Effects of precipitate morphology on the notch sensitivity of ductile fracture in heat-treatable aluminum alloys

Precipitate morphology evolution (PME) has been achieved in an Al–Mg–Si alloy by using two-step aging treatment. The PME from spherical gradually to rod-/needle-like allows us to track the mechanical properties to the precipitate morphology. For comparison, precipitate content evolution (PCE) was also developed in this alloy using one-step aging treatment, where the shape of precipitates was kept almost the same of needle-like. Systematic experiments have been carried out to compare the effects of PME and PCE on the notch sensitivity of the ductile fracture. It is clearly shown that the alloys with PME exhibit ductility and fracture toughness much sensitive to the triaxial stress than those with PCE. Interesting phenomena have been observed that the PME causes ductility and fracture toughness to change in an inverse way, i.e., fracture toughness increasing with decreasing ductility. While the PCE causes ductility and fracture toughness to vary in the same way. The difference in notch sensitivity of ductile fracture between PME- and PCE-containing alloys has been rationalized by considering a competition effect between precipitate–dislocation interaction and precipitate–matrix deformation discrepancy as the dominant strain localization mechanism, which, as revealed from quantitative assessments, is modulated by the precipitate morphology.