MSE Seminar: Tailoring Damage Pathways via Interfacial Control
Wednesday, March 30, 2022
2110 CHE & via Zoom
Speaker: Gianna Valentino, Ph.D., Postdoctoral Fellow/Materials Scientist, John Hopkins Applied Physics Lab
Title: Tailoring Damage Pathways via Interfacial Control
Material selection is the cornerstone of all engineering applications. In a perfect world, materials would not suffer from the classical tradeoff between competing properties, but would instead possess a synergistic balance for optimal material performance. For example, the ultimate structural material would have both high strength and ductility, but the majority of conventional materials fall short of this. Metals engineered with particular interfaces (nanotwins) have shown simultaneous ultrahigh strength and ductility, but are limited to a relatively small subset of materials that have low stacking fault energy such as Ag, Cu, and stainless steels.
This seminar will discuss results from the synthesis of nanotwinned nickel-molybdenum tungsten (Ni-Mo-W) alloys that possess ultrahigh strength, highly anisotropic plasticity, low electrical resistivity, and low thermal expansion that stem from the finely spaced growth twins that form during the deposition process. Combinatorial techniques were employed to sputter deposit a compositional spread of NissMoxW1s-x, alloys and to investigate their physical and mechanical properties as a function of alloy chemistry. The addition of Mo and W was shown to significantly decrease the coefficient of thermal expansion and provide a route for tailoring the coefficient of thermal expansion and its temperature dependence with compositional control. Microscale mechanical testing elucidated the ultrahigh tensile and compressive strengths of Ni Mo-W alloys, underpinned by the presence of highly-aligned nanotwins and their effectiveness as obstacles to dislocation motion.
Taken as a whole, this study highlights the balance of physical, thermal, and mechanical properties for Ni-Mo-W, driven by nanoscale twin formation. However, the widespread use of nanotwinned metals is greatly limited by their small form factors. Additive manufacturing (AM) exposes components to far-from-equilibrium cooling rates and provides a great opportunity to introduce nanotwins into materials. Although still in its infancy, a study to develop AM nanotwins in low-stacking fault energy metals is ongoing and the results will be discussed, along with a spotlight of other novel AM materials, such as shape memory alloys.
Dr. Gianna Valentino is a materials scientist with a background in mechanics of materials and metallurgy. Her research has focused on fabricating structural materials with high strength, ductility, and thermal stability via far-from-equilibrium processing techniques and understanding their deformation mechanisms for use in extreme environment applications. She has led numerous research efforts in the development of novel metallic systems, including nanotwinned metals, refractories, and shape memory alloys. Gianna received her B.S. in Physics from Saint Joseph's University in 2014, and her M.S. and Ph.D. in Mechanical Engineering from Johns Hopkins University in 2016 and 2019, respectively. She joined the Johns Hopkins Applied Physics Laboratory in 2020 as a postdoctoral fellow and holds a visiting scientist position with Johns Hopkins University.