Exceptional hardness in multiprincipal element alloys via hierarchical oxygen heterogeneities

David C. Beaudry, Michael J. Waters, Gianna M. Valentino, Daniel L. Foley, Elaf Anber, Yevgeny Rakita, Charlie J. Brandenburg, Jean Philippe Couzinié, Loïc Perrière, Toshihiro Aoki, Keith E. Knipling, Patrick G. Callahan, Benjamin W.Y. Redemann, Tyrel M. McQueen, Elizabeth J. Opila, James M. Rondinelli, Mitra L. Taheri