Tensile Properties and Fracture Behavior of a Powder-Thixoformed 2024 Al / SiC p Composite at Elevated Temperatures

In the present work, the tensile properties and fracture behavior of a 2024Al composite reinforced with 10 vol % SiCp and fabricated via powder thixoforming (PT) were studied at temperatures ranging from 25 ◦C to 300 ◦C with a strain rate of 0.05 s−1, as well as the PT 2024 alloy. The results indicated that the tensile strengths of both the PT materials were all decreased with increasing the temperature, but the decrease rate of the composite was smaller than that of the 2024 alloy, and the composite exhibited higher tensile strength than that of the 2024 alloy at all of the employed testing temperatures due to the strengthening role of SiCp. Increasing temperature was beneficial for enhancing the ductility of materials, and the maximum elongation was reached at 250 ◦C. The elongation decrease over 250 ◦C was attributed to the cavity formation due to the debonding of the SiCp/Al interface and the fracturing of the matrix between SiCp. The fracture of the composite at room temperature initiated from the fracture of SiCp and the debonding of the SiCp/Al interface, but that at high temperatures was dominated by void nucleation and growth in the matrix besides the interface debonding.


Introduction
Owing to their low density, high specific strength and stiffness, good wear resistance, and good thermal and electrical properties, aluminum alloy matrix composites reinforced with SiC particles (Al/SiC p composites) have attracted increasing attention in the fields of aerospace, automotive, military, electronics, and sports [1][2][3][4].Among the many fabricating methods, powder metallurgy (PM) is the most promising route because of the uniform distribution of SiC p , controlled SiC p /Al interface, and flexible design of constituents [5,6].However, the wide application of PM is largely limited by some challenges such as high production cost and great difficulty in synthesizing components with complex shapes and compact microstructures [7].However, thixoforming is just appropriate for producing components with such characteristics [8,9].The authors combined the merits of these two technologies and proposed a new fabrication method of powder thixoforming (PT) [10].For this method, a green composite compact is first prepared by using the mixing and cold pressing steps of PM.Then a PT composite component is fabricated via partial remelting and subsequent thixoforming.
So far, various aspects of Al/SiC p composites fabricated via PM have been studied extensively, especially in terms of the mechanical properties at room temperature and elevated temperatures.For example, Angers et al. studied the effects of SiC p volume fraction and milling time on the microstructure and mechanical properties of 2024Al/SiC p composite prepared by PM [11], and Xiao et al. reported the effects of testing temperature and strain rate on hot deformation characteristics of a 2009Al/SiC p composite fabricated by PM [12].Our previous studies revealed that the microstructures of the PT materials were significantly different from those fabricated by PM [10,13].Hence, the resulting tensile properties and corresponding fracture behavior will also differ from those prepared by PM.PT is a novel processing method, and the previous studies focused only on the semisolid microstructural evolution during partial remelting and the effects of processing parameters (such as the mold temperature, reheating temperature and time, and volume fraction of reinforcements) on the microstructure and room-temperature mechanical properties of the resulting Al/SiC p composites [10,[14][15][16].Additionally, the effect of solution heat treatment has also been investigated [17].The experimental results revealed that the Al/SiC p composites with excellent mechanical properties could be fabricated using PT.Unfortunately, there has been no study on the elevated temperature mechanical properties of the PT Al/SiC p composites.
Therefore, in this work, a 2024Al matrix composite reinforced with 10 vol % SiC p was successfully fabricated via PT route and its mechanical properties, strengthening mechanisms, and the corresponding tensile fracture behavior in a temperature range from 25 • C (room temperature) to 300 • C were investigated.

Materials and Methods
The matrix alloy used in present work was gas-atomized 2024Al powders, and the chemical composition (in wt %) was 4.22Cu, 1.35Mg, 0.5Si, and the balance Al.The SiC particles (SiC p ) were used as reinforcements.Figure 1 indicates that the morphologies of the SiC p and Al p were angular and spheroidal, respectively.The average size of the 2024Al powders was 20 µm, and that of the SiC p was 15 µm (Figure 2).To enhance the wettability between the matrix alloy and SiC p as well as to avoid the formation of undesired interfacial reaction products that were always detrimental to the mechanical properties of the composites, the SiC p have been oxidized at 930 • C for 5 h prior to incorporation into the 2024Al powders.
Metals 2017, 7, 408 2 of 13 a 2009Al/SiCp composite fabricated by PM [12].Our previous studies revealed that the microstructures of the PT materials were significantly different from those fabricated by PM [10,13].
Hence, the resulting tensile properties and corresponding fracture behavior will also differ from those prepared by PM.PT is a novel processing method, and the previous studies focused only on the semisolid microstructural evolution during partial remelting and the effects of processing parameters (such as the mold temperature, reheating temperature and time, and volume fraction of reinforcements) on the microstructure and room-temperature mechanical properties of the resulting Al/SiCp composites [10,[14][15][16].Additionally, the effect of solution heat treatment has also been investigated [17].The experimental results revealed that the Al/SiCp composites with excellent mechanical properties could be fabricated using PT.Unfortunately, there has been no study on the elevated temperature mechanical properties of the PT Al/SiCp composites.Therefore, in this work, a 2024Al matrix composite reinforced with 10 vol % SiCp was successfully fabricated via PT route and its mechanical properties, strengthening mechanisms, and the corresponding tensile fracture behavior in a temperature range from 25 °C (room temperature) to 300 °C were investigated.

