Effects Of Heat Treatment On Superplastic Deformation Microstructure Of TC21 Alloy

The TC21 alloy is a new type of titanium alloy which is developed by our country. The high strength and toughness of basketweave microstructure could be attained by isothermal forging and heat treatment, which have better damage tolerant. Recent researches have shown that the TC21 alloy have excellent superplasticity. During the process of superplastic forming, the microstructure of recrystallization would merge together and grow up especially at high temperature and low strain rate, which is different from the microstructure of isothermal forging. The rule of microstructure evolution isn’t clear yet in the process from the superplastic deformation to the heat treatment, therefore, whether the basketweave microstructure with high strength and toughness can be attained after heating treatment needs to be researched.In this paper, the microstructure of the TC21 alloy has improved through different heat treatment after superplastic deformation, and the effect of heat treatment on superplastic deformation microstructure has investigated. The main research contents and conclusions are as follows:(1) Superplastic deformation of the TC21 alloy has performed by the constant strain rate method. The behavior in the superplastic deformation and the regulation of microstructure evolution has studied, and the mathematical model of rheological mechanics has established.During the process of superplastic forming, the dynamic recrystallization occurred obviously. The content of α phase reduces gradually, the recrystallization grain merged together and the elongation of TC21 alloy decreases with the increase of deformation temperature or the strain rate decreases. The flow stress curve tends to be smooth and steady with lower strain rate and the higher deformation temperature. The main softening mechanism is characterized by dynamic recovery, which shown better superplastic. The superplastic deformation activation energy and the corresponding stress exponent of the TC21 alloy in(α+β) region are 329.20 k J/mol and 2.3677 respectively. The constitutive equation of rheological stress-strain has established by the Arrhenius model, which modified by the 1st Opt software. The maximum error is 11.22% and the average error is 3.51%.(2) The triple heat treatment of the TC21 alloy has performed after superplastic deformation, and the effect of heat treatment paramenters on the microstructure has studied.The first heat treatment temperature has influence on the grain boundary α phase in the final microstructure. The grain boundary α became larger and thicker relatively with the increase of the first heat treatment temperature, and the grain boundary α phase reduces when the first heat treatment temperatures near the β transns. The size of the secondary α phase grows longer and thicker with the increased of the second heat treatment temperature rises or the cooling rate declines, which presents needle-shaped. The secondary α phase became thick with the increase of triple heat treatment temperature. When the cooling rate is slow, the platelete α colonies precipitats in the grain boundaries and inside the grains, and the microstructure is the widmanstatten structure. The long acicular martensite α’’ and metastable β phase can be attained at faster cooling rate. The metastable β phase precipitates lots of small acicular α phase after the triple heat treatment and the microstructure is basketweave structure.(3)The basketweave can be attained from the microstructure of superplastic deformation in β region or(α+β) region through triple heat treatment, but some grain boundary α exist in the microstructure after the triple heat treatment. In this paper, the optimum heat treatment process of superplastic deformation of the TC21 alloy is:(940~970) ℃/1h AC +(900~930) ℃/1h AC + 590℃/4h AC.