542 Zr基バルクアモルファス合金の疲労破面における結晶化(OS7-2 微視構造を有する材料の変形と破壊,OS7 微視構造を有する材料の変形と破壊)

Abstract

The present study has investigated deformation-induced crystallization on fatigue fracture surfaces 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 bulk amorphous alloy 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 observed 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 of cracks 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.