Localized grain boundary engineeringGrain boundary engineering is often limited by routes that require bulk plastic deformation or strong segregation chemistries. Here we introduce thermal-elastic processing (TEP), which combines a room‑temperatureTemperature elastic preload with a sub‑solidus dwell to drive micron‑scale interfacial transformations while preserving bulk shape. Using coarse‑grained polycrystalline aluminumAluminum ingot (average grain sizeAverage grain size about 280 μm), a 10 MPa preload followed by a 30 min dwell at 473 K produces boundary serrationSerration/faceting without measurable grain growth. A mechanistic model quantifies stress amplification and dislocation pile‑up adjacent to boundaries, predicting a ~ 1 to 3.5 μm plastically affected zone; Arrhenius grain boundary diffusionDiffusion over the same annealing time gives Ld ≈ 35 μm at 473 K, so diffusionDiffusion is not rate‑limiting. DiffusionDiffusion creep bounds (Coble) predict < 10–7 bulk strain in 30 min for d = 280 μm, consistent with shape retention. TEP offers a general route to local grain‑boundary morphology control without macroscopic plasticity.