EDS points analysis on the FE-scanning electron microscopy (SEM) images of (a) Al 2 O 3 and (b) Al 2 O 3 –SiO 2 fibers. 

Contexts in source publication

Context 1

... advancement of reactivity between SiO 2 sol and molten Al alloy can lead to full densification of Al matrix to the internal minute spaces of preform, whereas the bad wettability between Al 2 O 3 fibers and molten Al alloy has been expected to generate imperfect infiltration around the fibers. Thus, the reaction is also expected to increase the relative density of the composites. The microstructures of composites using different content of SiO 2 sol and different types of alumina fibers are shown in Figure 3. There exists diverse sizes and directions of alumina fibers, Si phase surrounding fibers and some pores around those fibers. In addition, each alumina fiber has been seen two types of fiber cross- sectional structures: single and double-layered fiber regardless of SiO 2 content, but the content of Al, O and Si is not similar and not so far from the theoretical values, as shown in Figure 1. Figure 3(a) and (b) shows the microstructure of the composites reinforced with Al 2 O 3 and Al 2 O 3 –SiO 2 fibers, respectively, with the same content of SiO sol (8 mass%). Both images represent dense morphology with their high relative density of 97.4 and 97.7%, even though some pores exist at the extremely narrow spaces between fibers. Although the composites reinforced with Al 2 O 3 –SiO 2 fibers have slightly higher density than those reinforced with Al 2 O 3 fibers, it is negligible. It means that the SiO 2 content in Al 2 O 3 –SiO 2 fibers is expected to improve wettability, as shown in Figure 1, but such content of SiO 2 did not give remarkable influence to the densification of composites, due to much amount of SiO 2 sol which was covered on the surface of the alumina fibers widely. On the other hand, the composites containing 2 mass% sol exhibit large amounts of pores around Al 2 O 3 fibers seemed to be caused by imperfect infiltration of the molten Al alloy. Some of imperfect infiltration parts at the narrow spaces between fibers (B in Figure 3(c)) might be caused by bad wettability between molten Al and Al 2 O 3 fibers by lack of SiO 2 sol content. However, increase of the SiO 2 sol content enables to fill such narrow spaces with the sol, as shown in Figure 3(b) A. The composites containing 2 mass% sol also present relatively lower density than those containing 8 mass% in Table 2. Therefore, it is obvious that the content of sol contributes to the densification of the composites rather than chemical composition of fiber. The role of SiO 2 sol was not only improving the wettability but also reacting with matrix. Figure 4 exhibits the EDS point analysis of matrix (Figure 4(a)) and the X-ray diffraction (XRD) result of casting alloy of matrix and SiO 2 sol. The detected amount of oxygen was not considerable with low content of Si in Al matrix around fibers in Figure 4(a). The amounts of Mg, Ni and Cu detected were the original elements contained in Al matrix (A366). Besides, the XRD result of matrix (Figure 4(b) indicates that some amount of Al in matrix reacted with SiO 2 sol as written in equation (1) and generated some amount of Al 2 O 3 and Si. That is, the SiO 2 sol surrounding fibers in Figure 2 reacted with Al alloy and generated Al O near the fibers. Such ceramic reactant in matrix can contribute to the improvement of the strength of the matrix and mechanical properties of composites as well. The stress–strain (S–S) curves by tensile test of composites are shown in Figure 5. The 0.2% offset yield strength of composites was influenced by the type of fibers and content of SiO sol (fiber type/sol content). The composites with Al 2 O 3 fibers/8 mass% retained higher strength of 153 MPa than those with Al 2 O 3 – SiO 2 fibers/8 mass% of 129 MPa and Al 2 O 3 –SiO 2 fibers/2 mass% of 104 MPa. The differences of yield strength are related to the strength differences of matrix and fiber. The reason of Al 2 O 3 fiber-reinforced composites possessing higher yield strength, compared with the Al 2 O 3 –SiO 2 fiber-reinforced composite under same matrix with same 8 mass% SiO 2 sol content is attributed to the strength differences of the fibers. Besides, the yield strength differences of Al 2 O 3 –SiO 2 fibers-reinforced composites under different SiO 2 sol addition are related to the strength of the matrix. That is, the high content of SiO 2 sol in matrix can generate high-strength Al 2 O 3 precipitates by reaction of Al and SiO 2 in matrix more than low content of the SiO 2 sol, and thus increases the strength of matrix. The maximum strength of composites