Collaborative Research: Fatigue Crack Formation and Growth in the Presence of Reversible Martensitic Transformation in High Temperature Shape Memory Alloys

新发现的高温形状记忆合金可以通过其晶体结构的可逆转变在高温下恢复大的变形。这使得它们在航空、汽车和能量转换应用中用作固态致动器引起了极大的兴趣。这些致动器可以潜在地提供超高能量密度致动，并且将允许多功能使用，由于简化的设备设计而导致更低的成本和维护。该研究项目的目的是通过阐明寿命限制疲劳机制来加快这些材料的开发周期。这些合金的疲劳过程的知识将有助于在工程应用中更广泛地接受表现出可逆马氏体相变的材料。此外，通过实验和建模及其多学科性质的协同结合，这项研究将培养下一代多功能相变材料专家。它还将通过与波音公司、NASA和国际合作者的合作，支持代表性不足的群体参与研究和专业经验。这项研究将涉及多尺度热机械测试和微观结构表征以及复杂的建模，以确定在可逆马氏体相变和相关不可逆过程存在下，作为微观结构属性函数的缺陷动力学。虽然最近发现的镍-钛-铪（NiTiHf）合金将被用作模型材料系统，但许多实验发现和模型预计也可转移到其他相变材料。实验工作将依赖于原位热机械测试扫描电子显微镜和非原位透射电子显微镜，以及数字图像相关和同步辐射X射线断层扫描，以监测缺陷的形成/生长，并表征微观结构，变形结构和断裂表面。建模工作将基于疲劳裂纹形成和扩展的相场方法。这种方法将提供优于依赖于传统断裂力学的自相似裂纹生长假设的当前方法的优点，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查进行评估，被认为值得支持的搜索.

Newly discovered high-temperature shape memory alloys can recover large deformations at elevated temperatures through a reversible transformation of their crystal structure. This has warranted significant interest in their use as solid-state actuators in aeronautics, automotive, and energy-conversion applications. These actuators could potentially provide ultra-high energy density actuation and would allow multifunctional use, resulting in lower cost and maintenance due to simplified device designs. The aim of this research project is to accelerate the development cycle of these materials by elucidating the life-limiting fatigue mechanisms. The knowledge of fatigue progression in these alloys will facilitate the wider acceptance of materials exhibiting reversible martensitic transformation in engineering applications. Furthermore, through the synergistic combination of experiments and modeling and its multidisciplinary nature, this research will train the next generation of experts in multifunctional, phase transforming materials. It will also support the involvement of underrepresented groups in research and professional experiences via partnership with the Boeing Company, NASA, and international collaborators.This research will involve multiscale thermomechanical testing and microstructural characterization as well as sophisticated modeling to identify defect kinetics as a function of microstructural attributes in the presence of reversible martensitic transformation and associated irreversible processes. While recently discovered Nickel-Titanium-Hafnium (NiTiHf) alloys will be used as a model material system, many of the experimental findings and models are expected to be transferable to other phase transforming materials as well. The experimental work will rely on in-situ thermomechanical testing in scanning electron microscope and ex-situ transmission electron microscopy, as well as digital image correlation and synchrotron X-ray tomography to monitor defect formation/growth and to characterize microstructure, deformation structure, and fracture surfaces. The modeling work will be based on a phase field approach to fatigue crack formation and growth. This approach will provide an advantage over current methods that rely on self-similar crack growth assumptions of conventional fracture mechanics, which break down due to the variability of the underlying microstructure and their complex influence on the mechanical fields close to the crack tip.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.