Effect Of Microstructure On Fracture Toughness And Fatigue Crack Growth Rate Of TC4 Titanium Alloy

TC4 titanium alloy has been widely used in biomedical,shipbuilding and aerospace fields because of its high specific strength,good plasticity,corrosion resistance and high temperature performance.TC4 titanium alloy has been subjected to alternating load for a long period of time in service,which is prone to fatigue failure,causing deformation or even rupture.However,microcracks of the metallic material inevitably exist in the smelting process.When the applied load exceeds a certain critical value,the crack will be unstably expanded.Therefore,it is very important to measure the fracture toughness and fatigue crack growth rate of the material accurately.There are many methods for measuring the fatigue crack growth of TC4 titanium.At present,crack of displacement(COD)method is commonly used,but this method is not suitable foe M(T)specimen under high temperature.Direct current potential drop(DCPD)method has the advantages of high precision and automatic measurement by computer automatic acquisition system,and can be used for high temperature and corrosive environment.Therefore,this paper sets TC4 titanium as the research object,using DCPD method to study the effect of microstructure on fatigue crack growth rate and its mechanism,comparing with the results obtained by COD method,providing a basis for the development of DCPD method test system under high temperature and corrosion conditions.In this paper,equiaxed structure,bimodal structure and lamellar structure were obtained by three kinds of heat treatment systems for TC4 titanium:800?×2 h(AC),950?×1h(AC)+540?×5 h(AC),1020?×1h(AC)+940?×1h(FC)+540?×5 h(AC).Tensile test was performed on the universal testing machine.The Kq value was obtained from the fracture toughness test on Shimadzu EHF-F1 hydraulic servo fatigue tester.The DCPD method and the COD method for fatigue crack growth rate were test on the MTS810 hydraulic servo fatigue testing machine,obtaining double logarithmic curve and Paris constant from different tissues.Laser scanning confocal microscopy was used to observe the fatigue crack propagation path along the fracture surface.The tensile fracture and fatigue fracture morphology were observed by scanning electron microscopy.Research indicates:using double annealing system,equiaxed ? organization can be obtained while TC4 titanium is annealed at the ?-phase transition temperature.Part of the primary ?-phase transforms into beta sheet to form a dual-state structure.Using multiple heat treatment system,the original ? grains and lamellar structure consisting of internal a sheet bundle can be obtained.The yield strength,tensile strength,fracture toughness of equiaxed,bimodal,lamellar TC4 titanium alloy are increasing in turn,while the elongation decreasing.Dislocations are prone to plugging at the interface of the lamellar which increasing the strength,while the spherical ?-phase deformation is well coordinated and the bimodal structure has good strength and plastic matching.The curve of da/dN-?K of TC4 titanium alloy with different microstructures measured by DCPD method is in good agreement with COD method.The results of the simulation of Paris formula in the stable growing area show that for equiaxed and bimodal structure,the fatigue fracture has a small fluctuation and the main fatigue crack growth path is relatively straight.The data of COD method and DCPD method is not very different.Comparing with COD method,the data of DCPD method is more intensive,and DCPD method is working well in high tempreture.The results of COD method and DCPD method show that the TC4 titanium alloy with lamellar structure has a lower fatigue crack growth rate and a higher threshold value,while the fatigue crack growth rate of the bimodal structure is slightly lower than that of equiaxed structure.The falling and rising of TC4 titanium alloy with equiaxed and bimodal structure is smaller,with the fatigue crack propagation path flatter.The fatigue crack can be cut along the equiaxed ?-phase or along the ?-phase interface.The falling and rising of TC4 titanium alloy with lamellar structure is larger and the direction of main crack change can be accompanied by the birth of secondary crack.The ?-sheet bundle with different orientation can hinder the crack growth,which forces the crack turn to make the expanding path twists,and the closure of the crack front is getting increase,which causes the fatigue crack growth rate going decrease.