Journal of Materials Science & Technology  2019 , 35 (7): 1345-1353 https://doi.org/10.1016/j.jmst.2019.02.003

Orginal Article

The improved mechanical properties of Al matrix composites reinforced with oriented β-Si3N4 whisker

Chenxu Zhangaab, Yu-Ping Zenga*, Dongxu Yaoa, Jinwei Yina, Kaihui Zuoa, Yongfeng Xiaa, Hanqin Lianga

aState Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
bUniversity of Chinese Academy of Sciences, Beijing 100049, China

Corresponding authors:   *Corresponding author.E-mail address: yuping-zeng@mail.sic.ac.cn (Y.-P. Zeng).

Received: 2018-09-1

Revised:  2018-11-21

Accepted:  2018-12-21

Online:  2019-07-20

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

More

Abstract

The β-Si3N4 whiskers (β-Si3N4w) reinforced Al matrix composites were first fabricated by hot pressing, then treated through hot extrusion. The microstructure characterization demonstrated the preferred orientations of both β-Si3N4w and Al grains in the as-extruded composites. It indicated that β-Si3N4w were aligned along the extrusion direction and Al grains exhibited a distinct <111>Al texture. The interface between β-Si3N4w and Al was in a good bonding status without voids and reaction products. Effects of extrusion process on the mechanical properties of composites were also investigated. The results indicated the extrusion process had a prominent strengthening effect on the mechanical properties of composites. The maximum yield strength and ultimate tensile strength of composites reached up to 170 and 289 MPa, respectively, accompanied by a 12.3% elongation at fracture when the whisker fraction was 15 vol.%. This improvement was collectively attributed to the densification of composites, the strong interface, and the preferred orientation inside composites. The yield strength of the composites reinforced with 5 vol.% β-Si3N4w corresponded well with the theoretical value from different strengthening mechanisms.

Keywords: Metal-matrix composites (MMC) ; β-Si3N4 whiskers; ; Preferred orientation ; Interface ; Mechanical properties ; Extrusion

0

PDF (4860KB) Metadata Metrics Related articles

Cite this article Export EndNote Ris Bibtex

Chenxu Zhanga, Yu-Ping Zeng, Dongxu Yao, Jinwei Yin, Kaihui Zuo, Yongfeng Xia, Hanqin Liang. The improved mechanical properties of Al matrix composites reinforced with oriented β-Si3N4 whisker[J]. Journal of Materials Science & Technology, 2019, 35(7): 1345-1353 https://doi.org/10.1016/j.jmst.2019.02.003

1. Introduction

Driven by the demand for energy saving and efficiency promoting in engineering, structural metallic materials are advancing towards higher strength and lower weight, and this trend is poised to be accelerated with the applications of new metal matrix composites (MMCs) in the future [1,2]. Among the MMCs, Al matrix composites (AMCs) are one of the indispensable composites for numerous industrial and military applications due to its distinctive properties such as high specific strength and stiffness, increased wear resistance and enhanced high-temperature performance as well as fatigue properties [[3], [4], [5]]. Moreover, AMCs usually possess excellent workability, thereby ensuring its applicability of almost all the metallurgical processes. In the past few decades, the increasing interests in AMCs have led to a great development of different reinforcement types and various processing routes.

Reinforcements used in AMCs are usually in types of continues fibers, particulates and whiskers or short fibers. As recognized, the AMCs reinforced by continues fibers have the potential in achieving the highest performances of composites, but not only the costs of them are high, but also the fabrication processes are difficult [6]. The AMCs reinforced by particulates have a wide range of applications due to its competitive cost-effectiveness, but the strengthening effect is limited when particulates content is low, and the particle agglomerations are usually formed when the particulates content is high [7]. However, short fibers or whiskers have both merits of the above two reinforcement types, which are reflected in the good dispersion properties and the significant difference in length and diameter. Therefore, AMCs reinforced by whiskers or short fibers (SFAMCs) have the advantages such as ease of preparation, low-fabrication cost, flexible designability of microstructure and general applicability to the conventional machining process.

