| Popis: |
The effect of the structural change of metallic glasses on fracture is studied by observing the fracture surfaces of as-cast and annealed Fe78B13Si9 metallic glasses on the nanometer scale using atomic force microscopy (AFM). X-ray diffraction (XRD) was applied in studying the structural change. Metallic glass ribbons were fractured by the tensile test machine at room temperature. In the as-cast specimens, the fracture surface displays a characteristic vein pattern macroscopically. AFM observations of the fracture surface of the as-cast specimen show nanometer-scale steps and grooves, suggesting that the fracture has occurred associated with plastic deformation. However, in the fracture surfaces of the specimens annealed at and above 723 K, AFM observations reveal many pyramid-like features, although the fracture surface is featureless macroscopically. The pyramid-like features appear to be crystallites produced during heating, because the XRD indicates a crystalline structure. Since the pyramid-like features are not observed in the original annealed surface, they are found to be formed by fracture. This suggests that the fracture of the specimens annealed above 723 K has occurred along the crystallite facets, i.e. a kind of cleavage fracture. |