Materials and Methods
The matrix alloy used in present work was gas-atomized 2024Al powders, and the chemical composition (in wt %) was 4.22Cu, 1.35Mg, 0.5Si, and the balance Al.The SiC particles (SiCp) were used as reinforcements.Figure 1 indicates that the morphologies of the SiCp and Alp were angular and spheroidal, respectively.The average size of the 2024Al powders was 20 μm, and that of the SiCp was 15 μm (Figure 2).To enhance the wettability between the matrix alloy and SiCp as well as to avoid the formation of undesired interfacial reaction products that were always detrimental to the mechanical properties of the composites, the SiCp have been oxidized at 930 °C for 5 h prior to incorporation into the 2024Al powders.The preparation of the 2024Al/SiCp composite by PT mainly includes four procedures.First, the 2024 powders were blended with the oxidized SiCp by a ND7-21 planetary ball mill (Nanjing Levinstep Technology Co., Ltd., Nanjing, China).The ball milling time, ball-to-powder weight ratio, and rotation speed were maintained at 30 min, 5:1, and 100 r/min, respectively.Second, the mixed powders were compacted into green composite compacts with dimensions of Φ45 mm × 20 mm.Subsequently, the green compacts were partially remelted to obtain semisolid ingots.Finally, the semisolid ingots were thixoformed, and some composite products with 50 mm of diameter and 16 mm of thickness were fabricated.The detailed processing procedures have been reported in our previous studies [16,18].For comparison, some PT 2024 alloy components were fabricated by pressing of the as-received 2024 powders, partial remelting, and subsequent thixoforming processes.The preparation of the 2024Al/SiC p composite by PT mainly includes four procedures.First, the 2024 powders were blended with the oxidized SiC p by a ND7-21 planetary ball mill (Nanjing Levinstep Technology Co., Ltd., Nanjing, China).The ball milling time, ball-to-powder weight ratio, and rotation speed were maintained at 30 min, 5:1, and 100 r/min, respectively.Second, the mixed powders were compacted into green composite compacts with dimensions of Φ45 mm × 20 mm.Subsequently, the green compacts were partially remelted to obtain semisolid ingots.Finally, the semisolid ingots were thixoformed, and some composite products with 50 mm of diameter and 16 mm of thickness were fabricated.The detailed processing procedures have been reported in our previous studies [16,18].For comparison, some PT 2024 alloy components were fabricated by pressing of the as-received 2024 powders, partial remelting, and subsequent thixoforming processes.Tensile specimens, as illustrated in Figure 3, were machined from the center region of each product parallel to the direction of the applied pressure.Tensile tests were performed using a WDW-100D universal material testing machine (Jinan HengXu Testing Machine Technology Co., Ltd., Jinan, China) equipped with a heating furnace.The specimens were held at the testing temperatures for 10 min before loading in order to achieve a uniform temperature in the specimens.Tests were carried out at 25 °C (room temperature), 100 °C, 150 °C, 200 °C, 250 °C, and 300 °C, respectively, with a strain rate of 0.05 s −1 .The fractured samples were quickly water-quenched to preserve the deformed microstructures.Three tensile specimens were tested for each temperature.The fracture surfaces were observed using a QUANTA FEG 450 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) and analyzed by energy dispersive X-ray spectroscopy (EDS, FEI, Hillsboro, OR, USA).The side views of the typical fracture surfaces were examined by the SEM to clarify the fracture mechanisms at different testing temperatures.To study the effect of the testing temperature on the strengthening mechanisms, the dislocation density was calculated using D8 ADVANCE X-ray diffraction (XRD, Rigaku, Tokyo, Japan).The microstructure prior to tensile testing was also carried out, and the dimensions of the metallographic specimens are 10 mm × 10 mm × 16 mm.To quantitatively examine the primary particle size and eutectic θ-phase concentration, the related SEM images were analyzed by Image-Pro Plus 5.0 software (Media Cybernetics Company, Silver Spring, MD, USA).At least three SEM images were examined.

