Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging

    Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging K.B. Nie, K.K. Deng, X.J. Wang, T. Wang, K. Wu PII: DOI: Reference:

S1044-5803(16)31254-2 doi:10.1016/j.matchar.2016.12.006 MTL 8492

To appear in:

Materials Characterization

Received date: Revised date: Accepted date:

1 August 2016 26 November 2016 11 December 2016

Please cite this article as: Nie KB, Deng KK, Wang XJ, Wang T, Wu K, Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging, Materials Characterization (2016), doi:10.1016/j.matchar.2016.12.006

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ACCEPTED MANUSCRIPT Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging

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College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

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a

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K.B. Nie a, *, K.K. Deng a, X.J. Wang b, T. Wang a, K. Wu b

*Corresponding author. Tel.: +86 351 6010533. Fax: +86 351 6018051 E-mail address: [email protected] (K.B. Nie); [email protected] (X.J. Wang)

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Abstract

In this study, low volume fraction SiCp/AZ91 magnesium matrix nanocomposites billets

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intended for structural applications were synthesized using semisolid stirring assisted

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ultrasonic vibration, leading to the dispersion of SiC nanoparticles. Both the AZ91 alloy and

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nanocomposite billets were then subjected to isothermal multidirectional forging (IMDF). Micrographic observations illustrated that the mean grain size of the developed

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nanocomposite showed an initial increase after 3 IMDF passes, followed by a decrease after 6 IMDF passes compared to the AZ91 alloy. This indicated that the effect of dispersed SiC

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nanoparticles on the inhibition of the dynamic recrystallization grains growth was impaired due to the present high IMDF temperature. The improved yield strength of the nanocomposite could be attributed to Orowan strengthening effect related to the dispersed SiC nanoparticles as well as the second phases precipitated far from the dynamic recrystallization grains. KEYWORDS:

Magnesium

matrix

composite;

Nano

multidirectional forging; Microstructure; Mechanical properties

SiC

particles;

Isothermal

ACCEPTED MANUSCRIPT Influence of SiC nanoparticles addition on the microstructural evolution and mechanical properties of AZ91 alloy during isothermal multidirectional forging

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College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, PR China

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T

K.B. Nie a, *, K.K. Deng a, X.J. Wang b, T. Wang a, K. Wu b

*Corresponding author. Tel.: +86 351 6010533. Fax: +86 351 6018051 E-mail address: [email protected] (K.B. Nie); [email protected] (X.J. Wang)

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Abstract

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In this study, low volume fraction SiCp/AZ91 magnesium matrix nanocomposites billets intended for structural applications were synthesized using semisolid stirring assisted

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ultrasonic vibration, leading to the dispersion of SiC nanoparticles. Both the AZ91 alloy and

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nanocomposite billets were then subjected to isothermal multidirectional forging (IMDF).

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Micrographic observations illustrated that the mean grain size of the developed nanocomposite showed an initial increase after 3 IMDF passes, followed by a decrease after 6

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IMDF passes as compared to the AZ91 alloy. This indicated that the effect of dispersed SiC nanoparticles on the inhibition of the dynamic recrystallization grains growth was impaired due to the present high IMDF temperature. The improved yield strength of the nanocomposite could be attributed to Orowan strengthening effect related to the dispersed SiC nanoparticles as well as the second phases precipitated far from the dynamic recrystallization grains. KEYWORDS:

Magnesium

matrix

composite;

Nano

multidirectional forging; Microstructure; Mechanical properties

1.1 Introduction

SiC

particles;

Isothermal

ACCEPTED MANUSCRIPT Magnesium and its alloys have been regarded as ideal structural materials for both selection and use in automobile, sports-related and electronics industries, in order to reduce

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energy consumption [1-2]. However, the availability of limited number of slip systems

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related to magnesium and its alloys results in poor ductility and low formability at room temperature. Thus, it is attractive to develop new magnesium based materials targeting for good mechanical properties. The technology behind magnesium matrix composites involves

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adding strong reinforcements into magnesium matrix [3, 4]. By introducing selected matrix

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metal, reinforcements as well as composite fabrication method, the properties of magnesium matrix composites can be enhanced or tailored [4-6]. With the development of

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nanotechnology, research has shown that the use of inexpensive ceramic nanoparticles

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targeting composites can simultaneously improve the strengths and ductility of the

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magnesium with inappreciable weight gains [7-8]. The selection of fabrication methods is of prime importance to realize a homogeneous distribution of nanoparticles and expected

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property enhancement of magnesium matrix nanocomposites. In our previous work, the nanoparticles have been incorporated and dispersed uniformly in magnesium matrix using stirring and ultrasonic dispersing [9]. Despite the extensive conducted researches focused on fabricating magnesium matrix nanocomposites, there are just a few studies which have been directed towards thermo mechanical processing of magnesium matrix nanocomposites [10-13]. Our previous work has found that large scale dynamic recrystallization occurred in the SiCp/AZ91 nanocomposite after hot extrusion, resulting in a fine matrix microstructure [10]. Choi et al. has also reported that the yield strength, ultimate tensile strength, and ductility values of the extruded

