Ductility brittle-ductile transition temperatur

LME is enhanced by an increase in grain size. At the same time the brittle to ductile transition temperature is increased by an increase in grain size. 

The temperature at which a sample changes from brittle to ductile can be called the brittle-ductile transition temperature (Tgg). 

FIG. 13.66 Rate of strain dependence upon brittle-ductile transition temperatures of various solids. According to Vincent (1962). 

There are several parameters that affect the brittleness and the brittle-ductile transition temperature, such as molecular weight, presence of cross-links, crystallinity and the presence of notches. A schematic way, following Fig. 13.75 to depict the influence of the various parameters, is shown in Fig. 13.76. 

On the other hand, it is expected that the strain rate also influences Tj. It has been found that while brittle fracture is hardly affected, the yield stress changes significantly with the strain rate. As shown in Figure 14.26, when the strain rate increases, Oy increases. Therefore the brittle-ductile transition temperature increases, as does the strain rate. This is easily illu-... 

The brittle-ductile transition temperature depends on the characteristics of the sample such as thickness, surface defects, and the presence of flaws or notches. Increasing the thickness of the sample favors brittle fracture a typical example is polycarbonate at room temperature. The presence of surface defects (scratches) or the introduction of flaws and notches in the sample increases Tg. A polymer that displays ductile behavior at a particular temperature can break in the brittle mode if a notch is made in it examples are PVC and nylon. This type of behavior is explained by analyzing the distribution of stresses in the zone of the notch. When a sample is subjected to a uniaxial tension, a complex state of stresses is created at the tip of the notch and the yield stress brittle behavior known as notch brittleness. Brittle behavior is favored by sharp notches and thick samples where plane strain deformation prevails over plane stress deformation. 

Moreover, the brittle-ductile transition temperature depends on the molecular structure and morphology of the polymer sample. The correlation between chemical structure and fracture behavior is not yet well understood. It is recognized that entanglements control the fracture behavior of glassy... 

Conversely, nodules can significantly increase the amount of energy dissipated during deformation of notched specimens (Fig, 5., 6.). They contribute to decrease brittle to ductile transition temperature. Non-reactive nodule can toughen amorphous PET to a certain extend, being nevertheless always less efficient than reactive one. On the contrary, only the reactive nodules exhibit a certain level of efficiency in semi-crystalline PET, provided that concentration is at least 21 %. 

At room temperature, PP is close to its Tg(0-25°C) and well above its normal brittle-ductile transition temperature ( -30°C). However the presence of surface cracks in the photo-oxidized film is apparently sufficient to promote brittle failure at room temperature. According to the Griffith crack theory, once a critical crack length has been exceeded, a critical crack velocity is required to propagate the crack. If this velocity is not exceeded, cold drawing of the amorphous zones ensues. 

However, the monolithic compounds possess a lack of room temperature ductility and toughness because of their complex lattice structures and sessile superdislocations with large Burgers vectors. The brittle-ductile transition temperatures of these silicides are quite high of the order of 800 to 1050 °C, respectively. 

Steels killed with silicon, such as ASTM A515 plates, tend to have a coarse grain structure usually with a silicon content of 0.15 to 0.30 wt%. They characteristically have relatively high brittle-ductile transition temperatures, making them unsuitable for applications requiring low-... 

Low-temperature embrittlement occurs in carbon and low-alloy steels at temperatures below their brittle-ductile transition temperature range. The effect is reversible when the alloy is heated above the transition range, ductility is restored. This embrittlement is avoided by following the Charpy impact test requirements of the relevant engineering codes. The need to test depends primarily on the material, its thickness and the minimum design temperature. 

Duplex stainless steels are susceptible to 885°F (475°C) embrittlement and to sigma-phase formation, and they are usually not selected for temperatures above 650°F (345°C). Because of their ferrite content, they are susceptible to low-temperature embrittlement. However, the duplex stainless steels tend to have relatively low brittle-ductile transition temperatures. The engineering codes typically require the duplex stainless steels to be qualified for low-temperature service by impact testing. They can be susceptible to hydrogen embrittlement, but are less susceptible than are the ferritic and martensitic stainless steels. 

As a consequence of the available slip systems, the strength and ductility are highly anisotropic with a hard <100> direction and soft <110> and <111 > directions (Darolia et al., 1992 b Glatzel etal. 1993b Takasugi etal., 1993a). NiAl with the hard orientation shows practically no ductility - in spite of indications of local plastic deformation (Vehoff, 1992) - below the brittle-to-ductile transition temperature (BDTT) which is of the order of 350°C - corresponding to 0.33 (r =... 

Up to now the fracture toughness at room temperature is 2-4 MNm (see data in Sauthoff, 1990 a, b) for these alloys, and the brittle-to-ductile transition temperature is between about 500 °C and 700 °C depending on composition and preparation method. Analysis of the microstructures of the deformed alloys has shown that in the brittle-to-ductile transition range cracks in the Laves phase are stopped at the Laves phase-NiAl interface by producing a plastic zone in the NiAl phase ahead of the crack (Machon, 1992 Wunderlich etal., 1992). 

Since the transition metal elements in these ternary Laves phases can substitute for each other freely, and since the distribution of Laves phase can be controlled by thermomechanical treatments there are possibilities for optimizing such NiAl alloys with Laves phases with respect to creep resistance and the brittle-to-ductile transition temperature (BDTT) by controlling the composition and phase distribution, and this is being studied presently... 

Figure 33. Brittle-to-ductile transition temperature BDTT as a function of strain rate in bending (referred to the specimen edge) for various two-phase NiAl-(Ta,Nb)NiAl alloys with 23.5 vol.% Laves phase Tal0Ni45A145 (x), Ta9NblNi45A145 (-I-), Ta7.5Nb2.5Ni45A145 (o), TalNb9Ni45A145 ( ), and Nbl0Ni45A145 ( ) (Sauthoff, 1991a Zeumer etal., 1991 Zeumer and Sauthoff, 1992).

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