Collaborative Research: Traversals in Transformation Strain Space and Microstructure Design for High Performance Ferroelastic Materials

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NONTECHNICAL SUMMARYThis award supports theoretical and computational research to investigate new materials design concepts enabled by a new theory and computer simulations. The approach will be focused on an important class of smart materials – ferroelastic smart materials including superelastic metals and shape memory alloys (SMAs). Crystals can change their structures in response to an applied field, such as temperature, pressure or stress, electric or magnetic fields. Crystals have the property that sets of operations on a crystal leave the crystal looking the same. For example, 90-degree rotations around specific axes of a cubic crystal rotate atoms into the same positions previously occupied by atoms; the crystal is thus the same under such symmetry operations. Because of this crystal symmetry, changes associated with structural phase transformations can lead to the generation of multiple equivalent structural states. These states are interconnected by multiple equivalent forward and backward phase transformation pathways. These pathways can be represented pictorially as a graph that forms a web dubbed phase transformation graphs (PTGs). How a crystal traverses a PTG dictates all the “live” characteristics of structural phase transformations that underpin the practically important properties of a ferroelastic smart material. PTG analysis offers new opportunities to engineer smarter microstructures, the structure of crystals on scales larger than the atomic scale and able to be seen under modest magnification. Microstructures are connected to properties, particularly mechanical properties of materials. The PIs aim to develop microstructure designs that lead to materials with unprecedented properties. This research project will utilize the PTG "gene networks" in the design algorithms to "breed" new internal microstructures for improving functionality and performance of ferroelastic smart materials. The outcome of this research could benefit numerous advanced technological applications in automotive, aerospace, micro-electromechanical systems, and biomedical implants. The PTG analysis, just like phase diagrams in thermodynamics, is a fundamental tool in smart materials design and it can enrich undergraduate and graduate curricula in materials science and engineering. The intuitive nature of smart materials and their cool applications will help to encourage middle- and high-school students to enter science and engineering disciplines. The new alloy design strategies, PTG analysis and computer simulation techniques will be broadly disseminated at conferences, online tutorials, and in academic journals. TECHNICAL SUMMARYThis award supports theoretical and computational research to investigate new materials design concepts enabled by a new theory and computer simulations. It has yet to be recognized that the properties and performances of smart materials based on diffusionless transformations are dictated not only by the symmetry of the individual crystal structures involved and symmetry-breaking along a single phase transformation pathway (PTP), but also by the topology and symmetry of their phase transformation graphs (PTGs). The latter tells us how the multiple structural states of the parent and product phases are interconnected and what structural states could be visited by the system during multiple transformation cycles. The PIs will explore alloy design ideas and will address scientific issues by using a combination of PTG analysis, ab initio calculations, kinetic Monte Carlo, and phase field simulations. Specific scientific issues that will be addressed include: (a) Quantifying the connected pathways and free-energy barriers of transitions, including the symmetry-dictated non-PTPs that could alter the topology of PTGs and change the fundamental characteristics of the structural transformations and hence the functionality and performance of the smart materials; (b) Seeking answers for the following questions: What is the consequence of a biased random walk on PTG for microstructural evolution and functional fatigue? After dispersal on PTG, is there an effective way to “reset” the dispersed strain states at various spatial locations back to their original state and recover the original microstructure? (c) Making use of proper concentration modulations to regulate martensitic transformations and make linear super-elastic materials with large elastic strain limit, vanishing hysteresis, and ultralow pseudo-elastic modulus; (d) Characterizing the temperature- and rate-dependences of these transformations by predicting their activation strain-volume and pre-dominance of shuffling. Success of the project holds promise to transform ferroelastic materials design.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.