Many attempts conducted by researchers have verified that the mechanical behaviors of SFAMCs are strongly influenced by the fiber content and the interfacial bonding between the fiber and matrix [2,[8], [9], [10]]. Moreover, the orientation of short fibers also plays a significant role in determining the mechanical properties of composites [[11], [12], [13], [14]]. Gorsse and Miracle [15] found that the Ti matrix composites reinforced with aligned TiB whiskers demonstrated higher tensile strength and Young’s modulus than those reinforced with randomly distributed whiskers. Badini [16] studied the mechanical properties anisotropy of SiC whiskers reinforced 6061 Al matrix composites, and their result showed that the compressive strength of composites along the longitudinal direction was remarkably higher than that along the transverse direction. Similarly, Xin et al. [17] prepared highly oriented SiC nanowires reinforced Al matrix composites, and realized great improvements in both tensile properties of composites when compared with that reinforced with randomly distributed nanowires.

The β-Si3N4 whiskers (β-Si3N4w), known for its superb mechanical properties and excellent thermal stability [18], are promising reinforcement candidatesfor AMCs. Its hane low density (3.2 g/cm3) which are close to that of Al, making it easy to fabricate lightweight and homogenous distributions β-Si3N4w reinforced AMCs. The microstructure and mechanical properties of β-Si3N4w reinforced AMCs (β-Si3N4w/Al composites) prepared by a hot pressing method have been reported [[19], [20], [21]]. The β-Si3N4w were randomly dispersed in the matrix and a distinct enhancement in tensile strength was achieved with the addition of whiskers [19]. And, the tensile strength was further enhanced by modifying the whisker surface statement [20,21]. However, the porosity of β-Si3N4w/Al composites was increased with increasing the volume fraction of whiskers, thereby leading to the deterioration in tensile performance of composites when the whisker content is high.

Hot extrusion is a practical method for strengthening the mechanical properties of AMCs by modifying the microstructure and increasing the relative densities composites [11,15,22]. Usually, a clearly preferred orientation of reinforcements is obtained after extrusion, especially for the short fibers or whiskers, which have certain aspect ratios. Also, the orientation of metal grains will be influenced due to the deformation of composites [23]. A lot of researches have been conducted to explore the effect of hot extrusion treatment on the AMCs reinforced with different types of reinforcements [13,17,22,[24], [25], [26]]. However, up to now, no systematic studies have been conducted on the extruded β-Si3N4w/Al composites.

The aim of the present work is to investigate the effects of hot extrusion treatment on the microstructure and mechanical properties of composites. In this work, the β-Si3N4w/Al composites reinforced with various volume fractions β-Si3N4w are firstly prepared by a hot pressing method and then treated through a hot extrusion process. Both the preferred orientation and the interfacial structure in composites were characterized. Meanwhile, the strengthening effects determined by the extrusion and different strengthening mechanisms were discussed in detail.

2. Experimental procedures

2.1. Raw materials

Commercially available atomized high-purity Al powders (purity ≥ 99.95 wt%) with an average diameter of 25 μm were selected as the raw material of matrix. The β-Si3N4w, which were fabricated in our laboratory by pressureless sintering, were used as the reinforcements. More details on the preparation methods of β-Si3N4w have been documented in the previous report [27]. The morphology of the as-prepared β-Si3N4w whiskers is shown in Fig. 1. The average length and diameter of the whiskers were 3.64 μm and 0.38 μm, respectively.

Fig. 1.   SEM micrograph of β-Si3N4w.

2.2. Preparation of β-Si3N4w/Al composites

The β-Si3N4w/Al composites with various volume fractions of whiskers from 5 to 15 vol.% were fabricated by using a hot pressing method. The composites were labeled as 5-β-Si3N4w/Al composites, 10-β-Si3N4w/Al composites and 15-β-Si3N4w/Al composites, respectively. With the purpose of making the whiskers uniformly dispersed, the Al powders and as-prepared β-Si3N4w were fully ball milled before sintering. Then, the mixtures were preformed in a graphite die. The details of the mixing and preforming processes were described in our previous work [19]. The sintering temperature, holding time, single axial pressure and atmosphere chosen in this work were 550 °C, 2 h, 30 MPa and Ar atmosphere, respectively. In order to study the effect of hot extrusion, the as-fabricated composites were divided into two sets. One set of specimens was kept unchanged without any treatment. The other set was processed by a hot extrusion process. The specimens in the extruded group were machined to the specific dimension (a cylinder with height of 40 mm and diameter of 37.4 mm) first. Then, the cylinders were put into an extrusion die and held at 500 °C for 20 min before extrusion. The speed and the extrusion ratio of hot extrusion were 5 mm/s and 14:1, respectively. Finally, the composite rods with diameter of 10 mm were obtained. The specimens of pure Al were also extruded for comparison.

