A. Yousefiani et Fa. Mohamed, Correlation between microstructure evolution and cavity stringer formationin the superplastic Zn-22 wt% Al alloy, PHIL MAG A, 79(5), 1999, pp. 1247-1262
In the present investigation, microstructure evolution during the superplas
tic deformation of Zn-22 wt% Al was examined to provide insight into the or
igin of cavity stringers that form parallel to the tensile axis in the allo
y. Substructural data show that groups of fine alpha (Al-rich) and beta (Zn
-rich) phases are encompassed by former alpha boundaries (F alpha Bs) which
consist of fine elongated alpha grains and which divide the microstructure
into equiaxed domains (F alpha B domains). By examining the correspondence
between the behaviour of these boundaries and the development of cavity st
ringers, it is suggested that the alignment of cavities along the tensile a
xis can be explained in terms of two processes: firstly the preferential nu
cleation of cavities at F alpha Bs during the initial stages of deformation
, and secondly the tendency of F alpha Bs and their associated cavities to
change their orientation and align along the tensile axis during superplast
ic deformation. Evidence in support of this explanation is provided by pres
ent experimental results on firstly the correlation between the overall num
ber of aligned F alpha Bs and the total length of cavity stringers, secondl
y the effect of impurities on both the average size of F alpha Bs domains a
nd the total length of cavity stringers, and thirdly the direct corresponde
nce between the morphology of F alpha Bs and that of cavity stringers in no
tched specimens which, upon deformation, exhibit a distinctive local strain
gradient.