6061Al composites prepared by spark plasma sintering (SPS)

Journal of Alloys and Compounds 763 (2018) 822e834

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Effects of pulse conditions on microstructure and mechanical properties of Si3N4/6061Al composites prepared by spark plasma sintering (SPS) Han Jiang a, Zhongguo Xu a, Ziyang Xiu a, b, **, Longtao Jiang a, b, *, Huasong Gou a, Chang Zhou a, Gaohui Wu a, b a b

School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, PR China State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 April 2018 Received in revised form 1 June 2018 Accepted 3 June 2018 Available online 5 June 2018

In this paper, 30% Si3N4/6061Al composites were prepared by SPS process to study the influence of different pulses on the microstructure and mechanical properties of the composites. The results showed that there was a sintered connection of the reinforcement Si3N4 in the composite. Moreover, with the decrease of the value of pulse condition ton: toff, the increased current would lead a rise of the local temperature in the composites, thus the connection of the Si3N4 particles became serious. At the same time, the interfacial reaction became serious, and the interface morphology changed from smooth to steep and then to serration. The results of mechanical properties analysis showed that different pulse conditions had little effect on the tensile strength of composites, but they had a greater effect on the elastic modulus and plasticity. When the pulse condition was ton: toff ¼ 2:1, the composite had the best mechanical properties and was superior to the performance given in many literature currently. This paper attempts to provide a certain experimental basis for the preparation of aluminum matrix composites by SPS. © 2018 Elsevier B.V. All rights reserved.

Keywords: Spark plasma sintering Pulse conditions Microstructure Mechanical properties Aluminum matrix composites

1. Introduction In addition to requiring higher specific stiffness, specific strength, thermal conductivity, and low density in specific structural parts of certain new aircrafts, the coefficient of thermal expansion (CET) that match the requirement (12.5e15  106  C1) and plasticity are also very important requirements. As can be designed to meet the required requirements by the type and content of reinforcements, particle reinforced aluminum matrix composites (PRAMC) are the most suitable materials for the preparation of such parts among numerous structural materials [1,2]. At present, composites satisfying the above-mentioned application requirements are mainly high volume fraction Al matrix composites, but excessively high volume fraction makes the composites brittle,

* Corresponding author. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, PR China. ** Corresponding author. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, PR China. E-mail addresses: [email protected] (Z. Xiu), [email protected] (L. Jiang). https://doi.org/10.1016/j.jallcom.2018.06.024 0925-8388/© 2018 Elsevier B.V. All rights reserved.

and the reliability is difficult to guarantee. Therefore, it is necessary to find other suitable reinforcements to meet the needs, especially the coefficient of thermal expansion and the plasticity. Due to its excellent properties such as high mechanical strength, good thermal and chemical stability, and low density, Si3N4 is an ideal reinforcement in particle reinforced aluminum matrix composites [3e5]. In particular, because it has a lower thermal expansion coefficient compared to other reinforcements of particle reinforced aluminum matrix composites, by adding smaller content of Si3N4, the composites can obtain the thermal expansion coefficient required for the application conditions, which will increase the plasticity and ensure the reliability of the composites. Therefore, Si3N4/Al composites have a wide range of application prospects in certain spacecrafts and other applications. Yang et al. [3] fabricated 36% Si3N4/2024Al and pointed out the thermal conductivity of annealed Si3N4p/2024 composite was 94.997 W/(m$K) at room temperature. Xiu et al. [4] fabricated Al matrix composites reinforced with 45% Si3N4 particles with average particle size of 1.5 mm and pointed out the tensile strength and elastic modulus of Si3N4p/2024Al composite was 360 MPa and 168 GPa, respectively.

