Fracture of a 08KP Steel in Nano-Fragmented State
V.A. Yermishkin, M.P. Tamayo, N. Muñoz Aguirre
Instituto Politécnico Nacional, MX
Keywords: crack, dislocations, HVTEM, nano-fragmentation
Abstract:The evaluation of the resistance to the fracture toughness of metals and metal composites that work in the plastic regime require of the using of highly complex experimental methods and analysis which are not at all standard ones. The increment in the parameters describing the mechanical properties is certainly associated to the origin of internal barriers inside the material which are capable of stopping the free movement of the dislocations and their packing on specific sites of the crystalline structure. The dislocations play an important role on the transmitted energy in the metal during the deformation process. In this work it is analyzed the cryogenic hardness mechanism by deformation, and the features which influence such mechanism. The effectiveness of the hardness treatment of the metal is determined by the two factors: 1) the capacity of the dislocation barrier in order to support the stress between dislocations; 2) the distance between the barriers. Using high voltage transmission electron microscopy-HVTEM, the capacity of the barriers for stopping the dislocations is determined measuring the stress over them and the shift of the distance between the barriers. Also, it is evaluated the maximum specific elastic energy for a state of the structure, and an equation for a critic volume is proposed: (1) where: - specific elastic energy of the metal volume; - volume covered an originated crack; γ – specific surface energy of the crack; - area of the crack. Finally, a 3D analytical solution which gives the required energy in order to a crack unfold inside of an asymmetric volume is presented. The cryogenic thermo-mechanic treatment resulting in the nano-fragmentation process improved the mechanical properties of the 08KP steel to the best in the world. The standard mechanical properties of the studied material are show in table 1.