ACCEPTED MANUSCRIPT Mg-1%SiC nanocomposites increased as compared to the alloy [11]. Liu et al. indicated that the tensile yield strength of the SiC/AZ31 magnesium matrix nanocomposite after hot rolling

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was significantly improved by more than 200% compared with the as-cast nanocomposite

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[12]. Khosroshahi et al. reported that the addition of SiC nanoparticles to the cast AZ80 Mg alloy resulted in the best combination of strength and ductility in comparison with Al2O3 and TiO2 particles [13]. In short, the quality of magnesium matrix nanocomposites has been

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further improved with the application of conventional thermo mechanical process. The large

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numbers of research has demonstrated that exceptional grain refinement for bulk solids to the submicrometer or nanometer level can be achieved by the application of severe plastic

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deformation (SPD) [14]. Among the SPD procedures multidirectional forging that can be

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suitable for industrial applications, is of particular interest for its potential for scaling up of

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relatively large samples [15, 16]. The shape of the material does not change significantly during multidirectional forging, thus overcoming reduction of thickness and diameter of

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metallic materials caused by conventional processing such as forging, rolling or extrusion. Due to the high deformation temperature and the small pressure applied, multidirectional forging can be applied to brittle materials. Moreover, because the load is applied in three directions during multidirectional forging, the degree of deformation in each direction is substantially the same which avoids anisotropy easily generated in the conventional processing method. So far, multidirectional forging has been successfully used to achieve fine grains for pure metals or metallic alloys [17-23]. With respect to magnesium matrix composites, reports on the effect of multidirectional forging on microstructure and mechanical properties are very limited, and most research has focused on the application of

ACCEPTED MANUSCRIPT multidirectional forging to conventional magnesium matrix composites reinforced with micro-particles [24, 25]. As the sizes of reinforcements are less than 1 m, particles promote

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dynamic recrystallization (DRX) of magnesium matrix during hot deformation is unlikely

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[26]. The expected effect of nanoscale particle addition on the microstructural evolution and mechanical properties of magnesium matrix during multidirectional forging is thought to be different from micro-particles. The results of the literature review, however, revealed that

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there has been little systematic study on the influence of nanoscale particle addition on room

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temperature strength and formability of AZ91 alloy during isothermal multidirectional forging (IMDF).

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Accordingly, in the present work the effect of nanoscale silicon carbide particles addition

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on room temperature mechanical properties and microstructures of AZ91 magnesium alloy

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during IMDF is investigated. The nanocomposite materials were synthesized by semisolid stirring assisted ultrasonic vibration followed by homogenization. The mechanical properties

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were related to the microstructures and deformation behaviors of the nanocomposites.

1.2 Material and Methods The matrix material used was a commercial AZ91 alloy with nominal composition of Mg-9.07Al-0.68Zn-0.21Mn, while the reinforcements used were SiC nanoparticles with an average diameter of 60 nm and volume fractions (vol.%) of 1%. AZ91 alloy was supplied by Northeast Light Alloy Company Limited, China. SiC nanoparticles were supplied by Hefei Kaier Nanometer Energy & Technology Company Limited, China. The semisolid stirring assisted ultrasonic vibration method was used to synthesize the SiCp/AZ91 nanocomposites.

ACCEPTED MANUSCRIPT The detailed description of the fabrication process was described in Ref. [9]. The SiC nanoparticles were added into the AZ91 alloy using semi-solid stirring (AZ91 alloy was

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melted and cooled to 590 0C where the matrix alloy was in semi-solid condition) under a gas

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mixture of CO2/SF6. Then the melt was reheated to 700 0C and ultrasonically processed. At last, the melt was cast into a preheated steel mould (450 0C) and allowed to solidify under a 100 MPa pressure. It should be noted that ultrasonic vibration was only used in molten

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nanocomposite. For comparison, AZ91 alloy ingot was also cast under the same conditions.

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Before IMDF, as-cast ingots specimens were solution treated at 415 °C for 24 h to minimize the influence of Mg17Al12 phase.

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A rectangular billet with dimensions of 30 mm×30 mm×60 mm was cut from the ingots

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and used for IMDF. The IMDF was carried out at a temperature of 400 ℃ and at a pressing

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speed of 15 mm s-1, using a press with a 2000 kN load limit. The dimensional ratio of 1:1:2 of the billets was maintained during IMDF, but the press direction was turned by 90° from pass

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to pass [24, 25]. Billets were heated in an electrical resistance furnace at deformation temperature. The imposed strain of each IMDF pass was 0.693 [27]. A graphite-based mixture was used as high-temperature lubricant during IMDF. Their monolithic counterpart was fabricated by the same process for comparison. Microstructural analysis and determination of the evolution of the AZ91 alloy and nanocomposite during IMDF were carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD, Cu-Kα, Y-2000). Samples for microstructure analysis were cut from the central part of specimens parallel to the last compression axis and prepared by the conventional mechanical

ACCEPTED MANUSCRIPT polishing and etching using acetic picral [5 ml acetic acid + 6g picric acid + 10 ml H2O + 100 ml ethanol (95%)]. The grain sizes were determined using Image-Pro Plus software.

