【作者】 赵敬伟；

【导师】 丁桦；

【作者基本信息】 东北大学 ， 材料加工工程， 2009， 博士

【摘要】 钛合金加工成形比较困难,使得加工过程的制造成本过高,在一定程度上限制了其应用。钛合金热氢处理技术,它通过氢的可逆合金化作用,将氢作为临时性元素加入到钛合金中,可以达到改善钛合金工艺性能的目的。热氢处理技术的应用,能够解决钛合金高温下的氧化和难成形问题,从而可以大大降低钛合金的加工成本,在一定程度上将促进钛合金,尤其是高温钛合金的扩大应用。本文介绍了前人在热氢处理理论方面的研究成果,重点综述了近年来有关热氢处理对钛合金微观组织和力学性能影响方面的理论和试验研究进展。在此基础上,本文以Ti6Al4V和Ti600合金为研究对象,系统研究了钛合金热氢处理后的组织演变及高温变形行为,分析了钛氢化物生成的热力学过程,建立了钛合金动态再结晶过程的元胞自动机模型,取得的研究成果如下:（1）研究了热氢处理对Ti6Al4V合金组织结构、微观缺陷及显微硬度的影响。通过显微组织观察及物相分析等手段,研究了Ti6Al4V合金热氢处理后的组织演变规律,用正电子湮没方法分析了氢含量对微观缺陷数量及类型变化的影响,通过显微硬度测试研究了置氢对显微硬度的影响,利用电子探针研究了置氢对合金元素扩散的影响。研究结果表明:Ti6Al4V合金置氢后,当氢含量达到0.3%时,发现了面心立方结构的δ氢化物。在α相和β相中均能析出δ,当氢化物在β相析出时,δ与β具有以下取向关系:[011]_δ//[012]_β,（02（?））_δ//（200）_β。Ti6Al4V合金置氢后,Al、V等合金元素在合金中获得了重新分布。随着氢含量的增加,Ti6Al4V合金的缺陷类型由“空位+位错”逐渐过渡为位错;Ti6Al4V合金内部的缺陷数量,置氢0.1%后显著降低,之后,随着氢含量的增加又逐渐增加。Ti6Al4V合金置氢后,Al、V等合金元素在合金中获得了重新分布。α相和β相的硬度均随着氢含量的增加而升高,在相同氢含量条件下,β相的硬度高于δ相的硬度。置氢Ti6Al4V合金经真空除氢处理后,原始α晶界消失,β相变得细小、破碎,原轧制态组织获得了细化。细化组织的获得是相变和再结晶两种机制共同作用的结果。（2）研究了热氢处理对Ti600合金组织演变及宏观硬度的影响。通过显微组织观察及物相分析等手段,研究了Ti600合金置氢后的组织演变规律,分析了氢含量对硅化物析出规律的影响,通过宏观硬度测试研究了置氢对Ti600合金宏观硬度的影响。研究结果表明:Ti600合金置氢后,在氢含量为0.35%和0.5%的试样中均发现有面心立方结构的δ氢化物析出,并且,随着氢含量的增加,氢化物趋于细化。热氢处理后,在基体中析出具有四方结构的硅化物粒子S₃（0.35%H）和六方结构的硅化物粒子S₁（0.5%H）。热氢处理能显著提高Ti600合金的硬度,随着氢含量的增加,其硬度值升高。氢化物、硅化物粒子、晶格缺陷以及马氏体α′的存在是导致硬度升高的主要因素。（3）计算了钛氢化物生成的热力学函数。采用修正的Miedema生成热模型,计算了钛氢化物TiH_x（1≤x≤2）的标准焓变;采用统计热力学的方法,计算了TiH₂生成的标准熵变,分析了TiH₂生成的热力学过程。计算结果表明:TiH_x的标准焓变值随着x的增加呈线性关系减小。T=298K时,计算得到的TiH₂生成的标准焓变、熵变及Gibbs自由能分别为-137.46kJ·mol^-1、-143.0J·mol^-1·K^-1和-94.85kJ·mol^-1。当温度低于925K时,反应Ti（s）+H₂（g）→TiH₂（s）倾向于自发进行,而温度高于925K时,反应将朝相反方向进行。随着温度的升高,TiH₂的平衡氢压逐渐升高,其稳定性逐渐降低。（4）通过热模拟试验研究了Ti600合金热氢处理后的高温变形行为,通过显微组织观察研究了高温变形后的组织演变规律,分析了氢致Ti600合金的高温改性机理。结果表明:氢含量小于0.3%时,Ti600合金高温变形时的流变应力、应变硬化率及应变能密度均随着氢含量的增加而减小。从流变应力角度考虑,0.3%的氢可以降低Ti600合会热压缩温度至少80℃,或提高应变速率约2个数量级。氢含量一定时,Ti600合金的变形激活能随着应变的增加而逐渐降低。真应变为0.6时,Ti600合金未置氢及置氢0.1%、0.3%和0.5%后的变形激活能分别为648.4、459.0、324.3和420.0kJ·mol^-1。氢含量处于0～0.3%范围内时,氢含量越低,变形激活能越小。（5）建立了置氢Ti600合金高温变形的本构关系。为消除多重共线性对回归模型的影响,通过共线性诊断、变量筛选等过程合理地选取了影响Ti600合金流变应力的“最优”自变量子集。然后,采用偏最小二乘法分别建立了氢含量为0、0.3%和0.5%时Ti600合金高温变形时的本构关系。（6）应用元胞自动机方法模拟了Ti6Al4V合金在β单相区的动态再结晶过程,并分析了组织演变过程的动力学特征。模拟结果表明:在应变速率一定的情况下,动态再结晶分数随着应变的增加而显著增加。当应变量足够时,应变速率越大,得到的动态再结晶晶粒尺寸越细小,动态再结晶越不充分。动态再结晶过程的动力学分析表明:动态再结晶过程中同时存在恒定速率形核和位置过饱和形核两种形核方式。Avrami指数介于2.4～2.9之间,且Avrami指数随着应变速率的增加而升高。更多还原

【Abstract】 The manufacturing cost of titanium alloys is high for its poor workability,limiting the application in this way.Thermo hydrogen treatment（THT） of titanium alloys,or the use of hydrogen as a temporary alloying element due to the reversible reaction of hydrogen with titanium,can improve the processing properties of titanium alloys.THT for titanium alloys can solve the oxidative problem at high temperature and improve the hot workability of the alloys,consequently,lowering the manufacturing cost.The use of THT will give an impulse to the application of titanium alloys,especially high-temperature titanium alloys to some extent.The research findings on THT theory put forward by former researchers are introduced in this paper,especially,the theoretical and experimental development in aspect of influence of