Transitions ductile-fragile

Body-centered cubic (bcc) metals, such as ferritic steels as well as some hexagonal close-packed (hcp) metals, undergo a ductile-to-brittle transition in their fracture behavior. Fractures that arise from this phenomenon have an especially insidious character because they may occur under static loading at stresses well below the yield strength and without warning.

Brittle fracture is induced by low temperatures, high strain rates, and a state of triaxial tensile stresses, such as that produced by a notch. Only two or three need be present to initiate brittle fracture. In many cases, brittle is induced by low temperature and a state of triaxial tensile stresses at a critical location in the structure.

The ductile-to-brittle transition can be demonstrated quite readily by the Charpy V-notch impact test. The transition from ductile to brittle fracture is manifested geometricaly by a decrease in the amount of lateral expansion on the compression side of the specimen and by elimination of the shear lip as the test temperature decreases. In addition, the usual appearance of the fracture surface changes from that of a rough  ductile fracture to a smooth, planar brittle fracture. On a microscopic level the ductile fracture is characterized  by microvoid coalescence dimples of various orientations, and transgranular cracking, whereas the low-temperature brittle fracture consists of transgranular quasi-cleavage. At temperatures within the transition range, crack propagation occurs by both microvoid coalescence and quasi-cleavage, with the latter being confined predominantly to the central region of the fracture surface.

Face-centered cubic alloys, such as AISI type 316 stainless steel, do not exhibit a ductile-to-brittle transition. The existence of a ductile-to-brittle transition in low-carbon steel is related to the structure and mobility of dislocations in the bcc ferrite; the strong temperature dependence of the flow stress and dislocation-interstitial atom interactions are specific aspects of this phenomenon.
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