High entropy effects from variable chemical order in multi-principal element solution alloys

Johns Hopkins University, Baltimore MD

Investigators

Abstract

Non-technical Abstract: This project advances the field by answering a fundamental materials science question, the structure (in this case, the local chemical order, LCO)-property relationship in the emerging high entropy alloys (HEAs, where multiple elements are mixed in equal amounts and the resulting alloy retains a single atomic structure). The focus is on the signature 'high entropy effect' that sets these HEAs apart from traditional solid solutions. The research highlights the wide variety of local chemical order on the atomic-to-nanometer level and therefore 'plenty of room at the bottom' for rich property possibilities. It will also deliver a powerful toolset (realistic interaction potentials) for the atomistic modeling community working on HEAs to gain atomistic insight into the underlying deformation mechanisms. This work also offers niche benefits for society and education. First, from the application standpoint, the LCOs in HEAs open a vast playground to tailored properties in solution alloys. For example, a random solution is amenable to extensive shaping at room temperature. After shaping, the part can be heated to an intermediate temperature to develop LCO over an extended period of time, gaining strength for load-bearing applications. Second, this research will improve the understanding of closely-related concentrated alloys, such as the very popular stainless steels. Third, the new alloys with partial chemical order will enrich the basic courses in materials science, such as Thermodynamics and Phase Transformations, so that future students can practice a real-world example of 'solution' in their hands, one that can be manipulated to go all the way from chemically random to highly ordered, and see for themselves what various enthalpy and entropy terms do to the starting random solution. The findings of new alloys and new properties will be broadly disseminated at international conferences and in first-class journals. Technical Abstract: Configurational entropy has been perceived as the key factor in stabilizing multi-principal element single-phase solid solutions (SS), the so-called "high entropy alloys" (HEAs). However, the role of entropy in phase selection was overrated, as these concentrated alloys have complex chemical interactions, such that the random SS is only metastable except at very high temperatures. The goal of this project is to highlight a hitherto less-noticed, and arguably more important, trait of 'high entropy', and illustrate its impact on the defect behavior and mechanical properties. It will be illustrated that HEAs are not really chemically disordered, but rather have large variability of local chemical order (LCO) of the various species on the lattice sites. This, while precluding the hypothetical extreme of ideal random solution, represents a vast range of possible structural arrangements that can be tailored to change HEA properties, beyond traditional solutions and intermetallics. Molecular dynamics simulations will be conducted with realistic empirical interatomic potentials developed specifically for HEAs, to systematically i) map out the LCOs possible in HEA at a given composition and its strong dependence on the processing temperature; ii) demonstrate that the partial chemical order sensitively and drastically changes generalized planar fault energy, not only in terms of its sample-average, but also its spatial variation; and iii) unveil how the different LCOs influence the behavior of dislocation, in particular its energy landscape and the activation parameters that govern the mechanical strength. 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.