Fig.
10: Fractographs of fatigued surfaces (a-d) for J2N, (e-h) for the G10
in air
(Ductile striation mechanism (DSM ) with secondary crack
(SC ))
The metal crumbs were formed in between branched and re-joined crack
fronts. It can also be observed that several small metal crumbs were
formed as the crack propagated in air and the occurrence increased with
increase in ∆K . In other words, the tendency for crack branching
and re-joining tends to increase with increase in the driving force for
crack propagation. The length of the branched crack is considerably
long, and the extent of bifurcation angle is substantially large. Fig.
11(c) is the magnification of the locations 1 to 8 marked with the
yellow boxes. It was observed that the branched crack fronts generally
followed the thin ribbon αHA phase as shown in
the magnified locations. This tends to suggest thatαHA is the path of least resistance. An important
feature of the crack growth is that generally the tip of the propagating
crack was always sharp as can be seen from the tip of the branched crack
front as shown in the magnified micrographs in Fig. 11(c).