Ceramics ductile behavior

Despite the similarities in brittle and ductile behavior to ceramics and metals, respectively, the elastic and permanent deformation mechanisms in polymers are quite different, owing to the difference in structure and size scale of the entities undergoing movement. Whereas plastic deformation (or lack thereof) could be described in terms of dislocations and slip planes in metals and ceramics, the polymer chains that must be deformed are of a much larger size scale. Before discussing polymer mechanical properties in this context, however, we must first describe a phenomenon that is somewhat unique to polymers—one that imparts some astounding properties to these materials. That property is viscoelasticity, and it can be described in terms of fundamental processes that we have already introduced. 

Lately, however, some surprising exceptions have been found to the general rule of low plasticity in ceramics. One is the perovskite oxide strontium titanate, SrTiOs. Recent studies on single crystals have revealed a transition from nonductile to ductile behavior in this material not only at temperatures above 1000°C, but again, below 600°C. Even more unexpectedly, it reached strains of 7 percent at room temperature with flow stresses comparable to those of copper and aluminum alloys. At both the high and low temperatures, the plasticity appears to be owing to a dislocation-based mechanism (Gumbsch et al., 2001). 

To summarize, we can say that granulated materials should behave in a brittle manner, similar to a ceramic, in the sense of having small yield strains and small critical displacements but their behavior should also be ductile, similarto polymers, in the sense of having large process zones. The consequence of having large process zones is important since it implies that... 

The stress-strain curves for cortical bones at various strain rates are shown in Figure 5.130. The mechanical behavior is as expected from a composite of linear elastic ceramic reinforcement (HA) and a compliant, ductile polymer matrix (collagen). In fact, the tensile modulus values for bone can be modeled to within a factor of two by a rule-of-mixtures calculation on the basis of a 0.5 volume fraction HA-reinforced... 

Unlike clay-based systems, modem ceramics require additives, termed binders, to provide the plasticity required for ductile-forming methods to be used. These organic additives serve to modify the rheological behavior of the ceramic suspensions and impart handling strength to the green, as-formed ceramic bodies. Their... 

Unless materials are chemically stable in service environments, their otherwise useful properties (strength, ductility, magnetic and electronic behavior, etc.) may be lost. This section describes research opportunities and needs associated with metastable metallic alloys, metal-matrix composites, electroactive polymers, and high-performance ceramics. 

Despite many recent advances in material science and engineering, the performance of ceramic components in severe conditions is still far below the ideal limits predicted by theory. Modem ceramics have been primarily the products of applied physics and parallel the developments of physical metallurgy. The emphasis on the relation between behavior and microstructure has been fruitful for ceramic scientists for several decades. It has been recently realized, however, that major advances in ceramics during the next several decades will require an emphasis on molecular-level control. Organic chemistry, once abhorred by ceramic engineers trained to define ceramics as inorganic-nonmetallic materials, has become a valuable source of new ceramics. It has recently become known that as the stmctural scale in ceramics is reduced from macro to micro and to nano crystalline regimes, the basic properties are drastically altered. A brittle ceramic material has been shown to be partially ductile, for example. 

The stress-strain behavior of ceramic polycrystals is substantially different from single crystals. The same dislocation processes proceed within the individual grains but these must be constrained by the deformation of the adjacent grains. This constraint increases the difficulty of plastic deformation in polycrystals compared to the respective single crystals. As seen in Chapter 2, a general strain must involve six components, but only five will be independent at constant volume (e,=constant). This implies that a material must have at least five independent slip systems before it can undergo an arbitrary strain. A slip system is independent if the same strain cannot be obtained from a combination of slip on other systems. The lack of a sufficient number of independent slip systems is the reason why ceramics that are ductile when stressed in certain orientations as single crystals are often brittle as polycrystals. This scarcity of slip systems also leads to the formation of stress concentrations and subsequent crack formation. Various mechanisms have been postulated for crack nucleation by the pile-up of dislocations, as shown in Fig. 6.24. In these examples, the dislocation pile-up at a boundary or slip-band intersection leads to a stress concentration that is sufficient to nucleate a crack. 

Material / This has a high Young s modulus, high failure stress, low ductility, low toughness, and fractures without significant plastic deformation. This behavior is characteristic of many ceramics. 

Do ceramics experience a ductile-to-brittle (or the converse) transition and is it important Ceramics can exhibit both types of behavior over different temperature ranges. Figure 16.8 illustrates the temperature dependence of strength for ceramics. 

Region C Appreciable plastic flow occurs, with strains of the order of 10 prior to failure. This behavior is rarely observed in ceramics, even in ductile polycrystalline ceramics. 

Most metals are malleable, which means that they can be hammered into thin sheets, and ductile, which means that they can be drawn into wires ( FIGURE 12.10). These properties indicate that the atoms are capable of slipping past one another. Ionic and covalent-network solids do not exhibit such behavior. They are typically brittle and fracture easily. Consider, for example, the difference between dropping a ceramic plate and an aluminum cooking pan onto a concrete floor. 

Important amorphous ceramics are the glass-Uke materials. A schematic stress-strain curve is indicated in Fig. 1.1b and shows a lower overall stress—strain curve. However, it would be difficult to enumerate aU the many types of glass with a wide variety of compositions, ranging from the most common window glass to the various metallic glasses. In general however, internal and external factors influence the performance of ceramic materials even to the point at which ductility can be induced. Here are some of these factors, especially those that have critical effects on ceramic (and glass) behavior ... 

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