2.3. Microstructure characterization and mechanical properties evaluation

The phase compositions of β-Si3N4w/Al composites were characterized by a high-resolution X-ray diffractometer (D8 discover, Bruker, Germany). The microstructures of composites were imaged by a field emission scanning electron microscope (FESEM, Magellan 400, FEI, America), which was equipped with an electron back-scattered diffraction (EBSD) system. The specimens for EBSD detection were polished by Ar ion beam polishing (IM4000, Hitachi, Japan). The interfacial structure between β-Si3N4w and Al matrix was identified by transmission electron microscopy (TEM JEM-2100 F, JEOL, Japan). The specimens for TEM test was prepared by an ion milling system (691, Gatan, USA). The fracture surfaces of the as-extruded composites caused by tensile failure were observed by scanning electron microscopy (SEM, S-8220, Hitachi, Japan).

To verify the strengthening effects of hot extrusion on the composites, the tests of relative density, hardness, and tensile properties were carried out. The densities of composites were calculated by Archimedes method. The hardness of composites was measured on a Vickers hardness tester (Tukon-2100B, Instron, USA) with a load of 0.1 kg. The specimens of the as-extruded composites were taken from two sections: transverse section (TS) and longitudinal section (LS). Six points for each sample were measured to obtain the average value. The tensile tests were conducted on a universal testing machine (Instron-5566, Instron, USA) at room temperature. The strain rates were controlled at 0.003 s-1. The tensile specimens were machined into “dog-bone” type. The specimens taken from the as-fabricated composites were 4 mm in diameter and 24 mm in original gauge length, whose tensile axes were parallel to the hot pressing direction. The specimens taken from the as-extruded composites were 6 mm in diameter and 36 mm in original gauge length, whose tensile axes were parallel to the extrusion direction. In each case, six valid specimens were tested to ensure the reproducibility of the tests.

3. Results and discussion

3.1. Characterization of preferred orientation in the as-extruded composites

Fig. 2(a) shows the XRD diffraction patterns of the as-fabricated β-Si3N4w/Al composites reinforced with different fractions whiskers. According to our previous work, the as-fabricated composites are isotropous with whiskers randomly dispersed in the matrix [16]. β-Si3N4 is the only phase besides Al. The intensities of β-Si3N4 diffraction peaks increase gradually with increasing fraction of whiskers. The XRD analyses of the as-extruded 15-β-Si3N4w/Al composites from the LS and the TS are also carried out, as shown in Fig. 2(b). Here, the β refers to β-Si3N4. It can be found that there is a clear difference in the relative peaks intensities between the two sections. The peak of (111)Al rises significantly while the other peaks of Al drop markedly when the result of the LS is compared with that of the TS. Thus, it can be inferred that a preferred orientation of Al grains is formed after extrusion, which is consistent with the research of Zhao et al. [22]. Moreover, a preferred orientation of β-Si3N4w in Al matrix is also verified. Fig. 2(c) shows a magnified image of Fig. 2(b) to distinctly reveal the opposite features of β-Si3N4 peaks from two sections. The relative peaks intensities of (11 $\bar{2}$ 0)β, (20 $\bar{2}$ 0)β and (21 $\bar{3}$ 0)β of the LS are much stronger than that of the TS. However, the intensities of (0002)β and (10 $\bar{1}$ 1)β of TS are much stronger than those of the LS.

Fig. 2.   XRD patterns of β-Si3N4w/Al composites: (a) the as-fabricated composites with different volume fractions of β-Si3N4w; (b) the LS and the TS of the as-extruded 15-β-Si3N4w/Al composites; (c) a magnified view of (b).

