Correlation Between Fracture Mechanical and Atom Probe Investigations on Metallic Glass Ribbon
- PDF / 620,005 Bytes
- 4 Pages / 417.6 x 639 pts Page_size
- 92 Downloads / 195 Views
CORRELATION BETWEEN FRACTURE MECHANICAL AND ATOM PROBE INVESTIGATIONS ON METALLIC GLASS RIBBON CALVO M., MENAND A.*, OSTERSTOCK F. AND CHERMANT J.L. Equipe Mat~riaux-Microstructure de I'U.A. 251, ISMRa-Universit6, 14032 Caen Cedex, France U.A. 808, Facult6 des Sciences et Techniques de * Equipe Microscopie Ionique, Rouen, 76130 Mont Saint-Aignan, France ABSTRACT Metallic glasses have good mechanical and magnetical properties. However they embrittle after annealing and/or low temperature testing. In this paper, we describe the rupture of atom probe tips using fracture mechanical measurements. We found boron rich areas which seem to be interconnected and more numerous after annealing and act as crack nuclei. The good correlation between the calculated and the measured size of the critical defect seem to confirm that the boron segregations are on the origin of embrittlement and gives good ground for further straightforward investigations. EMBRITTLEMENT IN TERMS OF FRACTURE MECHANICS In a previous study [E], we have shown that metallic glass ribbons follow perfectly the concepts of fracture mechanics in plane stress. SEM observations have allowed us to link the decrease of the toughness, and so the embrittlement during heat treatment, with the change of the fracture surface and the rupture conditions from plane stress to plane strain [2] [3]. The continuous evolution from plane stress to plane strain means an embrittlement and thus a decrease of the plastic zone size at the crack tip. Its size and shape can be calculated from the K values [1][4]. The embrittlement is probably to be attributed to short or lsng range rearrangements inside the material during the heat treatment. EMBRITTLEMENT IN TERMS OF FIELD ION MICROSCOPY AND ATOM PROBE Atom probe (AP) provides a method for quantitative microanalysis with very high spatial resolution. A metallic specimen, prepared in the shape of a very thin tip, can be then analysed atom per atom. Such analysis have been achieved in the last few years on metallic glasses [5][6]. Piller and Haasen [5] have studied a FeNiB alloy in field ion microscopy (FIM), performing also some mechanical experiments. They have detected boron rich areas and an increase in size and number of these areas with increasing embrittlement. The boron content on the fracture surface of the tip is higher (Fig. 13 of [5]). The authors have concluded to a phase separation, with a (FeNi) B amorphous phase, which volume fraction increases with annealing time and t~mperature. These areas should be responsible for the embrittlement. We have also detected such areas of high boron content (often more than 25 at. %). These areas seem to be more numerous after heat treatment, and also interconnected, as it can be observed in comparing Fig. la and lb. Fig. 2 reveals a regroupment of such areas, detected for an as-annealed specimen. The AP available in Rouen has made it also possible to detect variations in iron and nickel contents. The high boron content observed after the tip rupture in the AP indicates the important rol
Data Loading...
