3341 Zrバルクアモルファス合金の疲労き裂進展挙動(S18-5 アモルファス合金,S18 材料の強度と組織)

Abstract

The present study has investigated fatigue fracture behavior and deformation-induced crystallization in a Zr-based bulk amorphous alloy. Fatigue crack growth tests were performed on center-cracked plate specimens of 55at%Zr-10at%Al-5at%Cu-30at%Ni at room temperature in air. Tension to tension fatigue loads were applied at stress ratios of R=0.05 to 0.3 at a loading frequency of 5Hz. The plastic replication technique was used for observing crack growth behavior. Crack branching, crack deflection and non-propagating branches frequently occurred at specimen surfaces of the amorphous alloy. Thus, the crack growth rate vs. stress intensity factor range diagrams in which projected crack length was used fluctuated widely. This anomalous growth behavior can be attributed to the fact that internal crack surfaces were very rough. Roughness-induced crack closure was observed at stress ratios lower than 0.3. Scanning electron microscopy revealed striation-like patterns in some portions of the rough crack surfaces. Spot X-ray diffraction analysis with a focal diameter of 30 micrometer of crack surfaces were also made and detected localized crystallization in the form of Zr_2Cu. The localized crystallization may have been brought about by severe deformation in the vicinity of crack tips. The striation-like patterns were produced in some portions of crack surfaces so crystallized. Sparks were observed emitted from growing fatigue cracks just before fracture by the high-speed camera.