  Cleavage crack propagation has been investigated in a low-carbon
lath-martensitic steel using electron back-scattered diffraction technique. The
ability of different martensitic boundaries within prior-austenite grain, such
as sub-block, block and packet boundaries to resist cleavage crack propagation
has been estimated in terms of Kurdjumov-Sachs crystallographic variants.
Crystallographic study of crack path indicated that block boundaries are more
effective in cleavage crack deviation as compared to packet boundaries, whilst
sub-block boundaries are ineffective in that respect. Moreover, characterizing
the boundaries in terms of misorientation angle (angle-axis pair) may be
misleading if their effectiveness in retarding cleavage crack propagation is
considered.
