Atomic Scale Study Of Precipitate / Matrix Interfaces in a Metallic Alloy
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atomic scale reconstructions is the atom probe microscope. When a high field is applied to a sharp conductive specimen, surface atoms ionise and are projected in a radial direction away from the charged tip. Atoms are identified through time-of-flight mass spectrometry, positioned horizontally from the location of the ion impact on a detector plate and vertically from the evaporation sequence. A thorough review of the OPoSAP' used in this study and atom probe microscopy in general has been written by Miller, Cerezo, Hetherington, and Smith.2 In this paper, the OPoSAP is used to study the internal interfaces of cobalt precipitates in the Cu-Co system in conjunction with computer simulations. The dilute Cu-Co system has been chosen because it has been widely studied. Small precipitates in this system are believed to be isotropic spheres with high cobalt concentrations coherently embedded in a matrix of nearly pure copper. The in-situ TEM misfit strain was found to be 1.0-1.4%.3-4 A review of phase transformation studies on the Cu-Co 5 system has been written by Aaronson.
521 Mat. Res. Soc. Symp. Proc. Vol. 481 01998 Materials Research Society
EXPERIMENTAL METHODS Specimen blanks were cut from bulk Cu- 1.Oat% Co stock to an approximate size of 1 mm by 1 mm by 15 mm. The blanks were sealed into silica tubes evacuated to less than 10-6 Torr and backfilled with a fraction of an atmosphere of argon gas to prevent oxidation. The heat treatments consisted of: homogenization at 1273K for 48 hours, step quench and aging in a molten salt bath at either 823K or 723K for various times, and a water quench in which the silica tubes were immediately smashed into cold water after extraction from the salt bath. Specimens aged for 24 hours were placed within a furnace instead of a salt bath. The 823K and 723K heat treatment produced precipitates with densities of 1023/m 3 and 2 x 1024/M3. respectively. Specimen needles were formed by electropolishing in a 3:1 solution of water and phosphoric acid. Polished specimens produced field ion images at approximately 3-5 kV in neon and were gradually smoothed to a voltage of 7-10 kV before analysis. Some specimens showed evidence of hydride contamination which results in copper ions being evaporated as copper hydride species, but since both species can be accurately identified the positioning accuracy is not affected. Figure 1 is a typical reconstruction of the 723K heat treatment. POSITIONING ACCURACY In order to show that the interface widths measured are real and not just artifacts of the Figure 1: This figure is a measurement technique errors in positioning accuracy reconstruction of the must be examined. Systematic errors stemming from cobalt atoms within a Cu I %Co asymmetry in the specimen tip or the analysis direction alloy aged at 723K for 24 hrs. being off-axis can be avoided if care is taken with Box size 1 : x 1x7n specimen preparation and alignment. The horizontal scatter is a random error that can not be corrected for experimentally, but its effects are only on the order of 0.2
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