Abstract: The micro process of crack initiation and propagation in H68 brass with low stock fault energy was observed by in situ tensile test in TEM. The results show that many dislocations are emitted from a loaded crack tip and move forward. A dislocation free zone(DFZ) is formed between crack tip and the inversely pile-up dislocations when the dislocation emission and motion reach equilibrium. Mechanical twin forms in the thicker area ahead of crack, and then the micro-crack initiates and propagates in the mechanical twin, which result in a zig-zag crack propagation. The micro-twin forms in the DFZ and the micro-crack initiates in the micro-twin, resulting in the crack propagation discontinuously. Sometimes the micro-crack propagates from a blunted crack tip continuously.
The micro process of crack initiation and propagation in H68 brass with low stock fault energy was observed by in situ tensile test in TEM. The results show that many dislocations are emitted from a loaded crack tip and move forward. A dislocation free zone (DFZ) is formed between crack tip and the inversely pileup dislocations when the dislocation emission and motion reach equilibrium. Mechanical twin forms in the thicker area ahead of crack, and then the microcrack initiates and propagates in the mechanical twin, which result in a zigzag crack propagation. The microtwin forms in the DFZ and the microcrack initiates in the microtwin, resulting in the crack propagation discontinuously. Sometimes the microcrack propagates from a blunted crack tip continuously.
Fig.2 Mechanical twin ahead of crack and zig-zag crack propagation (a) —Mechanical twin and quadrilateral crack; (b) —SADP of mechanical twin; (c) —Index of SADP; (d) —Crack slip and propagation along twin boundary
图3 无位错区中微孪晶及其对裂纹形核与扩展的影响
Fig.3 Micro-twin in DFZ and effect on micro-crack initiation and propagation (a) —DFZ ahead of crack tip; (b) —Micro-twin (Tm) in DFZ and micro-crack (Ct) ; (c) —Micro-crack propagation; (d) —Micro-crack connecting with main crack, and micro-twin forming in DFZ again