Microstructural effects on fracture toughness in AA7010 plate

The influence of recrystallization and quench rate after solution treatment on the fracture toughness of 7010 aluminum plate has been studied in longitudinal-transverse (L-T) and short-longitudinal (S-L) orientations for T76-type heat treatments. Extensive fractographic analysis was carried out to identify the failure mechanisms, including simultaneous scanning electron microscope (SEM) observation of fracture surfaces and underlying microstructures. A slow quench rate was strongly detrimental because it modified the dominant failure mode from a relatively high energy primary void growth mechanism to lower energy transgranular shear and grain boundary ductile failure in the L-T and S-L orientations, respectively. Low energy failure was associated with coarse eta precipitation during the quench in both L-T and S-L orientation tests, with intragranular and intersubgranular particles contributing to L-T quench sensitivity, and intergranular particles contributing to S-L sensitivity. Partial recrystallization was generally detrimental, with recrystallized grains being shown to be a preferential crack path. The commonly supposed susceptibility of recrystallized grains to intergranular failure did not explain this behavior, particularly in fast quench materials, as recrystallized grains primarily failed by transgranular void growth from the large intermetallics with which they were intrinsically associated. Exceptional S-L orientation quench sensitivity was observed in unrecrystallized material and attributed to a synergistic interaction between heterogeneous boundary precipitation and the specific location of coarse intermetallics along grain boundaries in the unrecrystallized condition. Quantitative assessment of individual contributions to overall fracture resistance is discussed for cases where multiple failure mechanisms occur, highlighting the importance of interacting and noninteracting mechanisms.