Effect of non-isothermal creep aging on the microstructure, mechanical properties and stress corrosion cracking resistance of 7075 alloy

The effect of non-isothermal creep aging (NICA) on the microstructure, mechanical properties, and stress corrosion cracking (SCC) resistance of 7075 alloy was investigated. The results showed that the tensile strength of the alloy increased to 565 MPa when the alloy was heated to 210 degrees C (CH210) and reached 580 MPa when it was subsequently cooled to 120 degrees C (CC120). Simultaneously, the SCC susceptibility of rtf increased from 50.8 % to 98.4 %. As compared with traditional creep aging process [1], a large strength increment with excellent stress corrosion resistance have been obtained by NICA. The microstructure revealed that a lot of dislocations have been introduced by creep during the heating stage which could improve the precipitates volume fraction and accelerate the diffusion of solutes; while during the cooling stage, eta ' was greatly refined, and GPI and GPII were re-precipitated from the matrix due to the decreased solid solubility and increased critical radius R*; both of them are responsible for the continuous strength increase during NICA. Moreover, the width of the precipitate free zone (PFZ) was narrowed from 46.1 nm (CH210) to 28.6 nm (CC120). The microchemical analysis reveals that solutes were more homogenously distributed in grain boundary precipitates (GB-ppts), matrix precipitates, the PFZ, and the matrix with the help of creep. The narrower PFZ and homogeneous solute distribution are responsible for improving the SCC susceptibility in the CC120 alloy.