The effects of precipitate free zone(PFZ) and of Mn-dispersoid on the fatigue crack propagation(FCP) behavior in two Al-4.2Zn-1.6Mg-0.15Zr-xMn(x = 0.10, 0.79) alloys has been investigated in air at room temperature. The width of PFZ has been varied by two step aging method.
The effect of Mn addition in Al-Zn-Mg-Zr alloy is to form Mn-dispersoids whose size ranges from 0.05 to 0.5㎛ and to increase strength due to dispersion hardening particle without sacrificing ductility due to homogeneous deformation.
In the case of 0.10wt%Mn-bearing alloy containing no Mn-dispersoid, the rate of FCP was increased with increasing the width of PFZ. It is believed that PFZ provides a preferred path of crack propagation since this region is softer than the precipitated matrix.
As compared with 0.10wt%Mn-bearing alloy, a role of Mn-dispersoid on the FCP behavior in 0.79wt%Mn-bearing alloy is to deflect propagating crack so that high resistance of FCP was obtained.
Meanwhile, in the case of 0.79wt%Mn-bearing alloy containing many Mn-dispersoids, the PFZ effect on the FCP behavior could not observed. This phenomenon may be believed that the effect of Mn-dispersoid play a predominant role in this system as compared with that of PFZ.
