Atomic-scale Investigation On The Ti₂AlN/TiAl Interface Models And Deformation Mechanisms

Due to their excellent properties such as low density,high melting point,high specific strength and good creep resistance,TiAl alloys have achieved enormous interest in the high-end equipment field of aerospace,military equipment and automobile industries.However,the industrial applications of TiAl alloys have been limited by their intrinsic brittleness at room temperature.In recent years,many researchers have found that the Ti2AlN/TiAl composite can effectively improve the plastic deformation capacity of TiAl alloys while maintaining the advantage of low density and high strength.However,the current challenge for Ti2AlN/TiAl composite is how to establish effective microstructure design criteria so that the comprehensive performance of Ti2AlN/TiAl composite can be further optimized and improved.Therefore,it is urgent to explore and understand the nature of Ti2AlN/TiAl composite from the microscopic aspect.In particular,it is needed to disclose the binding properties and micro-deformation mechanism of Ti2AlN/TiAl interface.Selecting the Ti2AlN/TiAl composite as research object,with the help of advanced computational simulation and experimental characterization technology,the present work systematically studied the Ti2AlN/TiAl interface microstructure and bonding properties,clarified the atomic-scale deformation mechanism of Ti2AlN/TiAl interface and disclosed the correlation between interface microstructure and mechanical behavior of composite.The present work cannot only provide the theoretical guidance for tuning the strength and plasticity of Ti2AlN/TiAl composite,but also has scientific significance for the development and utilization of Ti2AlN/TiAl composite.The main research results are as follows:The first-principles calculation results of surface properties show that the most stable Ti2AIN surface varies with different environment.Under the condition of Ti-and Al-rich environments,the Ti2AIN(1013)and Ti2AIN(0001)surface energy is very close,which means that the Ti2AIN(1013)and Ti2AIN(0001)surface can simultaneously form the interface with TiAl in the Ti2AlN/TiAl composite.The first-principles calculation results of interface properties show that the Ti2AlN(0001)/TiAl(111)coherent interface has a relatively high adhesive work(2.99J/m2),the interfacial bonding is mainly contributed from Ti-Al covalent and Ti-Ti metallic interactions.The Ti2AIN(1013)/TiAl(111)incoherent interface shows a lower adhesive work(2.44J/m2).The interface bonding characteristics are multiple and inhomogeneous depending on local atomic configurations,forming both the strong and weak interface interactive regions.The TEM observation results show that the first-principles calculation could predict the binding properties of Ti2AlN/TiAl interface models.The interphase boundaries between elliptical or polygonal Ti2AIN particles and TiAl matrix in the composite are mainly Ti2AIN(10T3)/TiAl(111)incoherent interfaces.The hexagonal and small plate Ti2AIN particles in the composite have a Ti2AIN(0001)/TiAl(111)coherent interface with TiAl matrix.An interatomic potential that could accurately describe the interactions between Ti,Al and N atoms in Ti2AlN/TiAl composite system has been developed by fitting to the lattice parameters,heat of formation and elastic constants of L10-TiAl and Ti2AIN on the basis of 2NN MEAM formalism.The reliability and transferability of present developed 2NN MEAM potential are evaluated by comparing MD simulation results of the surface energy and thermal expansion coefficient with relevant experimental and DFT calculations data.Based on the ternary Ti-Al-N 2NN MEAM potential developed in this work,we investigated uniaxial tensile behavior of single crystal TiAl and single crystal Ti2AIN,Ti2AlN/TiAl interface atomic structure and adhesive work using MD simulations.The tensile and compressive MD simulation results show that different interface configurations have different interface-micro crack and interface-dislocation interactive mechanisms,and thus show different fracture failure characteristics.It is found that the Ti2AIN(0001)/TiAl(111)coherent interface configuration shows tensile brittle fracture behavior due to the fact that the micro-crack forms in the interface and rapidly propagates along the interface during the tensile process.The Ti2AIN(1013)/TiAl(111)incoherent interface configuration shows tensile ductile fracture behavior,because the dislocations can nucleate at the tip of micro-crack and effectively blunt the propagation of micro-crack along the incoherent interface.When applied compressive loading,the Ti2AIN(0001)/TiAl(111)coherent interface configuration shows quasi-brittle fracture behavior due to the fact that the atomic-scale ripples appear in the Ti2AlN(0001)basal plane near the interface,couple with each other and form the kink bands.While the Ti2AIN(1013)/TiAl(111)incoherent interface could firstly provide the source for the nucleation of primary dislocations and then act as the sink for annihilation of secondary dislocations during the compressive loading.Thus it can be concluded that Ti2AIN(1013)/TiAl(111)incoherent atomic structure is beneficial to increasing the compressive ductility of Ti2AlN/TiAl composite without lowering its strength.The nanoindentation MD simulation results show that the introduction of Ti2AIN can cause the hardening effect of composite,The misfit dislocations in the Ti2AlN(0001)/TiAl(111)coherent interface can interact with the dislocations nucleated under the indenter,forming the Stair-rod dislocation and dislocation tangle.Thus,the Ti2AlN(0001)/TiAl(111)coherent interface is more beneficial for the hardening effect of Ti2AlN/TiAl composite.The Ti2AIN(1013)/TiAl(111)incoherent interface has some ability to annihilate the dislocations,and can also facilitate the dislocation propagation.Therefore,the Ti2AIN(1013)/TiAl(111)incoherent interface can simulataneously benefit the hardness and ductility of Ti2AlN/TiAl composite.The nanotribology MD simulation results show that the introduction of Ti2AIN and reasonable Ti2AlN/TiAl interface design can realize the goal of anti-friction and wear reduction for Ti2AlN/TiAl composite.In the initial stage of friction,the Ti2AIN(0001)/TiAl(111)coherent interface can hinder the propagation of dislocation and cause the hardening effect,thus the friction coefficient of coherent interface model is the lowest.In the latter stage of friction,the Ti2AIN(1013)/TiAl(111)incoherent interface can annihilate the dislocation,and thus the dislocation density in the incoherent interface model is the lowest,causing the lowest friction coefficient and wear rate for Ti2AIN(10T3)/TiAl(111)incoherent interface model.The in-situ TEM compression results of Ti2AlN/TiAl interface micro-region show that the MD simulations results obtained in this study can well describe and predict the experimental deformation behavior of Ti2AlN/TiAl interface.