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Microstructural features of the AZ91 alloy and SiCp/AZ91 nanocomposites were identified

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using energy dispersive spectrophotometric (EDS) analysis. Specimens for TEM were prepared by grinding-polishing the sample to produce a disk of less than 50 m in thickness. Specimens 3 mm in diameter were then punched out from the cut disk and ion beam thinned.

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The tensile properties at room temperature before and after IMDF were determined using an

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Instron-1186 tension machine at an initial strain rate was 8.33×10-4 (s-1). Flat dog-bone tensile specimens (15 mm gage length, 6 mm gage width and 2 mm gage thickness) were machined

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perpendicular to the last compression axis by electrical discharge machining. The average of

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three tensile tests was adopted as the final tensile strength in the present work.

1.3 Results and Discussion

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1.3.1 Microstructural evolution during IMDF The result of X-ray diffraction analysis conducted on the AZ91 alloy and SiCp/AZ91 nanocomposites after IMDF is given in Fig. 1. It can be observed that peaks related to α-Mg and β-Mg17Al12 can be found both in the AZ91 alloy and SiCp/AZ91 nanocomposites added with a small amount of SiC nanoparticles. In addition, the strong impact coupled with local high temperatures introduced by ultrasonic vibration can clean the nanoparticle surface. Thus, no new phase was found from the XRD pattern. Because of the low percentage of SiC nanoparticles in the matrix of the present nanocomposite, SiC nanoparticle was also not found from the XRD pattern.

ACCEPTED MANUSCRIPT Fig. 2 show optical microscopic images of the AZ91 magnesium alloy and its nanocomposites after IMDF at 400 ℃. The grain size distribution of the AZ91 alloy and its

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nanocomposite with different IMDF passes are shown in Fig. 3. After 1 IMDF pass, as shown

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in Fig. 2(a), the inhomogeneous microstructure of the AZ91 alloy is characterized by the original grains surrounded by newly formed smaller grains. Faraji et al. has reached similar observation about AZ91 alloy after other SPD processing under elevated temperatures and

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medium strain levels [28]. As compared to AZ91 alloy, the degree of DRX is slightly

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increased in the nanocomposite after 1 IMDF pass (Fig. 2(b)). As shown in Figs. 3 (a) and (b), inhomogeneous grain size distribution can be found in both the AZ91 alloy and its

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nanocomposite after 1 IMDF pass. The maximum grain size of AZ91 alloy and its

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nanocomposite is about 110 m and 90 m, respectively. As described in our previous work,

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recrystallization became much fuller in the micron-SiCp/AZ91 composite after 1 IMDF pass than that of the present nanocomposite [22, 24]. As the sizes of reinforcements are less than 1

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m, particles promote DRX of magnesium matrix during hot deformation is unlikely [26]. Thus, the degree of DRX for the present nanocomposite after 1 IMDF pass was different from that of micron-SiCp/AZ91 composite. However, pinning effect of the nanoparticles in the grain boundary could generate during the hot deformation process [26, 29]. Hence, in the current work, the degree of recrystallization for the nanocomposite after 1 IMDF pass is improved compared with the AZ91 alloy. With increasing the number of IMDF passes the accumulated strain values increase, the homogeneity of microstructure for the AZ91 alloy is improved compared with that after 1 IMDF pass as shown in Figs. 2(c) and (e). The degree of recrystallization in the

ACCEPTED MANUSCRIPT nanocomposite with the addition of SiC nanoparticles continues to increase, leading to a fully recrystallized after 6 IMDF passes as given in Figs. 2(d) and (f). As given in Figs. 3(c) and (d),

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the mean grain size of the AZ91 alloy decreases to ～22.1 m while the nanocomposite

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decreases to ～23.8 m after 3 IMDF passes. Further IMDF to 6 passes leads to a slight refinement of grains, as shown in Figs. 3(e) and (f), average grain sizes of the AZ91 alloy and its nanocpmposite are determined to be ～18.7 m and ～18.5 m, respectively. There is no

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significant difference between grain size of alloy and its nanocomposite after 3 and 6 passes.

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This can be attributed to the following reasons: on the one hand, with increasing the number of IMDF passes the accumulated strain values increase, enough strain imposed on each

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section of the sample can lead to fully recrystallized structure and further microstructure

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refinement for both the AZ91 alloy and its nanocomposite. On the other hand, the addition of

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SiC nanoparticles to the matrix alloy may inhibit the recrystallized grain growth in the nanocomposite, which can cause a refiner grain size than that of the AZ91 alloy. However, the

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present high IMDF temperature favors the growth of recrystallization grains, which impair the effect of dispersed SiC nanoparticles on the inhibition of the DRX grains growth in the nanocomposite. Compared with our previous studies [30], the degree of grain refinement after IMDF passes at 400 ℃ in the current research is lower than that after IMDF passes at 350 ℃. In addition, for both the AZ91 alloy and its nanocomposite, cumulative thermal after cyclic heating for a long time can cause static recrystallization and grain growth during reheated IMDF process. On the basis of the above factors, the difference in grain size between AZ91 alloy and its nanocomposite is not obvious after 3 and 6 IMDF passes. In order to analyze the distribution of the SiC nanoparticles and the phase constitution of

ACCEPTED MANUSCRIPT black area analyzed by microstructure observation, SEM test is carried out to investigate the AZ91 alloy and SiCp/AZ91 nanocomposites after different IMDF passes at 400 ℃, as shown

