. The composite containing 16.4wt.% AlN particles shows excellent properties: the ultimate tensile strengths can be up to 518MPa at room temperature and 190MPa at 350 °C. This peculiar performance results from the novel spatial distribution of nano-scale AlN particles. Our findings in this work would help to develop a potential candidate for high-performance heat resistance light-metal based materials.

Scientific RepoRts | 6:34919 | DOI: 10.1038/srep34919 Electron diffraction result (Fig. 1e) of AlN p exhibits a typical diffraction pattern of [0001] zone axis, while electron energy loss spectroscopy (EELS) (Fig. 1f) proves the existence of N element in this particle. High-Resolution transmission electron microscope (HRTEM) image further proves that the in-situ synthesized AlN p embedded in the matrix with a clean and close AlN p /Al interface with atom bonding. This can be attributed to the method we adopted, which avoid the oxidation and hydrolysis of AlN p . The well-defined interface between AlN p and Al matrix can effectively transfer the mechanical load from matrix to ceramic particles. Due to the super thermal stability of AlN p , the interfaces with Al matrix have no pernicious reaction even at high temperatures. Moreover, the nanometric AlN p tends to provide superior properties than the bulk ceramic 32 .

Properties of the in-situ synthesized AlN p reinforced Al matrix composites.
To determine the strengthening effect of AlN p , the mechanical properties of the composites from RT to elevated temperatures have been tested.
The mechanical properties of three samples at RT are presented in Fig. 2a, which displays that the UTS and hardness of the AlN p /Al composites increased rapidly with higher AlN p contents. The tensile strength and hardness of 16.4 wt.% AlN p /Al (all compositions quoted in this work are in wt.% unless otherwise stated) are up to 518MPa and 124 HBW, respectively, 6 times higher than those of pure Al. Besides, the 16.4% AlN p /Al composite yields at 460MPa, while the sample without nanoparticles does only at 42MPa. Moreover, the elongation of 16.4% AlN p /Al can be kept at 9.5%.
In order to meet the requirement for heat resistance materials, the properties of the composites at high temperatures were also investigated, as shown in Fig. 2b-d. It is found that the UTS of samples are markedly elevated by in-situ synthesized AlN p . At 350 °C, the UTS of composites are all above 110MPa. With increasing AlN p amount, UTS of 16.4% AlN p /Al can even achieve as high as 190MPa (Fig. 2b). The influence of AlN p on thermal expansion behavior of the composites has been demonstrated in Fig. 2d. Due to the high strength and low linear expansion factor of AlN p at temperatures ranging from RT to 500 °C, the expansion coefficient of the aluminum matrix is limited, thereby the expansion behavior of the composites has been restricted by the higher dimensional stability. The linear expansion coefficient of the 16.4% AlN p /Al composite is 19.5 × 10 −6 K −1 at 350 °C. Under the same testing condition, the value of Al is 25.6 × 10 −6 K −1 , which is 31% higher than that of 16.4% AlN p /Al composite.
As shown above, the fabricated Al-16.4% AlN p composite possesses excellent properties especially at high temperatures. The value is as high as 171MPa at 350 °C, much higher than the common heat resistant Al-Si-Cu alloy 9 . This kind of material has paved a possible way to improve the high temperature mechanical properties.
Based on the experimental data we got, there are mainly two reasons for the fantastic performance of the AlN p /Al composite at high temperatures: one is the high thermal stability of AlN p (at high temperature it can also perform as nano scale hard ceramic); the other one is related to the spatial distribution of AlN p throughout the Al matrix.
To study the formation of nano scale AlN p , the reaction mechanism in 16.4% AlN p /Al composite system has been investigated. At early stage, the initial interfacial reaction is described as follows 32 : Differential scanning calorimetry (DSC) analysis of the AlN p /Al composite system was conducted and the results show that there is an exothermal reaction between Al and BN starting at 580 °C in the heating curve as shown in Fig. 3, which is correspond to the formation of AlN p . That is to say, AlN p is generated through solid-solid reaction. Considering the low solid solubility and the slow diffusion rate of N atoms in solid Al, AlN p tends to be small and forming near the raw material BN in this circumstance. Then, at around 660 °C, there is an endothermic peak for Al melting with a more evident peak. As temperature continues to increase, the nano scale particles become bigger and cling to each other forming a closed and distorted circle in spatial (Fig. 3b). These spatial circles form a network structure throughout Al matrix.
At 800 °C, the boron atoms dissolve into aluminum to make up about 2.2% of the aluminum, as shown in Al-B phase diagram. When below 1030 °C, the remaining boron atoms form AlB 2 following the expression 32 : 2 When cooled down, there is only the exothermal peak of Al solidification, proving that the reaction products are thermally stable during this process. The DSC is in accordance with the reaction mechanism described above.
