Lattice Distortions in Concentrated Metallic Alloys

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Non-technical SummaryMost metallic alloys are based on a single principal element with other elements added in small amounts. Common structural steel, for example, is mostly iron with less than two percent of other elements present. Recently, complex concentrated alloys in which multiple elements are present in large amounts, with no single majority element, have been the subject of much research interest because they show promising properties. The precise reasons why the properties of these complex alloys are different from those of simpler alloys are the subject of intense debate. One idea is that the atomic-scale structure of complex alloys is heavily distorted by the presence of so many different kinds of atoms, each of different size. This project seeks to test this idea by careful experimental measurement and detailed computer simulations of structural distortions in complex alloys. An improved understanding of the atomic-scale structure of these novel alloys could lead to the development of new materials with superior mechanical and functional properties. A related educational aspect of the project is to validate a new tool for assessing how well students learn concepts related to the structure of materials, which will contribute to the development of better teaching methods in materials science and engineering.Technical SummaryMultiple-principal element alloys (MPEAs) have been the subject of intense research over the past fifteen years due in part to their promise in structural and functional applications. Early on it was suggested that the crystal lattices of these alloys should be highly distorted due to the need to accommodate atoms of various sizes. Although the lattice distortion hypothesis is widely invoked to explain some of the novel properties of high entropy alloys, evidence in support of it is sparse and contradictory. This program uses diffuse x-ray and neutron scattering studies of MPEA single crystals together with total scattering experiments on polycrystals to provide unparalleled insight into the nature of lattice distortions, and structural disorder more generally, in complex concentrated alloys. The experiments are supplemented and cross-validated with density functional theory calculations of structure. By comparing disorder in complex (four- and five-component) alloys with compositionally similar but simpler (two- and three-component) alloys the hypothesis that adding components results in a different degree or type of disorder can be tested. In a complementary set of educational activities, a concept inventory for use in undergraduate courses in Structure of Materials is being completed and validated to show that it measures expert thinking by students. When complete, the validated concept inventory will be disseminated to instructors for use in their courses.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.