Effect of retrogression temperature on the fracture toughness of 7136 aluminum alloy

7136 aluminum alloy was treated by retrogression and re-aging (RRA) at different retrogression temperatures in this study. The effect of retrogression temperature on the fracture toughness was investigated and the influence mechanism was discussed in detail. When the retrogression temperature is 175 °C, the K_IC value of 7136 aluminum alloy reaches a peak of 47.3 MPa m^1/2, increasing 59.2% compared to 171 °C. The fracture of the RRA-treated 7136 aluminum alloy is a combination of trans-granular crack and inter-granular crack, while the width of fatigue strips and the volume fraction of dimples are different. Al₇Cu₂Fe particles distributing along the grain boundary reduce the fracture toughness by weakening the interface bonding force and cracking themselves. TEM results show that retrogression temperature affects the fracture mode by changing the strength difference between the grain boundary and the grain. The grain boundary with a complex dislocation structure makes the dislocation movement more difficult, which is benefit to the fracture toughness of 7136 aluminum alloy.