【作者】 邓希光；

【导师】 惠松骁；

【作者基本信息】 北京有色金属研究总院 ， 材料科学与工程， 2014， 博士

【摘要】 自二十世纪七十年代以来,随着装甲与反装甲武器系统减重需求的日益增长,以及材料动态性能测试方法的完善和普及,越来越多的研究人员开始关注钛合金的动态性能和动态变形行为。已有研究主要针对Ti-6A1-4V等常用α+β型钛合金以及部分β型钛合金进行宏观动态性能表征、动态性能影响因素分析和绝热剪切带形成后的显微组织观察等,本文则重点关注α型钛合金的动态变形行为,主要包括孪生行为、动态剪切变形失稳现象和相关影响因素分析。首先,以Gr.2工业纯钛为研究对象,通过分离式霍普金森压杆实验测量材料在冲击载荷作用下的动态力学响应,并对变形组织进行显微观察。对比Gr.2工业纯钛在准静态(10-3s-1)和动态(6000s-1)压缩变形过程中的应变硬化速率和变形组织中的孪生密度,发现动态变形过程中孪生密度的急剧增加伴随着应变硬化速率的大幅提高。这表明应变速率的提高将影响α钛合金变形组织中的孪生密度,并通过Hall-Petch效应改变宏观应变硬化效果。为了分析α钛合金的孪生行为,利用EBSD分析技术标定出Gr.2工业纯钛在εp=0.36准静态压缩变形组织中含有两种类型的孪晶：{1012}型和{1122}型,而在εp=0.22动态压缩变形组织中含有三种类型的孪晶：{10T2}型、{1112}型和{1121}型,而且晶粒中孪晶的类型与晶粒基体受压应力时的晶体取向有关。为解释此相关性本文引入了“孪生Schmid因子”,并通过在α[钛的HCP单胞中定义三维球坐标系计算出上述三类孪生的Schmid因子分布图。将计算结果与EBSD标定出的孪晶类型和晶粒基体欧拉角相结合,证明了孪生Schmid因子是决定孪生系开动与否的重要参数。若进一步考虑材料变形时的应力状态可统计出Gr.2工业纯钛中{1012}型、{1122}型和{1121}型孪晶形核所需的临界分切应力值分别为321-361MPa、367-393MPa、525-538MPa,且受应变速率的影响不明显。此外,单晶纯钛压缩实验表明一种孪生类型对应的六个孪生变体中Schmid因子最大的孪生变体优先开动。为了分析α钛合金的动态剪切变形失稳现象,利用本文提出的动态剪切性能测试装置和方法测量出Gr.2工业纯钛在剪切应变速率为18000s-1时的动态剪切抗力为475MPa,临界剪切应变为1.1。为了验证和分析该实验结果,将Johnson-Cook本构方程与绝热温升方程耦合在一起计算出了应变速率在10s至104S-1范围内的绝热剪切应力-应变曲线。若将曲线顶点作为塑性变形失稳时的状态,可知动态剪切失稳时的临界剪切应变范围为0.9-1.3,与之相对应的剪切应力范围为395-480MPa,而且应变速率越高,剪切失稳时的临界应变值越小,对应的应力水平则越高。计算结果符合实验结果,说明本文提出的计算方法能够合理地描述动态剪切过程,适用于分析α钛合金动态剪切失稳的临界条件。在此基础上,通过单独调整计算过程中涉及的Johnson-Cook参数和热转换系数p分析了材料本征特性对动态剪切失稳临界条件的影响,发现提高应变硬化系数B、热软化系数m和热导率是降低α钛合金绝热剪切倾向性的有效途径。然后,以具有等轴组织特征的Ti-xAl(x=2,4,6)合金为研究对象,利用上述总结出的动态性能测试方法、变形组织观察方法和数据处理方法,探讨了A1含量对α钛合金动态性能和变形行为的影响。结果表明,Al含量由2wt.%升高至6wt.%会导致等轴Ti-xAl(x=2,4,6)合金的室温热导率降低约58%,室温准静态屈服强度提高约144%,{1012}型孪生临界分切应力提高约100%,εp≈0.07处的室温应变硬化速率降低约50%。这些变化导致Ti-xAl合金的动态剪切失稳临界条件由动态剪切抗力=398MPa,临界剪切应变=2.3～2.7变化至动态剪切抗力=536MPa,临界剪切应变=0.4-0.46。最后,以具有不同晶粒尺寸(32μm、119μm、336μm、587μ,)和明显织构特征的多晶纯钛为研究对象,探讨了等轴晶粒尺寸和加载方向对α钛动态性能的影响。结果表明,提高晶粒尺寸不会明显改变α钛的动态剪切抗力,但会显著降低临界剪切应变。当多数晶粒的c轴趋向于垂直剪切载荷方向时,材料具有较优的动态强度和塑性配比。更多还原

