EVALUATION OF STRESS-CORROSION RESISTANCE AND CORROSION-FATIGUE FRACTURE-BEHAVIOR OF ULTRA-HIGH-STRENGTH P M AL-ZN-MG ALLOY/

Citation
K. Minoshima et al., EVALUATION OF STRESS-CORROSION RESISTANCE AND CORROSION-FATIGUE FRACTURE-BEHAVIOR OF ULTRA-HIGH-STRENGTH P M AL-ZN-MG ALLOY/, International journal of fatigue, 20(10), 1998, pp. 757-766
Citations number
15
Categorie Soggetti
Material Science","Engineering, Mechanical
ISSN journal
01421123
Volume
20
Issue
10
Year of publication
1998
Pages
757 - 766
Database
ISI
SICI code
0142-1123(1998)20:10<757:EOSRAC>2.0.ZU;2-9
Abstract
Quasi-static tensile tests in air and slow strain rate tests (SSRTs) i n a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al -Zn-Mg alloy fabricated through powder metallurgy. Attention is also p aid to fatigue strength and fatigue crack growth behavior in laborator y air and in a 3.5% NaCl solution. The alloy has extremely high streng th of about 800 MPa. However, elongation at break remains small, at ab out 1.3%. The final fracture occurs by a macroscopically flat crack no rmal to the tensile axis, with little reduction in area and little she ar lip on the periphery of a smooth sample. However, it fails microsco pically in a ductile manner, with dimples. Dimple size is less than 1 mu m, because the grain size of the alloy is extremely small. Strength ening mechanisms operating in the alloy are: small grains, sufficient metastable eta' phase in a matrix, and intermetallic compound acting a s a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution dec reases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sampl e surface acts as a stress concentrator. Crack initiation site of quas i-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improveme nt of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al-Zn-Mg alloys fabricated by ingot metal lurgy, because the fatigue fracture toughness, or ductility, of the al loy is inferior to other Al alloys, and intergranular cracking promote s crack growth. However, no influence of 3.5% NaCl solution on corrosi on fatigue crack growth is observed, although an investigation is requ ired into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are dis cussed. (C) 1998. Published by Elsevier Science Ltd. All rights reserv ed.