THT on microstructures and mechanical properties of titanium alloys.On this basis, Ti6Al4V and Ti600 alloys are employed in this paper to study the microstructural evolution and high temperature deformation behavior of them after THT,systematically,including analysis of thermodynamic process of formation of titanium hydride and simulation of dynamic recrystallization（DRX） process of titanium alloy using a cellular automaton approach.The main conclusions to be drawn are as follows:（1） Influence of THT on microstructure,microdefect and microhardness of Ti6Al4V alloy is researched.The microstructural evolution of Ti6Al4V alloy after THT is investigated by microstructure observation and phase analysis.The influence of hydrogen content on the amount and type of microdefect is analyzed by positron annihilation technique（PAT）.The influence of hydrogenation on alloying diffusion and microhardness is studied by electron probe microanalysis（EPMA） and microhardness testing,respectively.The results indicate that titaniumδhydride（fcc structure） can precipitate from both a andβphases,and the orientation relationship betweenβandδfollows[011]_δ//[012]_β,（02（?））_δ//（200）_βwhenδprecipitates fromβphase.As the hydrogen content increases,the type of microdefect is at first "vacancy+dislocation" and then it is mainly dislocation.The amount of microdefect shows a dramatic decrease after 0.1%H is charged,and then increases with increasing of hydrogen.After hydrogenation,the alloying elements Al and V redistribute in Ti6Al4V alloy. The hardness values ofαandβphases of Ti6Al4V alloy increase synchronously with increasing of hydrogen,and the hardness ofβis higher than that of a at the same hydrogen content.After vacuum dehydrogenation,αgrain boundary of hydrogenated Ti6Al4V alloy disappears,andβphase is broken up into very fine microstructure.The rolled microstructure of hydrogenated Ti6Al4V alloy is refined after dehydrogenation,which is a result of a combined action of two operating mechanisms of phase transformation and recrystallization.（2） Influence of THT on microstructural evolution and macrohardness of Ti600 alloy is researched.The microstructure of hydrogenated Ti600 alloy is investigated by microstructure observation and phase analysis,and the influence of hydrogen content on precipitation of silicide is analyzed.The influence of hydrogenation on macrohardness of Ti600 is studied by macrohardness testing.The results show thatδhydrides（fcc structure） exist in the specimens with 0.35%and 0.5%hydrogen,andδtends to be refined with increasing of hydrogen.There are two types of silicide precipitate in the Ti600 alloy after THT,one is S₃（0.35%H）,and the other is S₁（0.5%H）.The hardness of Ti600 alloy increases with increasing of hydrogen,and it is considered that hydride,silicide,lattice defects and martensiteα’ are the major factors.