In order to further study the preferred orientation in the composites, microstructures of the as-extruded 15-β-Si3N4w/Al composites from the LS (Fig. 3(a, b)) and TS (Fig. 3(c, d)) are examined by FESEM. All the figures are imaged by backscattering electron signals so that different phases can be distinguished by color. The black dots or rods are identified as β-Si3N4w, as indicated by the arrows. The white dots marked by the circle are Fe, which are introduced by ball milling. The other zones with different grayscales as Al grains with different orientations. It is noted that the β-Si3N4w are uniformly distributed in the matrix, and a quite dense microstructure without pores or whisker segregations are formed after extrusion. From the LS (Fig. 2(a, b)), it can be found that the rod-like whiskers are roughly parallel to the extrusion direction. However, when viewed from the TS, only the ends of whiskers, some of which shows hexagon shapes, are found. Therefore, the preferred orientation of β-Si3N4w inside the matrix has been confirmed, which is consistent with the XRD results. The reason for the preferred orientation of β-Si3N4w is because of the asymmetric force on whiskers during the extrusion process. The larger the length-diameter ratio of the whisker, the greater the impact of the force. The whisker tends to be aligned along the extrusion direction to minimize this impact. According to Ref. [18], β-Si3N4 has a hexagonal crystal structure and the axial direction of β-Si3N4 is determined to be [0002] direction, which means that the {0002}β planes are approximately parallel to the TS. Therefore, the phenomenon that the XRD peak intensity of (0002)β of the TS becomes more apparent in Fig. 3(c) is well explained.

Fig. 3.   Microstructures of the as-extruded 15-β-Si3N4w/Al composites: (a, b) from the LS and (c, d) from the TS. The insets in Fig. 3(a, c) are the schematics of β-Si3N4w distribution.

The preferred orientation of Al grains inside composites was investigated through the EBSD analysis, as demonstrated in Fig. 4. To simplify the description of orientations, two perpendicular transverse directions (TD and TD*), both of which are perpendicular to the extrusion direction (ED) at the same time, are introduced in this work, as displayed in the insets in Fig. 4(a) and (b). The orientation maps of Al grains on the LS and on the TS were projected towards the ED and the TD, respectively. There are obvious differences in grain morphologies between two sections. When observed from the LS, strip-like Al grains are formed along the ED. However, the Al grains shapes in the TS are equiaxed. It is widely accepted that the textures inside composites are significantly determined by the deformation process [23,28]. The initial equiaxed grains in the as-fabricated composites were severely deformed and stretched along the ED during extrusion on the basis of the study by Liao et al. [29]. In this work, the as-extruded composites exhibit a typical fiber texture of face-centered cubic. The grains on the TS are randomly oriented with various colors. However, a major <111> texture along with a minor <100> texture is exhibited on the LS. It means that the <111>Al and <100>Al directions are parallel to ED, which is one of the typical features of extruded Al [30]. Therefore, the aforesaid differences in XRD patterns of Al from the two sections (LS and TS) are determined by this distinct texture. Of course, both <111>Al and <100>Al directions are not precisely parallel to ED. In order to determine the preferred orientation of Al grains more accurately, pole figures are also applied to measure the textures from the LS (Fig. 4(c)) and the TS (Fig. 4(d)). It noticeable that the distribution of <111>Al directions are strongly enriched together and the center of these areas shows a 3° deviation from the standard orientation. In addition, the measured scope of <111>Al direction is in a range of about 26°, as shown in Fig. 4(d).

Fig. 4.   EBSD analysis of Al grains in the as-extruded 15-β-Si3N4w/Al composites: inverse pole figure maps of (a) the LS and (b) the TS; (c) pole figures based on (a) along with a standard stereographic projection with (011) as projective plane; (d) pole figures based on (b) along with a standard stereographic projection with (111) as projective plane. The purpose of the insets in (a) and (b) is to easily describe the preferred orientation of Al grains.

3.2. Microstructure characterization of the interfacial phases

As a junction between matrix and reinforcements, the interface plays a decisive role in determining the properties of composites [7,31,32]. The good interfacial connection can achieve high-efficiency transfer of stress, thereby, making the most of the strengthening effect of reinforcements. On account of the preferred orientation in the as-extruded composites, TEM examinations of composites from both sections (LS and TS) are carried out, as respectively exhibited in Fig. 5(a) and (c). It is obvious that the orientations of whiskers are perpendicular to each other when observed from two different perspectives. The whisker axis in the LS is parallel to the LS plane while that in the TS is perpendicular to the TS plane. The typical hexagon shape is shown in Fig. 5(c), which confirms the three-dimensional structure of β-Si3N4w is a hexagonal prism. To further characterize the interfacial structure between Al and β-Si3N4, the high-resolution transmission electron microscopy (HRTEM) images from the two sections are observed, as shown in Fig. 5(b) and (d). The phases of Al and β-Si3N4 can be easily distinguished according to their differences in lattice parameters [33,34]. The fast Fourier transform (FFT) patterns of the corresponding phases are inset in Fig. 5(b) and (d), respectively.

