Abstract
The redistribution of carbon atoms during the early stages of ageing and tempering of iron-carbon martensites has previously been studied only by indirect methods. The computer-controlled atom probe field ion microscope permits the direct, quantitative determination of carbon concentrations at the atomic level, and thus all the stages of the martensite decomposition process become amenable to direct study. Analyses of a low-carbon martensite, Fe-1.0 at. pct C, (Fe-0.21 wt pct C), water quenched and tempered for 10 min at 150 °C, showed a matrix carbon content of only 0.14 at. pct. Analysis of a 2 nm diam area centered on a lath boundary showed a local concentration of 2.01 at. pct C. There is some evidence that this carbon level is associated with the presence of a thin film of retained austenite at the boundary. In the case of a higher carbon martensite, Fe-0.64 at. pct Mn, 3.47 at. pct C, (Fe-0.65 wt pct Mn-0.78 wt pct C) water quenched and aged for approximately 24 h at room temperature, analysis of twinned regions showed a matrix carbon level of 2.7 at. pct and a concentration enrichment to 6.9 at. pct in a region 2 nm diam, centered on the coherent twin interface. Assuming the segregated carbon to be located in a single atomic layer at the twin interface, this result indicates that a carbon concentration of 24 at. pct exists locally at the boundary. These results appear to be the first direct demonstration of the segregation of carbon atoms to lattice defects in carbon martensites. Tempering of the higher carbon martensite for 1 h at 160 °C produced further segregation of carbon to the region of twin interfaces. The matrix carbon content fell to 1.5 at. pct and the average carbon content over a 2 nm diam region at the interface rose to 8.7 at. pct. The width of the carbon segregated regions also increased, which seems to imply that incipient carbide precipitation in the plane of the twin boundaries is occurring at this stage of the tempering process.
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Formerly with the Department of Metallurgy and Science of Materials, University of Oxford
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Miller, M.K., Beaven, P.A. & Smith, G.D.W. A study of the early stages of tempering of iron-carbon martensites by atom probe field ion microscopy. Metall Trans A 12, 1197–1204 (1981). https://doi.org/10.1007/BF02642333
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DOI: https://doi.org/10.1007/BF02642333