【作者】 欧阳德来；

【导师】 鲁世强；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2011， 博士

【摘要】 近年来,随着航空材料以传统满足单纯的静强度设计向现代损伤容限设计转变,对钛合金提出了损伤容限的要求。为了适应这种选材判据的变化,采取对钛合金β锻是一种主要技术途径。钛合金在β锻时因加热温度超过β相变点,β晶粒长大倾向特别大,极易形成粗晶组织,并导致随后的“组织遗传性”,而钛合金动态再结晶是细化β晶粒的有效手段之一。作者以锻态和铸态为初始组织的TB6钛合金以及双态为初始组织的TA15钛合金为研究对象,通过等温恒应变速率热模拟压缩试验,对其β锻过程中的动态再结晶行为及微观组织演变规律进行了研究,并构建描述其微观组织规律的相关模型。研究结果对TB6及TA15钛合金β锻过程中β晶粒细化及钛合金β锻的发展具有重要的理论指导意义和实际应用价值。论文系统性地研究了TB6（锻态和铸态）和TA15（双态）钛合金β锻过程中动态再结晶行为。结果表明,锻态TB6钛合金的动态再结晶形核仅出现在原始晶界附近,其对应的再结晶发生机制为不连续动态再结晶;铸态TB6钛合金的动态再结晶形核主要出现在原始晶界和β晶粒内部两类地点,其再结晶发生机制分别对应着不连续动态再结晶和连续动态再结晶;双态TA15钛合金的动态再结晶形核主要出现在原始晶界及变形带处,其对应的形核机制分别为晶界弓弯形核和亚晶旋转形核。锻态TB6钛合金在应变速率≥1s^-1时,β晶粒被拉长,基本未发生动态再结晶,以动态回复为主。在应变速率≤0.1s^-1时,以动态再结晶为主,且在应变速率为0.01s^-10.1s^-1,变形温度≤950℃时,动态再结晶发生较充分,动态再结晶晶粒长大不明显,能获得明显细化的变形组织;在应变速率≤0.001s^-1及变形温度≥1050℃时,虽然动态再结晶发生较充分,但动态再结晶晶粒长大明显,变形组织粗化严重。从获得均匀、细小的变形组织考虑,锻态TB6钛合金的热变形参数以应变速率≤0.1s^-1,变形温度≤950℃为宜。铸态TB6钛合金在应变速率≤0.001s^-1及变形温度≥950℃时,动态再结晶发生充分,并以晶内形核为主,变形组织均匀、细小;在应变速率≥1s^-1时,基本未发生动态再结晶,β晶粒被拉长,以动态回复为主;在其他变形条件下发生部分动态再结晶,再结晶体积分数不超过20%。从获得均匀、细小的变形组织考虑,铸态TB6钛合金的热变形参数以应变速率≤0.001s^-1及变形温度≥950℃为宜。双态TA15钛合金在应变速率≤0.01s^-1时以不连续动态再结晶为主,动态再结晶晶粒呈大小不一的混晶组织;在应变速率≥0.1s^-1时以连续动态再结晶为主,整个动态再结晶过程保持较高的形核速率,晶粒长大现象不明显,呈现形核为主的动态再结晶。探究了铸态TB6钛合金在热变形+快冷过程中产生的形变诱导马氏体的原因、形貌特征、相转变过程以及变形参数对形变诱导马氏体析出量的影响规律。证实了形变诱导马氏体主要是合金在微观上的成分偏析以及变形晶粒内部的内应力所致。形变诱导马氏体存在针状和锯齿状两类形貌特征,其晶体结构均为斜方马氏体（α"）,晶格常数为a=0.301nm,b=0.491 nm,c=0.463 nm。形变诱导马氏体组织转变模式是先形成近似平行的细条状或针状主干,后从主干中不断生长成树枝状,枝干之间可能发生交合、重叠,马氏体内部存在孪晶。形变诱导马氏体析出量随变形温度升高先增多后减少。应变速率对形变诱导马氏体析出量的影响规律与变形温度区间有关。在800℃⁹00℃时,马氏体析出量与应变速率的关系呈单峰特征,即在0.1s^-1时马氏体析出量达到峰值;在925℃¹000℃时,马氏体析出量与应变速率的关系呈双峰特征,即在0.01s^-1和1s^-1时出现峰值;在>1000℃时,马氏体析出量随应变速率增加先增多后减少,在1s^-1时达到最少,继续增加应变速率,则析出量又出现明显增多。在所有的实验变形参数范围内,马氏体析出量在925℃、1s^-1时达到最大,此时马氏体的体积分数约为50%。探讨了利用加工硬化率（θ）来确定钛合金动态再结晶临界应变的可行性,并提出了相关的数据处理技术及其方法。结果表明,钛合金发生动态再结晶时,其动态再结晶临界应变对应着θ^σ曲线拐点,且在–?θ/?σ^σ曲线上出现最小值。利用此拐点判据,确定出了锻态TB6和双态TA15钛合金在不同变形条件下的动态再结晶临界应变。这两类原始组织钛合金的动态再结晶临界应变随应变速率增加和变形温度降低而增大,且锻态TB6和双态TA15钛合金的临界应变（εc）与峰值应变（εp）之间分别存在εc/εp=0.62和εc/εp=0.504的相关性。通过引入Z参数较好地表征了这两类钛合金的临界应变与热变形参数之间的函数关系。基于数理统计方法确定出了锻态TB6和双态TA15钛合金的临界应变与Z参数的函数关系分别为εc=1.7129×10-2Z0.156和εc=1.72×10^-2Z0.0605。研究了锻态TB6和双态TA15钛合金的动态再结晶动力学行为和动态再结晶晶粒尺寸演变模型。结果表明,锻态TB6和双态TA15钛合金的动态再结晶体积分数随变形温度升高和应变速率降低而增大,且其动态再结晶动力学曲线均呈现“S”型。采用Avrami动力学模型建立了锻态TB6钛合金的动态再结晶动力学方程为1.1848其中误差分析表明,所建立的动态再结晶动力学方程具有较高的精度。基于动态再结晶晶粒生长驱动力分析,提出了动态再结晶等效反驱动力“抑制”晶粒长大的概念,在此基础上,构建出了动态再结晶晶粒尺寸演变模型为,并采用遗传算法确定出了模型中的参数。误差分析表明,所建立的晶粒尺寸模型具有较高的精度。更多还原

【Abstract】 In recent years, as the aeronautical materials with traditional strength design criterion to modern damage tolerance design criterion, titanium alloy has to be meet the requirement of the performance of damage tolerance. In order to adopt this change of material selection criterion, the beta forging on titanium alloy is a major technical way. Since titanium alloy in beta forging is heated upon the beta phase transition temperature, the diffusion coefficient of alloy elements and impurity element in the beta phase is big. Beta grain growth tendency is particularly obvious and leads to the subsequent "organization hereditary". Dynamic recrystallization occurred in titanium alloy is one of the effective means of refining beta grains. In this paper, the hot dynamic recrystallization behavior and microstructure evolution of titanium alloy TB6 with different original microstructures, namely as-cast and as forged, and of titanium alloy TA15 with original duplex microstructure, have been studied during the isothermal forging in theβphase field. The microstructure evolution models have also been estab1ished. The results are of great theoretical significance and practical application for grains refinement in beta forging of titanium alloys TB6 and TA15 and beta forging development.In this paper, dynamic recrystallization behavior of titanium alloy TB6 （as-forging and as-cast） and TA15 （double beta