H. Jiang et al. / Journal of Alloys and Compounds 763 (2018) 822e834

However, fracture of Si3N4p/2024Al composite was characterized by brittle fracture without elongation, which is unfavorable for the application of materials. Chen et al. [5] fabricated 45% Si3N4/2024Al composite and indicated that the CET of the composite was about 10.4  106  C1, which was consistent with the average value of Kener model and lower bound of Schapery model. Therefore, when Si3N4 is selected as a reinforcement, the composite can satisfy performance requirements by reducing volume fraction of Si3N4. The main manufacturing methods for particle reinforced aluminum matrix composites include stirring casting [6,7], squeeze casting [8,9], hot press sintering [10,11], hot isostatic pressing [12] and spark plasma sintering (SPS) [13e16]. Among these, SPS is a very attractive method because it can achieve rapid heating and cooling, as well as require a short holding time to make the material densified, which can prevent the growth of aluminum grains. In fact, some scholars have already adopted SPS technology to prepare PRAMC, and also have a certain researches on the mechanism of SPS process [16,17]. A more general understanding is that part of the heating power comes from the Joule heat generated by the die (which may account for a major part), and the other part comes from the Joule heating at the particle contact and the high energy spark plasma at the particle gap [18]. The plasma can destroy the oxide layer on the surface of the particles and promote the formation of sintered necks during the processing of the SPS [19]. These characteristics make SPS a more attractive alternative to traditional methods such as hot pressing, casting and extrusion. For application to aluminum matrix composites, several recent studies have shown that the SPS method allows many properties to be improved. Ehsan Ghasali et al. [20] prepared the bulk Al-SiC-TiC composites by SPS and conventional sintering. SEM investigations of SPS sintered samples showed the homogeneous distribution of reinforcement particles with neither voids nor cracks in the microstructure while conventional sintered samples were porous in some areas. The SPS compared to conventional sintering leads to good mechanical properties and fine microstructure at short sintering time. Kurita et al. [21] produced multi-walled carbon nanotube-reinforced aluminum matrix composites with an ultimate tensile strength that was 40% higher than that of pure aluminum with the conservation of remarkable fracture elongation of aluminum. Kiyoshi Mizuuchi et al. [22] prepared diamond particle dispersed aluminum matrix composites fabricated in solidliquid coexistent state by spark plasma SPS. Thermal conductivity of the composite containing 45.5 vol.% diamond reached 403 W/ mK, approximately 76% the theoretical thermal conductivity. Chen et al. [23] showed that the formation of an Al4C3 phase in CNTeAl composites prepared by SPS, even under a comparatively low sintering temperature of 500  C. What's more, high-performance CNTAl composites can be obtained by selecting suitable sintering conditions. In all, the generation of plasma will cause great changes in the microstructure and interfacial reaction between the materials prepared by SPS and traditional powder metallurgy, and then affect the performance of the composites. There are many factors in the preparation of composites by SPS, and the pulse conditions are one of the most critical influencing factors. As the value of the current ton: toff decreases, the intensity of the instantaneous current during sintering increases, which results in a rapid increase in the local temperature between powder particles, and even can reach a high temperature of thousands of degrees Celsius. This excessive temperature may adversely affect the performance of the composites. Gregory Lalet et al. [24] showed that the shorter ton: toff times allowed larger quantities of Al4C3 crystals to be formed at the Al/Cf interface, which was detrimental to composites' properties. Therefore, it is of great significance to study the influence of pulse conditions on the properties of Si3N4/Al composites prepared by SPS.

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In this paper, 30% Si3N4/6061Al composites prepared by ball milling and SPS method were present. The influence of different pulse conditions on the microstructure and mechanical properties of the composites was studied. The mechanism of the sintering of Si3N4 ceramics in composites was proposed. A composite with high strength and high elongation was prepared, whose performance was superior to many current literature. This paper attempts to provide a certain experimental basis for preparation of particle reinforced aluminum matrix composites by SPS. 2. Materials and methods 2.1. Materials Gas-atomized 6061Al alloy powders (Northeast Light Alloy Co.,Ltd. China) and Si3N4 particles (99.9% in purity, Fujian Schnorrer New Material Co., Ltd. China) were used as the matrix and reinforcement respectively. The chemical composition (wt.%) of 6061Al alloy is shown in Table 1. 6061Al particles exhibited near spherical morphology with an average diameter of 10 mm and Si3N4 particles shaped as hexagonal short rod, as shown in Fig. 1(a) and Fig. 1(b) respectively. As shown in Fig. 1(c), the particle size distribution of Si3N4 particles is between submicron and micron, most of which distributed between 2mm and 5mm. XRD result (Fig. 1(d)) shows that Si3N4 is b-phase. 2.2. Fabrication process 30% Si3N4/6061Al composite ingots (f100 mm  20 mm) used in this study were fabricated through ball milling and SPS sintering process. Meanwhile, the same volume fractions of composites were prepared using vacuum hot pressing (HP) for comparing certain phenomena that occurred in SPS. During the ball milling process, Si3N4 and 6061Al powders were mixed using a QM-3SP2 high energy ball milling machine. Ball milling was carried out in nitrogen at a rotating speed of 300 r/min for 120 min with ball-to-powder weight ratio of 10:1. Al2O3 balls, with diameters of 10 mm, 6 mm and 3 mm were used as grinding media. The SEM micrograph of mixed powder is shown in Fig. 2. It can be seen that the Si3N4 particles were distributed evenly in the 6061Al powder, and there is no obvious agglomeration and crushing phenomenon in the ball milling process. Subsequently, the mixed powders were placed in a graphite mold and prepressed in an atmospheric environment at a pressure of 5 MPa. The powder-filled mold was then placed in the SPS apparatus (FCT System Co. Ltd. (HPD 400)) and the initial pressure was also 5 MPa. When the equipment vacuum reached the required condition, it started to heat up. An infrared thermometer was used to measure the temperature of the upper pressure head of the mold and adjusted current and voltage for controlling heating and temperature during the entire SPS run. The sintering temperature was set to 570  C, and the specific temperature curve is shown in Fig. 3(a). It should be noted that Fig. 3(a) shows the actual temperature curve. As the infrared thermometer can not measure the temperature below 400  C, when the mold temperature is lower than 400  C, the actual displayed temperature value is always 400  C. As a result, in Fig. 3(a), the temperature value is maintained 400  C from 0 to 15 min. During the heating stage, the pressure first reached 20 MPa and was maintained. When the temperature rised

