Research On Damage Behaviors Of TC4-DT Titanium Alloys

Titanium alloys are widely used in the aerospace industry due to their low density, high specific strength, good high-temperature properties and excellent corrosion resistance. In recent years, with the developments of fracture mechanics and damage tolerance theory, the design criterion of airplane component transforms from still strength design to damage tolerance design. To adjust the damage tolerance design requirements, the moderate strength or high strength titanium alloys with higher fracture toughness and slower crack propagating rate have been valued. TC4-DT is a moderate strength, high damage tolerance titanium alloy, which have been widely used in aerospace industry. To enlarge the use level of titanium alloys in the aviation department and improve the stability and safety of aircraft, it is very necessary to research the TC4-DT titanium alloy in-depth. The mechanical properties depend on the microstructure feature. Therefore, it has an important practical significance to obtain the comprehensive property by adjusting the microstructures. The damage-tolerance behaviors including fracture toughness and crack propagation rate were investigated for different microstructure of TC4-DT titanium alloy in this paper. The main studying content and results are listed as follows:Influence rules and mechanism of different microstructure for TC4-DT alloy on fracture toughness are revealed. Heat treatment in α+β phase zone, primary α phase content decreases, and secondary α phase content increases, both of them have effect on fracture toughness, fracture toughness increases with decreasing of primary α phase content and increasing of secondary α phase content. Primary α phases are passageway of crack initiation and propagation. Increasing of secondary α phases can increase the interface amount, so that it is possible to improve fracture toughness. Cluster boundaries are mainly obstacle to prevent slipping, and cracks produce big plasticity deformation when they cross the different orientation cluster, which leads to the higher fracture toughness.Heat treatment in β phase zone, effect of different solution cooling rate, solution temperature on double-heat treatment condition and multi-heat treatment on fracture toughness is analyses. Different cooling rate in β phase zone can obtain different α lamellar thickness. Wider lamellar α phases can improve the fracture toughness, and the cracks propagate along phase boundary between lamellar α and aging β phase. The lamellar α phases have high length-width ratio, which can offer more phase boundary, and boundary resistance can make crack change direction frequently, so absorb more energy and its fracture toughness was improved. Increasing of β grain size can improve fracture toughness, which is related with coarser fractograph surface and bigger plasticity deformation in crack tip. Multi-treatment can adjust lamellar microstructure parameters. By controlling temperature and cooling rate, size and amount of lamellar α and secondary α phases are adjusted,which influences the mechanical property and improves the fracture toughness.Crack propagation rate under different test conditions are discussed when cycle load is fixed. For equiaxed microstructure, high primary α phases content have higher strain absorbed energy and can also delay the crack growth, so the material has higher crack growth resistance under the fatigue load and lower crack growth rate. For lamellar microstructure, wider lamellar α phases have little lamellar number under the same displacement in the crack growth steady state, which has weaker effect on hindering crack growth than thinner one under cycle loading condition, so the thinner lamellar has lower crack growth rate. Secondary α phases from multi-treatment can resistance crack growth in near-threshold region, and effect is decreased with the increasing of △K. The crack growth route mainly is influenced by lamellar α thickness and colony size under higher △K.Crack propagation route is investigated in different microstructures. Different loads produce different consequences when cracks meet primary α phases. The properties change under different load model for the primary α phase and transformed β microstructures. Under the uniaxial loading conditions, cracks grow turn the α phase, and cracks grow through α phase under cyclic loading model at low frequence and small loading. Two growth models co-exist when the loading and frequency are improved. Fatigue crack growth is mainly perpendicular or paralle to lamellar direction in coarse lamellar microstructure, while crack grows through the boundary directly in thin lamellar, and crack grows along cluster in grain.Combined with the microstructure observation(OM and SEM), crack growth plasticity zone for two microstructures are observed and analyzed. The result shows that the lamellar microstructure has bigger plasticity zone than equiaxed one, and the plasticity deformation zone increases with the increasing of cracks length. Crack tip exists long slipping in larger plasticity zone, and slipping direction is influenced by grains and colonies, so crack growth route is zigzag and fracture surface is coarse, which lead to lower crack propagation rate. Crack propagation route is straight in the equiaxed microstructure, and cracks propagate along the phase boundary between α and β phases, plasticity zone is small, and it doesn’t exist long slipping distance.Mechanism of fatigue crack formation and growth for different microstructures is explained. Shear band and interface defect produced during sliding deformation can impel crack initiation in lamellar microstructure, defect produced during α phases reversible slide can lead to crack initiation in equiaxed microstructure.