【作者】 宫旭辉；

【导师】 夏源明；

【作者基本信息】 中国科学技术大学 ， 固体力学， 2010， 博士

【摘要】 目前对α+β钛合金在高温准静态加载下的力学行为及变形机理的研究已开展得比较充分,而对α+β钛合金在高温动态加载下的力学行为及变形机理的研究相对少得多,且大部分集中在高温动态压缩加载方面,对于α+β钛合金在中应变率拉伸加载和高温动态拉伸加载下的力学行为及变形机理的研究近乎空白。本课题来源于国家自然科学基金（项目批准号10472110）,旨在对我国自行研制的新型α+β钛合金TC21在中应变率和高温动态下的拉伸力学行为和变形机理进行初步的探索。利用高温冲击拉伸试验技术对双态组织（BM）和魏氏组织（W）TC21在温度为298-1023K应变率为240s-1和1270s-1下（还包括室温500s-1）的拉伸力学行为进行了试验研究;在MTS809材料试验机上进行了相对应温度下应变率为0.001s-1和0.05s-1（还包括室温0.01s-1）的准静态拉伸力学行为进行了试验研究;利用中应变率拉伸试验技术对室温2s-1、18s-1和50s-1下的拉伸力学行为进行了试验研究;利用冲击拉伸复元试验技术对室温240s-1下的等温拉伸力学行为进行了试验研究。试验结果表明,相同工况下,BM TC21的屈服应力和断裂应变要高于W TC21,应变硬化率要小于W TC21;BM TC21和W TC21在试验温度和应变率范围内的拉伸力学行为具有类似的温度相关性和应变率相关性。准静态加载下,屈服应力-温度曲线出现转折点,转折点后屈服应力随应变率的升高急剧下降;动态加载下屈服应力-温度曲线在试验温度范围内并未出现转折点,表明转折温度随应变率的升高而增大。准静态和动态加载下,环境温度低于转折温度时相同当量氧含量的TC21与纯钛的屈服应力具有相似的温度相关性。298-873K时,动态下的屈服应力的应变率相关性要大于准静态,应变率相关性在中应变率范围内发生转折,屈服应力与应变率的对数之间近似呈双线性关系。温度为298-873K时,两种组织TC21同一温度下的硬化率随应变率的升高而减小。准静态加载下,同一应变率下的硬化率随温度的升高先增大后减小而后又增大;动态加载下,同一应变率下的硬化率随温度的升高先增大后减小,准静态和动态加载下应变硬化率最高时的温度均为673K。温度为298-673K时,同一温度动态下的失稳应变要小于准静态;温度为873-1023K时,同一温度下动态下的失稳应变要大于准静态,呈现高温高速韧性的特点。在试验应变率范围内,两种组织同一应变率下的失稳应变均随温度的升高先增大后减小。温度为298-873K时,两种组织TC21同一温度动态下的断裂应变要大于准静态;温度为973-1023K时,同一温度动态下的断裂应变要小于准静态。准静态加载下,同一应变率的断裂应变随温度的升高先减小后增大;动态加载下,同一应变率的断裂应变随温度的升高先增大后减小。室温240s^-1下的等温应力应变曲线均具有比相同工况下的绝热应力应变曲线更高的应变硬化率及失稳应变和断裂应变;等温应力应变曲线的应变硬化率是应变率无关的。SEM断口分析表明,BM TC21在所有工况下的断裂方式均为穿晶断裂,而W TC21在室温准静态下的断裂方式为穿晶断裂和沿晶断裂的混合断裂方式,其余工况下为穿晶断裂。两种组织所有工况下拉伸断口均未发现由绝热剪切造成的局部融化区域,断口附近正表面未发现绝热剪切带,即TC21的断裂并不是由绝热剪切引起的,而是由裂纹的形核和扩展造成的。TEM显微分析表明,两种组织中均存在α束域,束域内α相与相邻的β相的晶体取向近似满足Burgers关系。BM TC21的α束域要小于W TC21,使得BM TC21的屈服应力和断裂应变均要高于W TC21。两种组织TC21在试验温度和应变率范围内的变形机制主要为螺型位错的滑移。准静态加载下,温度低于转折温度时,两种组织的主要滑移类型均为<a>型滑移;温度高于转折温度时,两种组织的滑移类型为<a>型滑移和<c+a>型滑移,表明转折温度是与滑移类型的改变密切相关的。动态加载下,试验温度范围内两种组织均为<a>型滑移,并未发生滑移类型的改变,从而使得动态加载下并未出现转折点。相同工况下,BM TC21与W TC21的主要滑移系相同,从而使得两种组织的拉伸力学行为具有类似的应变率相关性和温度相关性。动态加载下的应变率相关性要高于准静态可能是由位错增殖速度的增大引起的。在试验研究和显微分析的基础上,首次提出了一个统一表征准静态至动态、室温至转折温度范围内TC21拉伸力学行为的温度和应变率相关的双线性本构模型。与J-C模型和KHL模型相比,该模型具有以下特点:（1）采用基于双威布尔分布的应变率强化乘子项,该乘子项能够统一表征准静态至动态、室温至转折温度范围内两种组织TC21屈服应力与应变率的相关性,即屈服应力与应变率对数之间的双线性关系,并可预报转折应变率;（2）在应变硬化乘子项中,不仅引入了应变硬化与应变率的耦合项,还引入了应变硬化与温度的耦合项,能够很好地表征两种组织TC21等温条件下应变硬化率与应变率无关的规律和同一应变率加载下应变硬化率随温度的升高先增大后减小的规律。同时,给出了一个确定参数的拟合方法—分解综合法,该方法包括分乘子初值拟合和非线性总体拟合两个步骤。由双线性本构模型和分解综合法所得到的拟合结果与试验结果吻合较好,表明该模型是有效的,确实能够较好地统一完整地表征两种组织TC21在室温至转折温度（298K-约1023K）和准静态至动态(应变率为0.001^-1270s^-1)范围内的,与温度和应变率相关的拉伸力学行为。更多还原

【Abstract】 Researches on the mechanical properties and deformation mechanisms ofα+βtitanium alloy under quasi-static loadings at elevated temperatures have been carried out adequately. Researches on the dynamic behaviors are mainly focused on the compressive loading conditions. However, the tensile responses ofα+βtitanium alloy at intermediate and high strain rates are less investigated. This research, supported by the National Natural Science Foundation of China （No.10472110）, aims to investigate the tensile properties and deformation mechanisms of TC21, a new designedα+βtitanium alloy in China, under intermediate and high strain rate loadings at elevated temperatures.The