Brittle-to-ductile transition temperature

When pressure tests are conducted at metal temperatures near the ductile-to-brittle transition temperature of the material, the possibility of brittle fracture shall be considered. 

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. 

Hardness and a ductile-to-brittle transition temperature (DBTT) have also been noted to follow a Hall-Petch relationship (Meyers, and Chalwa, 1984). Ductility increases as the grain size decreases. Decreasing grain size tends to improve fatigue resistance but increases creep rate. Electrical resistivity increases as grain size decreases, as the mean free path for electron motion decreases. 

TABLE 1.10. Parameters which Modify the Ductile-to-Brittle Transition Temperature [1.43]... 

FIGURE 6.11. Ultimate tensile strength and fracture elongation at different temperature and ductile-to-brittle transition temperature as dependent on the degree of working for W-2%Th02 [6.1,6.19]. 

Rhenium (Re) differs from the other refractory metals (Nb, Ta, Mo and W) in that it has an hep structure, and does not form carbides. Because it does not have a ductile-to-brittle transition temperature. Re retains its ductility from subzero to high temperatures. In addition, it can be mechanically formed and shaped to some degree at room temperature. It also has a very high modulus of elasticity that, among metals, is second only to those of Ir and Os. Compared with other refractory metals. Re has superior tensile strength and creep-rupture strength over a wide temperature range. 

Thermomechanical processing (TMP) has first been developed in the second half of the last century for decreasing the ductile to brittle transition temperature of construction steels. It has then progressively been extended to other categories of structural metallic materials, such as titanium alloys or nickel base superalloys for aircraft forged turbine parts. More recently, investigations have been carried out to assess the possibility and advantage of TMP for ferritic and austenitic stainless steels. 

Notched lzod Test. The influence of particle size was studied in a PA-EPDM blend (Figure 7) (9). The ductile-to-brittle transition temperature decreases with decreasing particle size. The S-shape of the curve is so uniform... 

CTL close tolerance DBTT ductile-to-brittle 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. 

The impact tests of the wrapper material after irradiation showed a small decrease of the upper shelf energy. The DBTT (Ductile to Brittle Transition Temperature) was hardly influenced. 

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