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in Fig. 4. It can be further confirmed that the grain size of the AZ91 alloy after 6 IMDF passes

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exhibits not obvious changes compared with that after 3 IMDF passes as shown in Figs. 4(a) and (c). With respect to SiCp/AZ91 nanocomposites, as shown in Figs. 4(b) and (d), SEM

micrographs demonstrate that the occurrence of fully recrystallization and the decreased

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grain size after 3 and 6 IMDF passes. Furthermore, after the same IMDF passes second

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phases can be observed in the AZ91 alloy while the SiC nanoparticle dense zones along with some second phase are distributed along grain boundaries in the nanocomposite. As shown in

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Fig. 5, EDS is used to investigate the phase constitution of the SiC nanoparticle dense zones

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in the nanocomposite for 6 IMDF passes at 400 ℃. Analyzing an area of SEM micrograph in

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Fig. 5(a), EDS of Mg K, Al K and Si K (Figs. 5(b), (c) and (d)) verify that composition of the particle is SiC nanoparticle. The uniform distribution of SiC nanoparticle outside SiC

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nanoparticle dense zones can be observed in the nanocomposite as indicated by the EDS of Si K. Besides that, the composition of second phase in the form of plates within the SiC nanoparticle dense zones is Mg17Al12 phase by analyzing the EDS of Mg K and Al K. Fig. 6 gives the TEM micrographs of AZ91 alloy after 6 IMDF passes at 400℃. As shown in Figs. 6(a), (b) and (c), nanoscale precipitated phases with different size can be found in the interior of the coarse DRXed grain. Electron diffraction of the precipitated phase confirms that composition of the precipitated phase is Mg17Al12 (Figs. 6(b) and (d)). Figs. 7 and 8 show TEM micrographs of the SiCp/AZ91 nanocomposite after 3 IMDF passes and 6 IMDF passes, respectively. It can be also observed that some SiC nanoparticles are evenly

ACCEPTED MANUSCRIPT dispersed within the nanocomposite after 3 IMDF pass as shown in Fig. 7(a). The study of ED demonstrates that composition of the particle dense zones is SiC nanoparticles, as shown in

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Fig. 7(b). In order to further observe the distribution of SiC nanoparticles in the

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nanocomposite after 3 IMDF pass, dark field image is given in Figs. 7(c) and (d). The black areas indicate magnesium alloy matrix while white areas indicate SiC nanoparticles. It can be found that SiC nanoparticles are distributed near fine DRX grains. As shown in Figs. 8(a) and

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(b), SiC nanoparticles distributed homogeneously near fine DRX grains can be also found in

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the nanocomposite after 6 IMDF passes. This further indicates that the SiC nanoparticles can inhibit the DRX grains growth. At higher magnification, some SiC nanoparticles are

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distributed within the DRX grains while a little are along the DRX grains boundaries as

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shown in Fig. 8(c). The amount of SiC nanoparticles is decreased while more second phases

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are precipitated far from the fine DRX grains. The formation of precipitated phases both in the AZ91 alloy and its nanocomposite in part can be attributed to the strain induced

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precipitation by strong disordering of the magnesium lattice during applied IMDF processing. In view of the low solid solubility of aluminum in magnesium (12.7 wt.% at 437℃) based on phase diagram of magnesium alloys, the present IMDF at 400℃ under constant temperature conditions would be less effective for dynamic precipitation of the second phases in the nanocomposites.

1.3.2 Mechanical properties before and after IMDF The engineering stress-engineering strain curves of AZ91 alloys and the SiCp/AZ91 nanocomposites before and after different IMDF passes at 400 ℃ are obtained from the

ACCEPTED MANUSCRIPT room temperature tensile tests, as shown in Fig. 9. It can be found that the addition of SiC nanoparticles has a significant effect on the tensile behavior of AZ91 alloy. Tensile properties

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from the related curves of AZ91 alloy and SiCp/AZ91 nanocomposites after different IMDF

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passes are summarized in Table 1. Compared with the as-cast counterpart the tensile strength is obviously improved after different IMDF passes for both the AZ91 alloy and the nanocomposites as shown in Table 1. It can be seen from Table 1 that the AZ91 alloy exhibits

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a simultaneous improvement in YS, UTS and elongation to fracture compared with as-cast

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counterpart. With increasing the IMDF passes, the YS of the AZ91 alloy are gradually increased. Similar effect of increasing applied IMDF passes can be also found on tensile

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strength of the nanocomposite (Table 1)，but the degree is different. Both the UTS and the

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elongation to fracture of the nanocomposites are increased significantly with increasing the

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IMDF passes till 6 passes. Compared with the AZ91 alloy under the same IMDF process, the YS of the nanocomposite including SiC nanoparticles act as reinforced phase is significantly

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improved. In particular, after 6 MDF passes the elongation to fracture of the nanocomposite is maintained compared with that of the AZ91 alloy. It is well known that the strength of magnesium with hexagonal closed pack structure depends strongly on grain size due to the lack of slip systems. As shown in Figs. 2(a), (c) and (e), alloy structures are gradually refined till 6 IMDF passes, resulting in the change in the YS of the AZ91 alloy. However, the UTS and elongation to fracture after 6 IMDF passes are decreased compared with that after 3 IMDF passes. This can be ascribed to small amount of the coarse second phase along the grain boundaries in the AZ91 alloy (Fig. 4(c)), which may be prone to crack during room-temperature uniaxial tensile test.

