In-Situ Observations of Performance Evolution in Shape Memory Alloys

What did the Scientists Discover?

Researchers led by Professor Aaron Stebner's group at the Colorado School of Mines were able to elucidate important functional fatigue behaviors in a shape memory alloy (SMA) by combining multiple high-energy diffraction techniques available at the F2 station at CHESS. The research team studied a phenomenon wherein special high-angle grain boundaries emerged inside a nickel-titanium SMA as it was thermally actuated in-situ. The researchers were able to reconstruct the location and morphology of newly nucleated grains which contribute to functional fatigue in 3-D as they formed in-situ.

Broader Impacts of this work?

Shape memory alloys see use in numerous aerospace and biomedical applications, but their wider use is limited by functional fatigue. Understanding the micromechanical origins of functional fatigue will advance the development of new microstructures that mitigate these effects and lead to wider adoption in industry.

Why is this important?

Solid-state shape memory alloy (SMA) actuators are compact, scalable, and capable of supporting and moving large mechanical loads while weighing a fraction of traditional actuators. Together these advantages make SMA actuators an attractive, and often enabling, technology for nano and micro electro-mechanical systems, biomedical, and aerospace actuation systems. The vast majority of SMAs, however, exhibit an inherent cyclic instability that causes the performance of an SMA to degrade with continued use. This instability is known as functional fatigue and is posited to originate from defect generation during actuation.

Why did this research need CHESS?

The high-energy X-ray techniques performed at CHESS are capable of non-destructively probing much larger volumes (on the order of mm3) than traditional destructive electron microscopy, allowing for in-situ studies such as this to be performed.

Collaborators:

A.N. Bucsek, Colorado School of Mines

L. Casalena, Ohio State University

D.C. Pagan, Cornell High Energy Synchrotron Source

P.P. Paul, Northwestern University

Y. Chumlyakov, Tomsk State University

M.J. Mills, Ohio State University

A.P. Stebner, Colorado School of Mines

Publication Citation

A.N. Bucsek, L. Casalena, D.C. Pagan, P.P. Paul, Y. Chumlyakov, M.J. Mills, and A.P. Stebner;  Three-dimensional in situ characterization of phase transformation induced austenite grain refinement in nickel-titanium, Scr. Mater. 162C, 361–366.(2019) https://doi.org/10.1016/j.scriptamat.2018.11.043

How was the work funded?