Mat. Res. Soc. Symp. Proc. Vol. 819 ? 2004 Materials Research Society N1.1.1 On the Role of Grain-Boundary Films in Optimizing the Mechanical Properties of
Silicon Carbide Ceramics
R. O. Ritchie,1,2 X.-F. Zhang 1 and L. C. De Jonghe 1,2
1 Materials Sciences Division, Lawrence Berkeley National Laboratory, and 2Department of
Materials Science and Engineering, University of California, Berkeley, CA 94720
ABSTRACT
Through control of the grain-boundary structure, principally in the nature of the nanoscale intergranular films, a silicon carbide with a fracture toughness as high as 9.1 MPa.m1/2 has been developed by hot pressing β-SiC powder with aluminum, boron, and carbon additions (ABC-SiC). Central in this material development has been systematic transmission electron microscopy (TEM) and mechanical characterizations. In particular, atomic-resolution electron microscopy and nanoprobe composition quantification were combined in analyzing grain boundary structure and nanoscale structural features. Elongated SiC grains with 1 nm-wide amorphous intergranular films were believed to be responsible for the in situ toughening of this material, specifically by mechanisms of crack deflection and grain bridging. Two methods were found to be effective in modifying microstructure and optimizing mechanical performance. First, prescribed post-annealing treatments at temperatures between 1100 and 1500o C were seen to cause full crystallization of the amorphous intergranular films and to introduce uniformly dispersed nanoprecipitates within SiC matrix grains; in addition, lattice diffusion of aluminum at elevated temperatures was seen to alter grain-boundary composition. Second, adjusting the nominal content of sintering additives was also observed to change the grain morphology, the grain-boundary structure, and the phase composition of the ABC-SiC. In this regard, the roles of individual additives in developing boundary microstructures were identified; this was demonstrated to be critical in optimizing the mechanical properties, including fracture toughness and fatigue resistance at ambient and elevated temperatures, flexural strength, wear resistance, and creep resistance.
INTRODUCTION
The nature of the grain boundaries, in particular the presence of nanoscale intergranular films, is of paramount importance in controlling the properties of many ceramics. This is especially important for silicon carbide ceramics, which despite offering many desirable properties, including a high melting temperature, low density, high elastic modulus and hardness, excellent wear resistance, and low creep rates, are rarely used structurally because of their poor toughness. Consequently, an imperative in the structural application of SiC is high fracture toughness, which for commercially available SiC is typically on the order of 3 MPa.m1/2, well below that of grain-elongated Si3N4 and yttria-stabilized ZrO2, for example.
The toughening of ceramic materials can best be induced by crack bridging mechanisms
(i.e., extrinsic toughening by crack-tip shielding), which can result from crack paths bridged
by unbroken reinforcement fibers in composite ceramics (ex situ toughening) [1], or by self-reinforcement from elongated grains in monolithic ceramics (in situ toughening) [1]. The latter approach was exploited in the present work by hot pressing SiC with Al, B and C additives (ABC-SiC [2]) to induce elongated grains decorated with brittle intergranular films.
To achieve this, two strategies, specifically post-annealing heat treatments and changes in the additive content, were used to optimize the microstructure.
In this paper, we review the microstructure and mechanical properties of in situ toughened SiC with particular emphasis on the role of the grain boundary films. In
particular, atomic-resolution TEM in conjunction with nanoprobe chemical analyses were employed to determine the structure and chemistry of the grain boundaries. Such atomic-scale characterization was correlated with materials processing and mechanical testing with the objective of optimizing the structural performance of ABC-SiC.
EXPERIMENTAL PROCEDURES
Submicron β-SiC powder was mixed with 3 wt% aluminum metal powder, 0.6 wt% boron, and 2 wt% carbon sintering. The slurry was stir dried, sieved, and uniaxially pressed at 35 MPa. The green bodies were hot pressed at 50 MPa at 1900°C for 1 hr in an Ar atmosphere. The final product, which was produced as 99% dense (3.18 g/cm3), 4 mm thick and 38 mm diameter disks of polycrystalline SiC, was compromise of predominantly 4H and 6H α-SiC phases, with a minor fraction of 3C β-SiC. Some as-hot-pressed ABC-SiC samples were further annealed in a tungsten mesh furnace under flowing Ar, at temperatures between 1000 and 1500o C, for times typically ranging from 72 to 168 hr. Structural and mechanical characterizations were performed for both as-hot-pressed and annealed samples.
Structural and chemical analyses were carried out in a 200 kV Philips CM200 transmission electron microscope equipped with a windowless detector and corresponding X-ray energy-dispersive spectroscopy (EDS) system. A spatial-difference methodology was developed to determine the concentration of the impurities in the SiC grain boundaries using a nanoprobe with a diameter varied between 3 and 20 nm [3,4].
Indentation hardness, bending strength, creep resistance, abrasive wear properties, R-curve fracture toughness, and cyclic fatigue-crack growth behavior at ambient and elevated temperatures were all evaluated for ABC-SiC. Experimental procedures for each of these property assessments are described in detail in the respective references quoted in this paper.
RESULTS
Some typical mechanical properties for as-processed ABC-SiC are summarized in Table 1. Of particular note is the fracture toughness, which has been measured to be as high as 7 to 9 MPa.m1/2; this is the highest K Ic value recorded for SiC to date.
