The Effect Of Microstructure On Mechanical Properties Of ?-TiAl By Nanoindentation

With the rapid advancements of the precision and ultra-precision processing of technology,the mechanical behaviors of metal materials at nano-scale has received widespread attention.?-TiAl alloy has been widely used in aerospace and automobile industries due to its superior properties.However,its deformation mechanism at nano-scale is still unclear.First,considering that the microstructure has a greater impact on the plastic deformation and mechanical properties of materials at nano-scale.On the other hand,the macroscopic experiments are limited by conditions and costs so that it is difficult to capture the evolution of the internal microstructure of materials.In this dissertation,the nanoindentation of ?-TiAl alloy with different grain sizes,twin boundary spacings and crystal structures was performed by using molecular dynamics method.The mechanical properties of ?-TiAl alloy were discussed under different conditions.The relationship between the deformation mechanism and mechanical properties of ?-TiAl alloy was exposed.The main contents of this paper are as follows:(1)The nanoindentation process of ?-TiAl with different grain sizes was studied.The effects of grain size on the mechanical properties and deformation behavior were revealed.The results show that the relationship between grain size and its hardness exhibits an inverse HallPetch when the grain size is less than 9.9 nm.Meanwhile,the grain boundary activity and dislocation sliding promote the plastic deformation of matrix,and the grain boundary activity plays a major role.However,the relationship between the grain size and its hardness conforms to Hall-Petch when the grain size exceeds 9.9 nm.The grain boundary has little effect on the plastic deformation,and the plastic deformation of matrix is dominated by dislocation.In addition,the stress transfer and deformation recovery of ?-TiAl were analyzed in the nanoindentation process,it was found that the dense grain boundary grid can effectively inhibit the indentation defects and the internal stress transfer to the matrix.The smaller the grain size,the more uniform the internal stress distribution under the indenter and the smaller the elastic recovery ratio along the indentation direction.(2)The nanoindentation process of nanotwinned ?-TiAl alloys with different twin boundary spacings was explored.The deformation mechanisms of the specimens with different twin boundary spacings were revealed.The results indicates that the average hardness of the matrix changes in inverse proportion to the twin boundary spacings in shallow indentation depth.The smaller the twin boundary spacing,the more significant the strengthening effect of twins.The critical twin boundary spacing of the matrix appears when the indentation depth is greater than 14 ?.The plastic deformation mechanism of the matrix is dominated by the softening of the twin boundary when the twin boundary spacing is less than 2.8 nm,while the strengthening of the twin boundary dominates the matrix plastic deformation when the twin boundary spacing exceeds 2.8 nm.In addition,the influence of temperatures on the mechanical properties of ?-TiAl alloy was analyzed,and it can be found that the hardness and elastic modulus are the linear decreasing functions of the temperature.(3)The nanoindentation deformation process of the single crystal,polycrystalline and nanotwinned ?-TiAl alloy was simulated and discussed,and the influence of the crystal structure change on the mechanical behavior of materials was further analyzed.The results show that the hardness,the elastic recovery ratio,the temperature and the potential energy of the single crystal,polycrystalline and nanotwined ?-TiAl alloys depend on the crystal structure.The plastic deformation of the single crystal ?-TiAl depends on the propagation and reaction of Shockley dislocations,the polycrystalline matrix relies on the interaction of dislocations and grain boundaries,and the nanotwinned matrix depends on the interaction of the dislocationtwin and the dislocation-grain boundary.The hardness of single crystal matrix is the largest,that of nanotwined matrix is the second,and that of polycrystalline matrix is the smallest.In addition,the stress transfer process of ?-TiAl alloy with different crystal structures was analyzed.It can be found that twin boundarys and grain boundaries could effectively absorb and hinder the stress transfer.