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 longi tudinal-transverse (L-T) and short-longitudinal (S-L) orientations for T76- type heat treatments. Extensive fractographic analysis was carried out to i dentify the failure mechanisms, including simultaneous scanning electron mi croscope (SEM) observation of fracture surfaces and underlying microstructu res. A slow quench rate was strongly detrimental because it modified the do minant failure mode from a relatively high energy primary void growth mecha nism to lower energy transgranular shear and grain boundary ductile failure in the L-T and S-L orientations, respectively. Low energy failure was asso ciated with coarse eta precipitation during the quench in both L-T and S-L orientation tests, with intragranular and intersubgranular particles contri buting to L-T quench sensitivity, and intergranular particles contributing to S-L sensitivity. Partial recrystallization was generally detrimental, wi th recrystallized grains being shown to be a preferential crack path. The c ommonly supposed susceptibility of recrystallized grains to intergranular f ailure did not explain this behavior, particularly in fast quench materials , as recrystallized grains primarily failed by transgranular void growth fr om the large intermetallics with which they were intrinsically associated. Exceptional S-L orientation quench sensitivity was observed in unrecrystall ized material and attributed to a synergistic interaction between heterogen eous boundary precipitation and the specific location of coarse intermetall ics along grain boundaries in the unrecrystallized condition. Quantitative assessment of individual contributions to overall fracture resistance is di scussed for cases where multiple failure mechanisms occur, highlighting the importance of interacting and noninteracting mechanisms.