Table 1: Room temperature mechanical properties of as-processed ABC-SiC
Sample ABC-SiC
4-Point Bending Strength (MPa) 691±12
Vickers Hardness – 10 kg (GPa) 24.1
Fracture Toughness (MPa.m1/2) 6.8-9.1 Using X-ray diffraction, 70 vol.% of the hot-pressed structure was identified as hexagonal 4H-SiC and the remaining 30% as cubic 3C-SiC. TEM studies revealed that the 3C-to-4H phase transformation, which is presumed to have occurred during hot pressing, promoted anisotropic grain growth [4], resulting in high area density of plate-like, elongated SiC grains (Fig. 1a); the length and width of these grains ranged from 3 to 11 μm and 1 to 3 μm, respectively, with an aspect ratio for 90 % of them between 2 and 5. Interlocking between the elongated SiC grains was often observed, as shown in Fig. 1b. Equiaxed 3C-SiC grains with a submicron size were also found in ABC-SiC. It is believed that the unusually
high toughness of ABC-SiC results from a combination of crack deflection and principally frictional and elastic bridging by the elongated SiC grains in the wake of the crack tip [2,5]. To optimize this toughening mechanism, an understanding of the grain-boundary properties and structure is crucial; consequently, extensive TEM studies were focused on the nature of the grain-boundary microstructure.
Fig. 1: (a) Bright-field TEM images showing elongated SiC grains in ABC-SiC, showing (b) the interlocked nature of the elongated grains.
Fig. 2: (a) High-resolution electron micrograph showing a typical intergranular film between two SiC matrix grains. An amorphous structure is observed with a width of about 1 nm. (b) Distribution of amorphous grain-boundary widths determined by high-resolution electron microscopy. The grain-boundary width ranges from 0.75 nm to 2.75 nm, with a mean of about 1 nm.
Fig. 2a shows a high-resolution TEM image of a typical grain-boundary area in as-pressed ABC-SiC. An amorphous intergranular film is seen between two adjacent SiC grains, about 1 nm in width. Using high-resolution electron microscopy, the statistical width of these films was determined; results are shown in Fig. 2b. Specifically, the amorphous grain-boundary films range in width from about 0.75 to 2.75 nm, with a mean of ~1 nm. This width is consistent with the values found in other ceramic materials [6,7], and with theoretical explanations developed by Clarke [8]. In some particularly wide films (e.g. ~2.75 nm), nanoscale crystallites were recognizable, indicating local ordering in the glassy films. Earlier work on SiC sintered with boron and carbon showed a solid-phase sintering procedure with no grain-boundary films being formed [9,10]. The formation of the current films was due to liquid-phase sintering promoted by the Al additives. The liquid phase allowed for densification of the SiC at temperatures roughly 200o C lower than Al-free compositions, i.e., at ~1900o C instead of ~2100o C. The amorphous intergranular films provide the preferred 1 μm a 0.5 μm
b
1 nm SiC SiC
a
crack path, which is an essential element for the development of crack bridging and hence toughening in ABC-SiC.
Nanoprobe EDS analyses revealed that the grain-boundary films contained substantial Al, O, Si, and C. The oxygen came mainly from the SiO2 surface oxidation of the SiC starting powder, and is one of the features responsible for difference in the behavior of different starting powders. Boron was below the detectability limit at the grain boundaries and in the interior of the SiC grains. In fact, boron was largely involved in forming Al8B4C7 secondary phases. Other secondary phases identified in ABC-SiC include Al2OC-SiC, mullite, Al2O3, Al4C3, Al4O4C, and Al-O-C-B crystalline phases. These secondary phases were submicron in dimension and were present as triple-junction particles [3,13]. Al in solution in the SiC matrix grains was also detected.
Effects of Post-annealing
The most significant phenomenon that we observed in the efforts of modifying microstructure of ABC-SiC was crystallization of amorphous intergranular films in post-annealing. While no significant change in the overall, large-scale microstructure was recognized, annealing at 1000o C for only 5-30 hrs was sufficient to transform about half number of the amorphous grain-boundary films into more ordered structures. Apparently, 1000o C was about the threshold temperature for activation of grain-boundary diffusion. Annealing at higher temperatures for prolonged hours fully crystallized the intergranular films [3]. For example, whereas about 90% of the grain boundaries were amorphous in as-hot-pressed samples, 86% of the intergranular films in the annealed material were found to be crystalline. Fig. 3a shows a high-resolution TEM image of a grain-boundary film crystallized after annealing at 1200o C for 500 hrs. In this image, the crystallized grain-boundary film is not readily distinguishable because the grain-boundary phase was strictly epitaxial with the 6H-SiC grain on the left-hand side. However, corresponding EDS detected substantial Al-O-Si-C segregations between the two SiC grains, confirming the existence of the boundary phase. An enlarged image from the framed area is shown in Fig. 3b for closer inspection. The two adjacent SiC grains in this image show very different orientations. Due to a large deviation from the [1120] type zone-axes, only (0004) lattice fringes, with the lattice spacing of 0.25 nm, could be resolved for the 4H-SiC grain on the upper-right side, while a two-dimensional [1120] zone-axis lattice can be recognized in the 6H-SiC grain on the other side of the grain boundary. Under the imaging conditions for Fig. 3, the black dots in the image correspond to cationic columns along the incident electron beam direction. Some black dots in the 6H-SiC grain were marked with black circles. The zigzag stacking of the dots correspond to the …ABCACBABCACB… hexagonal structure of the 6H-SiC. The closest layer spacing along the c-direction is 0.25 nm.
The grain-boundary layer outlined in Fig. 3b can be distinguished by the abrupt change in the atomic arrangement. Some lattice points in the boundary region are marked with white circles. The projection distance between adjacent lattice dots in the grain-boundary layers is very close to that in the neighboring 6H-SiC grain, and the characteristic zigzag lattice arrangement is also apparent in the boundary layer. These observations would suggest a grain-boundary film structure similar to 6H-SiC. However, instead of the …ABCACBABCACB… periodic stacking for every six layers in the 6H-type structure, the stacking period in the grain-boundary phase contains only two layers in one period, resulting in …ABABAB… stacking with an 0.5 nm repeat length, within a grain-boundary width of about 1.25 nm. In conjunction with the quantitative analysis of the grain-boundary composition and computer image simulations, we concluded that one of the intergranular crystalline phases was aluminosilicate with a Al2OC-SiC solid solution composition and a
2H-wurtzite structure (hexagonal unit cell, a = 3.1 ?, c = 5.0 ?) [3,12]. The crystallized structure usually has an epitaxial structural relationship with SiC matrix grains when the (0001) habit plane is available. As shown in Fig. 3, the typical grain-boundary width after crystallization remains about 1 nm.
