Understanding the interplay of precipitates and dislocations on the reversible martensitic transformation in cyclically actuated NiTiHf shape memory alloys
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NON-TECHNICAL SUMMARY Shape memory alloys (SMAs) can contract and extend, like an artificial muscle, upon cooling and heating through a process called martensitic transformation. Similar to muscles, shape memory alloys experience fatigue after a number of operation cycles. The fatigue of shape memory alloys is triggered and then exacerbated by the creation and accumulation of ultra-small crystalline defects called dislocations. Recent work has shown that the fatigue life of the SMAs can be dramatically improved by incorporating nano-sized particles. The purpose of this project is to provide fundamental understanding of how these nano-particles influence the fatigue of these SMAs by using advanced and in situ electron microscopy to watch the interactions of the nanoparticles with the defects in real time. These insights will accelerate the development of fatigue-resistant and durable super-elastic SMAs that may beneficially impact a wide variety of critical technologies, including aerospace, energy conversion, biomedical, defense, and transportation. This project will also provide multidisciplinary STEM educational and career advancement opportunities for underrepresented students through the Louis Stokes Alliance for Minority Participation program, as well as online STEM course material based on this research. TECHNICAL SUMMARY The goal of the project is to develop a fundamental understanding of the interplay between nanoprecipitates, dislocations, and martensitic transformation, during thermo-mechanically induced, reversible martensitic transformation. The new knowledge gained will elucidate how their interactions control fatigue performance (both functional and structural) in cyclically actuated high-temperature shape memory alloys (SMAs). The central hypothesis is that the nanoprecipitates will suppress the transformation-induced dislocation multiplication, glide, and rearrangement during thermal cycling, thus dramatically modifying granular-level microstructure, and consequently the fatigue performance. The experimental tasks to be undertaken include: 1) identify precipitate characteristics, associated elastic strain, and local chemistry, 2) determine the effect of precipitates on the transformation-induced dislocation generation and structure evolution, 3) assess the effect of precipitate-dislocation interactions on the austenite and martensite microstructure evolution and the resultant functional fatigue, and 4) identify the role of precipitate-dislocation interaction on persistent slip band formation and crack initiation and growth (structural fatigue) in cyclically actuated SMAs. The experimental results will be used to develop a detailed description of the cyclic reversible martensitic transformation and fatigue responses in precipitate-bearing SMAs. 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.