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
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.