In the present study, the tensile and fatigue properties of extruded 7075 Al alloys subjected to re-solution treatment and then T6, T73 and retrogression-reaging (RRA) tempers were reassessed based upon the microstructural analyses by means of electron backscattering diffraction (EBSD) and X-ray diffraction (XRD). The microstructural analyses indicated that fibrous grains having orientations close to <111> and <001> were preferentially aligned in the extrusion direction and that re-solution treatment increased the fraction of <111> grains. Further the as-received T6 specimens had very high dislocation density as well as fine subgrains, while the re-solution treatment decreased dislocation density considerably and increased grain size. These characteristics explained tensile properties well, by taking into account the effect of precipitates formed by tempers. On the other hand, fatigue strength decreased prominently in all of the specimens, as the relative humidity (RH) was increased from 25% to 85%. T73 and RRA treatments which improve the resistance to stress corrosion cracking (SCC) in static loading were not effective in the humidity-enhanced deterioration in fatigue strength.

In order to investigate the growth mechanism of a shear mode fatigue crack in an extruded bar of an age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens of the alloy in environments of controlled relative humidity of 25%, 50%, 75% and 85%, distilled water and nitrogen gas. Fatigue strength was decreased by high humidity. The growth mechanism of a fatigue crack was different depending on environment and stress level. Although most of fractures occurred by the growth of a tensile mode crack, a crack propagated in a shear mode accompanying with slip planes and voids at high stress levels in high humidity and at all stress levels tested in nitrogen gas. The shear mode crack was related to the marked texture of the alloy. Growth rates of the shear mode crack were higher in high humidity than in nitrogen gas. Growth mechanisms of the shear mode crack were different between environments in nitrogen gas and in high humidity. That is, the reason for the growth of a shear mode crack in nitrogen gas was the suppression to growth of a tensile mode crack by reversible slip due to absence of oxide film. On the other hand, in high humidity, the growth of a shear mode crack was promoted by the formation and coalescence of voids, suggesting that the acceleration of the growth rate of a crack may be assisted by hydrogen accumulated around precipitated particles on glide planes.

In order to investigate the effect of humidity on fatigue strength of an extruded and age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens in environments of controlled relative humidity of 25%, 50%, 75% and 85% and distilled water. The cross section of the alloy has a marked texture of (111) plane. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60% -70% and fatigue strength was largely decreased over the humidity. Both of initiation and propagation of a crack were accelerated by high humidity. In high humidity, a crack propagated in a shear mode macroscopically and it was ductile in company with many glide planes and voids microscopically. That is, the propagation was not a tensile mode with brittle facets even in water. The shear mode propagation inclined about 35° to the extruded direction and fracture surface was (100) plane, meaning that the shear mode propagation of a crack was mainly caused by the marked texture of the alloy. The propagation mode of a crack was affected by not only environment but also stress level.

In order to investigate fatigue properties of 18%Ni maraging steel at elevated temperature, rotating bending fatigue tests were conducted for plain and drilled specimens at room temperature (RT) and 673K in air. The specimens involved single-aged ones under an under-aging condition at 753K and double-aged ones in which the second aging was performed at 673K to the single-aged ones. Fatigue strength of the single-aged plain specimens was higher at 673K than at RT, though the static strength was inversely decreased at the elevated temperature. On the other hand, the single-aged drilled specimens exhibited nearly the same fatigue strength at both temperatures. It was found that hardness measured at RT increased with time in the single-aged plain specimens fatigued at 673K, which also appeared in the static aging at 673K. This hardness increase was considered to be attributed to precipitation hardening which might occur due to the existence of excess Mo solute atoms. On the other hand, crack observations revealed that crack initiation in the plain specimens was retarded markedly at 673K in comparison with that at RT, whereas crack propagation rate did not differ significantly between these temperatures and between the plain and drilled specimens. The marked delay of crack initiation in the plain specimens at 673K arose from the oxidation of their surfaces. It was also observed that the double-aging led to a large increase in fatigue strength at RT, but resulted in a slight increase at 673K. Based on these results, it was shown that the main reason of the increase in fatigue strength at 673K in the plain specimens was the suppression of crack initiation due to oxidation, while the increase in hardness observed at this temperature played a minor role.

Ultrasonic and rotating bending fatigue tests were carried out using plain specimens and specimens with a small blind hole for an extruded and age-hardened Al alloy 7075-T6 in different environments in order to investigate the effect of humidity on fatigue strength and fracture mechanism. Fatigue strength was decreased by high humidity under both tests. The effect of humidity on fatigue strength was larger in ultrasonic fatigue. The humidity affected both of crack initiation and propagation processes. Crack propagated in tensile mode then changed to shear mode macroscopically in all environments under ultrasonic fatigue, though it was only in tensile mode under rotating bending fatigue. These differences in fracture mechanism related to the difference in environmental effect on fatigue strength in both tests.