Research thrust

Broadly, the Shahani group employs synchrotron- and laboratory-based methods, including X-ray tomography and diffraction, to peer into the solidification and processing pathways of metallic alloys in real-time.  Such studies would not be possible without the great strides in data sampling and reconstruction, computer hardware and storage, and algorithms for excavating the Big Data in a massively parallel environment.  It is anticipated that our in situ and multimodal approach will provide a fresh lens for solving age-old conundrums in the field of physical metallurgy.  Below, we outline our particular interests. 

Growth of nature's forbidden crystals

Quasicrystals are sometimes called "nature's forbidden crystals" because their structure is ordered but aperiodic.  This means that quasicrystalline patterns (e.g., Penrose tiling shown at left) fill all available space, but in such a way that the pattern of its atomic arrangement never repeats.  Perhaps unsurprisingly, the nucleation and growth of quasicrystals from a liquid is a topic that is as controversial as their inherent crystallography: multiple growth models have been proposed, nearly all of which lack experimental verification.  The central issue in the field has been to understand whether quasicrystals are energetically stabilized or entropically stabilized via defects. 

We have much to learn in this realm.  For instance, is the growth of quasicrystals in some ways analogous to the growth of other faceted periodic crystals?  If so, to what extent can classical theories of crystal growth be applied to the study of quasicrystal growth?  Using synchrotron-based live imaging, we have the unique opportunity to watch the growth of quasicrystals from a liquid.  The three-dimensional reconstructions will help us identify whether structural defects (e.g., dislocations and phasons) stabilize the quasicrystalline lattice, and how they might influence the morphology of the solid-liquid interfaces during free growth and impingement.  Recently, we have harnessed real-time X-ray imaging to capture the equilibrium and growth shapes of an icosahedral quasicrystal; the growth and dissolution kinetics of a decagonal quasicrystal; as well as the growth of a periodic (approximant) phase that shares structural motifs with a decagonal quasicrystal phase. We have also captured through dynamic transmission electron microscopy the highly unusual, dendritic growth of icosahedral quasicrystals born from an approximant matrix. This research is funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award No. DE-SC0019118.

Growth of eutectic patterns

In certain alloy systems, a liquid of a fixed composition freezes to form a mixture of two different solid phases.  Such a mixture is commonly known as a eutectic.  In order to tune the eutectic patterns to technological demands, we are exploring the crystallization of eutectic alloys.  The morphology of the eutectic phases depends on the driving force for growth, the anisotropy in interfacial energy and interfacial mobility, as well as the chemical environment of the parent liquid phase. In the latter case, it is well known that trace impurities (e.g., Na) drastically change the morphology of the eutectic phases (e.g., Si), from a coarse to fine structure.  Recently, we have demonstrated (see left) that the metallic addition also leads to topological transitions in the eutectic microstructure during solidification.  Although the effects of eutectic modification have been well documented (see our review on the topic), there is no accepted explanation for the mechanism by which the microstructure changes so much upon adding trace amounts of a metal species.  Thus, our goal is to provide a unified picture of eutectic growth and chemical modification. Our preliminary efforts indicate that steady-state crystallization is impossible in chemically-modified alloys since the trace metal species “poisons” the available growth sites of the Si phase.  This research is funded by the National Science Foundation Faculty Early Career Development Program (CAREER) under Award No. 1847855.

We are also interested in understanding the origin and propagation of spiral eutectics during directional solidification.  This unique morphology has been reported in organic, metallic, and semi-metallic alloy systems.  Even so, it is unclear to-date what thermophysical conditions are required for spiral formation.  For instance, what are the interrelationships between the thermal gradient, interface velocity, lamellar spacing, number of spiraling arms, and the period of rotation of the spiral?  Why should the two phases wrap around each other to form a spiral? We are working to answer these open questions through laboratory and synchrotron-based X-ray nanotomography, which will enable us to visualize the relatively fine lamellar spacings (10-100 nm) of eutectic spirals (see, e.g., image at left). Thus far, our research has demonstrated that eutectic growth proceeds in a non-classical pathway, wherein crystallographic defects (screw dislocations) provide a template for spiral self-organization. We have also examined the stability of the eutectic spiral against doping and annealing, discovering in both cases a breakdown of the spiral structure.   This research was funded by the Air Force Office of Scientific Research Young Investigator Program (YIP) under Award No. FA9550-18-1-0044.

Growth of abnormal grains in polycrystals

Most engineering materials are commonly found in polycrystalline form.  The sizes, orientations, and boundaries of the grains influence many of the solid’s properties.  During thermal processing, normal grain growth occurs, wherein some grains enlarge while others shrink and disappear. A second possibility, termed abnormal grain growth, is that a very few grains grow at a much faster rate than the others and eventually consume the sample volume.  Abnormal grain growth has been observed in a vast array of metallic and ceramic systems, yet its origins have remained an enigma for at least the past seventy years.  In some cases, abnormal growth leads to a single crystal that is totally devoid of grain boundaries.  Interestingly, a vast majority of these alloys often contain second-phase particles; thus, an open question concerns the role of the particles on the ensuing abnormal grain growth.

To tackle the problem of abnormal grain growth, we employ a novel multimodal imaging platform that has been recently developed by our team. In our setup, laboratory-based diffraction tomography together with absorption tomography will allow for the direct interrogation of both grain boundaries and second-phase particles, respectively, as grain growth proceeds, under isothermal conditions.  An example of a typical grain map outputted from laboratory X-ray diffraction tomography is shown at left. Using this hybrid approach, we found that the incipient grain size is set by the local particle density; subsequently, abnormally large grains may “run away” from the grain size distribution. Our current focus is on depicting these results on an isothermal transformation diagram, thus generalizing our work over temperature-time space.  This research is funded by the Army Research Office Young Investigator Program (YIP) under Award No. W911NF-18-1-0162.

It is worth mentioning that abnormal grain growth may also occur in response to phase transformations, e.g., during or after the dissolution of the second-phase particles upon heating (i.e., non-isothermal annealing).  In this case, we must contend with a complex interplay between many physical effects (e.g., strain energy and particle pinning pressures). We are actively exploring the origins of abnormal grain growth under such dynamic annealing conditions through synchrotron-based high energy diffraction microscopy. This research is funded by the National Science Foundation under Award No. 2003719.