Microstructure of the As-Fabricated 2024Al/SiCp Composite
Figure 4 shows the microstructures of the thixoformed 2024Al/SiCp composite and the 2024 alloy, respectively.The microstructure of the 2024Al/SiCp composite consisted of fine spheroidal primary α-Al particles (indicated by the arrows A in Figure 4a), intergranular secondary solidified Tensile specimens, as illustrated in Figure 3, were machined from the center region of each product parallel to the direction of the applied pressure.Tensile tests were performed using a WDW-100D universal material testing machine (Jinan HengXu Testing Machine Technology Co., Ltd., Jinan, China) equipped with a heating furnace.The specimens were held at the testing temperatures for 10 min before loading in order to achieve a uniform temperature in the specimens.Tests were carried out at 25 • C (room temperature), 100 • C, 150 • C, 200 • C, 250 • C, and 300 • C, respectively, with a strain rate of 0.05 s −1 .The fractured samples were quickly water-quenched to preserve the deformed microstructures.Three tensile specimens were tested for each temperature.The fracture surfaces were observed using a QUANTA FEG 450 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) and analyzed by energy dispersive X-ray spectroscopy (EDS, FEI, Hillsboro, OR, USA).The side views of the typical fracture surfaces were examined by the SEM to clarify the fracture mechanisms at different testing temperatures.To study the effect of the testing temperature on the strengthening mechanisms, the dislocation density was calculated using D8 ADVANCE X-ray diffraction (XRD, Rigaku, Tokyo, Japan).The microstructure prior to tensile testing was also carried out, and the dimensions of the metallographic specimens are 10 mm × 10 mm × 16 mm.To quantitatively examine the primary particle size and eutectic θ-phase concentration, the related SEM images were analyzed by Image-Pro Plus 5.0 software (Media Cybernetics Company, Silver Spring, MD, USA).At least three SEM images were examined.Tensile specimens, as illustrated in Figure 3, were machined from the center region of each product parallel to the direction of the applied pressure.Tensile tests were performed using a WDW-100D universal material testing machine (Jinan HengXu Testing Machine Technology Co., Ltd., Jinan, China) equipped with a heating furnace.The specimens were held at the testing temperatures for 10 min before loading in order to achieve a uniform temperature in the specimens.Tests were carried out at 25 °C (room temperature), 100 °C, 150 °C, 200 °C, 250 °C, and 300 °C, respectively, with a strain rate of 0.05 s −1 .The fractured samples were quickly water-quenched to preserve the deformed microstructures.Three tensile specimens were tested for each temperature.The fracture surfaces were observed using a QUANTA FEG 450 scanning electron microscope (SEM, FEI, Hillsboro, OR, USA) and analyzed by energy dispersive X-ray spectroscopy (EDS, FEI, Hillsboro, OR, USA).The side views of the typical fracture surfaces were examined by the SEM to clarify the fracture mechanisms at different testing temperatures.To study the effect of the testing temperature on the strengthening mechanisms, the dislocation density was calculated using D8 ADVANCE X-ray diffraction (XRD, Rigaku, Tokyo, Japan).The microstructure prior to tensile testing was also carried out, and the dimensions of the metallographic specimens are 10 mm × 10 mm × 16 mm.To quantitatively examine the primary particle size and eutectic θ-phase concentration, the related SEM images were analyzed by Image-Pro Plus 5.0 software (Media Cybernetics Company, Silver Spring, MD, USA).At least three SEM images were examined.

Microstructure of the As-Fabricated 2024Al/SiC p Composite
Figure 4 shows the microstructures of the thixoformed 2024Al/SiC p composite and the 2024 alloy, respectively.The microstructure of the 2024Al/SiC p composite consisted of fine spheroidal primary α-Al particles (indicated by the arrows A in Figure 4a), intergranular secondary solidified structures (SSS) (indicated by the arrows B in Figure 4a), and SiC p that distributed homogeneously within SSS (indicated by the arrows Figure 4a), which is different from the coarsened microstructures produced by conventional processes [19,20].The microstructure of the 2024 alloy was similar to that of the composite in addition to the SiC p (Figure 4c).The SSS in these two materials included the secondary α-Al phases (indicated by the arrows C in Figure 4b), eutectic α-Al phase (indicated by the arrows D in Figure 4b), and eutectic θ-phase (indicated by the arrows E in Figure 4b).It can be found that the primary particle size (24.02µm) and eutectic θ-phase concentration (4.34%) of the composite was smaller and less than those (34.05µm and 5.15%) of the alloy, respectively (comparing Figure 4b,d).
Metals 2017, 7, 408 4 of 13 structures (SSS) (indicated by the arrows B in Figure 4a), and SiCp that distributed homogeneously within SSS (indicated by the arrows Figure 4a), which is different from the coarsened microstructures produced by conventional processes [19,20].The microstructure of the 2024 alloy was similar to that of the composite in addition to the SiCp (Figure 4c).The SSS in these two materials included the secondary α-Al phases (indicated by the arrows C in Figure 4b), eutectic α-Al phase (indicated by the arrows D in Figure 4b), and eutectic θ-phase (indicated by the arrows E in Figure 4b).It can be found that the primary particle size (24.02μm) and eutectic θ-phase concentration (4.34%) of the composite was smaller and less than those (34.05μm and 5.15%) of the alloy, respectively (comparing Figure 4b,d).The mechanisms for thixoformed microstructure have been detailedly clarified in our previous investigations [21].In short, the secondary α-Al phases preferentially attached to the surfaces of the primary α-Al phase, and then the equiaxed secondary α-Al phases nucleated in the regions far away from the primary particles upon achieving the requirements for heterogeneous nucleation.Finally, solidification was completed through the eutectic reaction of the residual liquid phase.It should be noted that the incorporation of SiCp into the 2024 alloy decreased the coarsening rate [18], thereby the primary particle size was smaller than that of the 2024 alloy.The uniform distribution of SiCp within the Al powders was achieved by ball mill procedure, and the eutectics partially dissolved towards the primary α-Al phase during ball milling because of the increased mixture temperature caused by ball-powder collision and ball-ball sliding [22], lowering the eutectic concentration of the milled Al powders compared with that of the as-received 2024 powders.Moreover, the ball-milled Al powders always possessed high-density dislocations due to the plastic deformation that occurred during ball milling [23], providing convenient channels for dissolution of θ phase during partial remelting.Besides, the effective liquid fraction of the composite was less than that of the 2024 alloy The mechanisms for thixoformed microstructure have been detailedly clarified in our previous investigations [21].In short, the secondary α-Al phases preferentially attached to the surfaces of the primary α-Al phase, and then the equiaxed secondary α-Al phases nucleated in the regions far away from the primary particles upon achieving the requirements for heterogeneous nucleation.Finally, solidification was completed through the eutectic reaction of the residual liquid phase.It should be noted that the incorporation of SiC p into the 2024 alloy decreased the coarsening rate [18], thereby the primary particle size was smaller than that of the 2024 alloy.The uniform distribution of SiC p within the Al powders was achieved by ball mill procedure, and the eutectics partially dissolved towards the primary α-Al phase during ball milling because of the increased mixture temperature caused by ball-powder collision and ball-ball sliding [22], lowering the eutectic concentration of the milled Al powders compared with that of the as-received 2024 powders.Moreover, the ball-milled Al powders always possessed high-density dislocations due to the plastic deformation that occurred during ball milling [23], providing convenient channels for dissolution of θ phase during partial remelting.Besides, the effective liquid fraction of the composite was less than that of the 2024 alloy because of the SiC p incorporation, decreasing the eutectic concentration.Therefore, the primary particle size was smaller and eutectic θ-phase concentration was less compared with the PT alloy.From this standpoint, the generation of the powder-thixoformed microstructures can be well interpreted.