dynamic tensile experiments of TC21 with two different microstructures, Bimodal （BM） and Widmanstatten （W）, were carried out in a temperatures range of 298K to 1023K and under strain rates of 240s^-1 and 1270s^-1 using the high temperature tensile impact technique. Also, the quasi-static tensile experiments under strain rates of 0.001s^-1 and 0.05s^-1 and corresponding temperatures were carried out using MTS809 testing system. The intermediate tensile experiments under strain rates of 2s^-1, 18s^-1 and 50s^-1 at room temperature were carried out using intermediate strain rate tensile technique. The dynamic tensile recovery experiments under 240s^-1 at room temperature were carried out using the tensile impact recovery technique. The test results indicate that the values of yield stress and fracture strain of BM TC21 are higher than those of W TC21 under the same loading conditions, while the strain-hardening rate of BM TC21 is lower than that of W TC21. The tensile properties of BM TC21 and W TC21 have similar dependence on temperature and strain rate. There exhibits a discontinuity in the yield stress-temperature curve under quasi-static loadings, above which yield stress drops sharply with increasing temperature. Under high strain-rate loadings, no discontinuity is found in the yield stress-temperature curve, indicating that the discontinuity temperature increases with increasing strain rate. Below discontinuity temperature, the temperature dependence of yield stress of TC21 under quasi-static and dynamic loading is similar with cp-Ti that has the same interstitial solute concentration. At 298-873K, the strain-rate sensitivity of yield stress under high strain rate loadings is higher than that under quasi-static loadings and the strain rate sensitivity changes in the range of intermediate strain rates. Yield stress exhibit a nearly bi-linear relationship with the logarithm of strain rate.At 298-873K, the strain-hardening rates of both microstructures decrease with increasing strain rate. Under quasi-static loadings, the strain-hardening rates increase firstly and then decrease and then increase with increasing temperature, while under dynamic loadings, the strain hardening rates increase firstly and then decrease. The temperature corresponding to the maximum value of strain-hardening rate is 673K for both quasi-static and dynamic loadings.At 298-673K, the unstable strains of dynamic loadings are less than those of quasi-static loadings, while at 873-1023K, the unstable strains of dynamic loadings are greater than those of quasi-static loadings, indicating a high-velocity ductility phenomenon. The unstable strains increase firstly and then decrease with increasing temperature within the whole strain rate range. At 298-873K, the fracture strains under dynamic loadings are larger than those under quasi-static loadings, while at 973-1023K, the fracture strains under dynamic loadings are smaller than those under quasi-static loadings. Under quasi-static loadings, the fracture strains decrease