Characterization of the nucleation and growth behavior of copper precipitates in low-carbon steels
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7/10/04
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Characterization of the Nucleation and Growth Behavior of Copper Precipitates in Low-Carbon Steels MICHAEL S. GAGLIANO and MORRIS E. FINE The nucleation and growth behavior of copper precipitates in ferrite was investigated both theoretically and experimentally for two low-carbon steels with and without niobium additions in samples cooled directly to the desired aging temperature from the austenitizing temperature. Theoretical nucleation and growth rate models were constructed using calculated thermodynamic data in conjunction with classical theories. The maximum nucleation and growth rates for Cu were experimentally determined to be 8.0 1021 nuclei/m3 s at 612 °C and 0.12 nm/s at 682 °C, respectively. Using an experimentally determined “effective” activation energy for the diffusion of copper, the theoretical nucleation rate curve compared very well with the hardness data for the first 5 minutes of aging. The growth behavior of the Cu precipitates was investigated through use of a conventional transmission electron microscope (TEM) for samples directly aged at 550 °C. For aging times up to 21 hours, the average precipitate size scaled with a time dependence of t1/2.
I. INTRODUCTION
PRECIPITATION of copper in iron has been studied extensively and has become the cornerstone for the development of high-strength, low-carbon steels.[1–9] Copper initially precipitates from supersaturated -iron as spherical, metastable, body-centered-cubic (bcc) clusters since the sizes of the iron and copper atoms, predicted from the pure metals, are nearly the same. Early research by Goodman et al.[10,11] indicated that the bcc Cu precipitates were comprised of about 50 at. pct iron and 50 at. pct copper for sizes up to about 2.5 nm; however, more recent atom-probe field ion microscopy and small-angle neutron scattering studies claimed that the bcc copper clusters contain only small amounts of iron (2 at. pct).[9] In the peak-aged condition, the coherent, bcc clusters generally have an average diameter between 1 and 5 nm[3,4,10,12–13] with about 50 at. pct of the copper remaining in solid solution.[6,14] As the clusters grow, a critical size is reached where a reduction in strain energy and interfacial free energy prompts a secondary transformation from the bcc structure to an intermediate, twinned and faulted, close-packed structure, known as 9R. The critical size is generally between 4 and 12 nm, depending on the aging conditions, and ultimately results in a decreased strengthening effect.[3,10,11,15,16] Thus, copper precipitation in -iron progresses through the following sequence: Supersaturated solution of Cu in -iron : bcc copper : 9R copper : fcc -copper Microalloying elements such as niobium are added to lowcarbon steels to refine the grain size and provide precipitation strengthening by forming a fine distribution of highly stable carbide precipitates. Niobium carbide may precipitate either within the austenite matrix, at the austenite/ferrite MICHAEL S. GAGLIANO, Senior Metallurgist, is with
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