Fig. 3: (a) High-resolution electron microscopy image for a grain-boundary area in ABC-SiC annealed at 1200o C for 500 hrs. Amorphous intergranular film is crystallized. (b) Enlarged image from the framed area in (a). Atomic stacking in 6H-SiC grain on the bottom-left side is marked by black circles. 2H-wurtzite atomic stacking in grain-boundary film (GB) are marked by white circles. Note the epitaxial orientation relationships between the grain-boundary film and the 6H-SiC matrix grain.
To further study the crystallization process in intergranular films at elevated temperatures, an ABC-SiC sample was heated in situ in a 300 kV JEOL 3010 transmission electron microscope equipped with a hot stage. Fig. 4 shows high-resolution images for the same intergranular film before and after in situ heating, respectively. The amorphous film prior to heating crystallized discretely after 25 hrs at 1200o C, as indicated by arrowheads in Fig. 4b. The crystallization tended to proceed epitaxially on the (0001) plane of the adjacent 6H-SiC matrix grain, with a 2H-wurtzite structure similar to that observed in ex situ annealed SiC samples. No discernable features could be identified as potential preferential sites for nucleation at the SiC/film interface. Presumably, local compositional fluctuations in the intergranular films serve as nucleation sites.
Another significant consequence of the thermal treatment was coherent nano-precipitation within SiC matrix grains, as seen in Fig. 5a. Although Fig. 5a was taken from a sample annealed at 1400o C, the plate-like nanoprecipitates first formed at 1300o C. The precipitates were uniform in size and shape, and dispersed within the SiC matrix grains. The projected dimension of the precipitates after 1300o C annealing is ~4 × 1 nm 2 with a volumetric number density of 5×1022/m 3. The precipitates coarsened with annealing temperature, accompanied by decrease in the number density. Detailed high-resolution electron microscopy characterization and nanoprobe EDS analysis determined an Al 4C 3-based structure and composition for the nanoprecipitates [13]. A comparison of 6H-SiC and Al 4C 3 structures projected along the [0001] direction is shown in Fig. 5b. The similarity between the two structures caused coherent precipitation with the (0001) habit planes. The formation and coarsening of the precipitates at 1300-1600o C was a consequence of lattice-a 2 nm GB
b 1 nm
diffusion-controlled nucleation and growth [13]. The diffusion of Al-rich species through the SiC lattice at 1300o C resulted in Al-enrichment in the boundary films, as revealed by EDS.
Fig. 4: (a) High-resolution image of an amorphous intergranular film in as-hot-pressed ABC-SiC. (b) The same film as in (a) but after in situ heating in a transmission electron microscope at 1200o C for 25 hrs. Arrowheads indicate the discrete, crystallized boundary segments.
Fig. 5: (a) Al 4C 3-based nanoprecipitate formed in a 6H-SiC grain annealed at 1400o C. The viewing directions are marked. (b) Atomic models for 6H-SiC and Al 4C 3 projected along the
[0001] direction. The similarity between the two structures can be seen.
The Al content in intergranular films as a function of annealing temperature was analyzed with EDS. The results are plotted in Fig. 6. It is apparent that Al solution in SiC grains decreased at 1100o C and especially above 1300o C, consistent with TEM observations that Al solutions formed nanoprecipitates exsolved from the SiC lattice. Not surprisingly, the Al site density in the grain boundaries (N Al GB ) changed as well upon annealing. Below 1200o C, the composition of intergranular films was virtually invariant, even while the intergranular films crystallized. The N Al GB value was doubled at 1300o C, which can be readily correlated with diffusion of the Al-rich chemical species into the grain-boundary films. The Al content in intergranular films after annealing at 1300o C is in agreement with Al 1.1Si 0.9OC, a solid solution between 2H-wurtzite Al 2OC and SiC [12]. At even higher annealing temperatures up to 1600o C, N Al GB changed marginally taking the standard deviation into account. SiC-6H
As-processed
c 5 nm a 1200 o
C-25 hrs SiC-6H
c b a = 3.33 ?
Si C Al
Al 4C 3, [0001]
Fig. 6: Plots of EDS-determined Al site density in grain-boundary films (N Al GB ) and in SiC matrix grains (Al/SiC, wt%) as a function of annealing temperature.
The structural evolutions in intergranular films during thermal treatment demonstrated a profound influence on mechanical properties. It was found that the high strength, cyclic fatigue resistance, and particularly the fracture toughness of ABC-SiC at ambient temperature were not severely compromised at elevated temperatures. For example, the fatigue-crack growth properties up to 1300o C were essentially identical to those at 25o C. Fig. 7 illustrates the variation in fatigue-crack growth rates, da/dN , with applied stress-intensity range, ?K , at a load ratio (minimum to maximum load) of R = 0.1 for ABC-SiC under different test conditions. It can be seen that at both 25o C and 1300o C, crack-growth rates display a marked sensitivity to the stress intensity but little effect is from change of loading frequency over the range 3 to 1000 Hz [14]. Mechanistically, the lack of a frequency effect in ABC-SiC is expected as crack advance occurs via predominantly intergranular cracking ahead of the tip, as shown in Fig. 8. Grain bridging in the crack wake is a common feature. However, the absence of a frequency effect at elevated temperatures is surprising, particularly since comparable materials, such as Si 3N 4, Al 2O 3 and silicide-matrix ceramics, show a marked sensitivity to frequency at above 1000o C [15-17]. In these later materials, softening of the intergranular films, grain-boundary cavitation and viscous-phase bridging are common. In contrast, TEM studies of regions in the immediate vicinity of the crack tip in ABC-SiC (e.g. Fig. 8) provided direct confirmation of fracture mechanisms which were similar at ambient and elevated temperatures, with no evidence for grain-boundary cavitation (creep) damage or viscous-phase bridging at temperatures as high as 1300o C. We conclude that the unique high-temperature mechanical characteristics of ABC-SiC appear to be a result of the thermal-induced crystallization of intergranular glassy films.