In-situ synthesized AlN p and AlB 2 fabricated in the matrix are thermodynamically stable, avoiding the wettability or aggregation problem as well as the reaction with H 2 O and O 2 . Thus, the interfaces of AlN p are clean and connected through atomic bonding (Fig. 4b-e).
The typical morphology of the nano scale AlN p with orientation of [0001] zone axis is hexagonal flake (Fig. 1e). The reason why the morphology shown in SEM images (Fig. 1a,b) is irregular is deduced that there is a certain conjunction among the nano scale AlN p , as proved in Fig. 4a. In order to find out the conjunction mode between AlN p , further analysis has been done. Figure 4b shows an exemplary conjunction in the AlN p /Al composite, which reveals that the two regions (section 1 and 2), separated by the boundary, characterizes a twin relationship. The electron diffraction shown in Fig. 4c is acquired from a selected area of section 2 marked by dotted line. The dominating diffraction spots with the incident beam parallel to the [1210] direction can be separated into two groups, which are rotated 63.26° to each other along the [1210] direction (Fig. 4b). The two regions, separated by the twin boundary, are related to each other by 180° rotation along the (1013) plane. Besides, for the P6 3 mc structure of AlN, the {000l} and hl {hh2 } reflections with l = odd are forbidden 33 . The appearance of the {000l} forbidden reflection with l = odd in the [1210] pattern can be attributed to double diffraction. For example, the combination of (1010) and (1011) can give rise to a (0001) reflection when the incident beam is parallel to the [1210] zone axis. Thus the twin boundary is a (1013) plane. Due to the twinning interlink, large amount of AlN p are connected to each other by atomic bonding. This circumstance is common in the conjunction in AlN p /Al composite fabricated in this work. Thus far, the conjunction of AlN p in nano chains is in atomic bonding and most of them are twinning.
As demonstrated in previous works 7,9,11 , the mechanical properties for particle reinforced composites will shift from brittle to ductile when temperatures are above 300 °C, leading to an increase of elongation. There is an anomalous phenomenon in this work: the elongation decreased with elevated temperatures. As can be seen from Fig. 2, the elongation of 16.4% AlN p /Al is 8% at RT and 3% at 350 °C. Based on the above solid-solid reaction mechanism and the outstanding mechanical properties at elevated temperatures, the spatial distribution of AlN p has also been investigated. Through the observation of the fracture surface of the 16.4% AlN p /Al composite, the nano chains of AlN p seem to be connected with each other forming a spatial structure throughout the Al matrix. However, SEM images are hard to reveal this clearly because of the nano scale of AlN p chains. In order to get a better understanding of the spatial structure of AlN p in the Al matrix, large amount of experiments were done. The AlN p on the fracture surface in Fig. 4f,g are consistent with the irregular morphology in Fig. 1. Thus a schematic representation of the 3D network of AlN p has been proposed by integrating SEM, fracture surface and HRTEM  (Fig. 4h). The color changes from blue to red means the structure remains stable from RT to high temperature. In general, they are connected to each other forming chains and build up a network of AlN p in 3D direction, which support the Al matrix like the skeleton to human body.

Discussion
In summary, the 3D network throughout the composites makes the soft Al matrix surrounded and strengthened by the in-situ synthesized hard AlN p framework, which is consistent with the H-S upper bounds. The network structure acts as the hard armour for the soft Al matrix in the AlN p /Al composites, hindering the propagation of cracks. Also, there is a synergistic strengthening effect -reinforcement by the in-situ nano AlN p and reinforcement by the 3D network structure of AlN p . The well interfacial bonding in AlN p /Al composite helps to transfer the stress homogeneously and avoid stress concentration. Such kind of framework shows good resistance to slip, as the stress required to push the dislocations through the particles barriers is high. The composite begins to yield when the stress is sufficient for the network barriers.
On the one hand, the nano chains of the network help to refine the aluminum grains while the soft aluminum around the hard AlN p network can improve the ductility. All of these aspects lead to a high performance of the composite during a wide range of temperatures.
The 3D AlN p network-reinforced Al matrix composites have a promising future. When the external stress is applied to the composite, the AlN p network can effectively release the stress and powerfully impede the movement of the dislocations. In order to have a better understanding of the strengthening behavior of the 3D AlN p network, the fracture characteristics of the AlN p /Al composite has been investigated. Due to the differences in load bearing temperature circumstance, the fracture characteristics showed distinct differences. The following discussion would focus on the typical temperatures of RT and 350 °C.
