Surface Sensitivity Effects with Local Probe Scanning Auger-Scanning Electron Microscopy
- PDF / 2,555,498 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 10 Downloads / 233 Views
is also known to affect the adhesive strength at interfaces [3]. However, in contrast to segregation at grain boundaries, only very few studies have addressed segregation in particular at metal/ceramic interfaces [4]. Especially the dissolution of 4 at.% of Sb in the Ag matrix showed according to HRTEM observations two major effects on the Mn 30 4 precipitates [5] : (i) a change from a precipitate sharply facetted by solely {1 11 } to a globular shape with sometimes also short (220 } and (002) facets and (ii) a partial reduction of Mn 30 4 to MnO for a part of the precipitates. Macroscopic properties of materials can be better understood if, and only if, the related mechanisms are thoroughly investigated at nanometer and/or subnanometer length scales. If these mechanisms can be controlled, future materials can be fabricated for specific applications. Therefore, we employ nanometer scale (lateral resolution of -15 nm) UHV- Auger Electron Spectroscopy (AES), combined with Scanning Electron Microscopy (SEM) to study segregation phenomena in Cu-Sb alloys. Since the zone where segregation effects are most pronounced extends only a few monolayers from the grain boundary and the escape depths of Auger electrons are in the nanometer range for all elements, Auger Electron Spectroscopy is a suitable technique for segregation studies. Polycrystalline Cu-Sb alloys were chosen as a model system for segregation studies at GBs, since Sb mainly segregates at GBs due to size effect (Sb atoms are larger than Cu atoms). At first sight Sb is expected to segregate preferably at GBs consisting of higher index planes 81 Mat. Res. Soc. Symp. Proc. Vol. 589 © 2001 Materials Research Society
because it will be more easily accommodated there than at more densely packed low index planes. On the other hand, the open (excess) volume in the GB or heterophase interface may be the most important factor for segregation sites, and low index planes in GBs tend to be low energy (low excess volume) boundaries. S, which appeared in our samples as contamination, also segregates to GBs but with a different mechanism. The solubility of S in Cu is limited because of the formation of the very stable intermediate phase Cu 2 S. A strong interaction between S impurities and vacancies in copper was observed previously, which effectively leads to the formation of highly stable and mobile sulfur-vacancy defect complexes. The S-vacancy and S-S pair interactions favor precipitation of Cu 2S at lattice defects like GBs and dislocations [6]. The presence of Cu-sulphide precipitates at GBs is rather undesirable due to structural transformations that occur with temperature, favouring, in combination with the mechanical properties of Cu-sulphide, crack nucleation and propagation at GBs under creep conditions. Our aim in the present work is to determine locally the amount of S segregation in connection with the corresponding surface morphology and orientation. EXPERIMENTAL Cu-Sb samples were prepared by melting together Cu and Sb in a graphite crucible at 11000 C (for 1
Data Loading...
