TRANSMISSION ELECTRON-MICROSCOPY INSITU DEFORMATION STUDY OF (111) ZIGZAG TRANSGRANULAR CRACKS IN THE L12 INTERMETALLIC COMPOUND NI3GE

Slip and fracture in polycrystalline thin foils of Ni3Ge with and with out 0.06 at.%B have been examined by in situ straining in a transmissi on electron microscope. In both alloys, crack propagation was preceded by intense slip activity and occurred predominantly in a zigzag trans granular manner. The crack plane was {111}, which was coincident with the slip plane. Tilting experiments and trace analysis showed that sli p was activated alternately on two major systems, (111)[101BAR] and (1 11BAR)[101BAR]. Secondary slip was observed to occur ahead of dislocat ion pile-ups which occurred on the primary slip system. Because of the thinning of material from the slip of dislocations, the crack propaga ted alternately on these two slip planes, and hence resulted in a {111 } zigzag transgranular crack path. Calculations of the stress fields s howed that the choice of the secondary slip system was dictated by the stress concentration close to the tip of the pile-up.