Effect of te on morphological transitions in Fe-C-Si alloys: Part ii. Auger analysis
- PDF / 2,275,182 Bytes
- 7 Pages / 613 x 788.28 pts Page_size
- 74 Downloads / 176 Views
I.
INTRODUCTION
THE directional
solidification studies of Part I tl] demonstrated that small amounts of Te in an Fe-C-Si alloy were very effective at suppressing growth of gray iron structures. Theoretical models t2,3~ often explain this type of result by postulating that the Te atoms adsorb preferentially upon the graphite/iron interface to inhibit the graphite growth rate at a given undercooling. It is possible that Te surface coverages on the graphite as small as a monolayer or even a partial monolayer could produce the dramatic reduction in growth rate which is observed. A potential experimental technique for evaluating this possibility is Auger electron spectroscopy, which is capable of detecting submonolayer surface coverages. Therefore, a study was made of the alloys of Part I by fracturing them within the ultrahigh vacuum (UHV) of the Auger chamber. If it can be shown that fracture occurs along the iron/graphite interfaces, then Auger analysis of these surfaces should be capable of determining if Te adsorbs at these interfaces and to what thickness. II.
EXPERIMENTAL TECHNIQUE
The 5-ram-diameter directionally solidified rods of Part I had a longitudinal flat ground on one side for microstructural examination. A V-notch was filed into the sample from a direction directly opposite the polished flat and at right angles to the rod axis. A sample was placed in the UHV chamber of a Perkin-Elmer Corp., PHI Model 600 scanning Auger microscope (SAM) and broken in the liquid nitrogen fracture stage at an indicated pressure of 5 • 10 -1~ tort. The fracture surface originated at the base of the V-notch and propagated transverse to the rod axis across the diameter of the sample rod. All studies were done on rods possessing the type D flake graphite morphology. The SAM was equipped with a secondary electron detector so that secondary electron images (SEI) could be produced just as in a scanning electron microscope (SEM). It was also provided with an argon ion sputter gun so that surfaces could be depth profiled. After samples were examined J.D. VERHOEVEN, Professor and Senior Metallurgist, A.J. BEVOLO, Physicist, and J.S. PARK, Graduate Student, are with the Department of Materials Science and Engineering and Ames Laboratory, Iowa State University, Ames, IA 50011. Manuscript submitted July 21, 1988. METALLURGICAL TRANSACTIONS A
in the SAM, they were removed, and additional examination of the fracture surfaces was done in an optical microscope and a standard SEM, which provided improved image resolution. III.
EXPERIMENTAL RESULTS
It is necessary to determine the nature of the fracture surface before the significance of the Auger results can be evaluated. Figure 1 presents an optical micrograph of the polished flat after removal from the SAM. It is apparent that the fracture surface does not go through the secondary austenite dendrite arms but runs along the interfaces between secondary dendrite arms and the gray iron matrix. Figure 2 is an SEI of the fracture surface directly above region A of Figure 1, and the cav
Data Loading...
