Molecular Dynamics Simulation Of The Effect Of Temperature On Microcrack Propagation In Polycrystalline Gamma-TiAl Alloy

Titanium aluminum alloy has been developed rapidly in recent years,and has been widely used in alloy materials.Because of its advantages of low density,high strength and high temperature resistance,it has attracted wide attention in the fields of aerospace and automotive industry.It is a new generation of ideal lightweight and high temperature resistant alloy materials.However,the titanium aluminum alloy also has its shortcomings,such as the brittleness at room temperature and the high crack growth rate.These defects can easily result in the failure of the material in the process of use,and the failure expansion of the crack is the main cause of the failure of material.Therefore,it is necessary to research in detail about the process of crack propagation and mechanism of fracture.In this thesis,the polycrystalline ?-Ti Al alloy is used as the research object.With the help of the open source command program Atomsk,the polycrystal model is established by Voronoi algorithm,and the plastic constitutive relation of the crystal is constructed.The molecular dynamics simulation of the uniaxial tensile of the polycrystalline ?-Ti Al alloy is carried out.The growth process of microcracks and intragranular microcracks on grain boundaries of polycrystalline ?-Ti Al alloys at five different temperatures are compared and studied.The results of the study show that:The microcracks on grain boundaries of polycrystalline ?-Ti Al alloy exhibit brittle cleavage extension at low temperature and normal temperature.The cracks propagate first along the grain boundary to grain boundary or stress concentration,producing holes in the grain boundary near the crack tip.The holes grow along the crystal growth to form subcracks,and the confluence of the subcracks and the main cracks leads to the failure of the material.At medium and high temperature,the micro cracks are mainly brittle creep fracture,and the crack propagates along the grain boundary first,and the resistance expands slowly,and the crack tip is passivated and cracked.The grains gradually form creep holes and grow continuously,and then form wedge cracks on the trigeminal boundary,resulting in intergranular fracture and material failure.The microcracks between the grains of polycrystalline ?-Ti Al alloy are also extended by brittle cleavage at low temperature and normal temperature.The cracks are mainly along the crystal fracture,the fracture is the grain shape,and the fracture mode also grows to form the subcracks of the microvoid.The fracture is formed by the propagation of the crystal along the main crack and the fracture of the material.Inmedium and high temperature state,the alloy has slow plastic deformation due to stress and continuous high temperature.The grain gradually grows,and the creep is void,and the grain boundary expansion of the void leads to fracture failure.At high temperature,the fracture surface of the alloy is brittle fracture along the grain boundary.The microcracks at different location have many similarities in the expansion process.The crack fracture mode at low temperature and normal temperature is mainly brittle cleavage fracture,and the expansion rate is faster.The fracture mode under high temperature is mainly brittle creep fracture,the expansion rate is slow and the alloy at high temperature also has some toughness.But at the beginning of the crack growth,the crack in the grain boundary spreads along grain boundary,the crack on the grain is transgranular expansion,the maximum stress along grain boundary is lower than the maximum on the grain,and the crack at the grain boundary is also lower at the same temperature,So polycrystalline grain boundaries play an important role in the propagation of cracks.Grain boundaries strongly impede crack propagation and dislocation motion.In addition,there are lots of vacancies,stacking faults and dislocations in all stages of crack propagation.