Table 1 The chemical composition (wt.%) of 6061Al alloy. Chemical elements

Cu

Mg

Mn

Fe

Si

Zn

Cr

Al

6061Al

0.22

1.01

0.15

0.097

0.58

0.026

0.11

Bal

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Fig. 1. The morphology of 6061Al powders and the morphology, size distribution and XRD analysis of Si3N4 used in this paper. (a) SEM image of 6061Al powders; (b) SEM image of Si3N4 powders; (c) Particle size distribution of Si3N4 powders; (d) XRD pattern of Si3N4 powders.

was performed at 175  C for 2.5 h in a conventional low temperature heating furnace. Fig. 3(a) also shows the displacement of the punch as a function of temperature and pressure during the preparation of the composites (blue dotted line). When the holding time is 15 min, the displacement curve has been flattened, which shows that the density of the material has reached a relatively high level, and no longer changed. The pulse conditions correspond to the time during the electrical pulses which were applied to the sample (ton) and the time when no current was sent (toff). The pulses used in this study were 10:1, 2:1, and 2:5; the first number corresponds to the time on and the second to the time off during the electrical pulses. The schematic of corresponding pulse form modulations is shown as Fig. 3(b). 2.3. Characterization and measurement details

Fig. 2. SEM image of mixture of 6061Al and Si3N4 powders after ball milling.

to 570  C, the pressure rapidly rised to 50 MPa in 2 min, and then the heat preservation and pressure maintaining stage started, which lasted for 15 min. In the case of HP process, the powder mixture was put into graphite die for prepressing at ambient temperature under 30 MPa. Then the sintering was heated to 570  C and vacuum hot-pressed by means of uniaxial pressing of 100 MPa for 3 h in the ZJ-300 high temperature vacuum infiltration furnace (Jinzhou Boda high temperature materials equipment manufacturing Co. Ltd.). The composite specimens were solution treated at 530  C in KNO3 salt-bath furnace for 1.5 h and were water quenched at room temperature, and then peak-aging treatment

The morphologies and microstructures of reinforcement particles and the composites were characterized by field emission scanning electron microscope (FE-SEM, Quanta 200FEG). Further microstructure observation of Si3N4/6061Al interface and Si3N4 morphology in the composites were performed on transmission electron microscopy (TEM, Talos F200X). The tensile properties were measured with Instron-5569 electronic tensile testing machine (Instron Co., USA) using the flat dog-bone shaped specimens. The speed of tensile tests was 0.5 mm/min. Samples with dimension of 3 mm  4 mm  36 mm were machined for elastic modulus. Elastic modulus was evaluated by an impulse excitation technique involving the analysis of the transient composite natural vibration on EMT-01 instrument (Zhuosheng Instrument Co., Ltd. China). The achieved densities of composites were tested using Archimedes' method. All the tests have been performed on at least five samples to improve the statistical significance of the results. The morphologies of fracture surface of Si3N4/6061Al composites was observed by emission scanning electron microscope (FE-SEM, Quanta

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Fig. 3. Actual temperature-time and displacement-time curves during preparation of SPS and the schematic of corresponding pulse form modulations. (a) Changing in the upper head displacement and in the temperature of the 30% Si3N4/6061Al composites as a function of sintering time during SPS; (b) the schematic of different pulse form modulations curves during SPS.

