Lattice instability and room temperature hyper-diffusion in metastable phase-transforming alloys

Non-technical Summary

Shape memory alloys (SMAs) are metals that can return to a pre-programmed shape upon heating. This ability, which is driven by a reversible change in the crystal structure, makes them capable candidates for morphing aircrafts, deployable medical implants, and solid-state actuators. Beyond the unique mechanical response, atoms in some SMAs inexplicably move up 10 billion times faster near room temperature than predicted by current diffusion models.

As a result, the alloy spontaneously grows stronger but also less ductile over a period of weeks to years when left undisturbed at room temperature. This project proposes a new theory for this unexplained behavior based on locally weakened bonding between atoms caused by instabilities of the crystal structure, which lowers the energy required for atomic motion along certain directions within the crystal.

The award supports a theory-driven experimental approach that augments the fundamental understanding of diffusion in crystalline solids. Broader impacts of the project include research experience for undergraduate students, K-12 students, and especially those from underrepresented minorities and backgrounds, community outreach through contribution to science and technology exhibitions and makerspaces, and a “living sculpture” artistic project using SMA technology in collaboration with the school of design and architecture.

Fundamental understanding gained from the project could lead to a new class of programmable materials whose properties change controllably with time over the course of months to years. Technical Summary

This project seeks to identify the origins of ultra-fast low-temperature diffusion observed in some shape memory alloys (SMAs), which cannot be explained with current diffusion models. The knowledge gained will establish the basic understanding for a new mode of enhanced diffusion in crystalline solids based on phase instability and lattice softening.

This work proposes a hypothesis based on reduction of activation energy of vacancy-based diffusion due to orientation-dependent lattice softening of the unstable austenite. The hypothesis will be tested experimentally in a single crystal model through inelastic x-ray scattering and electron microscopy. It will also explain how rapid room-temperature nucleation and coarsening of precipitates can be possible in some alloys despite being far from their melting points.

At the basic science level, the project yields information on the phonon dispersion relations in beta-titanium shape memory alloys, and identifies soft phonon modes and their relationships with both phase transformation and abnormal diffusion at room temperature. At an applied science and engineering level, the proposed work will clarify the origins of the room temperature aging effect in SMAs and identify strategies by which such instability can be eliminated.

From an education and outreach perspective, the project will engage students at the intersection of science and the arts through a living sculpture project that utilizes the shape-changing characteristics of SMAs, and introduce advanced materials to designers and architects as a new set of tools in their toolbox.

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.