Esham, Kathryn

BackgroundThe Ohio State University - College of Engineering

Kathryn grew up in Geneseo, New York. She earned her B.S. in Glass Science Engineering from Alfred University in 2014 where her research focused on the aqueous corrosion of nuclear waste glasses. She completed an REU at Northern Illinois University and spent a semester abroad at the University of Sheffield in England. Following graduation, she participated in the Walt Disney World College Program in Orlando FL, working at Disney’s Animal Kingdom as a Park Greeter. In January 2015 she joined the Anderson Group at Ohio State.

Research Project: The Dual Role of Nano-precipitates in High Temperature Shape Memory Alloys

The goals of this work aim to investigate the effect of precipitate size, shape and orientation on the microstructure, transformation temperatures, and actuation properties of shape memory alloys used in high temperature applications. Investigative techniques will combine phase field finite element modeling (through collaboration with the Ohio Supercomputing Center) with characterization at the Center for Electron Microscopy and Analysis (CEMAS).

Shape memory alloys are a class of materials that experience a diffusionless (martensitic) transformation. At high temperature, SMAs have a cubic (austenite) structure and upon cooling they transform to a monoclinic martensite structure as. The symmetry of the cubic-to-monoclinic transformation imposes 12 martensite variants (same structure, different orientation). Heating reverts martensite back to cubic austenite. This ability to cycle between structures is termed the “shape memory effect.” The most common SMA is near-equiatomic nickel-titanium (NiTi). NiTi has been studied and used extensively in applications including sensing and energy dissipation, biological stents, and aerospace. These applications require a variety of temperatures that are achieved through careful compositional control. NiTi can be used because the transformation temperature from austenite to martensite can be engineered. Additionally it is a lightweight actuator that is competitive with pneumatic systems in terms of work performance. It seems like NiTi would be a miracle actuator for aerospace– but there’s a catch. These SMAs cannot be used above about 100°C. They would remain in the austenite phase and unable to transform, work would not be done. If NiTi were modified so that the transformation temperature (TT) was increased to as high as 800°C, we could tackle a whole new class of applications!

Our first challenge is to bring SMAs to this new temperature threshold. Preliminary work suggests this can be accomplished by creating a ternary system by adding an element that chemically substitutes for Ni or Ti. It’s unclear why this raises the TT and we don’t know how high we can go. How important is the electronic structure of tertiary element? Does the local strain due to atomic misfit play a role?

Our second challenge is reliability. What allows ternary systems to actuate at higher temperatures? How are they stabilized? Generally, repeated actuation of SMAs causes residual strain resulting in ratcheting or elongation (functional fatigue). Unfortunately, this negative effect is augmented at high temperatures. One approach is to form nanoprecipitates. These nano-scale particles stop functional fatigue up to 240°C. This is where we can use a heat treatment in a ternary system to grow nanoprecipitates that will pin dislocations, thereby preventing ratcheting. This solution is not without challenges, as too high a volume fraction of these precipitates can hinder phase transformation.

These challenges have inspired us to explore the following questions:

  • How exactly do these nanoprecipitates work to suppress ratcheting while also encouraging the martensitic phase transformation?
  • What is the specific effect of these nanoprecipitates on:
    1. The type of martensite variants that form upon cooling and other microstructual features
    2. Transformation temperatures
    3. Work output in terms of actuation

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