Because of the combination of soft aluminum matrix and the in-situ fabricated hard 3D AlN p network at RT, the composites can exhibit high strength while in the meantime acquiring certain ductility. The twin-bonded network can effectively pin dislocation motions. When the composite is under tensile state, the dislocations would aggregate at the interfaces between AlN p network and Al matrix. The atomic bonding interfaces can effectively transfer the stress to AlN p . Therefore, the Orowan stress of AlN p can bear such great stress without initiating crack. While, the premature cracks could occur in the AlB 2 interlayer (Fig. 5a), which introduce defects to the composites and further lead stress concentration to the tip of the crack. Followed by crack accumulation and the subsequent linkage 34 in the matrix (Fig. 5b), yet the network of AlN p could effectively change the spread direction of the cracks and hinder the spread of the cracks to some degree. As mentioned above, AlB 2 particles are homogenously distributed throughout the matrix without aggregation, thus it would not provide an adverse path for cracking. Finally the premature crack would lead to fracture of the materials. As a result, the fracture microstructure shows numerous AlB 2 -terraces without appearance of AlN p . The higher content of the reinforcements, the more fractured AlB 2 -terraces would appear (Fig. 5c,d). The AlB 2 -terraces also indicate that the interfacial atomic bonding between Al matrix and the secondary phases is strong enough to overcome the stress concentration. Because of the fracture mode, the spread rate of the crack is slow, which is good for higher reliability of the material.
When the tensile test is performed at 350 °C, Al matrix becomes softer and has little resistance for the dislocation slip. While, the hard AlN p can effectively hinder the movement of dislocations and the hard 3D network can strengthen Al matrix like the skeleton to human body. When the external stress is imposed on the AlN p /Al composite, Al matrix and AlN p network performs differently. The soft Al matrix will have plastic deformation to offset the external stress. The network structure remains stable for hard strength and thermal stability of AlN p and can effectively hinder the dislocation movement. With the increased stress, the soft aluminum will produce much more plastic deformation. While at the meantime, the network structure still remains no change. The discordant speed of deformation lead to the void initiated at the interface between AlN p and Al matrix 35 along the tensile direction as shown in Fig. 6a. Point A and B in Fig. 6b are the experiment results showing the initiated voids corresponded to the discussion before. It wouldn't cause crack immediately for the high strength network of AlN p and the ductility of aluminum matrix. Then with the increased time and stress, the 3D network of AlN p suffers nearly all of the external stress along with the propagation and aggregation of the voids along the chain like point C shows in Fig. 6b. The amount of the void-zone continues to increase, and finally it will lead to fracture of the material. Figure 6c is the cross-sectional observation of the 16.4% AlN p /Al fracture at 350 °C, the void is more composites shows the crack mode is particle crack. The exposed AlB 2 -terraces is the evidence.
concentrated and close to the fracture. The voids are generated along the interface of AlN p , so there are plenty of AlN p exposed. Besides, there are some deep black holes in the fracture (Fig. 6d), which correspond to the void zone as shown in Fig. 6a-c. In conclusion, a 3D network of nano scale AlN p has been successfully in-situ built by a liquid-solid reaction method in the composites, leading to the observed increase in strength, especially at high temperatures. The ultimate tensile strengths of 16.4% AlN p /Al can be up to 518MPa at RT and 190MPa at 350 °C. The novel composites fabricated in this work may contribute to designing high-performance heat resistance materials for advanced structural applications.

Methods
AlN p /Al composites fabrication. The raw materials used in this work contain commercial Al powders (99.7%), hexagonal Boron Nitride powders (98.5%) and active carbon powders (99.0%). The mixture of powders was consolidated under Argon gas by liquid-solid reaction, and then the obtained ingot was extruded at about 500 °C with an extrusion ratio of 20:1, according to the CN105385902A patent. For convenience, the AlN p reinforced Al matrix composites are defined as 4.1% AlN p /Al, 16.4% AlN p /Al with different fractions of reinforcement particles in this work. The raw powders used for the large amount experiment was pretreamented.
Phase identification and microstructural characterization. X-ray diffraction (XRD, Rigaku D/max-rB) was used to identify the phases contained in the AlN p /Al composites. Phases identification and microstructures characteristic of the AlN p /Al composites were performed utilizing field emission scanning electron microscope (FESEM, model SU-70, Japan) equipped with an energy dispersive spectroscopy (EDS) detector and High-Resolution transmission electron microscope (HRTEM, ZEISS LIBRA200) assembled with electron energy loss spectroscopy (EELS). Thermal stability of the composites was investigated by means NETZSCH DSC 404C and NETZSCH DIL 402 C high temperature dilatometer at a heating rate of 10 K/min.
Mechanical property testing. The hardness was measured on a HB-3000C Brinell hardenss tester with parameters of HB5/250/15. Each value was an average of at least four separate measurements taken at random places on the surface of specimens. Tensile testing was conducted on the extruded composites after T2 heat treatment (250 °C, 3h) at temperatures of RT, 200 °C, 300 °C, 350 °C and 400 °C. These tests were conducted by assuring the specimens to stabilize at temperatures for about 30minutes prior to test using an extension rate of 2 mm/min, and the matrix alloy were also measured for comparison. In each case, the average data was acquired from at least four specimens. The large amount experiment was conducted on the AlN p /Al composite with pretreatment. The testing temperature was at room temperature.