In-situ Electron Microscopy Researches On The Crystal Orientation,the Initiation And Propagation Of The Cracks In Deformed Aluminum Alloys

The inhomogeneous microstructure caused by preferred orientation or multiscale second phase particles is the significant reason for fracture failure of the wrought aluminium alloy.To investigate the effects of orientation and multi-scale second phases on the initiation and propagation of microcracks,two typical wrought aluminum alloys,namely,the high ductility 1060 aluminum alloy（without second phase）and the high strength Al-Zn-Mg-Cu alloy（containing second phases）,were selected as the objects of study.The inherent mechanism and influence factors of microcracks initiation and propagation were investigated in micro-and nano-scales by in-situ electron back scatter diffraction（In-situ EBSD）,in-situ and ex-situ transmission electron microscopy（TEM）combining the trace analysis and Schmid factor calculation.Texture types of the 1060 alloy were cube texture and copper texture,the average grain size was 24.6μm.The results showed that the crystal orientation had important effects on the deformation behavior,microcrack initiation and propagation of the alloy.The copper oriented grains were easier to deform than the cube oriented grains in the process of deformation.The orientation rotation behavior of the selected 42 grains on the sample surface were discussed and compared with Sachs model,Taylor model and reaction stress model,indicating that the initial orientation and the number of activated slip system（s）were important factors affecting the orientation rotation behavior.When single slip system was activated,the direction of orientation rotation unchanged,and the single slip system has a maximum Schmid factor（depended on the initial orientation）,it was consistent with the results predicted by Sachs model.However,the direction of orientation rotation changed when multiple slip systems were activated by the stress concentration due to deformation inhomogeneity or cracking.The Schmid factors of 12 potential slip systems and the stress distribution at the crack tip in a cracked grain were calculated,the results showed that the nature and the located slip system of the emitted dislocations from the crack tip were determined by the stress distribution.When the emitted dislocations away from the tip,the influence of the stress distribution weakened gradually on the emitted dislocations while influence of external loading stress strengthened gradually.The microcracks in the 1060 alloy initiated by interfacial debonding at grain boundaries,slip bands and a few impurity particles,and propagated transgranularly along the slip plane of{111}in continuous and discontinuous ways.In the case of continuous propagation,the number of dislocations emitting from the crack tip were less than that of the discontinuous propagation.The discontinuous propagation process included the formation and growth of nano-voids in the dislocation-free region ahead of the crack tip,and then coalescence of the nano-voids with the main crack.And,this process was usually accompanied by changes in the propagation path,such as zigzag crack.High angle grain boundary（HAGB）hindered the continuous propagation,blunted the crack tip and induced activation of multi-slip,suggesting that HAGB was conducive to reducing the crack growth rate and improving the fracture toughness of the alloy.The crack tip blunting led to the increase of the stress intensity factor,which provided driving force for the formation of the nano-voids,while the disordered zone in the thinning zone and around the crack tip provided vacancy sources for the formation of the nano-voids.Microstructure of the Al-Zn-Mg-Cu alloy consisted of multi-scale second phase particles,<001>//RD and<111>//RD oriented grain colonies,the average grain size was24.6μm.The multi-scale second phase particles included submicro-to micro-sized constituent particles(Al₉Fe_0.7Ni_1.3 and Mg Zn₂),tens nanometers-sized precipitates（Mg Zn₂）,a few nanometer-to a dozen nanometers-sized dispersoid（Al₃Zr）and precipitates（ηphase）,and a few nanometer-sized precipitates（η′phase）.The results showed that the size,and the location of the second phase particles played important roles in the initiation and propagation of microcracks.In the aspect of the size,the larger of the second phase particles was,the easier the microcrack initiated.The nano-sized particles（Al₃Zr,η′andη）had limited influence on the microcrack propagation path,but these particles increased the resistance of dislocation movement through the cut-through mechanism,thus improving the resistance of microcrack propagation.In the aspect of the location,the particles(Al₉Fe_0.7Ni_1.3 and Mg Zn₂)distributed along grain boundaries failured by interfacial debonding and particle fracture,and became the main sites of microcrack initiation,this increased the fracture sensitivity of the alloy.The intragranular Mg Zn₂particles,however,contributed to the homogeneous slip of the matrix and avoided the microcracks initiation in the concentrated slip bands.Furthermore,the intragranular Mg Zn₂particles prompted deflection of the propagation path,which was beneficial to increase the fracture strength and decrease the crack propagation rate.The<001>//RD grain colony was easier to deform than the<111>//RD grain colony during the deformation of the Al-Zn-Mg-Cu alloy.The fracture modes of the alloy were mainly dominated by intergranular mode,with a small amount of transgranular.The intergranular fracture was mainly attributed to a large number of the second phase particles distributed along grain boundaries and the inhomogeneous deformation between the grain colonies.For intergranular fracture,the crack propagated along the grain boundaries within the<111>grain colony or the boundaries between the grain colonies.For transcrystalline fracture,the crack propagation path in a grain depended on the combined effect of the external stress loading direction and the activated slip systems of the grain.