J. Zhang et al Scripta Materialia 52 (2005)381-385 and Y,O3 was used with the molar ratio of Y2O3: Al,O3 as 3: 5. The dispersant was a secondary polyamine, pol ethylene imine(PEl, Acros Organics, M. W. 50-60000 The binder and the plasticizer were Polyvinyl alcohol 1788(Qidong Chemical Plant, China) and glycerol SiC (Analytical, Shanghai Chemical Reagent Corporation, China), respectively. Details about the tape casting proc- ess were reported in previous papers [11, 12 The green sheets were cut into a rectangular size (40 x 50 mm) and stacked in a graphite die. Binder re- moval was carried out under an argon atmosphere Samples were hot pressed at 1850C and 35 MPa in 303540455055606569 Ar atmosphere for 0.5 h Monolithic samples were also prepared by hot pressing and pressureless sintering for companson Tests of flexural strength were performed by three point bending from speci 36 Fracture toughness was determined by single-edge- notched beam(SENB) method at room temperature The microstructure of the specimen was investigated by SEM and TEM. Energy dispersive X-ray(EDX) spectroscopy was also used to determine local compo- nents at the grain boundary 3. Results and discussions Fig. I(a) XRD patterns of as sintered Sic samples with initial 3. Microstructure characterization Y2O3: Al2O3 ratio as 3: 5 and(b) TEM micrograph showing morpho XRD analysis of the resultant SiC samples showed hat YAlO3, other than Y3AlsO1 is formed as the sec ond phase after sintering although the initial Y2O3: Al_O3 molar ratio was 3: 5(Fig. la). TEM obser- vation confirmed the presence of YAlO3 at triple and multiple-grain junctions(Fig. Ib). The microstructures do not manifest any evidences of elongated grain growth Thin amorphous films were found at grain bounda ries by TEM observation. Spatially-resolved EDS anal ysis detected a strong segregation of aluminum cations to grain boundary, as revealed in Fig. 2. Like this boundary, most grain boundaries have Al-rich amor- phous film, where little or no yttrium element was found. Such amorphous phase may contain also Sio in addition to Al2O3[13-15]. This observation indicates a preferential diffusion of Al,O3 to grain boundary, in eement with the formation of YAlO3 at triple To confirm the departure of second phase from the designed composition, another Sic sample with the additive Y203: Al2O3 molar ratio as 3: 7 was prepared Distance fom grain boundary (nm under the same procedure. According to the mechanism for the diffusion of AlO3, Y3AlsO12 phase should form 15Eem】ev at the intergranular regions. XRD characterization did find this phase as the second phase in the new material gin boundary n sice a d n l ispibusion of dl y eleme nts across as seen in Fig. 3and Y2O3 was used with the molar ratio of Y2O3:Al2O3 as 3:5. The dispersant was a secondary polyamine, poly￾ethylene imine (PEI, Acros Organics, M.W. 50-60000). The binder and the plasticizer were Polyvinyl alcohol 1788 (Qidong Chemical Plant, China) and glycerol (Analytical, Shanghai Chemical Reagent Corporation, China), respectively. Details about the tape casting proc￾ess were reported in previous papers [11,12]. The green sheets were cut into a rectangular size (40 · 50 mm) and stacked in a graphite die. Binder re￾moval was carried out under an argon atmosphere. Samples were hot pressed at 1850 C and 35 MPa in Ar atmosphere for 0.5 h. Monolithic samples were also prepared by hot pressing and pressureless sintering for comparison. Tests of flexural strength were performed by three point bending from specimens of size 3 · 4 · 36 mm. Fracture toughness was determined by single-edge￾notched beam (SENB) method at room temperature. The microstructure of the specimen was investigated by SEM and TEM. Energy dispersive X-ray (EDX) spectroscopy was also used to determine local compo￾nents at the grain boundary. 3. Results and discussions 3.1. Microstructure characterization XRD analysis of the resultant SiC samples showed that YAlO3, other than Y3Al5O12 is formed as the sec￾ond phase after sintering although the initial Y2O3:Al2O3 molar ratio was 3:5 (Fig. 1a). TEM obser￾vation confirmed the presence of YAlO3 at triple and multiple-grain junctions (Fig. 1b). The microstructures do not manifest any evidences of elongated grain growth. Thin amorphous films were found at grain bounda￾ries by TEM observation. Spatially-resolved EDS anal￾ysis detected a strong segregation of aluminum cations to grain boundary, as revealed in Fig. 2. Like this boundary, most grain boundaries have Al-rich amor￾phous film, where little or no yttrium element was found. Such amorphous phase may contain also SiO2 in addition to Al2O3 [13–15]. This observation indicates a preferential diffusion of Al2O3 to grain boundary, in agreement with the formation of YAlO3 at triple pockets. To confirm the departure of second phase from the designed composition, another SiC sample with the additive Y2O3:Al2O3 molar ratio as 3:7 was prepared under the same procedure. According to the mechanism for the diffusion of Al2O3, Y3Al5O12 phase should form at the intergranular regions. XRD characterization did find this phase as the second phase in the new material, as seen in Fig. 3. Fig. 1. (a) XRD patterns of as sintered SiC samples with initial Y2O3:Al2O3 ratio as 3:5 and (b) TEM micrograph showing morphol￾ogy of the second phase. Fig. 2. (a) TEM image of an amorphous film about 2.5 nm thick at a grain boundary in SiC and (b) distribution of Al, Y elements across this boundary. 382 J. Zhang et al. / Scripta Materialia 52 (2005) 381–385