Effect of Temperature on Tensile Properties and Strengthening Mechanisms
Figure 5 shows the variations of tensile properties of the 2024Al/SiC p composite with the testing temperature, as well as the 2024 alloy.It was observed that the composite tested at room temperature had an ultimate tensile strength (UTS) of 388 MPa and 0.2% offset yield strength (YS) of 297 MPa, increased by 29.3% and 35.0%, respectively, compared to those of the 2024 alloy.In addition, both the UTS and YS were all obviously higher than those of the congeneric 2024Al/SiC p composites prepared by other methods.For example, the UTS and YS of the 10 vol % 10 µm SiC p reinforced 2024 composite prepared by extrusion were 308 MPa and 210 MPa, respectively [24], and those of the 15 vol % 10 µm SiC p reinforced 2024 composite prepared by PM were 320 MPa and 212 MPa, respectively [25].Furthermore, these two properties of the PT 2024 alloy were also significantly higher than those of the squeeze-cast 2024 alloy [26].All of these show that PT is a promising method to fabricate metallic materials, especially the particle reinforced metal matrix composites.As the testing temperature increased, decreases in the UTS and YS of both the composite and alloy were attained, but the UTS and YS of the composite were always higher than those of the 2024 alloy within the entire range of testing temperatures.Especially, the decrease rate of the composite was obviously slower than that of the alloy within 25 • C-150 • C. The UTS and YS of the composite were only decreased by 18.8% and 21.9% at 150 • C, while those of the 2024 alloy were decreased by 24.3% and 25.9%.That is, the composite exhibited better strength stability against temperature over the 2024 alloy.As expected, the elongation of the composite increased continuously as the temperature increased, but it slightly decreased when the testing temperature exceeded 250 • C.However, the elongation of the 2024 alloy always increased in the entire temperature range.In addition, the elongation of the composite was much lower than that of the alloy at each testing temperature, lowered by 63.5% and 24.4% compared with the 2024 alloy at 25 because of the SiCp incorporation, decreasing the eutectic concentration.Therefore, the primary particle size was smaller and eutectic θ-phase concentration was less compared with the PT alloy.From this standpoint, the generation of the powder-thixoformed microstructures can be well interpreted.

