Sehitoglu awarded NSF grants for unraveling deformation and fatigue behavior at atomic scales
MechSE professor Huseyin Sehitoglu received two National Science Foundation (NSF) grants this summer. The first grant is on generation of slip in shape memory alloys, and the second one is on the mechanics of fatigue in high entropy alloys. These grants are from the Mathematical and Physical Sciences (MPS) Directorate and the Engineering Directorate, respectively.
In the first project, the degradation of functionality is recognized as a limiting factor in shape memory materials. The accumulation of slip, which is a rather complex process, is the topic of study. The work involves the development of new analytical (atomistic simulations and anisotropic elasticity, Mohammed, ASK and Sehitoglu, H., Acta Materialia, 208 116716, 2021) and experimental tools (high resolution transmission electron microscopy). The analysis relates the misfit strains at austenite/martensite interfaces to the formation of dislocations (atomic line defects) which are localized and produce degradation of recoverability and ultimately crack nucleation. Previous research relied on empiricism was unable to resolve atomic scales.
Sehitoglu’s research is aimed at uncovering the origins of irreversibility, responsible for deterioration of shape memory, based on using the most advanced analytical and computational tools. The experimental portion of the work utilizes high resolution electron microscopy (F. Brenne, ASK Mohammed, H. Sehitoglu Ultramicroscopy 219 113134 2020) to determine the local displacement fields and to check the agreement between the theoretical predictions and experiment.
In the second project, Sehitoglu and his students are pursuing an advanced understanding of a new class of materials without a dominant principal element. In high entropy alloys, multiple elements are mixed and preferential segregation of certain elements near faults could change the energy landscapes. Their high strength is superior to conventional materials, and this capability can be harnessed in structures ranging from nuclear and civil to aerospace sectors where enhanced safety and durability are desirable.
The theoretical framework for analyzing slip-twin interactions and how it affects strengthening is being established first. The modeling will determine the key parameters responsible for strength elevation. The approach is fundamentally rooted in energy-minimization within a fully-anisotropic framework revealing the evolution of dislocation cores with progression of slip. Also, these materials are expected to exhibit high fatigue threshold stress intensity which is a key parameter in fatigue design. The determination of fatigue threshold level is complex and not achieved in previous work. The work will establish dependence of fatigue threshold on the strength, energy parameters and suggest the most promising high and medium entropy alloys.