Determination of Activation Energies and Modeling of Low Temperature Creep of Alpha, Alpha-Beta and Beta Titanium Alloys

这个项目旨在对两相钛合金的低温蠕变有新的认识，这些合金在能源、航空航天、海洋、化学工业、消费品和生物医疗植入物等各种技术中都有应用。最近对α钛在室温下孪晶蠕变行为的观察表明，需要一个更完整和基本的模型来预测这些结构的完整性。本研究的主要目标是：(1)确定α、α - β和β钛合金在298- 458K低温范围内蠕变机制的活化能；(2)利用有限元模型（FEM）对这些合金的蠕变行为进行建模；(3)将FEM预测的蠕变曲线和应变分布与实验值进行比较；(4)为提高钛合金的低温蠕变性能推荐最佳的化学和显微组织。为了达到这些目标，选择了两种不同的合金类别（Ti-Mn和Ti-V）进行低温变形测试，并结合SEM和TEM进行电子光刻，表面变形研究和微观结构表征，如位错密度和晶体结构分析。根据蠕变试验数据，确定了活化能，建立了蠕变过程模型。采用有限元方法，利用α相和β相的蠕变常数对两相α - β钛合金的蠕变行为进行了数值模拟。对α - β合金蠕变的有限元模拟可以根据α和β相的存在及其形态进行预测。该研究结果可直接应用于其他类似系统，如在能源和交通部门应用的锆和镁合金。此外，有限元模拟的结果适用于任何两相或复合材料。本研究对低温蠕变机理有了新的认识，其结果可用于设计新型钛合金和优化现有合金的显微组织以提高蠕变性能。这项研究也将有助于预测部件的性能。***钛合金具有高强度重量比、高断裂韧性、良好的高温强度、优异的耐腐蚀性和生物相容性等工程性能。由于这些特性，它们在各个领域都有应用，包括能源、航空航天、海洋、化学工业、消费品和生物医学植入物。在一些低温应用中，如环境温度，载荷被施加在组件上很长一段时间，蠕变成为一个重要的特性。在这方面，最近的研究表明，钛合金在室温下可以在95%屈服应力下蠕变，但解释蠕变发生的原因和方式的蠕变活化能尚不清楚。此外，没有简单的模型可以预测两相材料的蠕变行为，从各个相的蠕变行为的知识，他们的形态和体积分数。这项调查的主要目的是：(1)确定α、α - β和β钛合金在298- 458K低温范围内蠕变机理的活化能；(2)对a、a-b和b钛合金的蠕变行为进行建模，包括对a-b钛合金进行有限元建模；(3)将有限元预测的蠕变曲线和应变分布与实验值进行比较；(4)为提高钛合金的低温蠕变性能推荐最佳的化学成分和显微组织。在这些研究中，将使用三种Ti-Mn合金和三种Ti-V合金作为模型系统。拉伸试验将在298 -458 K的温度范围内进行，蠕变试验将在298 -458 K的温度范围内进行，应力水平范围为85 -100% YS。扫描电镜和透射电镜将用于电子光刻，表面变形研究和微观结构表征，如位错密度和晶体结构分析。根据蠕变试验数据，将确定活化能，并对蠕变过程进行建模。采用ANSYS有限元模拟程序，利用a相和b相的蠕变常数来模拟a-b两相钛合金的蠕变行为。预计提案工作可在三年内完成。这项工作的成功完成将具有重要的技术意义，因为它将对设计新的钛合金和优化现有合金的微观结构以提高蠕变性能产生重大影响。例如，活化能的测定提供了材料中存在的哪些种类（即元素）导致低温蠕变的线索，因此，可以设计新的合金以提高性能。对α - β合金蠕变的有限元模拟可以根据α和β相的存在及其形态进行预测。这也将有助于设计和预测组件的性能。尽管本研究使用钛合金作为模型系统，但与活化能相关的研究结果直接适用于任何其他类似系统，如锆合金。此外，一般情况下，与有限元模拟有关的结果将适用于任何两相或复合材料。这项工作将由PI， S. Ankem教授和两名研究生进行。PI在钛合金的物理和力学行为以及有限元建模方面有着丰富的经验。研究生将学习最先进的实验技术，如绘制基准线的电子光刻技术、扫描电镜和透射电镜技术以及计算机在预测材料行为方面的应用。

0102320AnkemThis project is aimed at new understanding into low temperature creep of two-phase titanium alloys that find applications in various technologies including energy, aerospace, marine, chemical industries, consumer goods and bio-medical implants. Recent observations on the creep behavior of alpha titanium at ambient temperature by twinning led to the need for a more complete and fundamental modeling for predicting the integrity of these structures. The main goals of this study are: (1) to determine the activation energies for creep mechanisms in alpha, alpha-beta and beta titanium alloys in the low temperature range of 298- 458K, (2) to model the creep behavior of these alloys including Finite Element Modeling (FEM), (3) to compare FEM predicted creep curves and strain distributions with experimental values, and (4) to recommend optimal chemistry and microstructures of Ti alloys for improved low temperature creep resistance. To reach these goals, two different alloy classes (Ti-Mn and Ti-V) are selected for deformation tests at low temperatures combined with SEM and TEM for electron lithography, surface deformation studies and microstructure characterization such as dislocation densities and crystal structure analyses. Activation energies are determined and the creep processes are modeled from the creep tests data. The creep constants of the alpha and beta phases are used to model the creep behavior of two-phase alpha-beta Ti alloys by FEM using ANSYS computer program. The FEM modeling of creep of alpha-beta alloys gives a predictive capability in terms of the alpha and beta phases present and their morphologies. The results of the study are directly applicable for other similar systems such as zirconium and magnesium alloys that find application in energy and transportation sectors. In addition, the results related to the FEM modeling are applicable to any two-phase or composite materials. %%%The research develops new understanding of the low temperature creep mechanisms and the results are applicable for designing new titanium alloys and in optimizing the microstructures of existing alloys for improved creep performance. The research will also help in predicting component performance.