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Communications of the American Ceramic Sociery 2865 Fig. 2. Cells of some crystal planes in fcc structure(a)1111, (b)(110), (c)(100), (d)(112))of the B-Sic whisker(() first layer atom, () second layer atom,(X) third layer atom,(A) fourth layer atom, (v) fifth layer atom, and(+)sixth layer atom). [lll]WHisker [l1llTWhisker axIs axIs T一 (a)Model I (b)Model ll Fig. 3. Side-surface crystal models of the B-SiC whiskers("T" denotes twins). observation via TEM was difficult; therefore, a SicaL composite The total surface energy (o can be calculated using the was fabricated using a squeeze-casting route. The thin lamellae of following equation the composite were cut by spark machining and then thinned via on milling. In the composite specimen, many Sic whiskers, (2) riented in various directions, were present and were suitable for TEM observation. Therefore, the microstructure and crystal de where ohk, is the surface energy of the (hk, plane and Shk is the B-SiC whiskers could be examined via TEM urface area of the (hkl) plane, The calculated results are shown in Table ll Table ll shows that Model I has the highest total surface energy II. Results and Diseussion and Model Ill has the lowest total surface energy. Note that Model Il is suitable only for whiskers with a high density of microtwins Figure I is a collection of TEM images showing the morphol- in the(111) plane perpendicular to the whisker axis. High gy of the transverse section of the B-SiC whiskers. The side resolution TEM observation results have shown that a high density urfaces of both types of Sic whiskers have been proved to be of microtwins exists in the hexagonal p-SiC whisker, as shown in parallel to the(112) fee crystal planes, as shown in Fig. 1. Fig. 4(b).Therefore, Model Ill is a possible side-surface structure However, in the fcc crystal structure, the (112)fcc crystal planes for the hexagonal B-SiC whiskers. However, for the triangular eph贴hcm图s盒saat0tMha,吗0mFg4mps lave a higher surface energy, in comparison with that of other lower-index crystal planes, langular B-SiC whisker to form its side the whiskers, should be dependent on the total surface en of that in Model Ill. Therefore, one can conclude that Model Il is a the whiskers possible side-surface structure for the triangular B-SiC whiskers 2 shows the cells of the (111ifce,(110 fcc,(100) ce, and From the above-given analysis, the side surface of the p-siC crystal planes of the SiC whisker. The surface energy per whisker should be a type of zigzag surface that is composed of unit area(o) for each crystal plane is given as anes such as i11 110}c,and{100}e It is difficult to confirm this fact directly via TEM observation n(UJ2) High-resolution TEM observation results have show that the side surface of the B-sic whisker is really a form of zigzag surface that 1g where U is the ener uired to break one atom bond(a is the lattice constant of the Sic whisker), S the area of the crystal cel d n the number of the broken atomic bonds in each crystal cel Table L. Calculation Results of the Surface Energy for The calculation results are shown in Table I. Table I shows that Four Types of Crystal Planes in the fee Structure of for the same area of the four types of crystal planes, fill) has β- SiC Whisker the lowest surface energy and the i fce has the highest surface Number of broken Figure I shows that the side surfaces of the B-siC whiskers Crystal plane Surface area, S crystal cell,n both hexagonal and triangular) should be parallel to the (112 rystal planes. In this case, there are three types of side-surface 111} (V3/2a2 6 3(Ua2) rystal models, labeled as Models I, Il, and Il, respectively(see 1 Fig 3: the length and width of the side surface are noted as / and 3v2(UJa) (v6/2a respectively)observation via TEM was difficult; therefore, a SiCw/Al composite was fabricated using a squeeze-casting route. The thin lamellae of the composite were cut by spark machining and then thinned via ion milling. In the composite specimen, many SiC whiskers, oriented in various directions, were present and were suitable for TEM observation. Therefore, the microstructure and crystal defects of the -SiC whiskers could be examined via TEM. III. Results and Discussion Figure 1 is a collection of TEM images showing the morphology of the transverse section of the -SiC whiskers. The side surfaces of both types of SiC whiskers have been proved to be parallel to the {112}fcc crystal planes, as shown in Fig. 1. However, in the fcc crystal structure, the {112}fcc crystal planes have a higher surface energy, in comparison with that of other lower-index crystal planes, such as {110}fcc, {100}fcc, and {111}fcc. The crystal planes, which compose the side surfaces of the whiskers, should be dependent on the total surface energy of the whiskers. Figure 2 shows the cells of the {111}fcc, {110}fcc, {100}fcc, and {112}fcc crystal planes of the SiC whisker. The surface energy per unit area () for each crystal plane is given as nUa/ 2 S (1) where Ua is the energy required to break one atom bond (a is the lattice constant of the SiC whisker), S the area of the crystal cell, and n the number of the broken atomic bonds in each crystal cell. The calculation results are shown in Table I. Table I shows that, for the same area of the four types of crystal planes, {111}fcc has the lowest surface energy and the {112}fcc has the highest surface energy. Figure 1 shows that the side surfaces of the -SiC whiskers (both hexagonal and triangular) should be parallel to the {112} crystal planes. In this case, there are three types of side-surface crystal models, labeled as Models I, II, and II, respectively (see Fig. 3; the length and width of the side surface are noted as l and w, respectively). The total surface energy (t ) can be calculated using the following equation: t hklShkl (2) where hkl is the surface energy of the {hkl} plane and Shkl is the surface area of the {hkl} plane. The calculated results are shown in Table II. Table II shows that Model I has the highest total surface energy and Model III has the lowest total surface energy. Note that Model III is suitable only for whiskers with a high density of microtwins in the (111) plane perpendicular to the whisker axis. Highresolution TEM observation results have shown that a high density of microtwins exists in the hexagonal -SiC whisker, as shown in Fig. 4(b). Therefore, Model III is a possible side-surface structure for the hexagonal -SiC whiskers. However, for the triangular -SiC whiskers, no microtwins exist in the (111) plane perpendicular to the whisker axis, as shown in Fig. 4(a), so it is impossible for the triangular -SiC whisker to form its side surface similar to that in Model III. Therefore, one can conclude that Model II is a possible side-surface structure for the triangular -SiC whiskers. From the above-given analysis, the side surface of the -SiC whisker should be a type of zigzag surface that is composed of low-energy crystal planes such as {111}fcc, {110}fcc, and {100}fcc. It is difficult to confirm this fact directly via TEM observation. High-resolution TEM observation results have show that the side surface of the -SiC whisker is really a form of zigzag surface that is similar to that shown in Fig. 5. Fig. 2. Cells of some crystal planes in fcc structure ((a){111}, (b) {110}, (c) {100}, (d) {112}) of the -SiC whisker ((F) first layer atom, (f) second layer atom, () third layer atom, (Œ) fourth layer atom, () fifth layer atom, and () sixth layer atom). Fig. 3. Side-surface crystal models of the -SiC whiskers (“T” denotes twins). Table I. Calculation Results of the Surface Energy for Four Types of Crystal Planes in the fcc Structure of -SiC Whisker Crystal plane Surface area, S Number of broken atomic bonds per crystal cell, n Surface energy per unit area, {111} (3/2)a2 6 23(Ua/a2 ) {110} a2 8 4(Ua/a2 ) {100} (2/2)a2 6 32(Ua/a2 ) {112} (6/2)a2 9 36(Ua/a2 ) November 2002 Communications of the American Ceramic Society 2865