Fig. 5.   TEM analysis of β-Si3N4w/Al composites: images of β-Si3N4w embed in the Al matrix when observed from (a) the LS and (c) the TS; (b) and, (d) the HRTEM images of the interfacial structure which are taken from the LS and the TS, respectively. The insets in (b) and (d) are the FFT patterns of Al and β-Si3N4w, respectively.

When observed from the LS (Fig. 5(b)), a typical interface bonding with an amorphous layer between Al and β-Si3N4 can be found. The β-Si3N4 oriented to [$\bar{2}$ 113] is at the lower of the figure and the Al is at the upper. This kind of interface which results from atomic diffusion is detailedly discussed in our previous study [19]. It shows good wettability without clear orientation relationship between the two phases. When observed from the TS (Fig. 5(d)), the β-Si3N4 whose atoms are arranged in a hexagonal structure can be clearly observed. The β-Si3N4 is oriented to [0002] zone axis ([0002]β∥ED) while the Al is oriented to [111] zone axis ([111]Al∥ED), which is in good agreement with the conclusions of the preferred orientation of β-Si3N4w and Al grains in the as-extruded composites.

In the light of the above, both the interfacial structure from the TS and the LS demonstrate good bonding status between the β-Si3N4 and Al. All the interfaces avoid the formation of detrimental reaction products. Thus, it can be inferred that the mechanical properties of composites will be considerably improved by the well bonded interface.

3.3. Improved mechanical properties of the as-extruded β-Si3N4w/Al composites

From the above, the preferred orientation of β-Si3N4w and Al grains accompanied by the well bonded interface between the two phases are formed in the composites after extrusion. But what specific effects of this phenomenon on the mechanical properties of composites are unknown. Therefore, the densification, Vickers hardness and tensile behaviors of the as-fabricated and the as-extruded composites are tested and compared with each other in this section.

Fig. 6(a) exhibits the relative densities of the as-fabricated and the as-extruded composites with various whisker fraction. It is obvious that the extrusion process has a positive effect on the densification of composites. The relative density of the as-extruded 15-β-Si3N4w/Al composites has reached 98.9%, which is at the same level as the as-fabricated pure Al. Generally, the relative density of composites has crucial influences on the mechanical properties of composites [35]. Low strength and unexpected failure are usually generated by the reason of low relative densities of materials. Besides, the raw pure Al powders are inevitably covered with thin natural oxide layers, which block the contact between Al and β-Si3N4w. However, hot extrusion process could effectively eliminate this barrier by deforming Al powders and result in the nano-metric Al2O3 dispersoids [36]. The content of the nano-metric Al2O3 dispersoids is too few to be detected in this work. Therefore, the hot extrusion process is an effective way for AMCs to achieve a dense structure.

Fig. 6.   (a) Relative density and (b) Vickers hardness of β-Si3N4w/Al composites before and after extrusion; (c) grain diameter distribution histograms of the as-extruded 15-β-Si3N4w/Al composites from two sections.

Fig. 6(b) shows the Vickers hardness of composites before and after extrusion. Because of the difference in textures of both sections (TS and LS), the hardness of the as-extruded composites from both sections are respectively measured. In line with the expected, all the specimens of the as-extruded composites exhibit obvious improvement in hardness. For the specimens from the as-extruded composites, it shows differences in hardness between the two sections and the maximum hardness value of each set is reached on the TS. This anisotropy in the mechanical properties of the as-extruded composites should result from the preferred orientation inside composites. As discussed in Section 3.1, the Al grains in the LS are elongated while those in the TS are equiaxed. Based on the results of EBSD (Fig. 4(a) and (b)), the grain size distributions from two sections are statistically measured, as depicted in Fig. 6(c). It can be seen that the average grain diameter is visibly smaller in the TS, where the highest peak of grain diameter is positioned at 0.7 μm. However, the grain diameter in the LS is mainly concentrated at 1.9 μm, and the grain size distribution is up to 9.9 μm. It is widely considered that the smaller size of grains determines better mechanical properties [27,28,37]. Moreover, according to Ref. [30], the anisotropy in mechanical properties strongly depends on the texture of Al. There is a phenomenon of texture hardening effect in the extruded Al alloys, and the maximum mechanical strength is obtained along the extrusion direction because of the <111>Al texture. In this case, it makes sense that higher mechanical properties of composites are achieved along the extrusion direction in this work.