Effect of Temperature on Tensile Properties and Strengthening Mechanisms
Figure 5 shows the variations of tensile properties of the 2024Al/SiCp composite with the testing temperature, as well as the 2024 alloy.It was observed that the composite tested at room temperature had an ultimate tensile strength (UTS) of 388 MPa and 0.2% offset yield strength (YS) of 297 MPa, increased by 29.3% and 35.0%, respectively, compared to those of the 2024 alloy.In addition, both the UTS and YS were all obviously higher than those of the congeneric 2024Al/SiCp composites prepared by other methods.For example, the UTS and YS of the 10 vol % 10 μm SiCp reinforced 2024 composite prepared by extrusion were 308 MPa and 210 MPa, respectively [24], and those of the 15 vol % 10 μm SiCp reinforced 2024 composite prepared by PM were 320 MPa and 212 MPa, respectively [25].Furthermore, these two properties of the PT 2024 alloy were also significantly higher than those of the squeeze-cast 2024 alloy [26].All of these show that PT is a promising method to fabricate metallic materials, especially the particle reinforced metal matrix composites.As the testing temperature increased, decreases in the UTS and YS of both the composite and alloy were attained, but the UTS and YS of the composite were always higher than those of the 2024 alloy within the entire range of testing temperatures.Especially, the decrease rate of the composite was obviously slower than that of the alloy within 25 °C-150 °C.The UTS and YS of the composite were only decreased by 18.8% and 21.9% at 150 °C, while those of the 2024 alloy were decreased by 24.3% and 25.9%.That is, the composite exhibited better strength stability against temperature over the 2024 alloy.As expected, the elongation of the composite increased continuously as the temperature increased, but it slightly decreased when the testing temperature exceeded 250 °C.However, the elongation of the 2024 alloy always increased in the entire temperature range.In addition, the elongation of the composite was much lower than that of the alloy at each testing temperature, lowered by 63.5% and 24.4% compared with the 2024 alloy at 25 °C and 300 °C, respectively.It is known that two types of the strengthening mechanisms were responsible for the enhanced strength in the composite over the unreinforced matrix alloy: direct and indirect strengthening [27][28][29][30].The direct strengthening operated by load transfer from the matrix to the reinforcements.The indirect strengthening resulted from the microstructural changes of the composite, such as It is known that two types of the strengthening mechanisms were responsible for the enhanced strength in the composite over the unreinforced matrix alloy: direct and indirect strengthening [27][28][29][30].The direct strengthening operated by load transfer from the matrix to the reinforcements.The indirect strengthening resulted from the microstructural changes of the composite, such as thermal mismatch strengthening (TMS) and geometrically necessary dislocations strengthening (GNDs).
According to the load-transfer strengthening (LTS) theory, the YS increment, ∆σ LTS , was given by [27]: where σ m and V c are the matrix YS and volume fraction of SiC p , respectively.The YS increment induced by TMS, ∆σ TMS , was given by [29]: where B is dislocation strengthening coefficient with a value of 1.25, G m is the shear modulus (26.4 GPa), ∆α is the mismatch in CTE between the matrix and SiC p (19.1 × 10 −6 K −1 ), ∆T is the difference between the processing temperature and testing temperature (∆T = 600 K), d c is the average size of the SiC p , and b is the Burgers vector (0.286 nm) [31][32][33].
The YS increment, ∆σ GNDS , can be calculated according to [30]: where υ is Poisson's ratio (0.33), and ε c is yield strain (0.2%).The contributions of LTS, TMS, and GNDs to YS increments calculating from Equations ( 1)-(3) were 11.0 MPa, 30.9 MPa, and 20.8 MPa, respectively.Ramakrishnan reported that both the additive and synergistic effects of different strengthening effects should be taken into account if several kinds of strengthening mechanisms operate simultaneously [34], and thus the YS of the composite can be predicted by superposing the individual contribution of the aforementioned strengthening mechanisms.The predicted results indicated that the calculated value is much closer to the experimental value of the composite (Figure 6), which quantitatively demonstrated that the load-transfer strengthening and dislocation strengthening contributed to strengthening the composite at room temperature.
According to the load-transfer strengthening (LTS) theory, the YS increment, ΔσLTS, was given by [27]: where σm and Vc are the matrix YS and volume fraction of SiCp, respectively.The YS increment induced by TMS, ΔσTMS, was given by [29]: where B is dislocation strengthening coefficient with a value of 1.25, Gm is the shear modulus (26.4 GPa), Δα is the mismatch in CTE between the matrix and SiCp (19.1 × 10 −6 K −1 ), ΔT is the difference between the processing temperature and testing temperature (ΔT = 600 K), dc is the average size of the SiCp, and b is the Burgers vector (0.286 nm) [31][32][33].
The YS increment, ΔσGNDS, can be calculated according to [30]: where υ is Poisson's ratio (0.33), and εc is yield strain (0.2%).The contributions of LTS, TMS, and GNDs to YS increments calculating from Equations ( 1)-(3) were 11.0 MPa, 30.9 MPa, and 20.8 MPa, respectively.Ramakrishnan reported that both the additive and synergistic effects of different strengthening effects should be taken into account if several kinds of strengthening mechanisms operate simultaneously [34], and thus the YS of the composite can be predicted by superposing the individual contribution of the aforementioned strengthening mechanisms.The predicted results indicated that the calculated value is much closer to the experimental value of the composite (Figure 6), which quantitatively demonstrated that the load-transfer strengthening and dislocation strengthening contributed to strengthening the composite at room temperature.Both the calculated YS and experimental YS of the composite decreased as the testing temperature increased (Figures 5 and 6), implying that the strengthening efficiency may increasingly become less effective.Dislocation strengthening was significantly dependent on the density of dislocations.To determine the strengthening contribution with temperature, the dislocation density was quantitatively calculate by XRD based on the broadening of the diffraction peaks.Both the calculated YS and experimental YS of the composite decreased as the testing temperature increased (Figures 5 and 6), implying that the strengthening efficiency may increasingly become less effective.Dislocation strengthening was significantly dependent on the density of dislocations.To determine the strengthening contribution with temperature, the dislocation density was quantitatively calculate by XRD based on the broadening of the diffraction peaks.Figure 7a shows that XRD patterns of the composite tested at different temperatures, indicating that the full width at half maximum (W hkl ) values of the diffraction peaks decreased as the temperature increased.According to the Williamson-Hall equation (WH), W hkl can be calculated with Equation (4) [35,36]: where θ hkl is the Bragg angle, λ is the wavelength (0.15406 nm), d is the crystallite size, and ε is the lattice strain.Plotting W hkl cosθ hkl as a function of 4sinθ hkl by performing linear regression analysis (Figure 7b), the ε and d values were determined by the slopes and intercepts of the fitted curves, respectively.The dislocation density, ρ, was given by Equation ( 5) [37]: shows that XRD patterns of the composite tested at different temperatures, indicating that the full width at half maximum (Whkl) values of the diffraction peaks decreased as the temperature increased.
According to the Williamson-Hall equation (WH), Whkl can be calculated with Equation ( 4) [35,36]: where θhkl is the Bragg angle, λ is the wavelength (0.15406 nm), d is the crystallite size, and ε is the lattice strain.Plotting Whkl cosθhkl as a function of 4sinθhkl by performing linear regression analysis (Figure 7b), the ε and d values were determined by the slopes and intercepts of the fitted curves, respectively.The dislocation density, ρ, was given by Equation ( 5) [37]: For the composite tested at 25 °C and 300 °C, the values of ρ decreased from 1.6 × 10 14 m −2 to 6.5 × 10 13 m −2 as calculated from Equation (5).Increasing deformation temperature promoted dislocation motion by enhancing dislocation cross slip and climb due to the high thermal activation effect, and thus decreasing the dislocation density.It has been generally accepted that the strengthening contribution from dislocations, Δσdis, was calculated with the Bailey-Hirsch equation [38]: where M is the mean orientation factor (3.06) and β is a coefficient (0.2) [39].Substituting Equation ( 5) into Equation ( 6), Δσdis decreased from 58.4 MPa to 37.3 MPa with increasing the temperature from 25 °C to 300 °C, which quantitatively indicated that decreasing the dislocation density weakened the strength of the materials.Therefore, high temperature impeded the contribution of dislocation strengthening to YS increment.On the basis of Equation ( 1), Δσlt decreased from 11 MPa to 2.8 MPa when the temperature increased from 25 °C to 300 °C, which confirmed that high temperature degraded the load-transfer strengthening.During the tensile tests, the stress concentration around the SiCp was formed.The SiCp/Al interfaces debonded and SiCp cracked when the stress concentration exceeded a certain level, as evidenced by Figure 8a that many cracked and debonded SiCp distributed homogeneously on the resulting fracture surface.The SiCp failure well verified that the SiCp could effectively bear the external load transferred from the matrix during the tensile tests.That is, the contribution of load-transfer strengthening to YS increment at room temperature was operative.As the temperature increased, the void nucleation in the matrix has become more extensive, and cracks preferentially initiated in and propagated along the matrix, giving rise to the premature fracturing of the composite before the load-carrying capacity of SiCp was operated completely.This can be demonstrated by the following experimental results shown in Section 3.3.Therefore, the For the composite tested at 25 • C and 300 • C, the values of ρ decreased from 1.6 × 10 14 m −2 to 6.5 × 10 13 m −2 as calculated from Equation (5).Increasing deformation temperature promoted dislocation motion by enhancing dislocation cross slip and climb due to the high thermal activation effect, and thus decreasing the dislocation density.It has been generally accepted that the strengthening contribution from dislocations, ∆σ dis , was calculated with the Bailey-Hirsch equation [38]: where M is the mean orientation factor (3.06) and β is a coefficient (0.2) [39].Substituting Equation ( 5) into Equation ( 6), ∆σ dis decreased from 58.4 MPa to 37.3 MPa with increasing the temperature from 25 • C to 300 • C, which quantitatively indicated that decreasing the dislocation density weakened the strength of the materials.Therefore, high temperature impeded the contribution of dislocation strengthening to YS increment.On the basis of Equation ( 1), ∆σ lt decreased from 11 MPa to 2.8 MPa when the temperature increased from 25 • C to 300 • C, which confirmed that high temperature degraded the load-transfer strengthening.During the tensile tests, the stress concentration around the SiC p was formed.The SiC p /Al interfaces debonded and SiC p cracked when the stress concentration exceeded a certain level, as evidenced by Figure 8a that many cracked and debonded SiC p distributed homogeneously on the resulting fracture surface.The SiC p failure well verified that the SiC p could effectively bear the external load transferred from the matrix during the tensile tests.That is, the contribution of load-transfer strengthening to YS increment at room temperature was operative.As the temperature increased, the void nucleation in the matrix has become more extensive, and cracks preferentially initiated in and propagated along the matrix, giving rise to the premature fracturing of the composite before the load-carrying capacity of SiC p was operated completely.This can be demonstrated by the following experimental results shown in Section 3.3.Therefore, the load-transfer strengthening to YS increment of the composite tested at elevated temperature was lower than that at room temperature.From the aforementioned standpoints, the variations of mechanical properties with respect to the testing temperature can be well interpreted.
Metals 2017, 7, 408 8 of 13 load-transfer strengthening to YS increment of the composite tested at elevated temperature was lower than that at room temperature.From the aforementioned standpoints, the variations of mechanical properties with respect to the testing temperature can be well interpreted.