was also influenced by the fiber types and SiO 2 sol content, as shown in Figure 5. The composites reinforced with Al 2 O 3 fibers represented 259 MPa, whereas those reinforced with Al 2 O 3 –SiO 2 fibers showed lower strength of 213 MPa under same 8 mass% SiO 2 sol content. The fracture surfaces of both composites in Figure 6(a) and (b) represent long pull-outs with average length of 35 and 20 m m, respectively. The composites with 8 mass% SiO 2 sol are expected to generate lots of Al 2 O 3 precipitates around fiber/matrix interfaces. The crack tip generated in matrix, when the composites were under loading, can be curved at adjacent fiber region by those precipitates, blocking the direct crack propagation to fibers. Thus, the composites exhibit relatively high maximum strength with sufficient fiber loading. In addition, the maximum strength of the composites reinforced with Al 2 O 3 –SiO 2 fibers is slightly lower than those reinforced with Al 2 O 3 fibers because of the differences of the fiber strength. In other words, the high-strength fibers have a positive effect on increasing pull-out lengths of fibers and maximizing the strength of the composites. These fracture behaviors were totally different with similar composites without SiO 2 sol in Ref. 14. It is reported that the high-strength Al 2 O 3 fiber-reinforced composites formed weak interfaces to curve the matrix crack easily along the longitudinal direction of short fibers, but the composites had lower strength than the composites reinforced with Al 2 O 3 –SiO 2 fibers. The report focused on the strong interfacial bonding which were formed by the reaction of matrix and SiO 2 in Al 2 O 3 –SiO 2 fibers and improved maximum strength of composites by sufficient fiber loading, even though the crack from matrix directly propagated to fibers without crack curves. Meanwhile, the introduc- tion of SiO 2 sol between fibers and matrix, in this study, played a key role to improve maximum strength of composites by reaction with matrix. The Al 2 O 3 precipitates by reaction around fibers prevented direct fracture of fibers from matrix crack and enabled sufficient loading of fibers. In the viewpoint of the maximum strength of composites depending on the different sol content under same reinforcing condition of Al 2 O 3 –SiO 2 fibers, the composites containing 2 mass% sol showed 198 MPa which were lower than those containing 8 mass% sol. Although there are long fiber pull-outs being observed in Figure 6(a) and (b), such fiber pull-outs were not shown in composites containing SiO 2 sol of 2 mass%, as shown in Figure 6(c). That is, the composites containing 2 mass% sol exhibit quite different fracture behavior with short fiber pull-outs of 12 m m, compared to those containing 8 mass% sol. The little amount of SiO 2 sol could not generate enough strength-improving Al 2 O 3 precipitates, and thus the crack propagation from matrix went through fibers directly without sufficient fiber loading and even fiber pull-outs. From these results, Al 2 O 3 fiber-reinforced Al alloy composites fabricated by low-pressure infiltration method can be competitive casting composites with some advantages as competitive strength, extremely low infiltration pressure, simplified apparatus and low cost, compared to the similar composites fabricated by conventional high-pressure casting method. The microstructure and mechanical properties of Al 2 O 3 and Al 2 O 3 –SiO 2 short fiber-reinforced A366 alloy composites fabricated by low-pressure infiltration method have been influenced by the chemical composition of fiber and content of SiO 2 sol as binder. The high content of SiO 2 sol contributed to the densification of composites by encouraging fine wettability between molten Al and Al 2 O 3 fiber, whereas the SiO 2 content in Al 2 O 3 fibers did not affect the relative density. The strength of composites was remarkably influenced by the strength of fiber and matrix. The composites containing 8 mass% SiO 2 sol showed strength improvement by gen- erating high-strength Al 2 O 3 precipitates around fibers by reaction of matrix and SiO 2 sol. The precipitates enabled sufficient fiber loading and fiber pull-out by curving matrix crack around fibers. Moreover, Al 2 O 3 fibers also contributed to improve the strength of composites more than Al 2 O 3 –SiO 2 fibers. However, the composites containing 2 mass% SiO 2 sol exhibited low strength with short pull-outs by insufficient Al 2 O 3 precipitates around fibers. Therefore, Al 2 O 3 fiber-reinforced A366 composites fabricated by low-pressure infiltration method can be one of the competitive MMCs, compared with the conventional high-pressure casting ...

