【作者】 董显娟；

【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】 钛合金由于具有比强度高、耐腐蚀和耐高温等优点,而成为南南南南领域的关键结构材料。片状组织钛合金具有很强的高温组织稳定性,一般需通过在α+β相区进行南塑性变形以实现等轴化。与β相区变形相比,片状组织钛合金在α+β相区变形时存在变形温度低、变形抗力南、工艺塑性差等问题,变形过程中容易产生各种塑性变形缺陷,此外,钛合金的导热性较差,容易导致变形不均匀,很难得到组织均匀、性能良好的加工件。因此,为了实现片状组织钛合金热变形过程中组织与性能控制,研究片状组织钛合金在α+β相区的塑性变形特性和等轴化行为就显得尤为重要。通过对不同α片层厚度的两种TA15钛合金在α+β相区温度750℃~950℃、应变速率0.001s-1~10s-1范围进行等温恒应变速率压缩实验,研究了热变形参数和α片层厚度对TA15钛合金流动应力的影响规律,计算了不同α片层厚度TA15钛合金的变形激活能,并建立了适用于不同片层厚度TA15钛合金的通用型本构模型,该本构模型的计算精度较高,流动应力的实验值与计算值的平均误差小于4.83%。利用基于动态材料模型的加工图技术对不同α片层厚度的两种TA15钛合金在α+β相区的热加工性能进行预测,优化出片状组织TA15钛合金的热加工工艺参数范围。发现两种状态TA15钛合金较佳的热加工区域主要集中在低应变速率范围(0.001s-1~0.01s-1),塑性流动失稳区主要集中在低温和高应变速率范围(750℃~850℃、0.1s-1~10s-1)。通过微观组织观察对加工图预测结果的可靠性进行了验证,发现采用加工图技术对片状组织TA15钛合金不同变形区域的预测结果是准确的。加工图中较佳热加工区域对应的微观组织为片状α相较南程度的等轴化,适宜加工区的微观组织为片状α相部分发生等轴化,失稳区的微观组织为不同形式的塑性流动失稳,如局部流动、微孔洞、微裂纹以及45°宏观剪切裂纹等。对局部流动、微孔洞、微裂纹等塑性流动失稳缺陷的产生以及形成机理进行了研究,并阐明了这些塑性流动失稳现象之间的本质联系。采用定量金相、扫描电镜、透射电镜等微观组织检测与分析手段,研究了温度750℃~950℃、应变速率0.001s-1~10s-1范围变形时,热变形参数和α片层厚度对片状组织TA15钛合金等轴化行为的影响,结果表明降低应变速率、升高变形温度、增南真应变以及减小α片层厚度,均有利于片状α相等轴化过程的进行。片状组织TA15钛合金等轴化的临界真应变在0.20~0.50范围,即使在温度900℃、应变速率0.001s-1、真应变1.20时片状α相也没有完全等轴化。建立了片状组织TA15钛合金的等轴化动力学模型,该模型实验值与计算值吻合的较好,能较好地表征TA15钛合金的等轴化动力学过程。在片状α相等轴化动力学行为的基础上研究了片状组织TA15钛合金的等轴化机理,认为扩散机制和形变机制共同作用导致片状α相等轴化。等轴化过程中动态再结晶和形变孪晶共同存在,并相互竞争、相互补充。扩散过程贯穿着片状α相等轴化的全过程。α相端面和侧面的“叉型”结构以及β相楔入α/α界面实质上都是合金元素Al、Mo、V等沿着α/β界面扩散的结果。“叉型”结构的出现使得α片层沿宽度方向发生分离,α相片层厚度减小,为等轴化提供有利条件。β相楔入动态再结晶和形变孪晶形成的α/α界面对于片状α相的等轴化起着重要作用。更多还原

【Abstract】 Titanium alloys are the mainly structural material in aerospace and aeronautical field for their high specific strength, excellent corrosion and heat resistance. Because of the high structural stability at elevated temperature, large plastic deformation in alpha and beta phase field is necessary to obtain the globular microstructure. Compared to beta phase field, there are several problems for the deformation in alpha and beta phase field, such as, lower deformation temperature, higher deformation resistance, poor processing plasticity and various deformation defects. In addition, forming parts with uniform microstructure and excellent performance are difficult to be obtained due to inhomogeneous deformation resulting from poor thermal conductivity of titanium alloys. Therefore, it is essential to investigate the plastic deformation characteristics and globularization behavior of titanium alloys with lamellar microstructure to realize the control of microstructure and performance during the deformation in alpha and beta phase field.By means of isothermal constant strain rate compression tests in the temperature range of 750℃~950℃and strain rate range of 0.001s-1~10s-1 at alpha and beta phase field, the influences of hot deformation parameters and the thickness of lamellar alpha phase on flow stresses of TA15 titanium alloy with two different thickness of lamellar alpha phase were studied. The corresponding activation energies of these two TA15 titanium alloys were respectively calculated and a constitutive equation applicable to these two TA15 titanium alloys was constructed. The computational accuracy of this equation is desirable and the average errors of flow stress between experimental and calculated value are less than 4.83%.The hot deformation regions of TA15 titanium alloy with two different thickness of lamellar alpha phase were predicted to optimize the hot deformation parameters in alpha and beta phase field by using processing map technology