The representative tensile stress‒strain curves of the as-prepared and the as-extruded β-Si3N4w/Al composites are shown in Fig. 7(a) and (b). The detailed ultimate tensile strength (UTS), yield strength (YS) and elongation at fracture of composites with different whisker fractions are shown in Fig. 7(c) and (d). It is noticeable that the tensile performances of both the pure Al and the composites are significantly improved after extrusion. The strengthening effect of β-Si3N4w in the as-extruded composites is much higher than that in the as-fabricated composites. The UTS of the as-extruded 15-β-Si3N4w/Al composites reaches up to 289 MPa, 32.6% higher than that of the as-fabricated composites and 122.3% higher than that of the as-fabricated Al matrix, respectively. The YS of the as-extruded 15-β-Si3N4w/Al composites is 170 MPa, which is more than three times of the as-fabricated Al matrix. Simultaneously, the ductility of the as-extruded 15-β-Si3N4w/Al composites is also improved a lot which reflects in the 101.3% increment when compared with that of the as-fabricated 15-β-Si3N4w/Al composites. It also can be seen that the improvement of strengthening effect increases with the whisker fraction increasing, which is consistent with the result of the relative density of composites (Fig. 6(a)).

Fig. 7.   Tensile behaviors of β-Si3N4w/Al composites: stress-strain curves of composites (a) before and (b) after extrusion; (c) UTS, YS and (d) elongation at fracture of the as-fabricated and the as-extruded composites.

Even though the relative densities of the as-extruded composites with different whisker fractions vary little, there is an evident decline of ductility of composites when the whisker fraction is 15 vol.%. This can be explained by the deformation mechanism of composites. The fracture of the materials results from micro-voids formation, growth, and consolidation. However, the whiskers inside matrix act as barriers for voids expansion. For the specimens with high whisker content, there is not much free space for cracks expansion. So more voids initiation areas will be formed at the same time when the stress reaches the maximum, thereby leading to a reduction of ductility of composites. Fig. 8 shows the fracture surface of the as-extruded β-Si3N4w/Al composites with various whisker fractions. It can be seen that all the fracture surfaces of the specimens are covered with dimples and tearing edges, which indicate the typical ductile fracture characteristics of composites. The average size of dimples of the 15-β-Si3N4w/Al composites (Fig. 8(c)) is much smaller than that of the 5-β-Si3N4w/Al composites (Fig. 8(a)), which demonstrates the fact that the growth of voids in the 15-β-Si3N4w/Al composites is more hindered during fracture. Moreover, from the magnified view of fracture surface (Fig. 8(d)), the whiskers marked by the arrows are firmly embedded in the matrix and only some of the whisker ends are clearly observed. This can be explained by the well bonded interface formed in the as-extruded composites, which is so strong that it makes the cracks propagate away from the interface.

Fig. 8.   SEM images of fracture surface of the as-extruded β-Si3N4w/Al composites with (a) 5 vol.%; (b) 10 vol.%; (c) 15 vol.% whiskers; (d) a magnified view of (c).

3.4. Strengthening mechanism

According to the previous discussion, the preferred orientation inside composites should also play a critical role in determining the mechanical properties of composites. For the Al grains, both the elongated grains and the <111>Al texture are beneficial for the improvement of mechanical properties along the extrusion direction. And these effects can be demonstrated by the improvement in strength of the as-extruded pure Al, as shown in Fig. 7(c). For the β-Si3N4w, on the basis of Ref. [[38], [39], [40], [41], [42]], there are four major mechanisms playing strengthening roles: load transfer, Orowan looping, grain refinement, and dislocations caused by thermal mismatch. The final enhancement effect is the sum of these four strengthening factors. Only when all the factors are taken into account can we get the theoretical calculations which are close to the experimental value [42]. These four mechanisms are assumed independent of each other and occur simultaneously [26,28]. The multiple strengthening mechanisms resulted from β-Si3N4w can be expressed in Eq. (1),

σCM+ΔσLT+ΔσOL+ΔσGR+ΔσTM (1)

where σC and σM are the yield strength of the composites and matrix, respectively. The experimental value of σM is 62 MPa. ΔσLT is the increased strength caused by load transfer, ΔσOL is the increased strength caused by Orowan strengthening, ΔσGR is the increased strength caused by grain refinement, and ΔσTM is the increased strength related to the dislocations caused by the thermal mismatch.

