The Superplasticity And Microstructure Of TC21Titanium Alloy After Thermomechanical Treatment

TC21with high strength and high damage tolerance is a kind of new α+β alloywhich is developed from China. And it is widely used in the field of aerospace,chemical and so on. The aggregation and growth of microstructure has influence onthe performance under the high temperature and low strain rate during superplasticforming process. Therefore the research of TC21superplastic forming and heattreatment on the microstructure evolution has a very important significance to theisothermal forging. This subject adopt the constant strain rate superplastic tensiledeformation experiment, the influence of deformation conditions on the flow stress isanalyzed, the microstructure evolution of alloys is researched by metallographicmicroscope, then expect to receive basketweave structure with much bettercomprehensive performance through the double annealing heat treatment to deformedstructure. The main research content is as follows:The isothermal constant strain rate tensile tests of the TC21titanium alloy wereconducted by SSAN-CMT4104electronic tensile testing machine at differenttemperature. Superplastic tensile deformation of the TC21titanium alloy wasperformed under constant strain rate.And the elongation was observed under differentdeformation conditions. After deformation,the alloy was treated with double annealing.Microstructure of the alloy was investigated. The experimental results showed thatTC21titanium alloy has good superplasticity at temperature of870℃~930℃underwide strain rate of3×10-4s-1~3×10-2s-1.On the optimal superplastic condition (910℃,3×10-4s-1), the maximum elongation is373.3％. During the superplastic tensiledeformation, the dynamic recrystallization occurred in deformation zone of specimen.The primary α-grains aggregated and merged to form irregular grains in the shape offlake or striation. As a result, the primary α-grains were fracture and some tiny grainsgenerated. After superplastic deformation and duplex-annealing, the specimen wasduplex microstructure,composed of primary α phase and different formsβ-transformedmicrostructures. However, the areas of plastic deformation showed significantdifference compared with the undeformed regions. The change law of flow stress andmicrostructure are also studied.The thermal activation energy of766.7kJ/mol isevaluated by analyzing the stress-strain curves. Constitutive equations of the maximumflow stress are established.