Secondary ion mass spectrometry method for distinguishing the state of carbon in steels using negative molecular ions
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INTRODUCTION
S I G N I F I C A N T contributions have been made to the scientific literature showing mass spectral differences between phases of similar stoichiometry and different crystal structure, but few studies have emphasized the analytical utility of systematic changes in spectra within a well-defined system, tl] In most secondary ion mass spectrometry (SIMS) studies, spectral differences between phases typically were reported, t2-6J but the results of these studies did not help to clarify whether signal differences were due to changes in composition, crystal structure, or other variables. As a result of this ambiguity in the published data, little credence has been given to the analytical utility of the negative molecular spectrum in SIMS. Several advantages exist for the use of the negative secondary ion spectra instead of the positive, particularly for the characterization of anionic species. In the negative spectrum, carbon, for example, generates both a very intense atomic ion signal and a plethora of easily detectable molecular ions. Second, in the authors experience and as hypothesized elsewhere, [7] there is a greater predominance of secondary ions generated by lattice fragmentation in negative molecular ion spectra than in positive spectra where recombination above the sample surface dominates. The present work describes a systematic effort conducted to relate the state of carbon ( e . g . , solid solution vs a compound) to the negative molecular spectrum. The Fe-C binary system was chosen as a model system, since considerable effort has already been spent in determining the phase stability using microscopic and analytical techniques.
II.
EXPERIMENTAL PROCEDURE
The present study was conducted in two parts. The first part was performed on samples with uniform and well-established microstructures. The second part of the study was aimed at demonstrating the analytical utility of the SIMS negative secondary spectrum on more complex microstructures. The samples chosen for this pur-
C.F. KLEIN, D.P. LETA, and R. A Y E R are with Exxon Research and Engineering Company, Corporate Research Laboratory, Annandale, NJ 08801. Manuscript submitted August 13, 1990. METALLURGICAL TRANSACTIONS A
pose consisted of a quenched steel bar with a gradually varying microstructure and the heat-affected zone (HAZ) of a single bead weld where the transformation is relatively more abrupt, particularly at the base metal/HAZ interface. For the first pa~rt, samples 2 mm in thickness of four plain carbon steels, 1008, 1018, 1050, and 1080 (Table I), were prepared by initially austenitizing them at 1175 K for 0.5 hours followed by water quenching. The heat treatments were done in an evacuated quartz tube to minimize oxidation. Upon removal of the tubes from the furnace, they were immersed in water and immediately broken to maxin~ize the quenching rate. Half of each of these specimens was then re-evacuated and tempered at 975 K for 1 hour to precipitate the carbon as Fe3C. In addition to plain carbon steels, samples of alloy A
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