Table 2 The relative packing densities of 30% Si3N4/6061Al composites under different pulse conditions of SPS and hot pressing sintering.

Theoretical density (g/cm3) Actual density (g/cm3) Relative density (%)

ton: toff ¼ 10:1

ton: toff ¼ 2:1

ton: toff ¼ 2:5

Hot pressing sintering

2.87 2.82 98.3

2.87 2.85 99.3

2.87 2.83 98.6

2.87 2.71 94.4

200FEG).

3. Results and discussion 3.1. Reinforcement morphology and distribution The relative densities of 30% Si3N4/6061Al composites prepared by SPS in different pulse conditions and HP process were tested, and the results are shown in Table 2. For composites prepared by SPS process, it can be seen that when the pulse condition was ton: toff ¼ 2:1, the relative density of the composite was the highest, and when the pulse condition was ton: toff ¼ 10:1, the relative density of

the composite was the lowest. It was worthy of note that relative density of SPS samples were all above 98% without great difference, which was much higher than the composites prepared by HP method. Fig. 4 shows the form of the reinforcement distribution in the composites prepared by SPS in different pulse conditions. It can

Table 3 The degree of sintering of Si3N4 in the composites under different pulse conditions.

A1/A0 (%) A1/A2 (%)

ton: toff ¼ 10:1

ton: toff ¼ 2:1

ton: toff ¼ 2:5

8.0 26.6

14.6 48.6

22.9 76.3

Fig. 4. SEM images of the microstructure of 30% Si3N4/6061Al composites with different pulse condition. (a) (d) ton: toff ¼ 10:1; (b) (e) ton: toff ¼ 2:1; (c) (f) ton: toff ¼ 2:5.

Fig. 5. SEM images of the microstructure of 30% Si3N4/6061Al composites. (a) (c) and (d) made by hot pressing sintering; (b) (e) made by SPS in the condition of ton: toff ¼ 2:1.

Fig. 6. Comparison of residual Si3N4 particles after etching the composite and original Si3N4 particles. (a) (b) the morphology of raw Si3N4 materials; (c) (d) SEM images of the Si3N4 morphology after etching the composites.

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Fig. 7. TEM images of sintered Si3N4 particles in the composite. (a) (b) The morphology of sintered Si3N4 particles; (c) Energy spectrum analysis of yellow box in (b); (d) HAADF image of yellow box in (b), the red dash lines indicate boundaries between Si3N4 particles. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

be seen that when the value of ton: toff was 10:1, a small amount of Si3N4 particles have agglomerated, as indicated in yellow circles shown in Fig. 4(a). Under high magnification, these agglomerations are found the fact that the Si3N4 particles connect with each other, and at the same time, there are certain pores at the junction of the Si3N4 particles, which reduce the relative density of the composite, as indicated by red arrow in Fig. 4(d). When the value of ton: toff decrease to 2:1, the Si3N4 particles are sintered together to a greater degree, and the distribution is more uniform. Besides, no occurrence that a large number of particles are connected to form a sheet and no obvious holes can be observed in the composite, as shown in Fig. 4(b) and (e). When the value of ton: toff is further reduced to 2:5, a large number of Si3N4 particles are sintered together as indicated by yellow circles in Fig. 4(c). The size of the sintered Si3N4 particles even reach to 20 mm, which are much larger than the raw Si3N4 (Fig. 1(b)). A smaller amount holes are also found in the local area, as indicated by red arrow in Fig. 4(f). Image-Pro Plus software was used to quantitatively calculate the degree of sintering in composites under different pulse conditions. In this paper, the area ratio is used to characterize the probability that a certain Si3N4 particle may sinter together with surrounding particles. Let A0 be the total area, A1 be the area occupied by the sintered Si3N4 particles, and A2 be the area occupied by all Si3N4 particle. The volume fraction of Si3N4 particles sintered in the composites can be represented by A1/A0. Since Si3N4 particles account for 30% of the volume fraction of the composite, assuming Si3N4 particles are evenly distributed in the matrix 6061Al, then A1/ A2¼ (A1/A0)/0.3, which represents the degree of sintered Si3N4

