Research On Superplasticity And Microstructure Evolution Of TC21 And LC4 Alloy

TC21 with high strength and high tenacity and high damage tolerance is a kind of α + β alloy, this alloy is a kind of new alloy which is developed from America Ti-6-22-22s, which make it optimal material for airplane structure. LC4 alloy have been used on aerospace industry and others widely, and it is used on main load-carrying structure of airplane, for example girder and bulkhead and junction etc.In this paper, TC21 alloy was pretreated via annealing at 900 ℃, and the superplastic tensile deformation behavior of TC21 alloy and structure evolution rule have been studied. The 70mm diameter original hot-rolled bars of LC4 alloy were pretreated via forging at 410℃ then annealing at 400 ℃ and annealing at 400 ℃ directly, the superplasticity and superplastic condition and microstructure evolution rule have also been studied in this paper.The experimental result of superplastic tensile test indicates that TC21 alloy have good superplasticity and broader temperature-strain rate(720℃~960℃, 5. 5×105s1~ 1. 1×102s1). Under the optimal superplastic condition (900 ℃, 3. 3× 104s-1), the mean elongation is 980% , the maximum elongation is 1309% and the mean flow stress is only 19.5MPa. During the superplastic tensile, the middle deformation zone of the specimen occurs obvious dynamic recrystallization, so the original primary strip a phases are refined and equiaxed. Under the optimal superplastic condition, the middle deformation zone of the specimen leads to intensive collective recrystallization because of high temperature stay and supergiant deformation length, so the recrystallization a phases merge and grow up irregular big flake, and the microstructure is coarsened obviously. When the tensile temperature is lower on two-phase region, the sample fracture is even and the thin and little dimple is more, with tensile temperature higher and superplastic deformation length longer, the recrystallization grains grow up and fracture surface shows great fluctuation. On the optimal superplastic temperature, the large and deep cavities are formed, the feature of intergranular sliding and fracture is obvious.The coarse crystalline LC4 alloy appear superplasticity under the proper deformation temperature and velocity. The result of superplastic tensile test in I programme indicates that the specimens present superplasticity in the range of temperature from 390℃ to 470℃ and strain rate from 1. 1× 10~1s~1 to 1.1×10 2s1, in which the elongation can reach 102. 4%~ 14.9. 5%.The specimens in II programme exhibit superplasticity in the range of temperature from 410℃ to 450℃ and strain rate from 5. 5×10 1s1 to 1.1×10-3s1, in which the elongation can reach 104. 7%~ 143.1%. So the better deformation temperature is 430℃ and the better strain rate is 1. 1× 10 2s '.The coarse crystalline LC4 alloy exhibit some superplasticity under lower temperature and higher strain rate, because it can create effective recrystallization refinement. The specimen fracture appear cavity nucleation and growth and linkageand intergranular fracture feature.