The structural evolutions in grain boundary and matrix grains at elevated temperatures also benefit other mechanical properties of ABC-SiC. For example, the steady-state creep rate at high temperature of 1500o C was still impressively low (5 × 10-9/s at 100 MPa), and the creep rates at 1200o C in ABC-SiC was about three orders of magnitude slower than in single-crystal Ni-base superalloy tested under the same conditions [18]. In addition, 80% strength loss at 1300o C was restored by post-annealing [13]. Abrasive wear resistance was improved as well by formation of nanoprecipitates and by structural and compositional changes in grain boundaries after annealing [19]. These structural and mechanical characterization results demonstrate that the prescribed annealing is an effective way in tuning the microstructure and in turn optimizing the mechanical properties of ABC-SiC.
1020304050020040060080010001200140016001800Annealing Temperature (o C)
A l c o n t e n t i n I G F s (N A l G
B (n m -3))0246810A l
C o n t e n t i n S i C G r a i n s (w t %
)
Fig. 7: Cyclic fatigue-crack growth rates, da/dN , in ABC-SiC as a function of the applied stress-intensity range, ?K , for the tests conducted at temperatures between 25 and 1300o C, load ratio R = 0.1, and frequencies between 3 and 1000 Hz.
Fig. 8: TEM images of the intergranular crack profiles at the crack-tip in ABC-SiC at 1300o C under (a) cyclic loading (25 Hz, R = 0.1), and (b) static loading. Arrows indicate the direction of crack propagation. No evidence of viscous boundary layers or creep damage. Effects of Additive Content
A second effort in tailoring boundary microstructures of ABC-SiC was in adjusting the contents of the sintering additives. A series of samples were prepared by changing content of one of the three additives. For example, the Al content was 3 wt% in most ABC-SiC samples. This was then increased to 4 up to 7 wt%, while boron and carbon contents remained at 0.6 and 2 wt%, respectively. The samples were referred to as 3ABC-, 4ABC, up to 7ABC-SiC, according to the weight percentage of the Al content. Structural characterizations showed that Al variations between 3 and 7 wt% did not reduce the densification of SiC samples under the same processing conditions. However, changing the Al content did alter the microstructure, as illustrated in Fig. 9 (only images for the 3ABC- 5ABC-, and 7ABC-SiC are shown). Although all samples were composed of plate-like, elongated and equiaxed SiC grains, the size, aspect ratio and area density of the elongated grains varied significantly with increasing Al content. The length of the elongated grains was found to be at a maximum in the 5ABC-SiC, with the aspect ratio increasing almost linearly up to 6 wt% Al. Compared to 3ABC-SiC, the aspect ratios in 4ABC- to 7ABC-SiC are much higher, but the area density of the elongated grains continuously decreases. A distinct bimodal grain distribution is seen in the 5ABC-SiC, with elongated α-SiC grains and submicron-sized equiaxed β
-SiC grains [20].
10101010Stress Intensity Range, 10-7-8-9C r a c k G r o w t h R a t e , d a /d N (m /c y c l e )?K (MPam 1/2)
Fig. 9: Morphologies of the 3ABC-, 5ABC-, and 7ABC-SiC are shown in images in (a) to (c), respectively. Note the significant changes in dimension and area density of the elongated SiC grains. Note also the bimodal grain systems with 5ABC-SiC.
Table 2: X-ray diffraction-determined polytipic SiC phases, corresponding volume fractions
and mechanical properties in 3ABC, 4ABC, 5ABC, 6ABC and 7ABC-SiC
Samples Phase and Volume Fraction 4-Point Bending Strength (MPa) Toughness (MPa.m 1/2) Hardness*
(GPa)
3ABC-SiC 70% 4H 30% 3C
691±12 6.8±0.3 24.1 4ABC-SiC 19% 6H, 23% 4H 58% 3C 561±13 6.3±0.7 23.9 5ABC-SiC 21% 6H, 11% 4H 67% 3C
480±30 8.9±0.4 22.7 6ABC-SiC 24% 6H 76% 3C
598±34 3.2±0.2 35.9 7ABC-SiC 20% 6H 80% 3C
533±58 3.9±0.4 30.1 *Vickers indentation, Load = 10 kg. E = 430 GPa (for calculation).
Changing of the Al content by 1 wt% not only caused a considerable change in the grain morphology, but also induced different degrees of 3C-to-4H and/or 6H-to-4H SiC phase conversion during hot-pressing. X-ray diffraction spectra were collected from polished surfaces of the 3ABC- to 7ABC-SiC samples and from the starting powder for comparison. The spectra were matched with standard 3C-, 4H-, 6H-, and 15R-SiC phase spectra; results are summarized in Table 2. It was found that the 3 wt% Al addition in the 3ABC-SiC was sufficient to transform all of the 20% preexisting 6H-phase seeds in 3C-dominanted starting powder into 4H-SiC, with more than 50% of the β-3C transforming into α-4H phase as well. However, a 1 wt% higher Al content resulted in a completely different phase composition by retarding the 3C-to-4H transformation. Further increases in Al monotonically decreased the extent of the 3C-to-4H transformation, and apparently inhibited the preexisting 6H phase to be transformed into 4H. These results imply that the 3C-to-4H transformation was mainly promoted by boron and carbon, and the transformation was retarded when boron and carbon were compensated by increasing the Al content. The experimental data also suggest close relationships between the phase composition and grain morphology, which would be expected if the β-to-α transformation promotes the grain elongation [4].