state） has been studied systematically during theβforging process. The results show that titanium alloy TB6 with original microstructures as-forging only nucleates near the grain boundaries, and the operation of discontinuous dynamic recrystallization as predominant mechanisms is anticipated correspondingly. Titanium alloy TB6 with original microstructures as-cast has two typical nucleation sites, namely, grain interiors and near grain boundaries, the operation of discontinuous dynamic recrystallization and continuous dynamic recrystallization as predominant mechanisms is anticipated correspondingly. Titanium alloy TA15 with original duplex microstructure has two typical nucleation sites, namely, deformation band and near grain boundaries, and nucleation mechanism is of grain boundaries bulging and subgrain rotation correspondingly. For titanium alloy TB6 with as-forging microstructures, dynamic recovery of appears andβgrain elongate with strain rate higher than 1s^-1. Dynamic recrystallization appears with strain rate lower than 0.1s^-1, and with strain rate range from 0.01s^-1 to 0.1s^-1 and deformation temperature lower than 950℃, since dynamic recrystallization are sufficient and dynamic recrystallization grain growth is not apparent, deformation microstructures are of refinement. With strain rate lower than 0.001s^-1 and deformation temperature higher than 1050℃, although dynamic recrystallization is sufficient, dynamic recrystallization grains grow up dramatically. Thus the feasible strain rates are not more than 0.1s^-1 and feasible deformation temperatures are not higher than 950℃as viewed from obtaining fined recrystallized microstructures. For titanium alloy TB6 with As-cast microstructure, dynamic recrystallization are sufficient and nucreate in grain interiors, so deformation microstructures are of refinement in the strain rates no more than 0.001s^-1 and deformation temperatures no lower than 950℃. In the deformation conditions of stran rates higher 1s^-1, dynamic recovery occurs withβgrain enlongating. In other deformation condition, partial dynamic recrystallization appears, and the value of dynamic recrystallization volume fraction is not more than 20%. Increasing deformation temperature and decreasing strain rate can promote dynamic recrystallization of titanium alloy TA15 with double state microstructures. In strain rate no more than 0.01^-1, discontinuous dynamic recrystallization occurs and dynamic recrystallization microstructures are non-uniform. In strain rate no lower than 0.1s^-1, continuous dynamic recrystallization occurs with high nucleation rate and not obvious grain growth, which presents dynamic recrystallization related to nucleation primarilyDue to the composition segregation and the inner stress in deformed grains, the deformation-induced martensite which was produced from as-casted TB6 Ti alloy in the hot working and following rapid solidification process was testified. In the research, although there are two microstructural morphology in the deformation-induced martensite: needle shaped and serrated martensite, the structure of both two type of martensite belong to the orthorhombic martensite whose lattice parameters are a=0.301nm,b=0.491 nm,c=0.463 nm. The production mechanism of deformation-induced martensite could be concluded with following steps. Firstly, the parallel wicker shaped or needle shaped main dendrites were formed in the hot working and following rapid solidification process. Then, the branch dendrites were gradually grown from the main dendrites, traverse and overlap with each other. In the dendrites of deformation-induced martensite, twins could be found. The precipitate quantity of deformation-induced martensite increase with the deformation temperature, then decrease gradually. The influence of strain rate on precipitate