  The deformation and crack initiation mechanisms were analyzed in a textured
AZ31B-O Mg alloy subjected to fully-reversed, strain-controlled cyclic
deformation along the rolling direction after 50 cycles (approximately 33% of
the fatigue life). Distinct deformation bands corresponding to pyramidal slip
or tensile twins were found in 538 grains out of 2100 grains. Slip trace
analysis showed that 72.3% were pyramidal slip bands and 18.4% were twin
boundaries. Both pyramidal slip and twinning was only found in 9.1% of the
grains with deformation bands. Cracking was widespread after 50 cycles. Grain
boundary cracks were found in approximately 15% of the small grains (< 20
$\mu$m) and they were mainly associated with high angle grain boundaries (>40
degrees). Cracking was also found to occur by transgranular cracks parallel to
the pyramidal slip bands or twin boundaries in large grains (>45 $\mu$m). The
majority (>60%) of these large grains presented transgranular cracks after 50
cycles.