ACCEPTED MANUSCRIPT The improvement in yield strength of developed nanocomposite can be attributed to basic strengthening mechanisms of metal matrix composites. The main strengthening

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mechanisms for metal matrix composites include Hall-Petch strengthening, Orowan

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strengthening, dislocation strengthening mechanism and load transfer mechanism [31]. The increase in tensile strength due to grain refinement can be explained by Hall-Petch equation [32], which is given by

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 Hall-Petch  k y (dnc 1/2  dm1/2 )

(1)

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where, k y is the Hall-Petch coefficient, and d nc represents for the grain of the matrix in the nanocomposite while d m represents for the grain diameter of the AZ91 alloy.

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In the case of nanocomposites where the hard and external SiC nanoparticles are added,

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it would be reasonable to assume that the Orowan strengthening mechanism would occur [32].

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The addition of SiC nanoparticles to the matrix alloy can prevents the dislocation migration. The dislocations need to bypass the nanoparticles using Orowan bow, so the Orowan

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strengthening mechanism is an important mechanism for metal matrix nanocomposite. For magnesium matrix nanocomposites, the enhancement in yield strength due to the Orowan strengthening mechanism is given by following equation [32-36]:

 Orowan =

dp 0.13Gmb ln( ) 1/3 d p [(1/ 2v p )  1] 2b

(2)

Where d p represents for diameter of the nanoparticles； Gm is shear modulus of the matrix alloy; b represents for Burgers vector of the matrix alloy; v p represents for the volume fraction of the nanoparticles. Dislocation strengthening mechanism related to thermal mismatch between the matrix and reinforcement is commonly used to evaluate the effect of dislocation density on the

ACCEPTED MANUSCRIPT strength of the composite. The increase in the yield strength of magnesium matrix composites

12(Tprocess  Ttest )( m   p )v p bd p

(3)

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 CTE  3 Gmb

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due to thermal mismatch for the composite can be expressed as [26, 32, 37-38]:

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Where the value of  is given as 1.25; Tprocess and Ttest are the temperature of material preparation and mechanical measurements, respectively.  m and  p are thermal

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expansion coefficients of the matrix and reinforcement, respectively.

The good interfacial bonding between the dispersed particles and the matrix contributes

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to the transfer of the load applied to the material to the reinforcement. Assuming that the SiC

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nanoparticles are equiaxed, the increase in the yield strength resulting from the load transfer

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effect in the magnesium matrix nanocomposites can be expressed as [34, 36-38]:

 l  0.5v p m

(4)

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Because only 1 vol.% of SiC nanoparticles is used, the improvement in the yield strength due to the load transfer mechanism is less than 1 MPa according to Eq. 4, which is relatively

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small compared to other factors. Thus, the load transfer mechanism can be ignored for the current nanocomposite. Substitution Eq. (1) to Eqs. (3) provides the yield strength increment caused by the SiC nanoparticles is about 81MPa. The maximun yield strength of AZ91 processed by IMDF is 158 MPa (see Table 1), which provides the SiC nanoparticles reinforced AZ91 composite processed by IMDF is 239 MPa. However, it was measured as 189 MPa (see Table 1), i.e., addition of 1 vol% SiC nanoparticles achieves almost 30 MPa increase of the yield strength, while the elongation is almost maintained. The discrepancy indicates that if competely homogenous distribution of SiC particles can be realized in the nanocomposite with the optimized IMDF parameters, the yield strength of the nanocomposite

ACCEPTED MANUSCRIPT could be further improved according to prediction. In addition, TEM observations do not show a significant interaction between SiC nanoparticles and dislocation (Figs. 7 and 8), so

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the improvement in the yield strength due to the dislocation strengthening mechanism may be

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not significant for the present nanocomposite compared with other factors. The grains of the nanocomposites are gradually refined and the homogeneity of grain size is increased till 6 IMDF passes as shown in Figs. 2(b), (d) and (f) as well as Figs. 3(b), (d)

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and (f), resulting in increase of the contribution of Hall-Petch effect on improving the YS. In

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addition, for nanocomposite with homogeneous reinforcement distribution, a strong internal stress must develop between SiC nanoparticles and the matrix under applied load, which

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inhibits slip in the matrix to increase the tensile strength. Thus, as shown in Figs. 5, 7 and 8,

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with increasing the IMDF passes the SiC nanoparticles and the precipitated phases become

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more evenly distributed in the nanocomposites, which also contributes to the increase of ultimate tensile strength and elongation to fracture. So while there is no obvious change on

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the grain size between the nanocomposite and the alloy, the addition of SiC nanoparticles causes the improved YS of the nanocomposite. However, the YS of the nanocomposites exhibit no obvious change with increasing the IMDF passes as shown in Table 1. This can be attributed to texture softening related to the rotation of (0002) basal planes [22, 30], which may impair other strengthening effects. As shown in Table 2, the mechanical properties of the SiCp/AZ91 nanocomposites before and after 6 IMDF passes were compared with other AZ91 composites reinforced with nano-size SiC particles [39-40], micro-size SiC particles [24, 41-42], bimodal size SiC particles [43] and individual particles [44]. It can be seen from Table 2 that the ductility of the

