Effects Of Crystal Orientation And Void On Deformation Behavior And Mechanical Properties Of ?-TiAl By Nanoindentation

With the excellent mechanical properties, ?-TiAl alloy shows great potential for application in micro-electromechanical systems,while its poor plasticity at room temperature still restricts its further development.Since the mechanical properties of nanoscale materials and macro materials is put in difference and there is the anisotropy in the single crystal,as well as defects usually exist in materials in practical engineering applications,the nanoindentation of ?-TiAl with different crystal surfaces and voids was performed by molecular dynamics method in this paper to reveal the effects of crystal orientation and void on the deformation behavior and mechanical properties of ?-TiAl at room temperature.The main contents of this paper are as follows:Nanoindentation of ?-TiAl with different crystal surfaces at room temperature: the nanoindentation processes of ?-TiAl with(100),((?)10)and(111)surfaces were simulated at 300 K,the load–depth curve and defect evolution behavior during nanoindentation were obtained,the hardness and elastic modulus were calculated,as well as the variation of strain energy,potential energy and temperature during nanoindentation were analyzed.The results show that there is no pop-in event in the load–depth curve when the initial plastic deformation of ?-TiAl occurs,because the dislocation nucleates before the first load-drop;while a peak appears in both the unloading curves of the((?)10)and(111)samples due to the dislocations motion and annihilation.External stacking faults,twining boundaries and vacancies are formed in all samples;however,interstitials are formed in the(100)sample,a stacking fault tetrahedron is formed in the(111)sample,two prismatic dislocation loops with different activities are formed in the((?)10)and(111)samples,respectively.It is also concluded that the values of the critical load,hardness,elastic modulus,strain energy and potential energy for the(111)sample are the maximum,and for the(100)sample are the minimum.Furthermore,the crystal orientation has little effect on the temperature change during nanoindentation.Nanoindentation of ?-TiAl with voids at room temperature: spherical voids were prefabricated in the(100)sample and their positions or dimensions are various,the nanoindentation processes of ?-TiAl with voids were simulated at 300 K,the evolution behavior of voids and other defects during nanoindentation was analyzed,the load–depth curve,hardness and elastic modulus were discussed,and the variation of strain energy,potential energy and temperature in the nanoindentation process were studied.It is found that there is also no pop-in event in the load–depth curve when the initial plastic deformation of ?-TiAl with voids occurs.The dislocation loops are formed in all samples during nanoindentation,while the twinning boundary is formed in the d=20 ? sample,the external stacking fault is generated in the r=14 ? sample.The closer the void is to the upper surface of the specimen or the larger the void size is,the easier it is to collapse and annihilate,the earlier the plastic deformation occurs,the smaller the hardness and elastic modulus of ?-TiAl will be.The void makes the potential energy and strain energy reduced during nanoindentation,the closer the void is to the upper surface of the sample,the earlier the strain energy reduces,while the bigger the potential energy in the void collapse process;the larger the void size,the smaller the overall potential energy in the indentation process,the earlier the strain energy decreases and the greater the reduction.In addition,the effect of the void on the temperature change during nanoindentation can be ignored.