Scientific Understanding of Interfaces in Transforming Materials

About This Grant

NON-TECHNICAL SUMMARY Shape memory alloys (SMAs) experience reversible transformations, shifting back and forth between different arrangements of atoms, which in term dictate their properties. Once stretched, they undergo a change and then upon removal of the load, they can return to their original shape. Such a phenomenon finds applications in cardiovascular stents, structural dissipation under impacts, and actuators for motion control. Consequently, these kinds of materials can be used in defense, healthcare, aerospace, and structural engineering. However, for these materials to be trusted as reliable, they need to function over many cycles. However, the inability to return to its original shape is termed an “irreversibility”. Irreversibilities can eventually result in fatigue cracks, compromised performance and reduced durability. This work focuses on improving the understanding of atomic level changes in shape memory alloys such as NiTi and NiMnTi. The aim is to mitigate these irreversibilities via changes to elemental composition, introducing precipitates, and modifying atomic arrangements to reduce the defects that lead to irreversibilities. The introduction of precipitates in these materials is particularly promising, as they have been shown to facilitate transformation, imparting additional reversible strains especially under high stress applications. In this proposal, the advances in science-based understanding of SMA interfaces is establishing high-accuracy results built upon experiments using lattice scales and supported by modeling efforts that drive innovation. Additionally, this work is revamping our educational efforts in materials science, by enhancing pedagogical understanding and inspiring a new generation of students through the completion of a textbook on phase change materials and participation in a senior design project focused on fatigue testing. Ultimately, these efforts serve the national interest in different economic sectors and align with the National Science Foundation (NSF) mission by promoting research at the forefront of materials innovation. The research alos trains graduate students, allowing them to interact with others in the field and prepare them for the future workforce. TECHNICAL SUMMARY The intellectual contribution of this work centers on an advanced understanding of reversible phase transformations in shape memory alloys (SMAs). The aim is to develop the foundations of strain accumulation by accounting for the complexity of interfaces, specifically focusing on four characteristic twin intersections. This complexity is being studied with advanced high-resolution transmission electron microscopy and atomistic simulations. The local strains at characteristic twin interfaces of the monoclinic martensite phase are being established with a level of accuracy and high physical fidelity from atomic displacements using template matching methods and developing advanced algorithms to characterize displacements at twin interfaces. The creation of defects such as dislocations at these interfaces as an outcome of displacement incompatibilities will be established. The ab-initio calculations are also providing the basis for the determination of energy barriers for transformation motion, martensitic twinning, and dislocation slip progression, leading to the calculation of Critical Resolved Shear Stress (CRSS) that is being compared to experimental measurement. Modeling is pinpointing the minimum energy interface configurations considering the lattice offsets at interfaces, whereas the imperfect mis-alignments are providing crucial understanding of slip-induced irreversibility in shape memory alloys (SMAs). The experimental findings are checking the validity of these interface configurations and critical stress levels are being corresponded to the motion of such interfaces. Ultimately, the proposal focuses on an in-depth understanding of the microstructure of NiTi and the promising NiMnTi SMAs with varying compositions, particularly regarding the unexpected transformation of the precipitates (which have an initial rhombohedral structure) in the quest to achieve further functionalities. In summary, by deciphering the evolution of internal twinning interfaces in martensite, the proposed study’s is charting out a route to advanced shape memory alloys by curtailing dislocation slip effects. Additionally, this work is revamping our educational efforts in materials science, by enhancing pedagogical understanding and inspiring a new generation of students through the completion of a textbook on phase change materials and participation in a senior design project focused on fatigue testing. The proposal is aligned with national interest and the NSF mission by contributing to the science of advanced materials and accelerating materials discovery in the field of transforming alloys. 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.