Compositional Dependence of Deformation Mechanisms in Concentrated FCC Solid Solutions

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Non-Technical SummaryDuring the past millennia, humans have learned how to make strong materials by mixing more than one metal, i.e alloying. The traditional method has involved adding small amounts of alloying elements to a base metal. This process led to creation of the most famous metal alloy, steel, which is responsible for the industrial revolution and consequently a giant leap forward in technological advances of the human civilization. Despite great progress in making advanced steels as well as other types of more modern alloys, such as Ni base alloys for high temperature applications and Al and Mg alloys for light weighting, there still remain numerous fundamental questions regarding the underlying mechanisms through which these materials respond to loading. The answer to this question is the most important consideration when designing load bearing structures, from buildings and bridges to cars and airplanes. Moreover, with the modern technological advances, there is an urgent need to design new materials, capable of enduring complicated and often contradictory conditions, such as strength, formability and exposure to extreme situations such as high temperature or corrosive environments. On the other hand, recent progress in metallurgy has opened up the possibility of creating multicomponent metal alloys with much more complex compositions than conventional alloys. This program seeks to establish a rigorous relationship between alloy composition, the way it is processed, and how it responds to loading, thereby providing a physics-based predictive path to design new alloys for tailored structural properties. The program combines advanced modeling and experimental approaches. Interesting compositions for study will be identified using cutting-edge computational techniques. Experiments are designed to create predicted alloy compositions using efficient methods to rapidly explore a wide range of compositions, and characterization of internal material defects induced during deformation. In addition, this program will provide an opportunity to engage and train a diverse population of students from high school, to undergraduate and graduate levels, through hands on projects utilizing state of the art experimental and computational techniques to contribute towards educating the next generation of STEM workforce.Technical SummaryNovel, high-throughput computations and experiments are proposed over a wide range of compositions to test the hypothesis that deformation mechanisms and consequent properties in multicomponent fcc-based alloys can be favorably tuned by composition. This proposal seeks to develop a comprehensive understanding of deformation mechanisms in a wide range of fcc solid solutions, including deviation from equiatomic compositions, and specifically to explore compositions for which twinning and hcp martensite effects may contribute to extraordinary strain hardening and ultimate strength potential. Thus, the research objectives are to: (1) employ novel and efficient computational and combinatorial experimental approaches for creating desirable alloys; (2) understand the effect of stacking fault energy and relative fcc/hcp stability on the competing deformation mechanisms; (3) determine the evolution of deformation substructure with strain and the connection to remarkable strain hardening; and (4) characterize and model the deformation mechanisms operative at elevated temperature in association with anomalous hardening and dynamic strain aging. An integrated computational/experimental framework will be applied to a broad range of CrCoNi ternary alloys, and to alloys beyond this ternary system, in order to make a direct, quantitative connection between the chemistry of fcc-based solid solutions, and active deformation mechanisms and mechanical behavior. A new Monte Carlo method based on density functional theory (DFT) recently developed by PI Ghazisaeidi will be employed to predict phase stability and segregation behavior. Guided by these computations, PI Mills will conduct high-throughput experiments in order to reveal the complex relationships between composition, stacking fault energy, phase stability, and deformation mechanisms. This proposal will challenge present measurements in low stacking fault energy alloys through in situ mechanical experiments which will enable improved correlation between experiment and DFT calculation. Directly linking composition to phase stability and deformation mechanisms is necessary, as evidenced by the fact that TWIP steels with similar or lower stacking fault energies (when compared with CrCoNi) do not exhibit deformation-induced martensite (as does CrCoNi). Study of these complex composition effects will be extended to higher temperatures where evidence for strong solute interaction is observed in mechanical response, such as anomalous hardening and serrated flow, but the chemical species and deformation mechanisms associated with these interactions have yet to be identified. The proposed research will be extensible to other multicomponent alloys, including many important commercial alloys such as IN825, MP35, and Alloy 28 that share compositional commonality with the recently emerging fcc-based high entropy alloys, but presently lack detailed deformation mechanism understanding.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.