Collaborative Research: Nanoscale Structural and Compositional Instability-Driven Ductility in Refractory High-Entropy Alloys

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NON-TECHNICAL SUMMARYThe engineering term for a material’s ability to be stretched by pulling, without breaking, is known as “ductility”. This collaborative research program is developing an atomistic understanding of the relationship between how refractory high-entropy alloys are made and their ductility response. These alloys, which have near equal amounts of multiple elements can have both high strength and high melting points and are ideal for elevated-temperature applications. A main drawback of these alloys however, is their low ductility at room temperature. This research project addresses this issue by seeking to increase the ductile strength of these alloys with twinning-induced plasticity and transition-induced plasticity. These approaches can result in changes to the crystal lattice known as distortions. To detect these lattice distortions as soon as possible and at the level of atoms, a data mining-based transmission electron microscopy approach is being developed. The study is assisted by a complimentary effort to make new materials, mechanically test these materials, and characterize their structure. Results are assisting in the design of new refractory alloys and the data mining microscopy technique can be applied to other materials as well, therefore a broad impact on other alloy research is expected. This project also integrates with an outreach effort across multiple levels tasked with contributing to a sustainable, adaptable, and globally competitive science, technology, and engineering workforce in the areas of alloy design and characterization. This research program is also seeking to increase public awareness of materials, data science, and related technologies, while simultaneously enriching the pool of underrepresented groups interested in science and engineering. TECHNICAL SUMMARYThis collaborative project explores the phase stability of group IV elements (Ti, Zr, and Hf) containing alloys and their related twinning-induced plasticity and transition-induced plasticity effects. Specifically, the research focuses on the precursors of phase transformation, in the form of structure-induced local lattice distortions and chemical short-range ordering in disordered multi-principal component alloys. The study is motivated by the promise of the high-entropy alloy concept for designing new alloys and the lack of understanding concerning the effects of chemical complexity on phase stability and physical properties. This research uses ductility in refractory high-entropy alloys as a model problem for the studies of body-centered-cubic (bcc) dynamic instability, and the effects of chemical disorder and lattice distortions on phase stability. This project is also further developing the cepstrum-based analysis of electron-diffuse scattering in electron nano-diffraction patterns. Correlation with lattice distortions and short-range ordering in selected alloys is being assisted by efforts to synthesize and characterize a collection of multi-principal element refractory alloys. Specifically, this study is answering the following questions: How can one determine local chemical ordering and its impact on dynamic instability? How do phase transformations manifest in a distorted lattice? How do the above two effects influence stress-induced martensitic transformation and deformation twinning? This project also supports the goal of contributing to a globally competitive workforce in STEM while also enriching the pool of underrepresented groups who will be participants in such a workforce.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.