Neutron diffraction study of austempered ductile iron |
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Authors: | C S Choi W Sharpe J Barker R J Fields |
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Affiliation: | (1) Armament Research Development and Engineering Center, United States Army, 07806 Picatinny Arsenal, NJ;(2) Materials Science and Engineering Laboratory, National Institute of Standards and Technology, 20899 Gaithersburg, MD |
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Abstract: | Crystallographic properties of an austempered ductile iron (ADI) were studied by using neutron diffraction. A quantitative
phase analysis based on Rietveld refinements revealed three component phases, α-Fe (ferrite), γ-Fe (austenite), and graphite
precipitate, with weight fractions of 66.0, 31.5, and 2.5 pct, respectively. The ferrite phases of the samples were found
to be tetragonal,14/mmm, with ac/a ratio of about 0.993, which is very close to the body-centered cubic (bcc) structure. The austenite phase had C atoms occupying
the octahedral site of the face-centered cubic (fcc) unit cell with about 8 pct occupancy ratio. A strong microstrain broadening
was observed for the two Fe phases of the samples. The particle sizes of the acicular ferrite phase were studied by using
small angle neutron scattering. The analysis suggested a mean rod diameter of 700 A. The scattering invariant predicts a ferrite
volume fraction consistent with the powder diffraction analysis. A textbook case of nodular graphite segregation, with average
diameters ranging from 10 to 20 μm, was observed by optical micrography. |
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