Fracture Toughness And Fracture Mechanism Of Bulk Nanostructured Materials

In recent years,the methods for preparing bulk nanostructured metals have experienced further development,the theoretical studies on the mechanical properties of these materials are also gradually improved.Numerous studies have shown that bulk nanostructured metals possess superior strength,hardness and wear resistance.However,studies are still rather limited concerning fracture toughness and fracture mechanism of bulk nanostructured metal.In this work,elastic-plastic fracture mechanics method was employed to investigate the fracture toughness and fracture mechanism of the ultrafine-grained(UFG)Cu,which was prepared by means of the equal channel angular pressing(ECAP).The fracture toughness and toughening mechanisms of heterostructure nanostructured 316L stainless steel produced by cold rolling and subsequent annealing were also studied.The main results are concluded as follows:(1)A custom-designed contactless crack opening displacement gauging system based on the digital image correlation technique was established to measure the fracture toughness of miniaturized samples.The system could measure the load-line displacement of the miniaturized specimen accurately.The occurrence of crack tunneling during crack extension was suppressed effectively by appropriate side-grooving treatment.(2)Compared with the significant orientation dependence of the fracture toughness,the uniaxial tensile response of the UFG Cu is much less anisotropic.The critical fracture toughness K_IC and crack growth toughness T_R depend more significantly on the crack direction than on the crack plane.The fracture toughness for crack propagation in tangential direction(TD)is higher than in normal direction(ND)and extrusion direction(ED).When crack extends in TD,significant crack tip blunting and crack deflection take place,which plays a dominant role in promoting the crack growth resistance.(3)The fracture toughness anisotropy in UFG Cu is primarily controlled by the grain elongation plain(GEP),where the fracture resistance is the lowest.Severe plastic deformation induced high-aligned metastable grain boundaries aligned with GEP,providing high-density micro-void nucleation sites,which makes cracking more favorable along them.The low fracture resistance along the GEP also enhances microscopic crack deflection and local plasticity during fracture processes,this effectively promotes the crack initiation and crack growth toughness in directions with high GEP inclination angles by reducing the local driving force for crack extension.Furthermore,the ductile fracture in UFG Cu involves thermal activated vacancy diffusion and condensation.The long-range vacancy diffusion along the grain boundaries in UFG Cu is depressed at cryogenic temperature,resulting in a suppressed void nucleation-coalescence and increased crack tip blunting.Therefore,the fracture toughness of UFG Cu at-150?(K_IC=78 MPa m^1/2)is higher than that at 25?(K_IC=54 MPa m^1/2).(4)A heterogeneous nanostructured 316L stainless steel was produced by 87%cold rolling.The heterogeneous structure was characterized with ultrafine/nanograins and nano-twin bundles.Fracture toughness tests indicate this steel exhibits a considerable fracture toughness(K_IC=113 MPa m^1/2),which originates from martensite/austenite phase boundary delamination.The thermal annealing of cold rolled sample at 750?for 5-25 min leads to partial recrystallization in the microstructure,resulting in a mixed microstructure of recrystallized grains,ultrafine/nanograins and nano-twin bundles.The yield strength and uniform elongation of 25 min annealed 316L sample are 1043 MPa and 12.4%,which exhibits a good combination of strength and ductility.Although the delamination fracture was suppressed,good fracture toughness(K_IC=111 MPa m^1/2)was maintained for 25 min annealed samples,which was attributed to the ductile fracture process and strain-induced martensitic transformation.