On the basis of the shear-lag model, the load transfer reflects in that the stress is transferred to the whiskers through shear stress from the Al matrix during deforming [26,43]. The load transfer efficiency is closely related to whisker orientation [42]. Ryu et al. [14] proposed a modified shear-lag model, which took the misorientation of whiskers into account, and the improved yield strength of composites caused by load transfer was calculated by Eq. (2):

$Δσ_{LT}=σ_{M}[V_{W}(\frac{S_{eff}+2}{2})+V_{M}]-σ_{M}$ (2)

where VW and VM are the volume fraction of whiskers and matrix, respectively. Seff is the effective aspect ratio of whiskers which is defined as follows:

$S_{eff}=Scos^{2}θ+(\frac{3π-4}{3π})(1+\frac{1}{S})sin^{2}θ$ (3)

where θ is the misorientation angles of whiskers inside matrix relative to the tensile direction. S is the average aspect ratio of whiskers (S=l/d, l = 3.64 μm, d = 0.38 μm). Thus, the strength is influenced by the θ when the whisker fraction is certain. Only if the whiskers perfectly align along the tensile direction (θ = 0), the composites can take the full advantage of load transfer efficiency of whiskers. By the way, the interface between the matrix and whiskers is assumed as an ideal bonding in this model. In our case, this can be justified by the strong interface between Al and β-Si3N4w after extrusion. In this work, although the oriented whiskers in the as-extruded composites are not exactly parallel to the tensile direction, the effective aspect ratio of these whiskers is much higher than that of the randomly oriented whisker in the as-fabricated composites. Since it is difficult to calculate the angles of all whiskers, we assume an ideal condition that all the whisker are parallel to the loading direction during tensile testing (Seff=S).

Orowan strengthening is induced by the bowing force, which results from dislocations bypassing the reinforcements in the matrix [41]. Li et al. [40] concluded that the Orowan strengthening could be effective when the reinforcement particles are dispersed inside the matrix grains. However, in this work, nearly all the β-Si3N4w are located at the Al grain boundaries, as shown in Fig. 3. The dislocations should be blocked at the grain boundaries before it bypasses the β-Si3N4w. Moreover, there is no plastic deformation before the limit of yield strength, so that no movement of dislocations occurs during this period. The Orowan strengthening is hereby not considered.

The addition of reinforcements results in the grain refinement of the matrix [[38], [39], [40],44], which reflects in the whiskers can interact with metal grain boundaries by acting as pinning points, retarding or stopping their growth. The size of grains decreases with increasing the volume fraction of reinforcements [39,40]. The contribution of this factor to the improvement of composites’ yield strength can be calculated by the Hall-Petch relation [40,45,46]:

$Δσ_{GR}=K_{GR}(D^{-1/2}_{C}-D^{-1/2}_{M})$ (4)

where KGR is a constant depending on the material (KGR = 100 MPa μm1/2 [42]), DC and DM are the average grain size, respectively. Due to the difference in grain sizes between the TS and LS, the D average grain size used in the calculation is defined as D = (DLS+ DTS)/2, where DLS and DTS are the average grain size of the LS and the TS, respectively. The average grain sizes of the as-extruded composites from both sections are listed in Table 1.

Table 1   Average size of grains of the as-extruded composites reinforced with various β-Si3N4w content.

DLS (μm)DTS (μm)
02.822.66
5 vol.%2.742.21
10 vol.%2.622.05
15vol.%2.011.79

New window

Dislocations are generated because of the mismatch in thermal expansion coefficients between the Al matrix (23.6 × 10-6 K-1 [28]) and the β-Si3N4w (2.84 × 10-6 K-1 [47]). This contribution to the increment of the yield strength can be estimated by Arsenault equation [46]:

ΔσTM=kGM1/2 (5)

where k is a constant of 1.25 [49], GM is the shear modulus of the Al matrix (GM = 27.67 GPa [28]), b is Burgers vector of the Al (b = 0.286 nm [49]), and ρ is the dislocation density, which can be calculated as follows [28,46]:

