热处理对TC21合金超塑性变形后显微组织的影响

发布时间：2018-05-12 17:54

  本文选题：TC21合金 + 超塑性变形　； 参考：《南昌航空大学》2015年硕士论文

【摘要】：TC21合金是我国自主研发的新型钛合金,经过适当等温锻造及热处理可获得高强韧的网篮组织,具有较好的损伤容限性。已有研究表明TC21合金具有优良的超塑性,但其在超塑性成形过程中,尤其高温及低应变速率下组织会发生聚集长大,组织不同于等温锻造,而TC21合金从超塑变形到热处理过程的组织演变规律尚未明确,超塑性变形后的组织能否通过热处理得到高强韧性的组织亟待研究。本文对TC21合金进行超塑性变形,再通过不同的热处理制度对超塑性变形后的组织进行改善,研究了TC21合金的超塑性变形行为和热处理对其显微组织的影响。主要研究内容和结论如下:(1)采用恒应变速率法对TC21合金进行超塑性变形,研究超塑性变形过程的流变应力行为及显微组织演变规律,建立相关流变应力数学模型。TC21合金在超塑性变形过程中组织发生明显动态再结晶,随着变形温度的升高,或应变速率的降低,α相含量逐渐减少,动态再结晶晶粒发生聚集长大,合金延伸率降低;当应变速率较低,变形温度较高时,流变应力曲线趋于平稳,软化机制以动态回复为主,材料表现出较好的超塑性。通过计算推导得到TC21合金两相区超塑性变形激活能和应力指数分别为329.20 kJ/mol和2.3677;采用Arrhenius模型建立了合金流变应力-应变本构方程,并通过1stOpt软件进行修正,最大误差为11.22%,平均误差为3.51%。(2)对TC21合金超塑性变形后的组织进行三重热处理,研究热处理参数对TC21合金超塑性变形后组织的影响。第一重热处理温度对TC21合金最终组织中的晶界α相产生影响。随着第一重热处理温度升高,长条晶界α相增多,较为粗大笔直;当第一重热处理温度在相变点附近时,最终组织析出的晶界α相较少。热处理过程析出的次生α相随着第二重热处理温度的升高或冷却速率的降低而尺寸增大,呈针状;二次生α相随第三重热处理温度的升高而粗大;当冷却速率较慢时(炉冷),α相呈集束片状在晶界和晶粒内析出,显微组织为粗大魏氏体组织;当冷却速率较快时,得到长条针状马氏体α''和亚稳β相,亚稳β相经第三重热处理后弥散析出细小针状α相,呈网篮状。(3)三重热处理可使TC21合金在β相区或两相区超塑性变形的组织大部分获得网篮组织,但热处理后的组织中仍会存在部分的长条晶界α相。本文改善TC21合金超塑性变形后组织的最佳热处理工艺为:(940~970)℃/1h AC+(900~930)℃/1h AC+590℃/4h AC。
[Abstract]:TC21 alloy is a new type of titanium alloy developed independently in China. After proper isothermal forging and heat treatment, high strength and toughness can be obtained, which has good damage tolerance. It has been shown that TC21 alloy has excellent superplasticity, but in superplastic forming process, especially at high temperature and low strain rate, the microstructure will gather and grow, and the microstructure is different from isothermal forging. However, the microstructure evolution of TC21 alloy from superplastic deformation to heat treatment is not clear, and it is urgent to study whether the microstructure of superplastic deformation can be obtained by heat treatment. In this paper, the superplastic deformation of TC21 alloy was carried out, and the microstructure after superplastic deformation was improved by different heat treatment systems. The superplastic deformation behavior of TC21 alloy and the effect of heat treatment on its microstructure were studied. The main contents and conclusions are as follows: (1) superplastic deformation of TC21 alloy is carried out by constant strain rate method, and the rheological stress behavior and microstructure evolution of superplastic deformation process are studied. The dynamic recrystallization occurred in TC21 alloy during superplastic deformation. With the increase of deformation temperature or the decrease of strain rate, the 伪 phase content gradually decreased, and the dynamic recrystallization grain aggregate and grow. When the strain rate is low and the deformation temperature is high, the rheological stress curve tends to be stable, the softening mechanism is mainly dynamic recovery, and the material shows better superplasticity. The activation energy and stress exponent of superplastic deformation in two-phase region of TC21 alloy are 329.20 kJ/mol and 2.3677, respectively. The constitutive equation of rheological stress-strain of TC21 alloy is established by Arrhenius model and modified by 1stOpt software. The maximum error is 11.22 and the average error is 3.51. The microstructure of TC21 alloy after superplastic deformation is treated with triple heat treatment. The effect of heat treatment parameters on the microstructure of TC21 alloy after superplastic deformation is studied. The first heat treatment temperature affects the grain boundary 伪 phase in the final microstructure of TC21 alloy. With the increase of the first heat treatment temperature, the grain boundary 伪 phase increases, and the grain boundary 伪 phase is less precipitated when the first heat treatment temperature is near the transition point. The size of the secondary 伪 phase increases with the increase of the secondary heat treatment temperature or the decrease of the cooling rate, and the secondary 伪 phase is coarse with the increase of the third heat treatment temperature, and the size of the secondary 伪 phase increases with the increase of the secondary heat treatment temperature or the cooling rate. When the cooling rate is slow (furnace cooling, 伪 phase precipitates in the grain boundary and grain, the microstructure is coarse Weichenite structure, when the cooling rate is higher, the long acicular martensite 伪 'and metastable 尾 phase are obtained. After the third heat treatment, the metastable 尾 phase precipitated fine needle-like 伪 phase, which appeared to be net-shaped. 3) Triple heat treatment could make the microstructure of TC21 alloy superplastic deformation in 尾 -phase region or two-phase region obtain most of the net structure. However, after heat treatment, there will still be a part of grain boundary 伪 phase in the microstructure. In this paper, the optimum heat treatment process for improving the microstructure of TC21 alloy after superplastic deformation is as follows: (1) 940 ~ 970 鈩，

本文编号：1879567