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Mulholland, Michael |
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Publication |
Co-precipitation Kinetic Pathways in a Blast Resistant Steel for Naval Applications |
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Book Whole |
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2011 |
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Abstract |
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Summary Language |
150 |
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The nanoscale co-precipitation of Cu and M2C carbides in a high strength, low carbon
quenched and tempered steel is characterized in detail along with the mechanical properties and
precipitated austenite. Correlations between the nanostructure and the mechanical properties are
drawn.
The co-precipitation of Cu, M2C (M is any combination of Cr, Mo, or Ti), and austenite
(f.c.c.) is characterized for 5 h isochronal aging times by synchrotron x-ray diffraction and 3-D
atom-probe tomography for a HSLC steel, BA (BlastAlloy) 160. High number densities, ca. 1023
m-3, of co-located Cu and M2C precipitates were observed. Only small austenite volume
percentages (<1.5%) were measured after aging at temperatures up to 625 °C for 5 h.
Nanoscale co-precipitation is studied in detail after isothermal aging.
Atom-probe
tomography is utilized to quantify the co-precipitation of co-located Cu precipitates and M2C (M
is any combination of Cr, Mo, Fe, or Ti) carbide strengthening precipitates. Coarsening of Cu
precipitates is offset by the nucleation and growth of M2C carbide precipitate, resulting in the
maintenance of a yield strength of 1047 ± 7 MPa (152 ±1 ksi) for as long as 320 h of aging time
at 450 °C. Impact energies of 153 J (113 ± 6 ft-lbs) and 144 J (106 ± 2 ft-lbs) are measured at -
30 °C and – 60 °C, respectively. The co-location of Cu and M2C precipitates results in non-
stationary state coarsening of the Cu precipitates. Synchrotron-source x-ray diffraction studies
reveal that the measured 33% increase in impact toughness after aging for 80 h at 450 °C is due
to dissolution of cementite, Fe3C, which is the source of carbon for the nucleation and growth of
M2C carbide precipitates. Less than 1 volume percent austenite is observed for aging treatments
at temperatures less than 600 °C, suggesting that TRIP does not play a significant role in the
toughness of specimens aged at temperatures less than 600 °C. Aging treatments at temperatures
greater than 600 °C produce more austenite, in the range 2-7%, but at the expense of yield
strength.
The differences in artifacts associated with voltage-pulsed and laser-pulsed (wavelength =
532 or 355 nm) atom-probe tomographic (APT) analyses of nanoscale precipitation in a high-
strength low-carbon steel are assessed using a local-electrode atom-probe (LEAP) tomograph. It
is found that the interfacial width of nanoscale Cu precipitates increases with increasing
specimen apex temperatures induced by higher laser pulse-energies (0.6-2 nJ pulse-1 at a
wavelength of 532 nm). This effect is probably due to surface diffusion of Cu atoms. Increasing
the specimen apex temperature by using pulse energies up to 2 nJ pulse-1 at a wavelength of 532
nm is also found to increase the severity of the local magnification effect for nanoscale M2C
metal carbide precipitates, which is indicated by a decrease of the local atomic density inside the
carbides from 68±6 nm-3 (voltage-pulsing) to as small as 3.5±0.8 nm-3. Methods are proposed to
solve these problems based on comparisons with the results obtained from voltage-pulsed APT
experiments. Essentially, application of the Cu precipitate compositions and local atomic-density
of M2C metal carbide precipitates measured by voltage-pulsed APT to 532 or 355 nm
wavelength laser-pulsed data permits correct quantification of precipitation.
Based on detailed three-dimensional (3-D) local-electrode atom-probe (LEAP)
tomographic measurements of the properties of Cu and M2C precipitates, the yield strength of a
high-toughness secondary-hardening steel, BA160, as a function of aging time is predicted using
a newly developed 3-D yield strength model. Contributions from each strengthening constituent
are evaluated with the model and superposition laws are applied to add each contribution.
Prediction of the yield strength entirely based on 3-D microstructural information is thus
achieved. The accuracy of the prediction depends on the superposition laws and the LEAP
tomographic measurements, especially the mean radius and volume fraction of M2C precipitates. |
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Northwestern University |
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Ph.D. thesis |
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Evanston, IL |
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