particles. In this way the probability values of particles that have sintered together in all Si3N4 particles can be calculated. Table 3 shows the degree (probability) of sintering of Si3N4 in the composites in different pulse conditions. It can be seen that as the value of ton: toff decreased, the value of A1/A2 increased, which indicated that as the value of ton: toff decreased, the sintering of Si3N4 in the composites became more and more serious. To confirm the sintering of Si3N4 particles in composites prepared by SPS, the microstructure of composites prepared by HP process are compared, and the results are shown in Fig. 5. Fig. 5(a) and (b) are the SEM images of HP and SPS in the condition of ton: toff ¼ 2:1 respectively. For the composites prepared by the HP process, most Si3N4 particles uniformly distribute in the matrix in the form of individual particle, and some Si3N4 particles appear to be sintered together, as shown in the blue box portion of Fig. 5(a). A higher magnification of blue box portion in Fig. 5(a), as shown in Fig. 5(c) and (d), indicates a clear boundary between these Si3N4 particles. For the composites prepared by the SPS process, similarly, the distribution of Si3N4 is co-existence with clusters and single forms, as shown in Fig. 5(b). However, no visible grain boundaries can be seen (Fig. 5(e)) in the clusters, which means that compared to HP, Si3N4 particles in the composites prepared by SPS have been sintered together. In order to verify whether the Si3N4 particles actually sintered, we etched the composite and observed the remaining Si3N4 particles after the etching. The composite was etched with 5% NaOH solution for 48 h. After the Al matrix was completely etched away, the remaining Si3N4 particles were observed for morphology and it is found that there are multiple

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Fig. 8. Schematic illustration of the sintering of Si3N4 particles under the action of spark plasma generated in the gap between Al particles.

Si3N4 particles linked together, as shown in Fig. 6(c) and (d), which confirms that the Si3N4 particles do sinter together instead of simple agglomeration compared to raw Si3N4 particles showed in Fig. 6(a) and (b). In order to confirm whether the Al matrix had an effect on the sintering of the Si3N4 particles, the composites were analyzed by TEM and the results are shown in Fig. 7. Fig. 7(a) shows two sintered Si3N4 particles with no apparent boundary at the junction, as indicated by red ellipse in Fig. 7(a). Fig. 7(b) shows several sintered Si3N4 particles, as shown in yellow box. The HAADF image of the yellow box region shows that, these Si3N4 particles are sintered with clean boundaries, as indicated by the red dotted line in Fig. 7(d). The energy spectrum results show that there is no Al or other elements between the Si3N4 particles, indicating that the Si3N4 particles are not connected due to the Al matrix, as indicated in Fig. 7(c). Therefore, it can be considered that the sintering of Si3N4 particles does occur during the preparation of Si3N4/6061Al composites by SPS, which was different from traditional preparation methods. Actually, although the ball milling process has already mixed the powder uniformly, as shown in Fig. 2, as Si3N4 particles were smaller than Al particles and the volume fraction of Si3N4 in the composites was relatively high, when putting the powder in the mold, the smaller Si3N4 particles between Al particles were simply close to each other. Therefore, in the HP process, we found that some Si3N4 particles were agglomerated and clear interfaces between them can be observed. While in the SPS process, we speculated that the sintering of Si3N4 particles was achieved by

the transient high temperatures brought by the pulsed current, therefore the results of this paper appear as shown in Figs. 4, Figs. 6 and 7. The schematic diagram of Si3N4 particles sintering mechanism in SPS process is shown in Fig. 8. Although the sintering mechanism of SPS is still in the stage of research and discussion, most scholars currently believe that the sintering mechanism of conductive and non-conductive materials is different [25,26]. For the explanation of the sintering mechanism of Si3N4 particles in the Si3N4/6061Al composite, it is believed that this is mainly due to the local high temperature in the sintering process of SPS and the presence of Al2O3 as the sintering additives of Si3N4 [27] on the surface of 6061Al powder. The specific analysis is as follows [28,29]: (1) Since most of the raw materials were conductive 6061Al powder, pulsed current could flow through the powder to generate an electric field inside the entire particle. Under the action of the electric field, plasma was generated between the particle gaps, and the highspeed plasma generated high temperature locally and promote atomic diffusion (Fig. 8). (2) SPS sintering used low voltage, high current sintering. Currents were up to hundreds or even thousands of amps. According to Ampere's law, a strong magnetic field was generated around a large current. Meanwhile, the intensity of the magnetic field was proportional to the current intensity. The current was deflected at the edges to generate a pulsed electromagnetic field when passes through the Si3N4 particles. The magnetic field generated by the deflection current was asymmetric and could

