Grain size refinement effects on the strengthening of metallic materials has been known since the works of Hall and Petch on pure metal or metallic alloys [1,2]. High Pressure Torsion (HPT) is a highlighted severe plastic deformation method for producing ultrafine grains metallic samples , with grain sizes in the range of 100 nm and below. Indeed, pure metallic bcc and fcc metals show high strengthening due to smaller grain size . While the strength of bulk metallic ufg samples increases, their fracture toughness is most often equal or smaller than their counterparts with larger grains size (> 1µm) . This is related to the large number of grain boundaries, serving as more favorable sites for a crack to advance. However, the fracture toughness is also observed to be strongly dependent on the direction of the notch introduced for a desired crack path with respect to the grain shape (pancake direction or elongated) due to the respective shearing and compaction directions presents in an HPT disc.
In this work, we present miniaturization of fracture experiments to the microscale for in situ SEM tests on ufg chromium samples obtained from the three different principal directions of an HPT disc. This will allow us to reproduce bulk properties and to monitor in situ in SEM the crack path with respect to the grain shape and sample orientation with for the three directions in an HPT disc. Moreover, miniaturization to in situ TEM tests will allow us to introduce a notch either along a crystalline grain or along a grain boundary and thus monitor in situ in the TEM dislocations processes involved prior to crack propagation. Latter’s path will also be investigated at the nanoscale with respect to the grain boundaries orientation (grain shape).
 Hall, E. O. « The Deformation and Ageing of Mild Steel: III Discussion of Results ». Proceedings of the Physical Society. Section B 64, no 9 (1951): 747.
 Petch. N.J « The cleavage strength of polycrystals». Journal of Iron Steel Inst. 174:25–28 (1953)
 Pippan et al. Saturation of Fragmentation During Severe Plastic Deformation. Annual Review of Materials Research Vol. 40:319-343 (2010).
 A. Hohenwarter et al. Fracture and fracture toughness of nanopolycrystalline metals produced by severe plastic deformation. Philos Trans A Math Phys Eng Sci. 373(2038) (2015)