*** Titanium alloys have attractive engineering properties including high strength to weight ratio, high fracture toughness, good high temperature strength, excellent corrosion resistance and bio- compatibility. Due to these properties, they find applications in various areas including energy, aerospace, marine, chemical industries, consumer goods and bio-medical implants. In some of these applications at low temperatures such as ambient temperature, loads are applied on the components for extended periods of time where creep becomes an important property. In this regard, it was recently shown that titanium alloys can creep at 95% yield stress at ambient temperature, but the activation energies for creep which can explain why and how creep occurs are not known. In addition, there are no simple models available to predict the creep behavior of two-phase materials from the knowledge of the creep behavior of individual phases, their morphology and volume fractions. The main objectives of this investigation are: (1) Determine the activation energies of creep mechanisms in alpha, alpha-beta and beta titanium alloys in the low temperature range of 298- 458K, (2) Model the creep behavior of a, a-b and b Ti alloys including Finite Element Modeling of a-b Ti alloys, (3) Compare FEM predicted creep curves and strain distributions with experimental values, and (4) Recommend optimal chemistry and microstructures of Ti alloys for improved low temperature creep resistance. For these studies, three Ti-Mn alloys and three Ti-V alloys will be used as the model systems. Tensile tests will be conducted in the temperature range 298 -458 K and creep tests will be conducted in the temperature range 298 - 458 K and in the stress level ranging from 85 -100% YS. SEM and TEM will be employed for electron lithography, surface deformation studies and microstructure characterization such as dislocation densities and crystal structure analyses. From the creep tests data, activation energies will be determined and the creep processes will be modelled. The creep constants of the a and b phases will be used to model the creep behaviour of two-phase a-b Ti alloys by Finite Element Modelling using ANSYS computer program. It is expected that the proposal work can be completed in three years. A successful completion of this work will be of great technological importance, as it will have a significant effect in designing new titanium alloys and in optimizing the microstructures of existing alloys for improved creep performance. For example, determination of activation energies gives a clue as to which species, i.e. elements, present in the material are responsible for low temperature creep and, accordingly, new alloys can be designed for improved performance. The FEM modeling of creep of alpha-beta alloys gives a predictive capability in terms of the alpha and beta phases present and their morphologies. This will also help in designing and in predicting component performance. Even though this investigation uses titanium alloys as the model system, the outcomes of the studies related to activation energies are directly applicable for any other similar systems such as Zirconium alloys. Furthermore, in general, the results related to the FEM modeling would be applicable to any two-phase or composite materials. This work will be carried out by the PI, Prof. S. Ankem and two graduate students. The PI has extensive experience in physical and mechanical behavior of Ti alloys and FEM Modeling. The graduate students will be trained in the state of the art experimental techniques such as electron lithography for drawing fiducial lines, SEM and TEM techniques and application of computers in predicting material behavior.