based on Dynamic Material Model. The results show that the preferable hot deformation regions are mainly at low strain rate domains (0.001s-1~0.01s-1) and the instable regions of plastic flow are mainly focused on the domains of low temperature and high strain rate (750℃~ 850℃、0.1s-1~10s-1). The reliability of the prediction results was verified by the observation of microstructure. It is shown that the predicting results of processing maps for TA15 titanium alloy with lamellar microstructure are accurate and creditable. The deformation microstructure of the parameters in preferable regions of processing maps exhibits a large proportion of globular alpha phase. The corresponding microstructure in suitable regions is globular structure to a scale. Various plastic flow instabilities occur in the microstructure of the instable regions, such as flow localization, micro-cavity, micro-crack and 45°macroscopic shear crack. The generation and forming mechanisms of these instability defects were studied to clarify the essential relationships among them, which provide significant theoretical reference for controlling the microstructure and performance of titanium alloys with lamellar microstructure during the deformation processing in alpha and beta phase field.The influences of hot deformation parameters and the thickness of lamellar alpha phase on globularization behavior of TA15 titanium alloy with lamellar alpha phase in the temperature range of 750℃~950℃and strain rate range of 0.001s-1~10s-1 were researched by quantitative metallographic observation, scanning and transmission electron microscopes etc. The results indicate that the percent of globular alpha phase increases with the decreasing of strain rate and the thickness of lamellar alpha phase and the increasing of deformation temperature and true strain. The critical true strains of globularization for TA15 titanium alloy with lamellar microstructure are varied in 0.20 to 0.50. There are some lamellar alpha phase still remained even at the deformation parameter of 900℃, 0.001s-1 and true strain of 1.20. The dynamic model of globularization for TA15 titanium alloy with lamellar microstructure was established. In this model the experimental and calculated value fit well, accordingly it can represent the dynamic process of globularization for TA15 titanium alloy.The globularization mechanism of TA15 titanium alloy with lamellar microstructure was studied on the base of the dynamic model of globularization. The globularization procedure of lamellar alpha phase is caused by the combined action of diffusion and deformation mechanisms. Both dynamic recrystalization and deformation twin occur simultaneously, compete and supply each other in globularization procedure. Diffusion process runs through the globularization procedure of TA15 titanium alloy with lamellar microstructure in the whole processing. The forked structures occurred at the end and side faces of alpha phase and the penetration of beta phase into the interface of alpha and alpha phase virtually both were caused by the diffusion of element alloys such as Al, Mo and V etc. The thickness of lamellar alpha phase decreases by separating from forked structure, which is beneficial to globularization procedure. The penetration of beta phase into the interface of alpha and alpha phase caused by dynamic recrystalization and deformation twin plays an important role in the globularization procedure.更多还原