PSI - Issue 30

M.Z. Borisova / Procedia Structural Integrity 30 (2020) 17–22 Borisova M. Z. / Structural Integrity Procedia 00 (2020) 000–000

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fracture surface absorbs only an exceedingly small work of breaking the bonds, for the cut and tear this work is the same. The difference by orders of magnitude between viscous and brittle fracture is caused by the work of the plastic deformation. The main difference between viscous fracture is the continuous generation of many new focal cracks in the field of plastic deformation, moving along with the macro-crack. Thus, the visible "smooth" areas on the fracture surfaces of the specimens of the steel in initial state are less energy-intensive than the pores. This can explain the lower values of steel impact strength in the initial state, than after ECAP.

Fig. 2. SEM images of fracture surfaces of the low-carbon steel specimens in initial state after impact tests at room temperature (+20°C): (a) x 60; (b) x 2000; (c) x 10000 and after impact tests at sub-zero test temperature (–20°C): (d) x 60; (e) x 2000; (f) x 10000.

Fig. 3. Brittle zone of the fracture surface of the low-carbon steel specimen in initial state after impact tests at –20°C.

ECAP and subsequent heat treatment led to an increase in the impact strength values at room and sub-zero temperatures. To see the reason for the toughness increase, look at the fracture surface of the samples (see Fig. 4). As for the initial state, for the steel after ECAP the destruction character is viscous. But there are no areas of brittle destruction. At large magnifications, it can be seen that the destruction surface is evenly covered with pores. At the same time, the size and depth of the pores after ECAP is greater than in the initial state. Which means a more homogeneous structure of the steel after SPD. Spherical carbide inclusions are clearly visible in centers of the pores, which serve as stress concentrators and induce the origin of cracks.