ACCEPTED MANUSCRIPT present nanocomposite was superior when compared with the AZ91-1vol.% nano-SiCp processed by extrusion and equal channel angular pressing and AZ91-1 wt.%SiC composite

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as reported in Ref. [39-40], which means the nanoparticle distribution is more homogeneous

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in the current nanocomposite. The elongation of developed SiCp/AZ91 nanocomposites in the present study were remarkably increased when compared with AZ91 magnesium-based composite reinforced with micron-size SiC particles an average diameter of 10m and

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volume fractions (vol.%) of 10% and the bimodal size SiCp/AZ91 composite containing 1

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vol.% of 0.2 m SiC particles and 9 vol.% of 10m SiC paticles (denote as ‘‘S-1 + 10-9’’), as reported in Ref. [24, 41-43]. Both with the addition of only small amount of inclusions which

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does not affect the lightweight properties of the matrix alloy, the present nanocomposites

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reinforced by SiC nanoparticles possess comparable mechanical properties to the

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AZ91-2wt.%CNT composite as reported in Ref. [44]. The fracture surfaces of the nanocomposite in the as-cast and after 6 IMDF passes

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achieved by room temperature tensile tests are shown in Fig. 10. It can be obviously observed that some dimples and tearing ridge exists in the as-cast nanocomposite as shown in Fig. 10(a). In contrast, for the nanocomposite processed by 6 IMDF passes at 400 ℃ (Fig. 10(b)), more dimples with different size and shapes corresponding to refined grains are found. The changes of fracture surface morphology are consistent with the total elongations (Table 1), which indicate that after IMDF the fracture mechanisms of nanocomposite changes from the mixture of ductile and brittle to the mainly dimpled rupture compared with the as-cast nanocomposite.

ACCEPTED MANUSCRIPT 1.4 Conclusions The as-cast AZ91 alloy and its nanocomposite have been successfully prepared by

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semisolid stirring assisted ultrasonic vibration and then subjected to IMDF. The main

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conclusions can be summarized as follows:

(1) After 1 IMDF pass the microstructures of both the AZ91 alloy and its nanocomposite were uneven. With increasing the number of IMDF passes the accumulated strain values increased,

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the homogeneity of microstructure and the degree of recrystallization continued to increase,

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leading to a fully recrystallized.

(2) Compared with the AZ91 alloy with prolonging the IMDF passes the mean grain size of

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the nanocomposite showed an initial increase after 3 IMDF passes, followed by a decrease

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after 6 IMDF passes. The present high IMDF temperature could impair the effect of dispersed

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SiC nanoparticles on the inhibition of the DRX grains growth. (3) Both the AZ91 alloy and the nanocomposites exhibited an obvious increase in the tensile

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strength after different IMDF passes compared with the as-cast counterpart. The effect of increasing applied IMDF passes on tensile strength of the nanocomposite was similar to the AZ91 alloy，but the degree was different. (4) The change in the YS of the AZ91 alloy depended strongly on grain size, which was in accordance with gradually refined microstructure characterization. The enhanced YS of the nanocomposites reinforced by the SiC nanoparticles could be mainly related to the grain refinement as well as Orowan strengthening effect.

ACCEPTED MANUSCRIPT Acknowledgments This work was supported by the National Natural Science Foundation of China [grant

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numbers 51401144, 51471059], and the “Natural Science Foundation of Shanxi” [grant

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number 2015021067].

References

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processed by cyclic closed-die forging, Mater. Sci. Eng. A 642 (2015) 49-56. [4] M. Gupta,W.L.E. Wong, Magnesium-based nanocomposites: Lightweight materials of the future, Mater. Charact. 105 (2015) 30-46. [5] L.Y. Chen, J.Y. Peng, J.Q. Xu, H. Choi, X.C. Li, Achieving uniform distribution and dispersion of a high percentage of nanoparticles in metal matrix nanocomposites by solidification processing, Scripta Mater. 69 (2013) 634-637. [6] Muhammad Rashad, Fusheng Pan, Wei Guo, Han Lin, Muhammad Asif, Muhammad Irfan, Effect of alumina and silicon carbide hybrid reinforcements on tensile, compressive and microhardness behavior of Mg-3Al-1Zn alloy, Mater. Charact. 106 (2015) 382-389.

