Research On Microcrack Propagation Of ?-TiAl Alloy

The unstable crack extension directly results in material failure,so the extension process is widely studied by scholars.On the micro level,the fundamental cause of crack propagation results from atomic bond breakage,which brings on many changes in the microstructure of material,such as the formation of vacancies and voids,dislocation nucleation,microcrack initiation and propagation etc.These changes cannot be directly observed by the experiment method,so these studies adopt computer simulation to investigate the mechanism of crack propagation.It can provide the theoretical guidance for the material design.In the paper,Molecular Dynamics is used to study the deformation process of single crystal ?-TiAl alloy under the cyclic loading or the uniaxial loading.The mechanical properties and deformation mechanism under different microcrack inclination angles,strain ratios and loading directions are discussed.The results are as follows.1.When the angles between microcrack and the loading are 0°,45°and 90°,stress-strain curve shows that the elastic modulus is the same,which indicates that the angle has no effect on the elastic deformation process.However,the greater the angle is,the smaller the ultimate strength of material is.It can be observed from the atomic map of crack propagation that the process of plastic deformation is different at different angles.For example,stacking faults are formed at different times and their density is different.In addition,the angle also has an effect on the time required for material failure.2.When the strain ratio changes from 0.5 to 0.9,the fatigue crack begins to expand at different times.The greater the strain ratio is,the sooner the crack begins to expand.However,the mode of crack propagation is the same under different strain ratios.Namely,the crack begins to expand in cleavage mode,then the disordered region is formed.The mode of crack propagation changes into noncleavage mode,in which the crack extends by driving the disordered region.Finally,the crack propagates in the sub-main crack propagation mechanism.The difference is that the change in the mode of crack propagation occurs at different times.During the plastic deformation,dislocations slipping results in the formation of different point defects under different strain ratios.During fatigue crack propagation,stacking faults are formed in different cycle times and the density changes along different the close-packed planes.3.When the loading direction along [010] and [001] crystal orientation respectively,the material exhibits different deformation mechanisms.Compared with the stress-strain curves,it can be found that the elastic modulus along the [010] is less than along the [001].When the loading direction is [010],there are dislocations emitted during the plastic deformation.Dislocations slipping results in the formation of vacancies and other point defects.When the loading direction is [001],there is no dislocation emitted in the early stage of crack propagation.In the latter period,dislocations are emitted from the crack tip.And there is no point defect formed during dislocation slip.Under cyclic loading,the plastic deformation is different along the different crystal orientations and different from the plastic deformation under uniaxial loading.