Deformation And Fracture Mechanism Of Single Crystal ?-TiAl

?-TiAl alloy has been widely used in aerospace and automobile industries due to its good properties.However,cracks and holes are easy to produce during the manufacture and application in practice.The stiffness,toughness,strength and life of ?-TiAl alloy would be decreased by the micro crack,and its propagation and combination under external factors would lead to the macro crack.In order to solve the problems of poor plasticity,low fracture toughness and high crack growth rate at room temperature,which limit the widespread use of ?-TiAl alloy.It is necessary to have a wider and deeper understanding of the plastic deformation,fracture mechianism and mechanical properties of ?-TiAl alloy under different conditions,such as internal structure and loading mode.It is difficult to capture the evolution process of the internal microstructure of materials at experimental research,due to the limitation of experimental conditions and costs.As a result,it is hard to explain the intrinsic deformation mechanism of ?-TiAl alloy thoroughly at macro scale,but the deficiency of experimental research can be made up by molecular dynamics simulation.In this thesis,the tensile behavior of ?-TiAl alloy with different Al contents,crack positions and twin boundary spacings and the shear behavior of ?-TiAl alloy with different orientations and twin boundary spacings are systematically simulated at the atomic scale by using molecular dynamics method.The macro mechanical properties are discussed and the deformation,fracture mechianism is revealed by analyzing the evolution process of microstructure under different simulation conditions.The results of the study would have important guiding significance for the mechanical properties optimization of ?-TiAl alloy,and would helpful to the design and manufacture of the alloy which can meet the practical requirements better.The main research contents and results are as follows:(1)The uniaxial tension of ?-TiAl alloy with different Al contents is simulated.The effects of Al content on the deformation mechianism and crack propagation are analyzed and the rule of mechanical properties are obtained.The simulation results confirm that the crack propagation in the form of mother-child crack.At first,holes occurring,then evoluting and combining with crack,and accompanying with dislocations,stacking faults and vacancies is the main deformation mechanism of ?-TiAl alloy with low Al content,while it is the crack cleavage propagation at high Al content.The propagation mode of crack is influenced by the Al content,but the fracture mode is rarely affected.It is brittle fracture for all the specimens with different Al contents.The elastic modulus of the specimens is almost unaffected by the Al content,while the plasticity increases with the decreasing of Al content.The discussion results of the defect evolution at the crack tip show that the plasticity can be improved by the dislocation behavior,vacancy,stacking faults and Lomer-Cottrell dislocation.(2)The deformation mechcanism of ?-TiAl alloy with crack and twin boundary are simulated and discussed.The intergranular crack propagation rules of different crack positions are found and the effects of crack positions on the mechanical properties of ?-TiAl alloy are analyzed.The results indicate that the deformation mechianism of the specimens with three crack positions are the reaction between dislocation-dislocation,dislocation-twin and twin-twin in essence,but the reaction between defects as well as the type and number of products after the reaction and the crack propagation are all affected by the crack positions.The ductile-brittle transition can be observed from the lower boundary crack and the center crack specimens during the crack propagation.The type and movement of dislocation,the generation of stacking faults and the behavior of hierarchical twin play important roles in the deformation and fracture process.In addition,the results also manifest that the mechanical properties of ?-TiAl alloy vary with the crack positions,the yield strength of the upper crack is the largest,followed by the lower crack,finally is the center crack.However,the elastic modulus of the specimens is not affected by the crack positions.And the quantitative comparison results show that the toughness of the specimen with upper crack is the best,while the center crack is the worst.(3)The interaction between defects of nanotwinned ?-TiAl alloy with different twin boundary spacings are investigated and the deformation mechianism and propagation mechanism of transgranular crack is revealed.The results indicate that the reaction of dislocation-twin boundary is the main deformation mechanism of ?-TiAl specimen with small twin boundary spacing,while it is the dislocation-dislocation reaction in the large twin boundary spacing specimen.Moreover,the dislocation density and behavior are different due to the difference of twin boundary spacings.In addition,the deformation mechianism of nanotwinned ?-TiAl alloy with crack are also discussed.The results show that the dislocation and twin behavior,the competition between dislocation and crack controls the main deformation mechcanism.The interaction between the twin boundary and the crack decreases with the increasing of twin boundary spacing.It can be found that the elastic modulus is not influenced by twin boundary spacing,while the yield stress increases with twin boundary spacing and the toughness has no difference for ?-TiAl specimen without crack.Both the yield stress and fracture toughness decrease with increasing twin boundary spacing for ?-TiAl specimen with crack.In addition,the method of twin boundary spacing controls the material performance is given.The results indicate that there are no obvious effects of two methods which is adjusting the number of twin boundaries and the size of the models respectively on the initial plastic deformation mechianism,while they have significant influences on the later deformation.Although ?-TiAl alloy can be strengthened and toughened by both of these two methods at the same time,there are still differences.And the reasons for the differences of the strength and toughness are given through comparative analyzing.Finally,the influence of temperatures on the mechanical properties of ?-TiAl specimen are revealed.It can be found that both the yield stress and elastic modulus are the linear decreasing functions of the temperatures,and the stability of the crystal is deteriorated under higher temperature.(4)The effects of orientation and twin boundary spacing on the deformation mechianism and mechanical behaviors of ?-TiAl alloy under shear are discussed.The deformation mechanisms under different orientations are revealed and the variations of the shear modulus and the yield stress of ?-TiAl alloy are obtained.The shear deformation mechanisms under different twin boundary spacings are found.The results indicate that the difference of the deformation mechanism under different twin boundary spacings are not obviously.It is beneficial to the material strengthening when the shear loading along the direction which is perpendicular to the twin boundary.The strain hardening caused by the reaction of dislocationdislocation and dislocation-twin boundary results in the failure of twin boundary coherent structure,which increases the toughness of ?-TiAl alloy.The critical twin boundary spacing for the changing of shear modulus and the critical twin boundary spacing which leads to the critical stress for twin boundary migration maximum and minimum are obtained respectively.In addition,the deformation mechanism and mechanical properties under tension and shear are compared.