A transmission electron microscope observation has been carried out on the crack propagation in 99.9% pure magnesium foils stretched at the inside and outside of an electron microscope. Optical and scanning electron microscope observations also have been made on fractured surfaces and cracks in holed-sheet test pieces. The results obtained are as follows:
(1) In many cases, cracking occurs at an edge of foils and is associated with the preceding slip on the basal plane. Then this initial crack, thought to be in the basal plane, changes its direction and takes a zigzag path in the deformed and thinned region ahead of the crack where heavy pile-ups of dislocations have been observed.
After this, the crack develops deviating largely from its initial direction.
(2) Sometimes, a large crack composed of two cleavage planes has been observed, for example, in a foil whose surface is pallerel to the (0001) basal plane.
(3) Cracks mentioned above can be explained by cleavage planes already reported.
(4) As a result of the heavy stress concentration due to crack propagation, the activation of slip, nucleation of a new crack, twin formation and crack propagation along the twin have been observed in adjacent grains, with grain-boundary sliding.
Nucleation of a new crack due to slip also has been observed at a distance from the tip of initial crack.
(5) The tip of crack often branches out in two fine non-crystallographic cracks which are thought to be at the top and bottom surfaces of the foil.
(6) In the holed-sheet test pieces, the mode of fracture changes from brittle to ductile according to the distance from the hole.
Dimples, in which smaller ones also have been observed, are measured to be in sizes comparable to the grain sizes.
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