Intergranular microcracks

FiK. 13. External surface of the FP fibre broken at I300°C exhibiting grain boundary decohesions and intergranular microcracks. 

Most directly, FE studies can be of use in the study of the fracture process in minerals. Since the emission processes are often localized on atomic scales, FE yields information about atomic scale processes, such as bond breaking, which accompany fracture. FE can yield information on microcracking, intergranular failure, and plastic deformation rather directly. FE can also reflect die role of voids and inclusions in the fracture process and the subsequent surface chemistry. 

Observations of microcracks and thin intergranular films in ceramics by transmission electron microscopy. J. Amer. Ceramic Soc., 63, 104-6. 

Comprehensive structural study of Ti-3Al-5Zr-Si-alloys, as-cast and deformed, confirmed the features found with the binary Ti-Si-system described above. The transition from polygonal to dendritic structure takes place between 2- and 4-wt.% Si. Alloy with 2-wt.% Si fails with intergranular (but ductile) mode whereas alloys with 4- and 6-wt.% fail with mixed mode where dendritic structure may be recognized. In any case, eutectic areas, in contrast to dendrite or polygonal bodies, which are of a-phase failing with cleavage microcracking, fail with ductile mode - with voids coalescence (Fig. 8). Hot plastic deformation transforms the alloys studied into ductile or semi-ductile materials, which fail only with ductile void coalescence mode [1],... 

The microstructure of zirconia grains is observed by TEM and analyzed by CBED (Fig. 6). The large intergranular grain is transformed and the twin domains and a microcrack formed in the mullite matrix due to the different expansion coefficient after transformation are visible. In this case, the intragranular tetragonal zirconia particles show only a small concentric stress field around them. 

The morphology of a single microcrack after tension-compression cycling (maximum stress, = 450 MPa) is shown in the sequence in Fig. 7.30. Crack growth is predominantly intergranular. Evidence of crack bridging by both uncracked matrix ligaments (Fig. 7.30b) and SiC whiskers (Fig. 7.30c, d) in the wake of the crack tip is clearly apparent. 

Pits that reach a critical depth can act as crack initiation sites if they lead to a higher local stress intensity. The crack initiation time in this case corresponds to the incubation time of pits of a critical size. Alternatively, precipitation reactions at the grain boundaries can render an alloy sensitive to intergranular corrosion. The preferentially corroded grain boundary then serves as initiation site of a crack. Inclusions, preexisting microcracks, or other structural defects are also likely crack initiation sites. The crack initiation time, in this case, is defined as the time required for a crack to reach a detectable size. Crack initiation may also be the result of hydrogen formed by a corrosion reaction that may cause embrittlement of the metal or of successive ruptures of a passive film or tarnish layer, but these mechanisms are more important for the propagation than the initiation of cracks. Because of the multitude of possible crack initiation mechanisms, and because of the statistical nature of the phenomenon, it is not possible to predict the crack initiation time from first principles. 

It is possible also for the caustic to concentrate at the waterline. Generally, the waterline area is always most sensitive to corrosion. Two types of failures are common in boilers and both are related to the effect of concentration of caustic. One which forms discontinuous microcracks and results in the bursting of tubes is called hydrogen damage, and the other which results in the formation of continuous microcracks leading to intergranular corrosion is called caustic cracking. Both are briefly discussed below ... 

The wear mechanisms of ceramics have also been widely discussed and several mechanisms have been suggested, which include microcracking [11], fincture [12,13], tribochemical reactions [13,14], transgranular and intergranular cracking [15] and polishing wear [16]. 

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