firstly and then increase with increasing temperature, while under dynamic loadings, the fracture strains increase firstly and then decrease with increasing temperature. The values of strain-hardening rate, fracture strain and unstable strain of the isothermal stress-strain curve at 240s-1 and room temperature are higher than those of the adiabatic stress-strain curve under the same conditions. The strain-hardening rate of the isothermal stress-strain curve is strain-rate insensitive.SEM fractographic observations show that the fracture mode of BM TC21 is transgranular fracture under all conditions, while W TC21 show a mixed manner of transgranular fracture and intergranular fracture under quasi-static loadings at room temperature and transgranular fracture under other conditions. No localized melting area due to adiabatic shear is found on the fracture surfaces of all tensile samples. Metallographic observations show that no adiabatic shear band is found in the perpendicular sections near the fracture surfaces. The void nucleation and growth is the main reason for the tensile fracture of TC21.TEM observations show that both microstructures exhibitαcolony, in which there is a near Burgers orientation relationship betweenαphase and neighboringβphase. The size ofαcolony of BM TC21 is smaller than that of W TC21, resulting in higher yield stress and fracture strain of BM TC21. The main deformation mechanism of both microstructures is the screw dislocation slip. Under quasi-static loadings, the main slip system is <a> type slip system below discontinuity temperature and the main slip system are <a> type and <c+a> type slip systems above discontinuity temperature, indicating that the discontinuity of the yield stress-temperature curve is associated with the change of slip type. Under dynamic loadings, the main slip system is <a> type in the whole temperature range, resulting in no discontinuity in the yield stress-temperature curve. The main slip systems of both microstructures are the same under all conditions, resulting in the same dependence of mechanical properties on temperature and strain rate. The increase of strain-rate sensitivity from quasi-static loadings to dynamic loadings is probably caused by the increase of the dislocation generation rate.A bi-linear constitutive model based on bimodal Weibull distribution is proposed to describe the tensile behavior of TC21 under different strain rates and temperatures. Compared with the J-C model and KHL model, the bi-linear relationship between the yield stress and the logarithm of strain rate can be well described and the transition strain rate corresponding to the yield stress can be predicted. Furthermore, the variation of strain-hardening rate coupling with temperature and strain rat can be well described. A decompose-synthetize method is used to identify the values of the model parameters. The fitted model results are in good agreement with the experimental data obtained within the present investigated temperature and strain rate range.更多还原