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Fig. 9. TEM images of the interfacial reaction morphology of 30% Si3N4/6061Al composites under different pulse conditions. (a) ton: toff ¼ 10:1; (b) ton: toff ¼ 2:1; (c) ton: toff ¼ 2:5.

not offset, which caused a local high temperature within the composite, so that diffusion and rapid sintering of the Si3N4 particles were achieved. (3) When the energy of accumulated pulse current was large enough between the Si3N4 particles, current would breakdown and flow through Si3N4 particles. As a result, the plasma acted on the Si3N4 particles to activate and promote the sinterability of the Si3N4 particles. (4) Assuming that a 10 nm oxide layer exists on the surface of the 6061Al spherical particle (~10 mm), the mass ratio of Al2O3 to Si3N4 can be calculated to be 2.3%. Al2O3, used as a sintering aid, reacted with the SiO2 on the surface of the Si3N4 particle to form a silicate liquid phase, which was beneficial

for the sintering of Si3N4. In all, the local high temperature of the SPS process and the sintering aid Al2O3 were possible reasons for the sintering of the Si3N4 particles. On the other hand, in the preparation of composites by SPS method, pulse conditions affected the local temperature in the composites. When the value of ton: toff was large, the current could be a small value since it existed for a relatively longer time in the composites, as shown in Fig. 3(b). Therefore, the phenomenon of pulse discharge between the particles was weak, and the temperature rise between the particles was not particularly high, so the sintering of reinforcement was less likely to occur, or some Si3N4

Fig. 10. Interfacial reactant analysis results. (a)TEM images of the 30% Si3N4/6061Al composites in the condition of ton: toff ¼ 2:1; (b) TEM-EDS analysis of the interfacial reactions; (c) The HRTEM image of the interfacial reactions; (d) The corresponding FFT patterns of the box in image (c).

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H. Jiang et al. / Journal of Alloys and Compounds 763 (2018) 822e834 Table 4 The tensile properties Si3N4/6061Al composites under different pulse conditions.

Tensile Strength (MPa) Yield Strength (MPa) Elastic Modulus (GPa) Elongation (%)

ton: toff ¼ 10:1

ton: toff ¼ 2:1

ton: toff ¼ 2:5

493 426 117.9 0.82

499 400 125 1.52

495 393 119.6 1.16

between the particles and leaded a sharp rise in temperature. In this condition, the free path of diffusion of Si3N4 particles became larger due to the increase of energy, which caused more adjacent Si3N4 to be severely sintered together. At the same time, because this process was almost instantaneous, the degree of sintering was not enough so that holes will appear between Si3N4 particles, as shown in Fig. 4(f). This shows that moderate ton: toff value is favorable for the distribution of Si3N4 particles and the relative density of the composites.

3.2. Interfacial reaction

Fig. 11. STEM analysis and linear scanning between Si3N4 particles and 6061Al composites.

particles were not completely sintered, resulting in a certain hole in the composite, as shown in Fig. 4(a). When the value of ton: toff was small, the momentary current needed to reach a very high value to achieve the temperature required for sintering, as shown in Fig. 3(b). The excessive current would cause severe discharge

Fig. 9 shows the interface morphology of composites under different pulse conditions. It can be seen that when the value of ton: toff ¼ 10:1, the interfacial reactants are relatively thin and layered along the interface. However, When decreasing the value of ton: toff to 2:1, the interfacial reactants begin to thicken and the surface of Si3N4 is stepped. When further reducing the value of ton: toff to 2:5, a more severe case of Al eroding Si3N4 particles occurs with the thicker and serrated morphology of interfacial morphology. Therefore, it can be inferred that 6061Al matrix and Si3N4 particles can form a tight interfacial bond through chemical reaction in SPS. The degree of interfacial reaction was mainly related to temperature. When the value of ton: toff decreased, the temperature at the interface between the reinforcement and the matrix increased, which was more conducive to the interfacial reaction and leaded to a more severe interfacial reaction. In order to study the interfacial reaction products, the composites in the pulse condition of ton: toff ¼ 2:1 was observed, as shown in Fig. 10. From the elements distribution in Fig. 10(b), it indicates that Mg element participates in the interfacial reaction, which will affect the interfacial reaction of the composites. High resolution image in Fig. 10(c) and the corresponding Fast Fourier Transform (FFT, Fig. 10(d) and (e)) images indicate two reaction products, namely AlN and Mg3Al2N4, produced in the Si3N4 and 6061Al matrix interface. Based on the above results, it was inferred that the following three reactions occurred at the interface: 12Mg þ Si3N4 ¼ 2Mg3N2 þ 3Mg2Si

