Quantitative characterization of gradient microstructures: A study on friction stir spot processing of pure cobalt

Journal Article · Thu Feb 12 00:00:00 UTC 2026 · Acta Materialia

DOI: https://doi.org/10.1016/j.actamat.2026.122018 · OSTI ID:3020307

Brown, Nathan  ^[1]; McBride, Michael  ^[2]; Hooks, Daniel E.  ^[2]; Jensen, Brian J.  ^[1]; Field, David P.  ^[1]

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1. Washington State University, Pullman, WA (United States)
2. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Heterogeneous microstructures in polycrystalline metals can enhance the strength and ductility, outperforming homogeneous structures of similar composition. This study investigates deformed cobalt via friction stir spot processing with varying dwell times to uncover the effects of plastic deformation and heat generation on the formation of morphological, phase, and grain boundary character gradients. A new approach to quantify the morphological gradients in materials, which describes grain morphology in terms of density followed by parametric regression, enables direct quantification of processing depth and gradient sharpness. Results show that longer processing times increase the steepness of morphological gradients and reduce the deformation depth for friction stir spot processing with low plunge depths and high tool rotational speeds. The amount of retained FCC is increased in the shorter processing conditions, primarily due to refined grain size, increased defect content, and reduced heat generation. Crystallographic texture analysis of the HCP phase indicated a dominant B-fiber described by (0001) ∥ shear plane normal in the extreme processing conditions and the formation of a P-fiber, shear direction ∥ ⟨11$$\bar2$$0⟩ for intermediate dwell times. The texture of the FCC phase for low processing times was a C texture {100}⟨011⟩ where longer processing times were dominated by a [001] fiber texture with a main {110}⟨100⟩ orientation and emergence of a slight [111] fiber in the longest processing condition. The approaches outlined in this work give insight into quantifying gradients and improve the understanding of highly deformed cobalt.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); Washington State University, Pullman, WA (United States)
• Sponsoring Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States); Los Alamos National Laboratory and Triad National Security, LLC,; USDOE National Nuclear Security Administration (NNSA); USDOE National Nuclear Security Administration (NNSA). Office of Defense Programs (DP) (NA-10)
• Grant/Contract Number: 89233218CNA000001; NA0004229
• OSTI ID: 3020307
• Report Number(s): LA-UR-25-25975
• Journal Information: Acta Materialia, Journal Name: Acta Materialia Vol. 308; ISSN 1359-6454
• Publisher: Elsevier BVCopyright Statement
• Country of Publication: United States
• Language: English