In addition to the grain morphology, a factor that may have a determining influence on mechanical properties of dense, polycrystalline ceramic materials is the grain-boundary structure and composition. High-resolution TEM showed that about 90% of the intergranular films examined in the 3ABC-SiC processed an amorphous structure. In contrast, this fraction
3ABC 5 μm a 5ABC b 7ABC
c
dropped to about 50% in the 5ABC-SiC, and all grain-boundary films examined in the 7ABC-SiC were crystalline in structure. EDS analyses indicated that increasing the Al content enhanced the Al concentration in the intergranular films, as well as in the bulk SiC grains, but the concentrations saturated in 5ABC-SiC. More than 5 wt% Al resulted in precipitation of excess free Al, as observed in the 6ABC-and 7ABC-SiC. It should be noted that crystalline boundaries were prevalent in as-hot-pressed 5ABC- to 7ABC-SiC without any post-treatment. It is plausible that Al facilitates the formation of crystalline intergranular films.
These changes in microstructure can be expected to affect mechanical properties. Table 2 lists various mechanical properties for the sample series. The mechanical data illustrate the tradeoff mechanical performance which is often encountered in developing advanced ceramic materials. While the highest strength was obtained in the 3ABC-SiC, the hardness was at maximum for the 6ABC-SiC. As for toughness, it is clear that 5 wt% Al resulted in the best toughness whereas 6 wt% or higher Al additions significantly degraded the toughness so that the materials became extremely brittle. Cyclic fatigue tests revealed that the 3ABC- and particularly the 5ABC-SiC displayed excellent crack-growth resistance at both ambient (25°C) and elevated (1300°C) temperatures. Again, the crack propagation in both 3ABC- and 5ABC-SiC was intergranular, and crack bridging was observed in the crack wake. No evidence of viscous grain-boundary layers and creep damage, in the form of grain-boundary cavitation, was seen at temperatures up to 1300°C. The substantially enhanced toughness in the 5ABC-SiC was associated with extensive crack bridging from both interlocking grains, as in 3ABC-SiC, and uncracked ligaments, which only occurred in 5ABC-SiC. No toughening by crack bridging was apparent in 7ABC-SiC, concomitant with a transition from intergranular to transgranular fracture [21].
Similar to the aluminum concentration variations, changes in boron and carbon contents also alter the microstructure and phase composition. Typical results are shown in Fig. 10. In this case, the Al content was fixed at 6 wt%, while either the B or the C concentrations were changed. The phase composition determined by X-ray diffraction is noted in each image. It is clear that at fixed Al (6 wt%) and B(0.6 wt%) contents, 1 wt% additional carbon promoted the formation of more elongated grains and enhanced the 3C-to-4H transformation (most of the 6H phase should originate from starting powder). This effect of carbon on 3C-to-4H transformation was also reported before by Sakai et al [22]. Growth of the equiaxed grains was found to be limited. With the Al and C contents kept constant, increasing the boron content from 0.6 wt% to 0.9 wt% largely increased the number density of elongated grains, but reduced their aspect ratio. In addition, boron promoted the 3C-to-4H and 6H-to-4H phase transformations more effectively than carbon. The positive effect of boron on the 6H-to-4H transformation is consistent with the previous observations of Huang et al [23].
The systematic processing and characterizations of a series of ABC-SiC described above also allowed for the determination of the roles of Al, B, and C additives in developing the microstructures of silicon carbide. The observed effects may be summarized as follows: in terms of developing phase composition, boron is more effective in promoting the β-to-αphase transformations than carbon. Aluminum retards the β-to-α phase transformation, but promotes the 6H-to-4H transformation. As for the developing grain morphology, aluminum and carbon both promote anisotropic grain growth, whereas boron tends to coarsen the volume fraction, but reduce the aspect ratio, of the elongated grains. It should be noted that during processing, the combined roles of the Al, B and C additives often override their individual roles. For example, B and C together favor of the β-to-α phase transformation associated with grain elongation; however, the final microstructure does not necessarily have strongly elongated grains as B and C have opposite effects on anisotropic grain growth. Actually, the B:C ratio determines the final grain configuration. Aluminum has a different
effect: when the B:C ratio favors the anisotropic grain growth, Al-rich liquid phase accelerates such growth so that the aspect ratio is further increased. However, if the Al:B and Al:C ratios are reduced, less liquid phases are expected to be present after the Al-B-C reactions so that the effects of Al are diminished. Further experiments have indicated that even at constant Al:B:C ratios, a change in total amount of additives can still alter the grain configuration and phase composition significantly. This emphasizes the fact that the optimization of the mechanical properties of many structural ceramics such as ABC-SiC is a strong function of the absolute and relative amounts of the sintering additives.
2 wt% C
5 μm76% 3C, 24% 6H3% 6H, 97% 4H
3 wt% C
36% 3C, 27% 6H, 37% 4H6% 6H, 94% 4H
Fig. 10: Microstructural changes with fixed Al (6 wt%) and adjusted boron or carbon contents. Phase compositions are marked in each image. Effects of boron and carbon additions in changing microstructure and phase composition are clear.
SUMMARY
ABC-SiC ceramics with unprecedented toughness have been developed by control of the grain-boundary structure. The high toughness is attributed to a mechanism of crack bridging by interlocked, elongated SiC grains, aided by the presence of nanoscale amorphous intergranular films. Two methods for modifying the boundary structure and chemistry were investigated using annealing and changes in the contents of Al, B, C sintering additives.