ACCEPTED MANUSCRIPT [7] M.J. Shen, X.J. Wang, M.F. Zhang, M.Y. Zheng, K. Wu, Significantly improved strength and ductility in bimodal-size grained microstructural magnesium matrix composites reinforced by bimodal

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microstructures and mechanical properties of SiC nanoparticles reinforced magnesium matrix

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[20] M.G. Jiang, H. Yan, R.S. Chen, Twinning, recrystallization and texture development during multi-directional impact forging in an AZ61 Mg alloy, J. Alloys Compd. 650 (2015) 399-409. [21] Juqiang Li, Juan Liu, Zhenshan Cui, Microstructures and mechanical properties of AZ61 magnesium alloy after isothermal multidirectional forging with increasing strain rate, Mater. Sci. Eng. A 643 (2015) 32-36. [22] K.B. Nie, X.J. Wang, K.K. Deng, F.J. Xu, K. Wu, M.Y. Zheng, Multidirectional forging of AZ91 magnesium alloy and its effects on microstructures and mechanical properties, Mater. Sci. Eng. A 624 (2015) 157-168. [23] Xiangsheng Xia, Qiang Chen, Zude Zhao, Minglong Ma, Xinggang Li, Kui Zhang, Microstructure,

ACCEPTED MANUSCRIPT texture and mechanical properties of coarse-grained Mg-Gd-Y-Nd-Zr alloy processed by multidirectional forging, J. Alloys Compd. 623 (2015) 62-68.

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[24] K.B. Nie, K. Wu, X.J. Wang, K.K. Deng, Y.W. Wu, M.Y. Zheng, Multidirectional forging of

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magnesium matrix composites: Effect on microstructures and tensile properties, Mater. Sci. Eng. A 527 (2010) 7364-7368.

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multidirectional forging on microstructures and tensile properties of a particulate reinforced

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magnesium matrix composite, Mater. Sci. Eng. A 528 (2011) 7133-7139. [26] C.S. Goh, J. Wei, L.C. Lee, M. Gupta, Properties and deformation behaviour of Mg-Y2O3

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[27] Q. Chen, D. Shu, C.K. Hua, Z.D. Zhao, B.G. Yuan, Grain refinement in an as-cast AZ61

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magnesium alloy processed by multi-axial forging under the multitemperature processing procedure, Mater. Sci. Eng. A 541 (2012) 98-104.

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[28] G. Faraji, M.M. Mashhadi, H.S. Kim, Microstructure inhomogeneity in ultra-fine grained bulk AZ91 produced by accumulative back extrusion (ABE), Mater. Sci. Eng. A 528 (2011) 4312-4317. [29] S. Hwang, C. Nishimura, P.G. McCormick, Compressive mechanical properties of Mg-Ti-C nanocomposite synthesised by mechanical milling, Scripta Mater. 44 (2001) 2457-2462. [30] K.B. Nie, K.K. Deng, X.J. Wang, W.M. Gan, F.J. Xu, K. Wu, M.Y. Zheng, Microstructures and mechanical properties of SiCp/AZ91 magnesium matrix nanocomposites processed by multidirectional forging, J. Alloys Compd. 622 (2015) 1018-1026. [31] Muhammad Rashad, Fusheng Pan, Muhammad Asif, Shahid Hussain, Muhammad Saleem, Improving properties of Mg with Al–Cu additions, Mater. Charact. 95 (2014) 140-147.

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nanocomposite, Mater. Sci. Eng. A 486 (2008) 56-62.

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[33] S.F. Hassan, M.J. Tan, M. Gupta, High-temperature tensile properties of Mg/Al2O3

[34] Z. Zhang, D.L. Chen, Contribution of Orowan strengthening effect in particulate-reinforced metal

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matrix nanocomposites, Mater. Sci. Eng. A 483-484 (2008) 148-152.

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[35] G. Cao, J. Kobliska, H. Konishi, X. Li, Tensile properties and microstructure of SiC nanoparticle–reinforced Mg-4Zn alloy fabricated by ultrasonic cavitation–based solidification

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processing, Metall. Mater.Trans. A 39 (2008) 880-886.

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[36] R. Kapoor, N. Kumar, R.S. Mishra, C.S. Huskamp, K.K. Sankaran, Influence of fraction of high

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angle boundaries on the mechanical behavior of an ultrafine grained Al–Mg alloy, Mater. Sci. Eng. A 527 (2010) 5246-5254.

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[37] M.D. Cicco, H. Konishi, G. Cao, H.S. Choi, L. Turng, J.H. Perepezko, S. Kou, R. Lakes, X. Li , Strong, ductile magnesium-zinc nanocomposites, Metall. Mater. Trans. A 40A (2009) 3038-3045. [38] L.H. Dai, Z. Ling, Y.L. Bai, Size-dependent inelastic behavior of particle-reinforced metal–matrix composites, Compos. Sci. Technol. 61 (2001) 1057-1063. [39] X.G. Qiao, T. Ying, M.Y. Zheng, E.D. Wei, K.Wu, X.S. Hu, W.M. Gan, H.G. Brokmeier, I.S. Golovin, Microstructure evolution and mechanical properties of nano-SiCp/AZ91 composite processed by extrusion and equal channel angular pressing (ECAP), Mater. Charact. 121 (2016) 222-230. [40] X. Zhou, D. Su, C.Wu, L. Liu, Tensile mechanical properties and strengthening mechanism of hybrid carbon nanotube and silicon carbide nanoparticle-reinforced magnesium alloy composites, J.

ACCEPTED MANUSCRIPT Nanomater. 2012 (2012) 1-7. [41] K.K. Deng, X.J. Wang, W.M. Gan, Y.W. Wu, K.B. Nie, K. Wu, M.Y. Zheng, H.G. Brokmeier,

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Isothermal forging of AZ91 reinforced with 10 vol.% silicon carbon particles, Mater. Sci. Eng. A 528

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(2011) 1707-1712.

