Plastic-to-brittle transition

Franks, G.V. and Lange, F.R, Mechanical behavior of saturated, consolidated, alumina powder compacts Effect of particle size and morphology on the plastic-to-brittle transition, Colloids Surf. A, 146, 5, 1999. 

All of these conclusions have recently been confirmed by others [158, 159]. Wan etal. [159] however, have ascribed the drop in Kic to a dramatic decrease in modulus at higher temperatures. In the present authors studies, however, no correlation was found between the plastic-to-brittle transition and a drop in modulus, for the simple reason that the modulus does not drop precipitously, but rather linearly with the same slope as below the transition temperature [50]. In measuring the modulus, care must be taken not to load the sample, as this can trigger the formation of IKBs that will result in apparently lower modulus values. 

The concept of a ductile-to-brittle transition temperature in plastics is likewise well known in metals, notched metal products being more prone to brittle failure than unnotched specimens. Of course there are major differences, such as the short time moduli of many plastics compared with those in steel, that may be 30 x 106 psi (207 x 106 kPa). Although the ductile metals often undergo local necking during a tensile test, followed by failure in the neck, many ductile plastics exhibit the phenomenon called a propagating neck. Tliese different engineering characteristics also have important effects on certain aspects of impact resistance. 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

Nozzle throat inserts of molybdenum and steel are most frequently used for short duration firings, while bulk graphite is much better for longer duration operations. When it is critical to maintain throat dimensions, a metal (like silver) infiltrated porous refractory (such as tungsten) is employed. All of these materials are heavy, however, and they possess certain other limitations. Molybdenum and mngsten are inherently brittle below their ductile-to-brittle transition temperatures. Graphites and carbides are brittle because their crystallographic structures preclude plastic flow at low temperatures. Moreover, the carbides are sensitive to thermal shock. 

As noted above, all MAX phases tested to date pass through a brittle-to-plastic transition (BPT) [98, 145, 158, 159]. The temperature of the transition varies from phase to phase, but for many Al-containing phases and Ti3SiC2, it is between 1000 and 1100 °C. The fact that Kicdrops above the BPT temperature (Figure 7.18a) [132], categorically rules out the activation of additional slip systems, as some have suggested, and this is why it is more accurate to label the transition as a BPT transition, rather than as the more common ductile-to-brittle transition. 

Figure 5.24 compares compression tests of MoSi2 and MoSi2 + 2.5 at% Re alloys. Compressive plasticity in MoSi2 is observed only above 900 °C, whereas in the MoSi2 + 2.5 at% Re alloy only above 1000 °C. This indicates the increase in ductile-to-brittle transition temperature due to the Re alloying. Typical dislocation structures of these two materials are compared in Fig. 5.25. Arrays of subgrains...

The transition-metal carbides have the ability to deform plastically above a given temperature, i.e., the ductile-to-brittle transition temperature. Below that temperature, the carbides fail in a brittle manner while above it, they show a ductile behavior and undergo plastic deformation. 1 1 The transition temperature is not a fixed value but depends on several factors such as grain size, composition, and impurity content.It is usually about 800°C. 

The mechanical behavior of polymers is quite different from metals and ceramics and depends greatly on their structure and operating temperature. Below their glass transition temperature, Tg (the temperature at which their covalent bonded chains can no longer move relative to one another), they are quite brittle and exhibit glass-like behavior. Above their Tg, they behave plastically. In this sense, they are similar to metals that exhibit ductile-to-brittle transitions, but for entirely different reasons. 

For thermoplastic polymers, both ductile and brittle modes are possible, and many of these materials are capable of experiencing a ductile-to-brittle transition. Factors that favor brittle fracture are a reduction in temperature, an increase in strain rate, the presence of a sharp notch, an increase in specimen thickness, and any modification of the polymer structure that raises the glass transition temperature (T ) (see Section 15.14). Glassy thermoplastics are brittle below their glass transition temperatures. However, as the temperature is raised, they become ductile in the vicinity of their T s and experience plastic yielding prior to fracture. This behavior is demonstrated by the stress-strain characteristics of poly(methyl methacrylate) (PMMA) in Figure 15.3. At 4°C, PMMA is totally brittle, whereas at 60°C it becomes extremely ductile. 

The widespread use of Izod and Charpy impact tests to evaluate plastics is, to an unprejudiced eye, rather difficult to justify. Many structural polymers us in load-bearing applications do show a range of fracture behaviour from ductile to brittle . Most thermoplastics can show either kind of behaviour, and may suffer an abrupt tough-to-brittle transition with any of a number of parameters — one of which is the rate of loading at a notch. In order to select a polymer for a specific application it may be important to know its sensitivity to this kind of impact embrittlement. However, it is difficult to see how one might learn this fiem conventional impact strength data. 

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