Prescribed post-annealing treatments at higher than 1000o C were found to activate grain-boundary diffusion and consequently caused the crystallization of most of the glassy intergranular films (not simply the “pockets” at the grain-boundary triple points). This led to superior high-temperature strength, creep and fatigue properties at elevated temperatures with no evidence of creep damage in the form of grain-boundary cavitation until temperatures above 1400o C were reached. Using atomic-resolution electron microscopy and quantitative nanoprobe EDS, one of the crystallized intergranular structures was identified as 2H-wurtzite aluminosilicate. Heating at 1300o C or higher also resulted in uniformly dispersed nanoprecipitates as a result of lattice diffusion. In addition, the lattice diffusion almost doubled the segregation of Al into the intergranular films. It is believed that this significant microstructural evolution, which occurs during post-annealing and is not characteristically observed in other advanced ceramics such as Si3N4, and Al2O3, is the origin of the many outstanding mechanical property attributes of ABC-SiC at elevated temperatures. These include high resistance to crack growth at temperatures between ambient and 1300o C, far superior steady-state creep resistance than in single-crystal Ni-base superalloys, enhanced resistance to abrasive wear, and high-temperature strength loss recovery.
Changing the nominal contents of the sintering additives was also used as an effective means to modify the microstructure. In particular, the area density and dimensions of the elongated SiC grains, their phase composition, and the grain-boundary structure were all found to be sensitive to small variations in additive content. Using this approach, a series of materials processing with systematic changes in sintering atom additions were used to develop optimal microstructures for ABC-SiC. Compositions with superior fracture toughness, or creep properties or abrasive wear resistance were all defined. Such methods, in conjunction with prescribed post-annealing heat treatments, permit the tailoring of microstructures in ABC-SiC to achieve and optimize a wide range of mechanical properties in a highly controlled manner.
ACKNOWLEDGMENTS
This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC03-76SF0098. Thanks are due to Da Chen, M. E. Sixta, Q. Yang, R. Yuan, J. J. Kruzic, and R. M. Cannon for their assistance and discussion in this work.
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一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991
●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the
高中英语~词性~句子成分~语法构成 第一章节：英语句子中的词性 1.名词：n. 名词是指事物的名称，在句子中主要作主语.宾语.表语.同位语。 2.形容词;adj. 形容词是指对名词进行修饰~限定~描述~的成份，主要作定语.表语.。形容词在汉语中是（的）.其标志是: ous. Al .ful .ive。. 3.动词：vt. 动词是指主语发出的一个动作，一般用来作谓语。 4.副词：adv. 副词是指表示动作发生的地点. 时间. 条件. 方式. 原因. 目的. 结果.伴随让步. 一般用来修饰动词. 形容词。副词在汉语中是（地）.其标志是：ly。 5.代词：pron. 代词是指用来代替名词的词，名词所能担任的作用，代词也同样.代词主要用来作主语. 宾语. 表语. 同位语。 6.介词：prep.介词是指表示动词和名次关系的词，例如：in on at of about with for to。其特征：
介词后的动词要用—ing形式。介词加代词时，代词要用宾格。例如：give up her（him）这种形式是正确的，而give up she（he）这种形式是错误的。 7.冠词：冠词是指修饰名词，表名词泛指或特指。冠词有a an the 。 8.叹词：叹词表示一种语气。例如：OH. Ya 等 9.连词：连词是指连接两个并列的成分，这两个并列的成分可以是两个词也可以是两个句子。例如：and but or so 。 10.数词：数词是指表示数量关系词，一般分为基数词和序数词 第二章节：英语句子成分 主语：动作的发出者，一般放在动词前或句首。由名词. 代词. 数词. 不定时. 动名词. 或从句充当。 谓语：指主语发出来的动作，只能由动词充当，一般紧跟在主语后面。 宾语：指动作的承受着，一般由代词. 名词. 数词. 不定时. 动名词. 或从句充当. 介词后面的成分也叫介词宾语。 定语：只对名词起限定修饰的成分，一般由形容
M A: Has the case been closed yet? B: No, the magistrate still needs to decide the outcome. magistrate n.地方行政官，地方法官，治安官 A: I am unable to read the small print in the book. B: It seems you need to magnify it. magnify vt.１．放大，扩大；２．夸大，夸张 A: That was a terrible storm. B: Indeed, but it is too early to determine the magnitude of the damage. magnitude n.１．重要性，重大；２．巨大，广大 A: A young fair maiden like you shouldn’t be single. B: That is because I am a young fair independent maiden. maiden n.少女，年轻姑娘，未婚女子 a.首次的，初次的 A: You look majestic sitting on that high chair. B: Yes, I am pretending to be the king! majestic a.雄伟的，壮丽的，庄严的，高贵的 A: Please cook me dinner now. B: Yes, your majesty, I’m at your service. majesty n.１．［Ｍ－］陛下（对帝王，王后的尊称）；２．雄伟，壮丽，庄严 A: Doctor, I traveled to Africa and I think I caught malaria. B: Did you take any medicine as a precaution? malaria n.疟疾 A: I hate you! B: Why are you so full of malice? malice n.恶意，怨恨 A: I’m afraid that the test results have come back and your lump is malignant. B: That means it’s serious, doesn’t it, doctor? malignant a.１．恶性的，致命的；２．恶意的，恶毒的 A: I’m going shopping in the mall this afternoon, want to join me? B: No, thanks, I have plans already. mall n.（由许多商店组成的）购物中心 A: That child looks very unhealthy. B: Yes, he does not have enough to eat. He is suffering from malnutrition.
意见应以事实为根据. 3 来自辞典例句 192. The bombers swooped ( down ) onthe air base. 轰炸机 突袭 空军基地. 来自辞典例句 193. He mounted their engines on a rubber base. 他把他们的发动机装在一个橡胶垫座上. 14 来自辞典例句 194. The column stands on a narrow base. 柱子竖立在狭窄的地基上. 14 来自辞典例句 195. When one stretched it, it looked like grey flakes on the carvas base. 你要是把它摊直, 看上去就象好一些灰色的粉片落在帆布底子上. 18 来自辞典例句 196. Economic growth and human well - being depend on the natural resource base that supports all living systems. 经济增长和人类的福利依赖于支持所有生命系统的自然资源. 12 1 来自辞典例句 197. The base was just a smudge onthe untouched hundred - mile coast of Manila Bay. 那基地只是马尼拉湾一百英里长安然无恙的海岸线上一个硝烟滚滚的污点. 6 来自辞典例句 198. You can't base an operation on the presumption that miracles are going to happen. 你不能把行动计划建筑在可能出现奇迹的假想基础上.
