Seminar: Jerry Floro, University of Virginia
November 7 @ 11:00 am – 12:00 pm
Eutectoid Disorder/Order Transformations in Co-Pt and Fe-Pd: Microstructures, Magnetism, and a New Old Phase
Abstract
Unusual microstructures can result from eutectoid decomposition in alloy systems where the product phases result from disorder/order transformations. In Au-Cu, Co-Pt, and Fe-Pd, the eutectoid reaction is A1 –> L10 + L12, where L10(2) is an ordered tetragonal (cubic) phase. The coexistence regions for the two ordered phases tend to be narrow, likely resulting from coherency strain. In Au-Cu and Co-Pt, the resulting microstructure is the beautiful nanochessboard, which forms a 2+1-D tiling that is quasi-periodic on the nanoscale. In Co-Pt, both product phases are ferromagnetic, producing exchange-coupled ferromagnetism between the “hard” L10 and “soft” L12 phases. We have explored the magnetic behavior using first-order reversal curve (FORC) analysis, micromagnetic simulations, and Lorentz transmission electron microscopy. As expected, the magnetism can vary from partial to complete exchange coupling, depending on the length scales of the microstructure. Fe-Pd eutectoid transformation process appears quite different, producing nanotwinned, plate microstructures similar to martensite. This system can order to a different tetragonal product phase, L1’, instead of L10. The L1’ phase was first theoretically predicted by William Shockley in 1938, and our work is the first direct observation of L1’ in any bulk alloy. Proving its existence is non-trivial, I will outline the burden of proof. Support by the National Science Foundation under grants DMR-110536 and DMR-170914 is gratefully acknowledged.
Biography
Jerry Floro investigates phase transformations, pattern formation, and process/structure relationships. His work spans metallic and semiconducting materials systems, both bulk and thin film, relevant to ferromagnetism, thermoelectrics, additive manufacturing, and superconducting logic. He earned his Ph.D in Materials Science from MIT in 1992, spent the following 14 years at Sandia National Labs, and joined the University of Virginia in 2006. He is a Manchester United FC supporter, even in these darkest times.
