In-situ transmission electron microscopy of microstructure formation during laser irradiation induced irreversible transformations in metals and alloys

University Of Pittsburgh, Pittsburgh PA

Investigators

Abstract

Non-Technical Abstract This activity develops scientific understanding of the formation of microstructures in multi-component metallic materials during solidification under far-from-equilibrium conditions. Binary and ternary model alloys will be studied using unique electron microscopy in conjunction with characterization of the microstructures. Direct correlation of local features in the solidification microstructure with the conditions of their formation will deliver experimental data sets unobtainable with other approaches that are suitable for validation of predictions from competing theoretical models of alloy rapid solidification. The research enhances scientific understanding of microstructure formation during solidification in complex alloys and contributes to the development of techniques for nano-scale resolved studies of materials. Understanding of microstructure formation during processing of engineering materials is a fundamental challenge of the field of materials science and engineering (MSE). It enables identification of strategies for property tailoring for optimal material performance in a technological application. Solidification is ubiquitous in fabrication of metallic materials, which are particularly critical to energy generation and transmission, advanced transportation, biomedical and information technologies. Research results will be emanated by journal publication and presentations at conferences. Integration with instructional resources and outreach module development will enhance the MSE undergraduate curriculum. Modules for targeted outreach will be developed collaboratively with student teams and sustain improved MSE outreach efforts at the University of Pittsburgh. Future members of the US science, technology, engineering and mathematics workforce will receive research training, advanced science and engineering education, leadership and mentoring opportunity. Collaboration with the Lawrence Livermore National Laboratory team provides synergy for project resources and multi-faceted professional preparation outside of academia. The activity will positively impact engineering education, promote lifelong learning, broaden participation of underrepresented groups in research and advance the scientific knowledge of transformations in metallic alloys under technologically relevant non-equilibrium conditions. Technical Abstract Solidification is a ubiquitous and fundamental process in materials fabrication. Under extreme conditions arising in rapid solidification processing the migration of solid/liquid interfaces is driven far-away from equilibrium and kinetic factors can become dominant over thermodynamic factors in determining the final microstructure. We use the movie-mode dynamic transmission electron microscope (MM-DTEM) for nano-scale spatio-temporal resolution in situ imaging observations and diffraction measurements of rapid solidification transformations in alloy thin films. Complementing the in situ studies with quantitative post-mortem micro-characterization delivers direct correlation of local features in the solidification microstructure with the conditions of their formation during the irreversible transformation under-far-from-equilibrium rapid solidification. Focusing on concentrated binary Al-Cu and Al-Ag alloys, and for ternary Al-Cu-Ag alloys the research will deliver accurate global and locally resolved information on the transformation interface, including average and local velocity, changes in these velocities, and the morphology associated with changes in crystal growth modes. Post-mortem analyses of solidification microstructures provide compositional gradients, crystal structures, as well as the local arrangements, size, shape and composition of the constituent phases, which may differ from those that would form at or near equilibrium conditions. The use of thin film alloy specimens enables study of rapid solidification transformation microstructure formation for unexplored regimes of composition (e.g. hypereutectics in Al-Cu) and very large transformation rate, suitable to elucidate details of transitions to banded morphology and partitionless alloy crystal growth for instance. Effects of atomic size misfit, faceting tendencies, chemical ordering and interfacial coherency, as well as Ag addition effects on two-phase solidification microstructure formation will be determined using Al-Cu and Al-Ag alloys Al-Cu-Ag ternaries. The proposed research will deliver unique experimental data sets and insights suitable to evaluate current rapid solidification models and will enhance scientific understanding of microstructure formation in solidification of multi-phase alloy systems.