英语造句大全English sentence 在句子中，更好的记忆单词！ 1、(1)、able adj. 能 句子：We are able to live under the sea in the future. (2)、ability n. 能力 句子：Most school care for children of different abilities. (3)、enable v. 使。。。能句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的句子：We must have the accurate calculation. (2)、accurately adv. 精确地 句子：His calculation is accurately. 3、(1)、act v. 扮演 句子：He act the interesting character. （2）、actor n. 演员 句子：He was a famous actor. (3)、actress n. 女演员 句子：She was a famous actress. (4)、active adj. 积极的 句子：He is an active boy. 4、add v. 加 句子：He adds a little sugar in the milk. 5、advantage n. 优势 句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n.
主语+谓语 1. 理解主谓结构 1) The students arrived. The students arrived at the park. 2) They are listening. They are listening to the music. 3) The disaster happened. 2．体会状语的位置 1) Tom always works hard. 2) Sometimes I go to the park at weekends.. 3) The girl cries very often. 4) We seldom come here. The disaster happened to the poor family. 3. 多个状语的排列次序 1) He works. 2) He works hard. 3) He always works hard. 4) He always works hard in the company. 5) He always works hard in the company recently. 6) He always works hard in the company recently because he wants to get promoted. 4. 写作常用不及物动词 1. ache My head aches. I’m aching all over. 2. agree agree with sb. about sth. agree to do sth. 3. apologize to sb. for sth. 4. appear (at the meeting, on the screen) 5. arrive at / in 6. belong to 7. chat with sb. about sth. 8. come (to …) 9. cry 10. dance 11. depend on /upon 12. die 13. fall 14. go to … 15. graduate from 16. … happen 17. laugh 18. listen to... 19. live 20. rise 21. sit 22. smile 23. swim 24. stay (at home / in a hotel) 25. work 26. wait for 汉译英： 1.昨天我去了电影院。 2.我能用英语跟外国人自由交谈。 3.晚上7点我们到达了机场。 4.暑假就要到了。 5.现在很多老人独自居住。 6.老师同意了。 7.刚才发生了一场车祸。 8.课上我们应该认真听讲。9. 我们的态度很重要。 10. 能否成功取决于你的态度。 11. 能取得多大进步取决于你付出多少努力。 12. 这个木桶能盛多少水取决于最短的一块板子的长度。
【it's time to和it's time for】 ——————这其实是一个句型,只不过后面要跟不同的东西. ——————It's time to跟的是不定式（to do）.也就是说,要跟一个动词,意思是“到做某事的时候了”.如： It's time to go home. It's time to tell him the truth. ——————It's time for 跟的是名词.也就是说,不能跟动词.如： It's time for lunch.(没必要说It's time to have lunch) It's time for class.(没必要说It's time to begin the class.) They can't wait to see you Please ask liming to study tonight. Please ask liming not to play computer games tonight. Don’t make/let me to smoke I can hear/see you dance at the stage You had better go to bed early. You had better not watch tv It’s better to go to bed early It’s best to run in the morning I am enjoy running with music. With 表伴随听音乐 I already finish studying You should keep working. You should keep on studying English Keep calm and carry on 保持冷静继续前行二战开始前英国皇家政府制造的海报名字 I have to go on studying I feel like I am flying I have to stop playing computer games and stop to go home now I forget/remember to finish my homework. I forget/remember cleaning the classroom We keep/percent/stop him from eating more chips I prefer orange to apple I prefer to walk rather than run I used to sing when I was young What’s wrong with you There have nothing to do with you I am so busy studying You are too young to na?ve I am so tired that I have to go to bed early
《The Kite Runner》追风筝的人--------------------------------美句摘抄 1.I can still see Hassan up on that tree, sunlight flickering through the leaves on his almost perfectly round face, a face like a Chinese doll chiseled from hardwood: his flat, broad nose and slanting, narrow eyes like bamboo leaves, eyes that looked, depending on the light, gold, green even sapphire 翻译：我依然能记得哈桑坐在树上的样子，阳光穿过叶子，照着他那浑圆的脸庞。他的脸很像木头刻成的中国娃娃，鼻子大而扁平，双眼眯斜如同竹叶，在不同光线下会显现出金色、绿色，甚至是宝石蓝。 E.g.: A shadow of disquiet flickering over his face. 2.Never told that the mirror, like shooting walnuts at the neighbor's dog, was always my idea. 翻译：从来不提镜子、用胡桃射狗其实都是我的鬼主意。E.g.:His secret died with him, for he never told anyone. 3.We would sit across from each other on a pair of high
一、翻译 1. The idea of consciously seeking out a special title was new to me., but not without appeal. 让我自己挑选自己最喜欢的书籍这个有意思的想法真的对我具有吸引力。 2.I was plunged into the aching tragedy of the Holocaust, the extraordinary clash of good, represented by the one decent man, and evil. 我陷入到大屠杀悲剧的痛苦之中，一个体面的人所代表的善与恶的猛烈冲击之中。 3.I was astonished by the the great power a novel could contain. I lacked the vocabulary to translate my feelings into words. 我被这部小说所包含的巨大能量感到震惊。我无法用语言来表达我的感情（心情）。 4，make sth. long to short长话短说 5.I learned that summer that reading was not the innocent(简单的) pastime(消遣) I have assumed it to be., not a breezy, instantly forgettable escape in the hammock(吊床)，( though I’ ve enjoyed many of those too ). I discovered that a book, if it arrives at the right moment, in the proper season, will change the course of all that follows. 那年夏天，我懂得了读书不是我认为的简单的娱乐消遣，也不只是躺在吊床上，一阵风吹过就忘记的消遣。我发现如果在适宜的时间、合适的季节读一本书的话，他将能改变一个人以后的人生道路。 二、词组造句 1. on purpose 特意，故意 This is especially true here, and it was ~. (这一点在这里尤其准确，并且他是故意的) 2.think up 虚构，编造，想出 She has thought up a good idea. 她想出了一个好的主意。 His story was thought up. 他的故事是编出来的。 3. in the meantime 与此同时 助记：in advance 事前in the meantime 与此同时in place 适当地... In the meantime, what can you do? 在这期间您能做什么呢？ In the meantime, we may not know how it works, but we know that it works. 在此期间，我们不知道它是如何工作的，但我们知道，它的确在发挥作用。 4.as though 好像，仿佛 It sounds as though you enjoyed Great wall. 这听起来好像你喜欢长城。 5. plunge into 使陷入 He plunged the room into darkness by switching off the light. 他把灯一关，房
