| Summary: | 碩士 === 國立中央大學 === 機械工程研究所 === 92 === A program of experimental and analytical tasks has been conducted to discuss the relationship between heat treatment and fatigue crack propa-
gation（FCP）in an aluminum aerospace alloy 7075 and 7050.
The experiment was investigated via different homogenization, solution, aging condition and load ratio. The influence of fatigue crack pro-
pagation was characterized and analyzed by using optical microscopy (OM), scanning electron mic-
roscopy (SEM), transmission electron microscopy (TEM) and Instron Model 8800 Testing System. The basis of airframe structural application is the result of experiment combining with tensile pr-
operties and stress corrosion cracking effect.
The result indicate that the alloys received step-homogenuzation(Step-H) have higher resistanc to fatigue crack growth with lower yielding st-
ress, incoherent precipitation and wider PFZ when a stress ratio (R) is 0.1 and △K is small. But it can get similar fatigue crack growth rates in all aged temper condition with increasing △K. When the alloys were given H470, it can obtain the highest fraction of recrystallization and
larger grain size. Alloys with larger grain size will get almost the same fatigue crack growth rates in all heat treatment and it also occur roughness-induced closure for the mis-match of fracture surface.The aluminum alloy 7075 will percipates coarse Al3Zr dispersoids and can bri-
ges the surface cracks resulting in retarding crack propagation, hence the alloy 7075 can get better resistance of crack propagation than the alloy 7050.
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