Elucidating the Impact of Nanoscale Strain and Concentration Fields on Martensitic Transformations in NiTiHf-based Shape Memory Alloys

Abstract NON-TECHNICAL SUMMARY

Shape memory alloys (SMAs) are a unique class of alloys that can recover their original shape. This feature makes SMAs a leading contender for many future commercial and industrial technologies such as medical devices, sensors, actuators, and components for aerospace vehicles. Shape recovery in these metals relies on a reversible change in atomic arrangements.

Designing and engineering their atomic structures can radically change their shape memory performance. Producing nano-sized particles, called precipitates, in SMAs offers a direct route to modifying their local atomic structure and chemistry, thereby achieving targeted shape memory properties. However, accurately pairing the characteristics and properties of nano-sized precipitates to shape memory performance remains a challenge due to a lack of high-precision, real-time measurements, which creates a roadblock to establishing a reliable alloy design for various applications.

The PIs address this challenge by developing atomic-resolution, in situ electron microscopy techniques to quantitatively measure the impacts of precipitates on shape memory behavior in SMAs. The gained knowledge can generate powerful alloy design rules for precipitation-engineered SMAs for applications requiring high-temperature operations and improved mechanical properties.

Moreover, the collaborative research activities between the PIs will be intertwined with educational programs and outreach activities through summer internships for underrepresented minority high-school students from local public schools, curriculum development targeting both on-campus students and distance-learning students (e.g., industry and military), and research training of graduate and undergraduate students with a strong emphasis on alloy design and materials characterization. These education plans promote student awareness about critical materials needs for new technologies and encourages diverse students to pursue careers in STEM.

TECHNICAL SUMMARY

Mechanical and chemical effects of precipitates play a crucial role in controlling martensitic transformation (MT) dynamics in shape memory alloys (SMAs). However, there is a distinct lack of systematic and quantitative experimental evidence that can support, or test, theoretical models of the physical mechanisms of a MT modified by non-transforming coherent precipitates within the microstructure.

This experimental program aims to quantify and elucidate the effects of nanoscale strain and concentration fields induced by coherent precipitates on the phase transformation and mechanical properties of NiTiHf-based SMAs. Specifically, the PIs seek to uncover and isolate the role of Heusler and Han phase precipitates (when they coexist), which have a distinctly different crystal structure and chemistry in NiTiHf-based SMAs.

To achieve the goal, the PIs utilize a unique set of expertise in atomic-scale materials characterization based on aberration-corrected electron microscopy, electron diffraction, and in situ heating applications, as well as alloy design/synthesis and thermo-mechanical properties characterization. This program focuses on: (i) the development of alloy synthesis routes that allow for co-precipitation of Heusler and Han phases with controlled microstructure and chemistry; (ii) the quantification of strain and concentration fields around precipitates using atomic-resolution electron microscopy, spectroscopy, and scanning electron diffraction to uncover their effects on the microstructure morphology of the matrix; (iii) the characterization of the MT mechanisms in precipitation-strengthened SMAs and the phase transformation pathways using in situ temperature-controlled experiments for determining the dependence of a MT on the designed properties of precipitates (i.e., strain and concentration fields).

This work provides a strong foundation for future SMA designs as well as in computational modeling by offering a quantitative, holistic understanding of the structure-composition-property relationships of SMAs and their dynamic response to realistic in-service environments.

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.