The secret to how steel hardens and shape-memory alloys snap into place lies in rapid, atomic-scale shifts that scientists have long struggled to observe in materials. Now, researchers at Cornell University are shedding light on these transformations, particle by particle, using advanced modeling techniques.
Through custom-built computer simulations, Julia Dshemuchadse, assistant professor of materials science and engineering at Cornell, and Hillary Pan, Ph.D. ’25, have visualized solid-solid phase transitions in unprecedented detail. Their work captures the motion of every particle in a theoretical material as its crystal structure morphs into another.
Their findings, published on July 23 in the Proceedings of the National Academy of Sciences, reveal not only classical transformation mechanisms but also entirely new ones, reshaping the scientific understanding of this fundamental process in materials science.
Unprecedented Detail in Crystal Transformations
“Most prior research either reports on the before and after stages of the transformations or discusses them from a theoretical perspective,” Dshemuchadse explained. “Our computational study is the first to fill the gap between these two more traditional approaches. We simulate the transformations directly, and we can track particle by particle how one structure forms from the other.”
The researchers focused on transformations between two of the most common crystal structures: face-centered cubic and body-centered cubic sphere packings. These structures are prevalent across a wide range of materials, from soft-matter systems like plastics to hard metals like iron and steel, where such transformations play a key role in industrial processes like metallic hardening.
“There’s no camera fast enough to capture the resolution you need to know what exactly is happening in between,” Pan remarked, “and X-ray diffraction techniques provide limited information about how the transformation is actually proceeding.”
Advanced Simulations and New Discoveries
Starting with small simulations of about 4,000 particles and then scaling up to more than 100,000 particles, the researchers designed the models to explore general transformation behavior using abstract, tunable particles. This approach allowed them to characterize multiple transformation pathways, including three well-known mechanisms proposed for atomic systems: the Bain, Kurdjumov-Sachs, and Nishiyama-Wassermann orientation relationships.
The simulations revealed systems in which the material’s microstructure and temperature dictate the transformation pathway taken, and they discovered a stable intermediate phase on the path from body-centered cubic to face-centered cubic.
One of the study’s most surprising discoveries was a completely new way the transformation could happen: Particles in the material shifted together in a coordinated, multiunit shearing motion that had not been predicted or seen before.
“Importantly, the study shows that the pathways taken are not clearly determined by comparing before and after configurations of the material,” Dshemuchadse noted. “This suggests that researchers have rightfully struggled with classifying these transformations when unable to observe them in action.”
Implications for Future Research and Industry
The pathways are linked to the shape of the underlying particle interactions, offering new insights that could help experimentalists interpret data from material systems by providing simulated templates for transformations that remain invisible in real time.
“It’s possible lab experiments could be designed to tune particle interactions in order to replicate the different transition pathways we’re seeing,” Pan suggested, adding that previous studies have hinted that hydrodynamics can play a role in pathway selection for soft materials.
The research was supported by the National Science Foundation and the Camille and Henry Dreyfus Foundation, highlighting the importance of collaborative efforts in advancing scientific understanding.
This development represents a significant leap forward in materials science, potentially influencing how industries approach the design and manufacturing of materials. As the understanding of these transformations deepens, the potential applications in fields ranging from aerospace to biomedical engineering could be vast.
Syl Kacapyr is the associate director of marketing and communications for Cornell Engineering.