The effective sentences:(improve the sentences!) 1.She hopes to spend this holiday either in Shanghai or in Suzhou. 2.Showing/to show sincerity and to keep/keeping promises are the basic requirements of a real friend. 3.I want to know the space of this house and when it was built. I want to know how big this house is and when it was built. I want to know the space of this house and the building time of the house. 4.In the past ten years,Mr.Smith has been a waiter,a tour guide,and taught English. In the past ten years,Mr.Smith has been a waiter,a tour guide,and an English teacher. 5.They are sweeping the floor wearing masks. They are sweeping the floor by wearing masks. wearing masks,They are sweeping the floor. 6.the drivers are told to drive carefully on the radio. the drivers are told on the radio to drive carefully 7.I almost spent two hours on this exercises. I spent almost two hours on this exercises. 8.Checking carefully,a serious mistake was found in the design. Checking carefully,I found a serious mistake in the design.
M1 U1 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： add up (to) be concerned about go through set down a series of on purpose in order to according to get along with fall in love (with) join in have got to hide away face to face 1 We’ve chatted online for some time but we have never met ___________. 2 It is nearly 11 o’clock yet he is not back. His mother ____________ him. 3 The Lius ___________ hard times before liberation. 4 ____________ get a good mark I worked very hard before the exam. 5 I think the window was broken ___________ by someone. 6 You should ___________ the language points on the blackboard. They are useful. 7 They met at Tom’s party and later on ____________ with each other. 8 You can find ____________ English reading materials in the school library. 9 I am easy to be with and _____________my classmates pretty well. 10 They __________ in a small village so that they might not be found. 11 Which of the following statements is not right ____________ the above passage? 12 It’s getting dark. I ___________ be off now. 13 More than 1,000 workers ___________ the general strike last week. 14 All her earnings _____________ about 3,000 yuan per month. 二.用以下短语造句： 1.go through 2. no longer/ not… any longer 3. on purpose 4. calm… down 5. happen to 6. set down 7. wonder if 三. 翻译： 1.曾经有段时间，我对学习丧失了兴趣。（there was a time when…） 2. 这是我第一次和她交流。（It is/was the first time that …注意时态） 3.他昨天公园里遇到的是他的一个老朋友。（强调句） 4. 他是在知道真相之后才意识到错怪女儿了。（强调句） M 1 U 2 一. 把下列短语填入每个句子的空白处（注意所填短语的形式变化）： play a …role (in) because of come up such as even if play a …part (in) 1 Dujiangyan(都江堰) is still ___________in irrigation(灌溉) today. 2 That question ___________ at yesterday’s meeting. 3 Karl Marx could speak a few foreign languages, _________Russian and English. 4 You must ask for leave first __________ you have something very important. 5 The media _________ major ________ in influencing people’s opinion s. 6 _________ years of hard work she looked like a woman in her fifties. 二.用以下短语造句： 1.make (good/full) use of 2. play a(n) important role in 3. even if 4. believe it or not 5. such as 6. because of
English sentence 1、(1)、able adj. 能 句子：We are able to live under the sea in the future. (2)、ability n. 能力 句子：Most school care for children of different abilities. (3)、enable v. 使。。。能 句子：This pass enables me to travel half-price on trains. 2、(1)、accurate adj. 精确的 句子：We must have the accurate calculation. (2)、accurately adv. 精确地 句子：His calculation is accurately. 3、(1)、act v. 扮演 句子：He act the interesting character.（2）、actor n. 演员 句子：He was a famous actor. (3)、actress n. 女演员 句子：She was a famous actress. (4)、active adj. 积极的 句子：He is an active boy. 4、add v. 加 句子：He adds a little sugar in the milk. 5、advantage n. 优势 句子：His advantage is fight. 6、age 年龄n. 句子：His age is 15. 7、amusing 娱人的adj. 句子：This story is amusing. 8、angry 生气的adj. 句子：He is angry. 9、America 美国n. 句子：He is in America. 10、appear 出现v. He appears in this place. 11. artist 艺术家n. He is an artist. 12. attract 吸引 He attracts the dog. 13. Australia 澳大利亚 He is in Australia. 14.base 基地 She is in the base now. 15.basket 篮子 His basket is nice. 16.beautiful 美丽的 She is very beautiful. 17.begin 开始 He begins writing. 18.black 黑色的 He is black. 19.bright 明亮的 His eyes are bright. 20.good 好的 He is good at basketball. 21.British 英国人 He is British. 22.building 建造物 The building is highest in this city 23.busy 忙的 He is busy now. 24.calculate 计算 He calculates this test well. 25.Canada 加拿大 He borns in Canada. 26.care 照顾 He cared she yesterday. 27.certain 无疑的 They are certain to succeed. 28.change 改变 He changes the system. 29.chemical 化学药品