$ρ=\frac{12ΔCΔTV_{W}}{bdV_{M}}$ (6)

where ΔC is the difference in thermal expansion coefficients between the Al and β-Si3N4w, ΔT is the difference in temperature between the extrusion process (500 °C) and the tensile test (25 °C), and d is the average size of β-Si3N4w. In this work, the d of β-Si3N4w, which is similar in shape to a cylinder, is derived by assuming a spherical model, i.e.:

$V_{S}=\frac{4πR^{3}_{s}}{3}\equiv V_{W}=πR^{2}_{W}l$ (7)

where VS and RS are the volume and radius of the spherical model (d = 2RS), respectively. VW is the average volume of whiskers, RW and l are the average radius and average length of β-Si3N4w, respectively (RW = d/2 = 0.19 μm, l = 3.64 μm). So the calculated RS is 0.46 μm.

Table 2 lists the contribution of the above-mentioned strengthening mechanisms to the increment of yield strength of the composites and the difference between the theoretical values and experimental values. The theoretical yield strength (σTheo.) is the sum of the yield strength of matrix (σM) and the total improvement (ΔσTotal) considering all the mechanisms. It is obvious that the dislocation strengthening caused by thermal mismatch play a dominant role in the improvement of yield strength. Furthermore, the impact of the load transfer becomes more and more noticeable with the increase of whisker content. Fig. 9 exhibits the comparison between the theoretical and the experimental values of the yield strength. It can be seen that the experimental yield strength of 5-β-Si3N4w/Al composites is in good agreement with the theoretical value with a variation of 3.2%. However, the yield strength of composites shows a relatively large deviation (17.8%) from the theoretical value as for the 15-β-Si3N4w/Al composites. The discrepancy between the theoretical value and the experimental value may attribute to two main reasons: (1) the actual whiskers’ orientation is not exactly parallel to the extrusion direction; (2) the experimental samples contain defects like pores and impurities, which results from the β-Si3N4w agglomeration and the preparation process, respectively.

Table 2   Contribution of respective strengthening mechanisms to the increment of yield strength and difference between theoretical values and experimental values.

Content of
β-Si3N4w (vol.%)
Theoretical contribution from different strengthening mechanisms (MPa)Total improvement
ΔσTotal (MPa)
Theoretical yield strength
σTheo. (MPa)
Experimental yield strength
σExp. (MPa)
ΔσLTΔσGFΔσOLΔσTM
0-----62.062
514.93.1-48.166.1128.1124
1026.75.0-69.9104.6166.6142
1544.512.1-88.1144.7206.7170

New window

Fig. 9.   Comparison between the theoretical and the experimental values of the yield strength.

4. Conclusions

The preferred orientation in the β-Si3N4w/Al composites was realized after the hot extrusion process. According to the microstructure characterization and mechanical properties examination, the following conclusions were obtained:

(1)The β-Si3N4w were aligned along the extrusion direction after extrusion and the Al exhibited an obvious <111>Al texture in the as-extruded composites.

(2)The interface between β-Si3N4w and Al was in a good bonding status without voids and reaction products.

(3)The hardness of the as-extruded composites exhibited better performance along the extrusion direction and this anisotropy was determined by the preferred orientation inside composites.

(4)The tensile performances both in strength and ductility of composites were significantly improved after extrusion. The maximum UTS could reach up to 289 MPa accompanied by a 12.3% elongation at fracture when the whisker fraction was 15 vol.%.

(5)The improvements in mechanical properties of composites after extrusion were mainly attributed to three main reasons: the densification of composites, the strong interface between the whiskers and Al, and the preferred orientation of whiskers and Al grains.

(6)The improvement in the yield strength of 5-β-Si3N4w/Al composites is in good agreement with the theoretical value predicted by different strengthening mechanisms. Whereas, the deviation between the theoretical value and the experimental value of 15-β-Si3N4w/Al composites is obvious because of the non-ideal state of whiskers orientation and the increased internal defects.

Acknowledgements

This work was supported by National Key R&D Program of China (Nos. 2017YFB0406200, 2017YFB0703200, and 2017YFB0310400), the National Natural Science Foundation of China (No. 51501215), Shanghai Sailing Program (No. 16YF1412900), Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (No. SKL201701), State Key Laboratory of New Ceramic and Fine Processing Tsinghua University (No. KF201806).

The authors have declared that no competing interests exist.


/