CAREER: Heterogeneous Nucleation in Reversible Martensitic Transformations
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Non-Technical Summary:Some solids undergo reversible phase transformations, which involve the rearrangement of atoms and the distortion of bonds, resulting in changes to the shape and properties of a material. This behavior can be used to develop compact and efficient actuators, sensors, and a variety of microelectronic devices. Structural transformations start near a defect or imperfection in a material, which facilitates the rearrangement of atoms. However, when considering transformations in small volumes of material, the probability of encountering such a rare defect is low, resulting in unpredictable and inefficient operation. In extreme cases, this can stop the transformation from occurring entirely. This project''s aims are to understand the role that defects play in initiating structural transformations, and to find an approach to introduce such defects into a material. This would allow the operation of devices at micrometer and nanometer length-scales. The broader objectives of this proposal include developing distributable interactive materials science experiments (''lab in a box'') that can be performed remotely to engage distance students. Additionally, this proposal will engage undergraduate (UG) students, and in particular transfer students from community colleges, through targeted UG research opportunities and through societally impactful engineering design challenges related to the proposed research.Technical Summary:Diffusionless solid-state phase transformations generally nucleate from sparse, high energy atomic-scale defects in the crystal lattice. One of the grand challenges in materials science is understanding how material heterogeneity and disorder can be harnessed to influence the properties of materials. In particular, while dislocation defect models of heterogeneous nucleation in martensitic transformations (MTs) are fairly well-developed, the role of clusters of point defects is very poorly understood, despite initial observations that irradiation induced defect clusters can serve as potent nucleation sites. The outcomes of this study will advance the science of nucleation of MTs 1) by measuring the number density of native nucleation sites as a function of thermodynamic driving force, and determining the role of lattice compatibility in governing size-dependent nucleation behavior, and 2) by experimentally quantifying the potency of irradiation-induced defect clusters and He nanobubbles. These observations will be used to develop and test a defect-cluster based MT nucleation model. The educational outcomes of this proposal will impact two paradigm shifts currently underway in UG engineering education: 1) broader adoption of active learning methods, and 2) effective and scalable approaches to distance education.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.