Dual-edged effect of strengthening on fatigue strength in 7xxx Al alloys
The relationship between tensile and fatigue properties in Al alloys remains vague because strengthening often affects fatigue damage in many aspects. In this study, 7xxx Al alloys were strengthened solely by varying the precipitate content while keeping both the overall microstructure and damage mechanisms consistent, so as to examine the intrinsic effect of strengthening on fatigue performance. The results show that there was an increment of 100 MPa in tensile strength, while the fatigue strength remained nearly unchanged. Further analysis indicates that the strengthening had a dual-edged effect: strengthening enhanced the whole resistance to plastic deformation, while also causing strain localization. Combining our previous models associated with tensile and fatigue properties, a relationship between yield and fatigue strengths is established, which shows a first increasing and then declining trend in fatigue strength with increasing yield strength, leaving a relatively stable region in between. This explains the plateau phenomenon of fatigue strength in a middle yield strength range for high-strength 7xxx alloys.