Abstract: In this paper, an approach for characterizing the evolving grain boundary morphology state of Al under fatigue loading conditions is demonstrated. Although grain boundaries have often been correlated with fatigue crack initiation, most past studies on fatigue-induced microstructure variations have been confined to surface or thin-film based characterization, leaving the internal morphological evolution of the material unknown. This study employs a method of Ga-induced liquid metal embrittlement (LME) to preserve the grain boundary (GB) morphology for scanning electron microscopy (SEM) characterization at various points in fatigue life. This enables indepth characterization of the GB structure at different stages of a material’s fatigue life. To demonstrate this new approach, the evolving GB topographical state and the associated dislocation configurations were investigated at different stages of low cycle fatigue life in high purity Al. GB deformation structures such as ledges, extrusions and voids were observed with SEM. Dislocation structures were characterized by transmission electron microscopy (TEM). It was found that the formation of ledges, extrusions and dislocation cells occur at approximately the same time and that their formation is sensitive to the stress amplitude, with GB ledges and triple junction extrusions forming after the first few cycles when loaded at high stress amplitudes.
Authors: Xueqiao Wang, Wade Lanning, Christopher Muhlstein, and Josh Kacher
Keywords: fatigue, liquid metal embrittlement, dislocation cells, grain boundary ledges, extrusions