Effect of Temperature on Tensile Fracture Behavior
The fracture surfaces of the 2024Al/SiCp composite tested at different temperatures were shown in Figure 8.The fracture of the composite was found to be brittle at room temperature, as evidenced by the quasi-cleavage facets on the fracture surface (Figure 8a).Many failed SiCp (including the cracked SiCp and debonded SiCp/Al interface) can also be observed (indicated by the arrows in Figure 8a).The primary α-Al particles basically maintained their original spheroidal morphology in the side view of fracture surface (comparing Figures 4a and 9a), which implies that no obvious plastic deformation occurred prior to fracture, and thus the ductility of composite was quite low.The visible SiCp can also be observed in the side view of the fracture surface (Figure 9a).During the tensile tests, a higher stress concentration will be generated surrounding the SiCp/Al interface due to the mismatch between the plastic deformation abilities of the SiCp (only elastic deformation) and Al matrix (plastic deformation), which induces the cracking or debonding of the SiCp if the concentrated stress reaches a critical value.The failed SiCp increased the points of crack initiation and propagation, and thus resulted in the premature fracture of the composite with a lower elongation.This has been evidenced by the in situ tensile testing in our previous work [16].That is, the fracture of the composite at room temperature was mainly initiated from the fractured SiCp and debonded SiCp/Al interface.However, a large number of small dimples distributed homogeneously in the fracture surface of the 2024 alloy as shown in Figure 10a, indicating a ductile fracture mode.