(1)

4Al(l) þ Si3N4 ¼ 4AlN þ 3Si(l)

(2)

Fig. 12. Tensile curves (a) and tensile properties (b) of the 30% Si3N4/6061Al composites under different pulse conditions.

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Fig. 13. Schematic illustrations of the distributions of Si3N4 particles under different pulse conditions. (a) ton: toff ¼ 10:1; (b) ton: toff ¼ 2:1; (c) ton: toff ¼ 2:5.

Fig. 14. Schematic diagrams of fracture types of individual particles and sintering particles in composites respectively. (a) (c) interface debonding; (b) (d) crack deflection.

2AlN þ Mg3N2 ¼ Mg3Al2N4

(3)

According to reports in the literature [30,31], Si3N4 reacted with liquid Al, which is the above mentioned reaction Eq. (2). When the reaction temperature was higher than 950  C, a stronger interface reaction occurred. Although the setting value of the sintering temperature in this paper was 570  C, it is entirely possible to reach and even higher than the temperature of the interface reaction because the spark plasma generated under the electric field as described above. However, other interfacial reaction products have appeared in this paper besides AlN. In the present work, the segregation of Mg at the interface between Si3N4 particles and 6061Al matrix has been revealed by STEM analysis, as shown in Fig. 11. Spark plasma generated in SPS and the local temperature rised instantaneously, Mg vaporized and contact with Si3N4 at the interface front due to its higher vapor pressure, leading to the reaction Eq. (1) occurred preferentially, then the molten Al liquid reacted with Si3N4 (Eq. (2)) to form AlN. However, the reaction did not end this time. Reaction Eq. (3) would continue, leading to the eventual ternary compounds produce at a very high local temperature. The literature [32] pointed out that AlN reacted with Mg3N2 to form a compound in the form of MgAlnNnþ2, which was

consistent with the compound form of Mg3Al2N4 produced here. Therefore, whether AlN was present or not in the reaction product may depend on whether sufficient Mg elements were involved in the reaction or not. If Mg elements at the interface were sufficient, AlN reacted with Mg3N2 to form Mg3Al2N4. Otherwise, there was a partial surplus of AlN.

3.3. Mechanical properties Fig. 12 and Table 4 show the influence of different pulse conditions on the tensile behaviors of the composites. It can be seen from Table 4 that as the value of ton: toff decreases, the tensile strength of the composites were all about 500 MPa and does not change too much. However, the yield strength decreases with the decrease of pulse value and when the value of ton: toff ¼ 2:1, the composite possesses the highest elastic modulus and elongation. The elongation of the composite with the pulse of ton: toff ¼ 2:1 increases by 85.4% and 31.0% compared to the composite with the pulse of ton: toff ¼ 10:1 and 2:5, respectively. Therefore, the improvement of the strength of the composites should be attributed to the increase in the plasticity.

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In the composites prepared in this paper, localized particles of Si3N4 appeared to sinter together in different degrees. The uniform distribution of the individual particles was replaced by co-existence with different degrees of clusters and single forms, and with the decrease of the value of ton: toff, the sintering of Si3N4 particles become serious, as shown in Fig. 13. Different sintering states would affect the mechanical properties of composites, which is mainly manifested in preventing crack propagation. When a crack propagated in the composites, there were three types of fractures when it encountered Si3N4 particles: (1) penetrating through the particles (particle fracture); (2) debonding along the interface between the Si3N4 and the 6061Al matrix (interface debonding); (3) deflecting near the particles into the matrix (crack deflection). For the case of cracks passing through the particles, it was considered that there would not be much difference in different conditions. According to the latter two types, the cracks would propagate through longer paths relative to the individual particles when they encountered the sintered particles, which is more conducive to the improvement of the plasticity of the composites, as illustrated in Fig. 14. Moreover, L.J. Huang [33] mentioned when damage does occur, clustering concentrates the damage in small regions which is likely to enhance crack formation resistance and thus enhance the ductility of the materials. In addition, the high interfacial bonding strength (explain above) and larger interfacial area of clusters than single rod particles are also the reasons for the higher strength and ductility of composites. It should be emphasized that the above analysis was based on the assumption that Si3N4 particles did not have holes, cracks, and other defects in the sintering process. However, in the actual process, there were certain large-sized voids inside the granules as a result of incomplete sintering when the value of ton: toff was 10:1, and these cavities would become crack nucleation sites under external stress. Furthermore, the cracks that passed through the

