Effect Of Nb On Micro Crack Propagation Of TiAl Alloy

Aerospace materials require comprehensive properties such as high strength,low density,high-temperature resistance and oxidation resistance etc.TiAl alloy,which has excellent performance at high temperatures,is widely used in the aviation industry.By increasing the content of Nb and lowering the content of Al,the formation of Ti-Al-Nb triad system is the main trend of TiAl alloy development.Improvement of deformation strength,heat capacity,effective means of room temperature ductility and oxidation resistance of TiAl based alloy could be achieved through adding Nb element.So adding niobium to the alloy becomes the research focus of material engineering.In this thesis,the effect of Nb on the crack propagation in TiAl alloy was studied by molecular dynamics theory.The results are as follows:(1)Comparison of crack propagation on ?-TiAl alloy and ?-TiAl alloy containing Nb was studied.Nb has a great impact on the performance of ?-TiAl,and ?-TiAl alloys with 3% Nb and without Nb have different crack propagation paths.Fracture time was extended after adding Nb at the same strain rate conditions,and the energy curve fluctuates with two summits: stress first increases and then decreases with the increase of strain;when crack growth becomes slow,the formation of the cross section is not smooth,and the crack shape changes.(2)Effect of Nb contents on the crack growth of ?-TiAl alloy was analyzed.TiAl alloys containing 2%,4%,6%,8% Nb were studied.Nb would improve the ductile strength of the alloy.With the increase of niobium contents,the cohesive force between the atoms of single crystal ?-TiAl alloy and the strength of the alloy also increase,which means an increase of the content of Nb improved the ductility of ?-TiAl alloy and reinforced the alloys' stability.(3)Effect of temperature on the crack growth in the ?-TiAl alloy containing Nb was studied.The principles of crack propagation and mutual influence were analyzed at four temperatures.The results showed that with the increase of the temperature,the higher the crack tip dislocation emission frequency,and the more serious the degree of crack tip passivation,and the longer the fracture extended trial time;increase of the temperatures leads to the decrease of the system's yield stress,so the material is more prone to plastic deformation;due to plastic deformation,crack propagation was hindered and becomes slow.(4)Effect of crack positions on the crack growth of ?-TiAl alloy containing Nb was analyzed.Crack propagation processes were studied on the single crystal ?-TiAl alloy containing 3% Nb.The results of the study showed that: ?-TiAl alloy with the crack on its center did not have concentrated stress at its initial tensile stage,then due to rupture of atomic bonds,holes were formed,and expansion of the holds suppressed crack.Therefore,crack needed to overcome greater resistance when expanding.The effects of the crack's positions(at the center or at the boundary of the ?-TiAl alloy)were different,and the boundary cracks were more harmful to the material.