Dynamic recrystallization during microindentation: A site-specific study of indents using a novel LaserFIB method

Grain-refinement by continuous dynamic recrystallization (cDRX) is generally known to require extreme deformation conditions—high strain/strain rates—like those observed in extreme deformation processes, e.g., microparticle impact and high-energy ball milling. It is not clear whether cDRX steps can be interrupted to circumvent the requirement for such extreme conditions; a phenomenon we term two-step cDRX or interrupted cDRX (icDRX). We hypothesized that simple microindentation of a predeformed metal could result in grain refinement by icDRX mechanism without the need for post-process heat treatment, as conventionally done. Microindention of friction stir processed (FSP) aluminum is conducted to test this hypothesis. To circumvent the limitations involved in the characterization of a large indent size and the problem with conventional focused ion beam (FIB) methods of accessing deep regions of interest such as the apex of an indent, we use a custom FIB lift-out grid and LaserFIB-method to extract a large TEM lamella from microindents for scanning/transmission electron microscopy (S/TEM). The results show an unusually high hardness value for aluminum that is explained by the transformation of prior dislocation cells to bonafide cDRXed ultrafine grains during microindentation to establish an icDRX grain refinement mechanism: prior dislocation cells develop into partitioned elongated subgrains at the upper corner of the indent ; evidence of ultrafine near-equiaxed grains that indicates the onset of cDRX emerges midway down the indent ; and complete grain refinement by icDRX occur at the indent apex . Conclusively, the microstructural observations are also consistent with the deformation map for aluminum, while the hardness/yield strength is reasonably modeled. These findings are particularly interesting for metal additive manufacturing (and allied) processes that develop dislocation cell structures; an additional simple plastic deformation step on these manufactured parts can drive the dislocation cells to completely refined grains by icDRX , thereby improving materials’ mechanical properties.