quantity of deformation-induced martensite is closely related to the deformation temperature region. When the deformation temperature region is between 800℃-900℃, the relationship of precipitate quantity of martensite with strain rate present a single-peak characteristics and reach the peak value at 0.1s^-1. When the deformation temperature region is between 925℃^-1000℃, the relationship of precipitate quantity of martensite with strain rate present a bipeaks characteristics and reach the peak value at 0.01s^-1 and 1s^-1. When the deformation temperature is higher than 1000℃, the precipitate quantity will increase firstly with strain rate and then decrease with the raise of strain rate. The minimum value of precipitate quantity will appeare when the strain rate is 1s^-1. However, with the strain rate further increasing, the precipitate quantity will increase obviously once again. In all deformation conditions, in this research, when the deformation conditions are 925℃and 1s^-1, the maximum precipitate quantity of martensite with volume fraction of about 50% could be obtained. The feasibility of determining dynamic recrystallization critical strain using work hardening rate was dicussed, and the relevant data processing techniques and methods were proposed. The results show that the inflection point and the minimum value will appear inθ^σcurve and–?θ/?σ^σcurve respectively, when the dynamic recrystallization of titanium alloys occur in the deformation process. Based on the inflection point inθ^σcurve that could be used as evidence to judge the critical strain of dynamic recrystallization, the critical strains of dynamic recrystallization in deformed TB6 and double state TA15 Ti alloy were confirmed. With the increase of strain rate and decrease of deformation temperature, the critical strain of dynamic recrystallization the two kind of Ti alloy increase obviously. For the critical strain of dynamic recrystallization, it is related to the peak strain that can be shown by some equations:εc/εp=0.62 for deformed TB6 Ti alloy andεc/εp=0.504 for double state TA15 alloy, respectively. In order to further inflect the relationship between critical strain and deformation conditions for both two Ti alloy, Z parameter can be used in the research. And the function between the two parameter can be shown as the following equations:εc=1.7129×10-2Z0.156 for deformed TB6 Ti alloy andεc=1.72×10-2Z0.0605 for double state TA15 alloy, respectively.The behavior of dynamic recrystallization kinetic and the evolution model of dynamic recrystallized grain size have been studied. The results show that dynamic recrystallization volume fraction of dynamic recrystallized grains for titanium alloy TB6 with as-forging and TA15 with double state increase with increase of temperature and decrease of strain rate. For both the two alloys, the kinetic curve of dynamic recrystallization looks like“S”. The kinetic model of dynamic recrystallization can be contributed to the structure of Avrami kinetic model, for titanium alloy TB6 with original as-forging microstructures: 1.1848（1 92R3T0.8） and titanium alloy TA15 with original double state: and by analysing the errors, the model of dynamic recrystallization kinetic shows a high accuracy. In this research, the drive force for the growth of dynamic recrystallized grains in Ti alloy was assayed in theory. Then, the theory that the driving force provided by interface energy and strain energy could stimulate grain growth, while the equivalent non-driving force restrain the grain growth is also be announced. By this basis, the evolution model of dynamic recrystallized grain size was established, which is of and the parameters in this model were confirmed by using inherit calculation method. By analysing the errors, the model of grain size shows a high accuracy .更多还原