Original paper(Vol.45 No.5 pp.537)
On tensile properties and SCC resistance of two Al-Zn-Mg-Cu system alloys having fully recrystallized fine-grained and un-recrystallized sub-grained structures
Matsuoka Hideaki; Hirose Yukio; Kishi Yoichi; Higashi Kenji
Abstract:Two types of Al-Zn-Mg-Cu system alloys with different microstructures, i.e., a fully recrystallized fine -grain and an un-recrystallized sub-grain, were prepared to investigate their tensile properties and stress corrosion cracking, SCC, resistance. The former is the superplastic material (SP), and the latter is a typical one of the commercial materials (CM). The effects of grain shapes and orientation on SCC resistance were discussed on the basis of the SCC test and fractography. The main results obtained in the present work are as follows:
(1) When the orientations, L and T, of specimens were in agreement to rolling direction, the values of mechanical Young's modulus, tensile strength and 0.2% proof stress of both SP and CM materials were nearly equal. However, the elongation of CM material was higher than SP one.
(2) The nucleation time and life of SCC on L-and T-directions of CM material were longer than those of SP one. SCC resistance in the CM material was superior to that in the SP material.
(3) Anisotropy of specimen orientation for SCC life was not observed in the SP material. In the CM material, on the other hand, SCC life on L-direction was longer than that of T-direction. The anisotropy on SCC life observed in the CM material seemed to be related to an easy separation of interfaces between the matrix and the inclusions having Fe and Si elements existing continuously along the grain boundaries by hot rolling.
Key Words:stress corrosion cracking, hydrogen embrittlement, grain boundary, crack nucleation, SCC life, grain size, tensile property