Effect of Temperature on Tensile Fracture Behavior
The fracture surfaces of the 2024Al/SiC p composite tested at different temperatures were shown in Figure 8.The fracture of the composite was found to be brittle at room temperature, as evidenced by the quasi-cleavage facets on the fracture surface (Figure 8a).Many failed SiC p (including the cracked SiC p and debonded SiC p /Al interface) can also be observed (indicated by the arrows in Figure 8a).The primary α-Al particles basically maintained their original spheroidal morphology in the side view of fracture surface (comparing Figures 4a and 9a), which implies that no obvious plastic deformation occurred prior to fracture, and thus the ductility of composite was quite low.The visible SiC p can also be observed in the side view of the fracture surface (Figure 9a).During the tensile tests, a higher stress concentration will be generated surrounding the SiC p /Al interface due to the mismatch between the plastic deformation abilities of the SiC p (only elastic deformation) and Al matrix (plastic deformation), which induces the cracking or debonding of the SiC p if the concentrated stress reaches a critical value.The failed SiC p increased the points of crack initiation and propagation, and thus resulted in the premature fracture of the composite with a lower elongation.This has been evidenced by the in situ tensile testing in our previous work [16].That is, the fracture of the composite at room temperature was mainly initiated from the fractured SiC p and debonded SiC p /Al interface.However, a large number of small dimples distributed homogeneously in the fracture surface of the 2024 alloy as shown in Figure 10a, indicating a ductile fracture mode.When the composite was tested at 200 °C, the fracture surface was characterized by tear ridges, and shallow and small dimples was observed in some local regions (Figure 8b), which was indicative of a ductile-brittle mixed fracture mode.As the temperature exceeded 200 °C, the fracture surfaces were characterized by large pits and heaves (Figures 8c,d and 10b), indicating a quite brittle fracture feature.However, the side views of resulting fracture surfaces indicate that the primary particles were elongated along the loading direction (comparing Figure 9b-d), implying that the composite experienced serious plastic deformation during tensile testing, and thus exhibiting a better ductility.This phenomenon was attributed to the softening of the matrix at elevated temperature.In addition, the side views of fracture surfaces indicate that many large-sized cavities  When the composite was tested at 200 °C, the fracture surface was characterized by tear ridges, and shallow and small dimples was observed in some local regions (Figure 8b), which was indicative of a ductile-brittle mixed fracture mode.As the temperature exceeded 200 °C, the fracture surfaces were characterized by large pits and heaves (Figures 8c,d and 10b), indicating a quite brittle fracture feature.However, the side views of resulting fracture surfaces indicate that the primary particles were elongated along the loading direction (comparing Figure 9b-d), implying that the composite experienced serious plastic deformation during tensile testing, and thus exhibiting a better ductility.This phenomenon was attributed to the softening of the matrix at elevated temperature.In addition, the side views of fracture surfaces indicate that many large-sized cavities were characterized by large pits and heaves (Figure 8c,d and Figure 10b), indicating a quite brittle fracture feature.However, the side views of resulting fracture surfaces indicate that the primary particles were elongated along the loading direction (comparing Figure 9b-d), implying that the composite experienced serious plastic deformation during tensile testing, and thus exhibiting a better ductility.This phenomenon was attributed to the softening of the matrix at elevated temperature.In addition, the side views of fracture surfaces indicate that many large-sized cavities nucleated in the SSS, especially around the SiC p /Al interface (indicated by the arrows in Figure 9c,d).Moreover, both the size and number of the cavities increased as the temperature elevated.It is known that SSS were the weak points of the thixoformed materials because shrinkage porosities and pores always existed in these structures [40].Meanwhile, its compatible deformation ability was poor due to the existence of hard and fragile eutectics and SiC p .During tensile testing, the stress concentration was easily generated in the SSS, especially the stress around the SiC p /Al interface was higher, resulting in the growth of the shrinkage porosities and pores as well as formation of microcracks surrounding the SiC p /Al interface due to the debonded SiC p .As the temperature increased, the enhanced plastic deformation ability of the primary particles induced larger stress concentration in the SSS, resulting in the further increased size and number of the cavities.These discontinuous cavities joined together as the loading continued, leading to the fracture of the composite.It was supposed that the large pits on the fracture surface evolved from these cavities and the heaves corresponded to the microstructure surrounding the pits that fractured due to serious deformation, and thus forming the uneven fracture surface.The debonding of SiC p /Al interface and void nucleation in the matrix contributed to crack initiation and propagation, limiting further increase in ductility for the composite with temperature.Thus, the elongation of the composite exhibited the maximum elongation at 250 • C, but that slightly decreased as the temperature further increased.By comparing Figure 8a-d, the number of the visible SiC p on the fracture surfaces decreased gradually with increasing the testing temperature, which was evident from the EDS analysis results of elemental Si on the resulting fracture surfaces (Figure 11).The quantitative results indicated that the visible SiC p fraction decreased from 16.86% to 3.51% as the temperature increased from 25 • C to 300 • C. A decrease in the visible SiC p fraction was attributed to the increased cavities that facilitated the crack propagation in the matrix.Therefore, the debonding of SiC p as well as void nucleation and growth were the main fracture mechanisms in the composite at high temperature, and increasing the testing temperature (less than 250 • C) induced relatively improved ductility of the composite, but the elongation decreased when the temperature exceeded 250 • C due to the extensive cavity formation.
Metals 2017, 7, 408 10 of 13 pores always existed in these structures [40].Meanwhile, its compatible deformation ability was poor due to the existence of hard and fragile eutectics and SiCp.During tensile testing, the stress concentration was easily generated in the SSS, especially the stress around the SiCp/Al interface was higher, resulting in the growth of the shrinkage porosities and pores as well as formation of microcracks surrounding the SiCp/Al interface due to the debonded SiCp.As the temperature increased, the enhanced plastic deformation ability of the primary particles induced larger stress concentration in the SSS, resulting in the further increased size and number of the cavities.These discontinuous cavities joined together as the loading continued, leading to the fracture of the composite.It was supposed that the large pits on the fracture surface evolved from these cavities and the heaves corresponded to the microstructure surrounding the pits that fractured due to serious deformation, and thus forming the uneven fracture surface.The debonding of SiCp/Al interface and void nucleation in the matrix contributed to crack initiation and propagation, limiting further increase in ductility for the composite with temperature.Thus, the elongation of the composite exhibited the maximum elongation at 250 °C, but that slightly decreased as the temperature further increased.By comparing Figure 8a-d, the number of the visible SiCp on the fracture surfaces decreased gradually with increasing the testing temperature, which was evident from the EDS analysis results of elemental Si on the resulting fracture surfaces (Figure 11).The quantitative results indicated that the visible SiCp fraction decreased from 16.86% to 3.51% as the temperature increased from 25 °C to 300 °C.A decrease in the visible SiCp fraction was attributed to the increased cavities that facilitated the crack propagation in the matrix.Therefore, the debonding of SiCp as well as void nucleation and growth were the main fracture mechanisms in the composite at high temperature, and increasing the testing temperature (less than 250 °C) induced relatively improved ductility of the composite, but the elongation decreased when the temperature exceeded 250 °C due to the extensive cavity formation.

