What failure mechanisms lead to the observation of cleavage and flat facets in fractographs of tensile samples?
The presence of cleavage and flat facets on fracture surfaces of tensile samples indicates brittle fracture behavior. They typically occur due to the presence of brittle phases such as carbides in the microstructure. You can refer to the below papers for more details.
1. doi:10.4028/www.scientific.net/DDF.395.95
2. doi:10.1016/j.msea.2012.01.130
3. Article Simultaneous enhancement of strength and ductility in cryoge...
The observation of cleavage and flat facets in fractographs of tensile samples is typically associated with brittle fracture mechanisms. These features arise due to the rapid propagation of cracks along specific crystallographic planes, resulting in a relatively flat fracture surface. The main failure mechanisms that lead to such observations are:
The previous two answers are spot on. Flat facets in hexagonal close-packed and especially body-centered cubic metals are invariably associated with transgranular cleavage fracture along low energy crystallographic planes. When brittle fractures like this were detected in the olden days, due to their shiny/sparkling nature the conclusion was often that the metal had failed because "it had crystallized", which is obviously pure rubbish?
However, other fracture modes can yield flat facets, in particular intergranular fracture along the grain boundaries. These facets are generally very smooth if the failure is due to hydrogen or where elements like S, P and "tramp elements" (Sb, Sn, etc.) segregate to the boundaries, but it is also seen in creep fracture where the intergranular surfaces are more "pock-marked" due to cavitation around particles in the boundaries.
ROR
It is great that Materials Science community comes back to such eternal topics as cleavage fracture. Sure new look on the problem is an engine of advance, but we should taken into account the contribution of researchers who were before. It is the honor to mention the following bright papers by the big names on cleavage fracture in metals:
1 C. GANDHI and M. F. ASHBY, Acta Metall. 27 (1979) 1565;
2 C. A. BROOKES, J . H. GREENWOOD and J . L. ROUTBORT,
J. Appl. Phys. 39 (1968) 2391;
3 P. HAASEN, H. HIEBER and B. L. MORDIKE, Zt. Metallkde
56 (1965) 832;
4 G. REINACHER, Metall 18 (1964) 731;
5 S . S . HECKER, D. L. ROHR and D. F. STEIN, Metall.
Trans. 9A (1978) 481;
6 S.P. LYNCH, Mater. Forum, 11 (1988) 268;
7 J . R. RICE, J. Mech. Phys. Solids. 40 (1992) 239.
Deformation modes (plastic slip, twinning, and fracture, for examples) in loaded materials decrease the energy of the system E (elastic energy and potential energy of the loading mechanism). A measure of the decrease of E during deformation evolution over time is given by what is called in fracture “energy release rate or crack extension force G per unit length of the crack front”. An expression for G depending on the shape of the crack front is available only under stationary conditions. For a crack with arbitrary front, it is necessary to consider an average (G averaged over all the positions on the crack front). Observed fracture systems fulfil the condition d=0 ( being function of crack-front configuration, applied loadings, material elastic constants…). Fracture is defined by a system {S}: S is the fracture plane; T the fracture propagation direction on S. The plane S may have different types of fracture propagation direction. Fractography of broken specimens is helpful in determining fracture systems. In laboratory experiments, we may have a tension σ22 in the vertical x2-direction and shears σ21 and σ23 in the horizontal Ox1x3-plane along x1 and x3, respectively. Changing the loadings may change the fracture systems. can be detected from the markings on micrographs. Please, for experiments see: “A study of the brittle fracture characteristics of CoSi2 using laser beam reflections”, Acta Metall. Mater. 43 (1995) 2275 – 2285. For theoretical prediction, see “Conoidal crack with elliptic bases, within cubic crystals, under arbitrarily applied loadings – III. Application to brittle fracture systems of CoSi2single crystals, ResearchGate, DOI: 10.13140/RG.2.2.34719.37282”.
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