（3） Thermodynamic calculation of formation of titanium hydride is operated.A modified Miedema model is employed to calculate the standard enthalpy of formation of titanium hydride TiH_x（1≤x≤2）.The standard entropy of formation of titanium hydride TiH₂ is calculated by statistic thermodynamics method,and the thermodynamic process of formation of TiH₂ is analyzed.The calculated results show that the values of standard enthalpy of formation of TiH_x decrease linearly with increasing of x.The calculated results of standard enthalpy,entropy and Gibbs free energy of formation of TiH₂ at 298K are -137.46kJ·mol^-1, -143.0J·mol^-1·K^-1 and -94.85kJ·mol^-1,respectively.The reaction of Ti（s）+H₂（g）→TiH₂（s） inclines to occur spontaneously when temperature is lower than 925K,and the reaction tends to take place in the opposite direction when temperature is higher than 925K.As temperature increases,the equilibrium hydrogen pressure of formation of TiH₂ increases,and the stability of TiH₂ falls,accordingly.（4） High temperature deformation behavior of Ti600 alloy after THT by hot simulation experiments is researched,and the microstructural evolution after deformation is investigated by microstructure observation,also,the hydrogen modified high temperature deformation mechanism is analyzed.The results reveal that the flow stress,strain-hardening rate and strain energy density decrease synchronously with increasing of hydrogen when hydrogen content is less than 0.3%.The addition of 0.3%hydrogen in Ti600 alloy can decrease the hot deformation temperature by 80℃or increase the deformation strain rate by two orders of magnitude in flow stress terms.The activation energy of deformation of Ti600 alloy decreases with increasing of strain at a given hydrogen content level.At true strain 0.6,the calculated values of activation energy of deformation of Ti600 alloy without and with 0.1%,0.3%and 0.5%hydrogen are 648.4,459.0,324.3 and 420.0KJ/mol,respectively,and the value of activation energy of deformation decreases gradually with increasing of hydrogen contents from 0 to 0.3%.（5） The constitutive relationship of hydrogenated Ti600 alloy during high temperature deformation is established.For eliminating the influence of multi-correlation,the "optimum" independent variable subsets influencing the flow stress of Ti600 alloy are determined by diagnosis of colinearity and selection of variables.Then,constitutive relationship is obtained using partial least squares regression method for high temperature deformation of Ti600 alloy with 0,0.3%and 0.5%hydrogen contents,respectively.（6） A cellular automaton（CA） model is employed to simulate the dynamic recrystallization（DRX） inβphase field of Ti6Al4V alloy,and the kinetics during DRX process has been analyzed.The simulation results show that the DRX volume fraction increases remarkably with increasing of strain at a given strain rate.If an adequate strain is given,the DRX grain size of Ti6Al4V alloy decreases with increasing of strain rate,and also the inadequate microstructure is induced.The results of kinetics analysis of DRX reveal that constant nucleation rate nucleation as well as site saturated nucleation behavior occurs during DRX.The Avrami exponent obtained in the present work is a variable ranging from 2.4 to 2.9, which increases with increasing of strain rate.更多还原