Conclusions
(1) The powder-thixoformed 2024Al/SiCp composite exhibited outstanding properties and was thereby favorable for prospective applications.(2) The microstructure of the composite was composed of fine spheroidal primary α-Al particles, intergranular SSS, and SiCp that were distributed homogeneously in the SSS.

Conclusions
(1) The powder-thixoformed 2024Al/SiC p composite exhibited outstanding properties and was thereby favorable for prospective applications.(2) The microstructure of the composite was composed of fine spheroidal primary α-Al particles, intergranular SSS, and SiC p that were distributed homogeneously in the SSS.(3) The incorporation of SiC p to the 2024 alloy brought considerable improvements, the UTS and YS of the PT composite at room temperature were increased by 29.3% and 35%, respectively, compared with the 2024 alloy, which was mainly attributed to the strengthening effects of LTS, TMS, and GNDs.(4) Owing to the degraded strengthening effects due to the decreased dislocation density and weakened load-bearing ability, the tensile strengths of both the PT materials were all decreased with increasing the temperature, while the composite exhibited higher tensile strength than that of the 2024 alloy at all of the employed testing temperatures, revealing better stability of its strength against temperature over the 2024 alloy.(5) A high temperature was beneficial for enhancing the ductility of the materials because of the increased softening effect upon increasing the temperatures, whereas the composite possessed the maximum elongation at 250 • C that decreased up to 300 • C because the formation of extensive cavities due to the debonding of the SiC p /Al interface and fracturing of the matrix limited further increase in ductility as the temperature increased.(6) The fracture of the composite at room temperature was initiated by failure of the SiC p , whereas void nucleation and growth as well as debonding between the SiC p and matrix were the main fracture mechanisms at high temperature.

Figure 2 .
Figure 2. Size distributions of the 2024Al powders and SiCp.

Figure 2 .
Figure 2. Size distributions of the 2024Al powders and SiC p .

Figure 2 .
Figure 2. Size distributions of the 2024Al powders and SiCp.

Figure 4
Figure4shows the microstructures of the thixoformed 2024Al/SiCp composite and the 2024 alloy, respectively.The microstructure of the 2024Al/SiCp composite consisted of fine spheroidal primary α-Al particles (indicated by the arrows A in Figure4a), intergranular secondary solidified

Figure 5 .
Figure 5. Variations of the mechanical properties with the testing temperature.

Figure 5 .
Figure 5. Variations of the mechanical properties with the testing temperature.

Figure 6 .
Figure 6.Comparison between theoretical calculation and experimental results.

Figure 6 .
Figure 6.Comparison between theoretical calculation and experimental results.

Figure 7 .
Figure 7. (a) XRD patterns and (b) Williamson-Hall (WH) plots of the composite tested at different temperatures.

Figure 7 .
Figure 7. (a) XRD patterns and (b) Williamson-Hall (WH) plots of the composite tested at different temperatures.

Figure 11 .
Figure 11.EDS surface scanning analysis of elemental Si on the fracture surfaces of the composite tested at temperatures of (a) 25 °C and (b) 300 °C.

Figure 11 .
Figure 11.EDS surface scanning analysis of elemental Si on the fracture surfaces of the composite tested at temperatures of (a) 25 • C and (b) 300 • C.