Study On Creep Crack Growth Of 2219 Aluminum Alloy By Synchrotron Radiation

With the rapid development of aeronautics and aerospace industry,more and more attention has been paid to the use of high strength and high temperature resistant aluminium alloys.Especially,the properties of high temperature aluminium alloys have got more attention from people,including creep crack growth behavior.However,most of the current related studies are focused on two-dimensional cracks because of the limitation of the observation methods.In this work,the creep crack growth behavior is studied,and the three-dimensional crack tip is studied by means of synchrotron radiation X-ray tomography.The general law of creep crack growth of aged 2219 aluminum alloys,was studied firstly by using the direct current potential method.It is found that the incubation period of crack growth is greatly shortened and the fracture of the specimen is accelerated with the increase of load.Two crack tip parameters,stress field intensity factor(K)and modified J integral(C*),are calculated and compared.The results showed that modified J integral(C*)can describe the crack growth behavior of CT specimens under different loads,and it is an appropriate crack tip control parameter.In addition,Under the same experimental conditions,the creep crack growth experiments of CT specimens with two kinds of microstructures showed that the microstructure with large grain size and less precipitated second phase exhibited better creep resistance,and the fracture mechanism gradually changes from intergranular fracture to transgranular fracture.Meanwhile,A suitable gallium penetration process was explored and determined: holding at 50℃ for 10 minutes,based on the phenomenon that liquid gallium metal can penetrate along the grain boundary of aluminium alloy matrix,for displaying the grain boundary of 2219 aluminium alloy matrix.As expected,the gallium penetration method was suitable for displaying the grain boundaries of three microstructures of 2219 aluminium alloys,namely rolling state,equiaxed state and aged state.Moreover we found that the grain boundary imaging result of equiaxed and ageing state was better than that of rolling state.The grain boundaries of equiaxed samples and aged samples showed different results,but it did not affect the analysis of grain boundaries and grain morphology.The creep crack growth behavior was divided into different crack growth stages,according to the creep crack growth curve(a-t).The crack tip samples at different stages were collected by interruption experiment and observed by synchrotron radiation X-ray tomography.It can be seen that the creep crack first forms and growths in the interior of the material,and its growth behavior is prior to the crack behavior on the surface of the material,by observing and analyzing the reconstructed slice diagram and 3D volume rendering diagram.In the first stage,the internal crack will also growth to the surface layer along the thickness direction,when it growths forwardly.In addition,in the second stage,the larger independent branch cracks formed.The number and volume of the independent branch cracks increased with time,and it will combine with the main crack,which can lead to the instability growth of the main crack.