【Abstract】 Since the1970s, with the growing demand for weight-efficient weapon system, more researchers began to focus on the dynamic performance of titanium alloys. Many studies have been performed on Ti-6A1-4V and other a+β type titanium alloys as well as some β type titanium, and attentions have been paid to the performance characterization, influencing factors analysis and microscopic observation of adiabatic shear band etc. The present study then focuses on the dynamic deformation behavior of a type titanium alloy, including twinning behavior, dynamic shear instability and the related influencing factors.Firstly, the dynamic mechanical response of Gr.2commercial purity titanium were obtained using split Hopkinson pressure bar by means of compression, and the deformed microstructure were observed by optical microscopic. By contrasting the strain hardening modulus and twinning fraction of quasi-static (10-3S-1) and dynamic (6000s’1) compression deformation process, it was found that a sharp increase in twinning fraction was accompanied by a substantial increase in strain hardening modulus. This indicates that increasing the strain rate will boost the density of twinning and enhance the strain hardening effect through the law of Hall-Petch.In order to analyze the twinning behavior, EBSD technique was applied to confirm that quasi-statically deformed structure with εp=0.36consisted of two types of twinning namely{1012} and{1122}, while dynamically deformed structure with εp=0.22consisted of three types of twinning namely{1012},{1122} and{1121}. Twinning type was dependant on the spatial relationship between crystal orientation and compressive direction. Schmid factor of twinning and spherical coordinates in HCP unit cell were induced to calculate the Schmid distribution. By combining the calculated results with twinning type and Euler angles of grain matrix, the Schmid factor was proved to be able to predict the activation of twinning under both quasi-static and dynamic states. With the help of the calculated SF map, the corresponding critical resolved shear stress range was determined to be321-361MPa,367-393MPa and525-538MPa respectively. Moreover, strain rate was found to have a minor effect on the value of CRSS of twinning. In addition, by means of single-crystal titanium compression it has been proved that among the six twinning variants of a specific type the one with the maximum Schmid factor will be activated.In order to investigate the phenomenon of plabtic instability in Gr.2titanium a dynamic shear loading system was proposed. According to the pattern of pre-gridded scratch lines on the deformed specimen surface, a critical shear strain of1.1was found to activate plastic localization, while the shear stress was475MPa at a strain rate of18000s-The critical condition can be analyzed by considering the competition between strain hardening and thermal softening. This was realized mathematically by coupling the Johnson-Cook constitute model and temperature rise equation, based on which adiabatic shear stress and strain curve in a strain rate range from10s"1to104S-1was constructed. According to peak points of calculated curves, the critical condition can be determined as0.9-1.3for critical shear strain and395-480MPa for maximum shear stress. The calculated result fits well with the experimental value of1.1and475MPa. The relationship between the critical condition and metallurgical factors were also discussed. In order to lower ASB propensity the most important material parameters are those indicating strain hardening capacity, thermal softening propensity and thermal conductivity.Then, the influence of Al on the dynamic performance and deformation behavior of Ti-xAl(x=2,4,6) alloys was explored through the above mentioned methods on dynamic performance test, the deformation microstructure observation and data processing. While Al content increased from2wt.%to6wt.%, the heat conductivity at room temperature was lowered by approximately58%, the quasi-static yield strength was increased by144%, the critical resolved shear stress of{1012} type twinning was increased by100%, and strain hardening modulus at εp-0.07was lowered by50%. These changes directed the dynamic shear instability critical condition of Ti-2A1along the forecasted path to that of Ti-6A1.Finally, polycrystalline pure titanium with different grain size and apparent texture feature was used to discuss the effect of equiaxed grain size and loading direction on the dynamic performance of a titanium. The result shows that increasing the grain size does not significantly change the dynamic shear resistance, however significantly reduces the critical shear strain. When the shear loading direction tends to perpendicular to the c-axis, a titanium will manifest a superior combination of dynamic strength and ductility.更多还原