Fig. 16. Comparison of tensile strength and elongation of several Al matrix composites. The data of relationship between tensile strain and tensile strength was drawn based on Ref. [4] (represented by symbol ), Ref. [30,35] (represented by symbol ), Ref. [36] (represented by symbol ), Ref. [37] (represented by symbol ), Ref. [38] (represented by symbol ), Ref. [39] (represented by symbol ), Ref. [40] (represented by symbol ), Ref. [41] (represented by symbol ), respectively.

granules significantly reduced the plasticity of the composites, as indicated in Fig. 15(a). When the value of ton: toff was 2:5, due to the large number of sintering-bonded particles, the internal grain boundaries of the large particles and the particle connection boundaries, when under stress, became weak zones, leading to the occurrence of brittle cleavage within the particles (Fig. 15(c)), which would also reduce the plasticity of the composites. It has been reported in the literature [34] that damage in the form of particle

Fig. 15. Fracture morphology of the 30% Si3N4/6061Al composites under different pulse conditions. (a) ton: toff ¼ 10:1; (b) ton: toff ¼ 2:1; (c) ton: toff ¼ 2:5; (d) SEM-EDS analysis of the particle surface in image (b).

H. Jiang et al. / Journal of Alloys and Compounds 763 (2018) 822e834

fracture prior to final fracture will probably results in a significant degradation of elastic modulus of the composites. This was agreed with the experimental results in this paper. For the case of when the value of ton: toff was 2:1, the particles had a moderate degree of sintering with fewer internal defects in the composites. Thus, it exhibited excellent comprehensive mechanical properties. In the fracture morphology shown in Fig. 15(b), the bare Si3N4 particles are analyzed by energy spectrum and it is found that 11 atom percent of Al elements are present, indicating there is still a part of Al adhesion on the surface of the particles, that is, the moderate interfacial reactions between particles and Al matrix improve the interfacial bonding strength, which is also beneficial to the improvement of the properties of the composites. Furthermore, Fig. 16 directly gives the tensile strength and elongation of several composites and our work. Compared with other aluminum matrix composites, the composites prepared in the pulse condition of ton: toff ¼ 2:1 exhibits more excellent performance. As shown in the figure, the strength and elongation of several composites in the literature [4,30,35e41] are below the green curve, while the 30% Si3N4/6061Al composites in T6 state is above the green curve, which shows better comprehensive performance compared with the literature's. In addition, the significance of this work is to give a new method to adjust the distribution of particle reinforcement in aluminum matrix composites. Different from the traditional way of adjusting the particle distribution through mixing powder process, by adjusting the pulse condition in the SPS process, the distribution form of the reinforcement can be improved, and then the mechanical properties of the PRAMC can be improved. 4. Conclusions

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[11] [12] [13]

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In summary, 30% Si3N4/6061Al composites were prepared by SPS in this paper. The influence of pulse conditions on the microstructure and mechanical properties of the composite was studied. The results were shown as follows: (1) During the preparation of SPS, Si3N4 particles appeared to be sintered together, and with the value of ton: toff gradually decreased, the sintering of Si3N4 particles were gradually serious. (2) When Si3N4/6061Al composites were prepared by SPS, there was an interfacial reaction and the interfacial reaction products were AlN and Mg3Al2N4. As the pulse conditions decreased, the degree of interface reaction became more severe. (3) When ton: toff ¼ 2:1, the Si3N4 particles had a moderate distribution and sintering status. There were no obvious voids inside the particles. In addition, the interfacial reaction was moderate, which improved the bond strength between the particles and the matrix. The composites had the best mechanical properties with a tensile strength of 499 MPa, an elongation of 1.52% and an elastic modulus of 125 GPa and was superior to the performance given in many literature currently.

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Acknowledgements [26]

The authors are grateful to the National Natural Science Foundation of China (Grant No. 51571069, No. U1637201 and No. 51203035).

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