[42] L. Li, M.O. Lai, M. Gupta, B.W. Chua, A. Osman, Improvement of microstructure and mechanical properties of AZ91/SiC composite by mechanical alloying, J. Mater. Sci. 35 (2000) 5553-5561.

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[43] Kunkun Deng, Juyan Shi, Cuiju Wang, Xiaojun Wang, Yewei Wu, Kaibo Nie, Kun Wu,

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Microstructure and strengthening mechanism of bimodal size particle reinforced magnesium matrix composite, Composites: Part A 43 (2012) 1280-1284.

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[44] Qiu-hong Yuan, Xiao-shu Zeng, Yong Liu, Lei Luo, Jun-bin Wu, Yan-chun Wang, Guo-hua Zhou,

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Microstructure and mechanical properties of AZ91 alloy reinforced by carbon nanotubes coated with

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MgO, Carbon 96 (2016) 843-855.

ACCEPTED MANUSCRIPT List of figure captions: Fig. 1. XRD patterns of the AZ91 alloy and SiCp/AZ91 nanocomposite after IMDF.

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Fig. 2. OM micrographs of the AZ91 alloy after (a) 1, (c) 3, (e) 6 IMDF passes at 400 ℃,and

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the SiCp/AZ91 nanocomposite after (b) 1, (d) 3, (f) 6 IMDF passes at 400 ℃.

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Fig. 3. Grain size distribution of the AZ91 alloy after (a) 1, (c) 3, (e) 6 IMDF passes at 400 ℃, and the SiCp/AZ91 nanocomposite after (b) 1, (d) 3, (f) 6 IMDF passes at 400 ℃.

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Fig. 4. SEM micrographs of the AZ91 alloy after (a) 3, (c) 6 IMDF passes at 400 ℃, and the SiCp/AZ91 nanocomposite after (b) 3, (d) 6 IMDF passes at 400 ℃.

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Fig. 5. Surface scan of SiCp/AZ91 nanocomposite for 6 IMDF passes at 400 ℃: (a)

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distribution of SiC nanoparticles; EDS of (b) Mg K, (c) Al K, (d) Si K.

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Fig. 6. TEM micrographs of AZ91 alloy for 6 IMDF passes at 400℃: (a)(b) DRXed grain and fine precipitated phase, (c) DRXed grain and coarse precipitated phase, (d) electron

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diffraction of coarse precipitated phase. Fig. 7. TEM micrographs of SiCp/AZ91 nanocomposite for 3 IMDF pass at 400℃: (a) bright

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field image of SiC nanoparticles and DRX grains, (b) electron diffraction of SiC nanoparticles, (c)(d) dark field images of SiC nanoparticles and DRX grains. Fig. 8. TEM micrographs of SiCp/AZ91 nanocomposite for 6 IMDF pass at 400℃: (a) bright field image of SiC nanoparticles and DRX grains, (b) dark field image of SiC nanoparticles and DRX grains, (c) SiC nanoparticles within DRX grains and precipitated phase. Fig. 9. The engineering stress-engineering strain curves before and after IMDF: (a) AZ91 alloy and (b) nanocomposite. Fig. 10. Fratograph of SiCp/AZ91 nanocomposites before and after IMDF: (a) as-cast, (b) 400-6P.

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Fig. 1. XRD patterns of the AZ91 alloy and SiCp/AZ91 nanocomposites after IMDF.

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ACCEPTED MANUSCRIPT Table 1 Results of tensile properties of AZ91 alloy and SiCp/AZ91 nanocomposites after different IMDF passes. Yield strength

Ultimate

(MPa)

tensile strength (MPa)

AZ91-as-cast

72±2.1

126±3.1

SiCp/AZ91-as-cast

86±2.4

184±5.5

AZ91-400-1P

147±3.6

249±6.2

AZ91-400-3P

152±3.9

304±7.9

AZ91-400-6P

158±4.1

292±7.6

SiCp/AZ91-400-1P

185±5.7

SiCp/AZ91-400-3P

182±5.4

SiCp/AZ91-400-6P

189±5.9

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Ductility (%)

5.4±0.3 2.4±0.2 14±1.1 7.8±0.4

248±6.1

2.8±0.2

294±7.7

8.1±0.4

302±7.8

9.2±0.6

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(MPa)

tensile strength

-184

SiCp/AZ91-400-6P

-179

-302

AZ91- 1vol.% nano-SiCp

-245

-320

AZ91-1 wt.%SiC

-150

AZ91-10vol.% 10m SiCp

-175

AZ91-10vol.% 10m SiCp

-238

AZ91-10vol.% 10m SiCp

-175

AZ91- S-1 + 10-9

-328

AZ91-2wt.%CNT

-197

-9.2

before IMDF [present work] after IMDF [present work]

-3.1

[1]

-220

-5.5

[2]

-267

-1.61

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-251

-0.81

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-1.28

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SiCp/AZ91-as-cast

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ACCEPTED MANUSCRIPT Research Highlights

>SiCp/AZ91 nanocomposite was subjected to multidirectional forging.

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>The SiC nanoparticles and precipitated phases could hinder the DRX grain

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growth.

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>The increase of strength can be attributed to different strengthening effect.