• PDF / 984,937 Bytes
• 8 Pages / 417.6 x 639 pts Page_size
• 53 Downloads / 243 Views

DOWNLOAD

REPORT

---

153

TECHNIQUES FOR INVESTIGATING STRUCTURE AND COMPOSITION WITH HIGH SPATIAL RESOLUTION JOHN B. VANDER SANDE Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge,MA 02139. ABSTRACT The techniques of scanning transmission electron microscopy and field iron microscopy/atom probe are briefly described. The advantages of these techniques for high spatial resolution compositional analysis are discussed and examples cited. INTRODUCTION One of the major benefits expected from rapid solidification processing is a refinement of the microstructure and a reduction in segregation in alloys processed in this way. The finer scale to the microstructure developed by rapid solidification processing places extreme demands on the techniques available for elucidating the details of these microstructures. This paper discusses two instrument types that are capable of high spatial resolution compositional analysis: the scanning transmission electron microscope (STEM) and the field ion microscope/atom probe (FIM/AP). After describing each technique, including a brief description of limitations. examples of microstructural analysis from each will be presented. STEM The field emission STEM used in this work was a VG Microscopes HB5 commercial instrument, fitted with an energy dispersive X-ray analysis system and an electron energy loss spectrometer. The electron source is a cold tungsten field emitter. There is a two-lens electron probe forming system, by which typically 2nA of lOOkV electrons can be formed into a probe approximately 1.5 nm diameter on the specimen. The vacuum in the specimen chamber is n'2x 10-9 torr, which minimizes any problems of specimen contamination. The X-ray detector is a slightly modified Kevex Si (Li) detector, with a Kevex 7000 multichannel analyzer and microcomputer system for spectral manipulation. Specimens in the STEM consist either of thin sections of bulk material, or carbon films supporting particulate material. While it is possible to image specimens many tens to several hundred nanometers thick, effects such as beam spreading, specimen fluorescence, and absorption become of increasing importance in modifying the resolution and sensitivity of X-ray analysis. Interpretation of energy-dispersive X-ray data requires a great deal of care. The usual way to consider the effects of X-ray production is to employ the sensitivity factor method due to Cliff and Lorimer (1), embodied in the equation: CA IAKA (1)

C

IBKBB

where Ca/CB is the ratio of the concentrations of the elements A and B in a specimelA. IA and IB are the integrated X-ray counts in chosen lines due to each element, and KA and KB are sensitivity factors. Often the ratio KA/KB is used as a relative sensitivity factor denoted by KAR, relating the sensitivities of the two elements. In this case, some eleme~t in the specimen is

154

chosen as the reference, and the relative sensitivity factors for each of the other elements are determined from measurements on specimens of known composition. It is thus p