*Article* **E**ff**ect of Natural Aging on the Stress Corrosion Cracking Behavior of A201-T7 Aluminum Alloy**

**Mien-Chung Chen 1, Ming-Che Wen 2, Yang-Chun Chiu 1, Tse-An Pan 1, Yu-Chih Tzeng <sup>3</sup> and Sheng-Long Lee 1,\***


Received: 2 November 2020; Accepted: 6 December 2020; Published: 10 December 2020

**Abstract:** The effect of natural aging on the stress corrosion cracking (SCC) of A201-T7 alloy was investigated by the slow strain rate testing (SSRT), transmission electron microscopy (TEM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), conductivity, and polarization testing. The results indicated that natural aging could significantly improve the resistance of the alloys to SCC. The ductility loss rate of the unaged alloy was 28%, while the rates for the 24 h and 96 h aged alloys were both 5%. The conductivity of the as-quenched alloy was 30.54 (%IACS), and the conductivity of the 24 h and 96 h aged alloys were decreased to 28.85 and 28.65. After T7 tempering, the conductivity of the unaged, 24 h, and 96 h aged alloys were increased to 32.54 (%IACS), 32.52 and 32.45. Besides, the enthalpy change of the 24 h and 96 h aged alloys increased by 36% and 37% compared to the unaged alloy. The clustering of the solute atoms would evidently be enhanced with the increasing time of natural aging. Natural aging after quenching is essential to improve the alloy's resistance to SCC. It might be due to the prevention of the formation of the precipitation free zone (PFZ) after T7 tempering.

**Keywords:** Al-Cu-Mg-Ag alloy; natural aging; stress corrosion cracking; SSRT; PFZ