Context 2

... advancement of reactivity between SiO 2 sol and molten Al alloy can lead to full densification of Al matrix to the internal minute spaces of preform, whereas the bad wettability between Al 2 O 3 fibers and molten Al alloy has been expected to generate imperfect infiltration around the fibers. Thus, the reaction is also expected to increase the relative density of the composites. The microstructures of composites using different content of SiO 2 sol and different types of alumina fibers are shown in Figure 3. There exists diverse sizes and directions of alumina fibers, Si phase surrounding fibers and some pores around those fibers. In addition, each alumina fiber has been seen two types of fiber cross- sectional structures: single and double-layered fiber regardless of SiO 2 content, but the content of Al, O and Si is not similar and not so far from the theoretical values, as shown in Figure 1. Figure 3(a) and (b) shows the microstructure of the composites reinforced with Al 2 O 3 and Al 2 O 3 –SiO 2 fibers, respectively, with the same content of SiO sol (8 mass%). Both images represent dense morphology with their high relative density of 97.4 and 97.7%, even though some pores exist at the extremely narrow spaces between fibers. Although the composites reinforced with Al 2 O 3 –SiO 2 fibers have slightly higher density than those reinforced with Al 2 O 3 fibers, it is negligible. It means that the SiO 2 content in Al 2 O 3 –SiO 2 fibers is expected to improve wettability, as shown in Figure 1, but such content of SiO 2 did not give remarkable influence to the densification of composites, due to much amount of SiO 2 sol which was covered on the surface of the alumina fibers widely. On the other hand, the composites containing 2 mass% sol exhibit large amounts of pores around Al 2 O 3 fibers seemed to be caused by imperfect infiltration of the molten Al alloy. Some of imperfect infiltration parts at the narrow spaces between fibers (B in Figure 3(c)) might be caused by bad wettability between molten Al and Al 2 O 3 fibers by lack of SiO 2 sol content. However, increase of the SiO 2 sol content enables to fill such narrow spaces with the sol, as shown in Figure 3(b) A. The composites containing 2 mass% sol also present relatively lower density than those containing 8 mass% in Table 2. Therefore, it is obvious that the content of sol contributes to the densification of the composites rather than chemical composition of fiber. The role of SiO 2 sol was not only improving the wettability but also reacting with matrix. Figure 4 exhibits the EDS point analysis of matrix (Figure 4(a)) and the X-ray diffraction (XRD) result of casting alloy of matrix and SiO 2 sol. The detected amount of oxygen was not considerable with low content of Si in Al matrix around fibers in Figure 4(a). The amounts of Mg, Ni and Cu detected were the original elements contained in Al matrix (A366). Besides, the XRD result of matrix (Figure 4(b) indicates that some amount of Al in matrix reacted with SiO 2 sol as written in equation (1) and generated some amount of Al 2 O 3 and Si. That is, the SiO 2 sol surrounding fibers in Figure 2 reacted with Al alloy and generated Al O near the fibers. Such ceramic reactant in matrix can contribute to the improvement of the strength of the matrix and mechanical properties of composites as well. The stress–strain (S–S) curves by tensile test of composites are shown in Figure 5. The 0.2% offset yield strength of composites was influenced by the type of fibers and content of SiO sol (fiber type/sol content). The composites with Al 2 O 3 fibers/8 mass% retained higher strength of 153 MPa than those with Al 2 O 3 – SiO 2 fibers/8 mass% of 129 MPa and Al 2 O 3 –SiO 2 fibers/2 mass% of 104 MPa. The differences of yield strength are related to the strength differences of matrix and fiber. The reason of Al 2 O 3 fiber-reinforced composites possessing higher yield strength, compared with the Al 2 O 3 –SiO 2 fiber-reinforced composite under same matrix with same 8 mass% SiO 2 sol content is attributed to the strength differences of the fibers. Besides, the yield strength differences of Al 2 O 3 –SiO 2 fibers-reinforced composites under different SiO 2 sol addition are related to the strength of the matrix. That is, the high content of SiO 2 sol in matrix can generate high-strength Al 2 O 3 precipitates by reaction of Al and SiO 2 in matrix more than low content of the SiO 2 sol, and thus increases the strength of matrix. The maximum strength of composites was also influenced by the fiber types and SiO 2 sol content, as shown in Figure 5. The composites reinforced with Al 2 O 3 fibers represented 259 MPa, whereas those reinforced with Al 2 O 3 –SiO 2 fibers showed lower strength of 213 MPa under same 8 mass% SiO 2 sol content. The fracture surfaces of both composites in Figure 6(a) and (b) represent long pull-outs with average length of 35 and 20 m m, respectively. The composites with 8 mass% SiO 2 sol are expected to generate lots of Al 2 O 3 precipitates around fiber/matrix interfaces. The crack tip generated in matrix, when the composites were under loading, can be curved at adjacent fiber region by those precipitates, blocking the direct crack propagation to fibers. Thus, the composites exhibit relatively high maximum strength with sufficient fiber loading. In addition, the maximum strength of the composites reinforced with Al 2 O 3 –SiO 2 fibers is slightly lower than those reinforced with Al 2 O 3 fibers because of the differences of the fiber strength. In other words, the high-strength fibers have a positive effect on increasing pull-out lengths of fibers and maximizing the strength of the composites. These fracture behaviors were totally different with similar composites without SiO 2 sol in Ref. 14. It is reported that the high-strength Al 2 O 3 fiber-reinforced composites formed weak interfaces to curve the matrix crack easily along the longitudinal direction of short fibers, but the composites had lower strength than the composites reinforced with Al 2 O 3 –SiO 2 fibers. The report focused on the strong interfacial bonding which were formed by the reaction of matrix and SiO 2 in Al 2 O 3 –SiO 2 fibers and improved maximum strength of composites by sufficient fiber loading, even though the crack from matrix directly propagated to fibers without crack curves. Meanwhile, the introduc- tion of SiO 2 sol between fibers and matrix, in this study, played a key role to improve maximum strength of composites by reaction with matrix. The Al 2 O 3 precipitates by reaction around fibers prevented direct fracture of fibers from matrix crack and enabled sufficient loading of fibers. In the viewpoint of the maximum strength of composites depending on the different sol content under same reinforcing condition of Al 2 O 3 –SiO 2 fibers, the composites containing 2 mass% sol showed 198 MPa which were lower than those containing 8 mass% sol. Although there are long fiber pull-outs being observed in Figure 6(a) and (b), such fiber pull-outs were not shown in composites containing SiO 2 sol of 2 mass%, as shown in Figure 6(c). That is, the composites containing 2 mass% sol exhibit quite different fracture behavior with short fiber pull-outs of 12 m m, compared to those containing 8 mass% sol. The little amount of SiO 2 sol could not generate enough strength-improving Al 2 O 3 precipitates, and thus the crack propagation from matrix went through fibers directly without sufficient fiber loading and even fiber pull-outs. From these results, Al 2 O 3 fiber-reinforced Al alloy composites fabricated by low-pressure infiltration method can be competitive casting composites with some advantages as competitive strength, extremely low infiltration pressure, simplified apparatus and low cost, compared to the similar composites fabricated by conventional high-pressure casting method. The microstructure and mechanical properties of Al 2 O 3 and Al 2 O 3 –SiO 2 short fiber-reinforced A366 alloy composites fabricated by low-pressure infiltration method have been influenced by the chemical composition of fiber and content of SiO 2 sol as binder. The high content of SiO 2 sol contributed to the densification of composites by encouraging fine wettability between molten Al and Al 2 O 3 fiber, whereas the SiO 2 content in Al 2 O 3 fibers did not affect the relative density. The strength of composites was remarkably influenced by the strength of fiber and matrix. The composites containing 8 mass% SiO 2 sol showed strength improvement by gen- erating high-strength Al 2 O 3 precipitates around fibers by reaction of matrix and SiO 2 sol. The precipitates enabled sufficient fiber loading and fiber pull-out by curving matrix crack around fibers. Moreover, Al 2 O 3 fibers also contributed to improve the strength of composites more than Al 2 O 3 –SiO 2 fibers. However, the composites containing 2 mass% SiO 2 sol exhibited low strength with short pull-outs by insufficient Al 2 O 3 precipitates around fibers. Therefore, Al 2 O 3 fiber-reinforced A366 composites fabricated by low-pressure infiltration method can be one of the competitive MMCs, compared with the conventional high-pressure casting ...