Copper precipitation during continuous cooling and isothermal aging of a710-type steels
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I.
INTRODUCTION
MANY studies of copper-precipitation-hardened steels preceded the advent of the transmission electron microscope.tt~] Based on hardness tests and X-ray diffraction resuits, Norton[6] concluded that a "pre-precipitation" phenomenon was largely responsible for the hardening of copper-containing steels and that precipitation itself was of secondary importance. Three decades later, Lahiri et al.[71 obtained results from modulus and Mrssbauer studies which provided strong support for a two-stage precipitation process where the first stage involved a body-centered-cubic (bcc) form of copper. Hornbogen and GlenntS] directly studied copper precipitation via transmission electron microscopy (TEM) examination of extraction replicas. They observed the e-copper phase in the form of fine, face-centered-cubic (fcc) precipitates with a lattice parameter of about 0.363 nm. Hornbogentg] later hypothesized that precipitation occurred by the "formation of copper-rich coherent bcc zones which transform in situ into fcc particles." Once a viable population of fcc precipitates was established, spherical particles grew to a diameter of about 30 nm, after which the particles evolved into a rodlike morphology with the rod axis along [110]~.~oppJ/[111],~_iron.[81 It was suggested that this new morphology was adopted based on strain energy considerations. Speich and OrianiVOJ examined Ostwald ripening of ecopper precipitates in a-iron. These authors showed that the precipitates adopted the Kurdjumov-Sachsin1 orientation S.W. THOMPSON, Associate Professor/ISS Professor, and G. KRAUSS, John Henry Moore Professor, are with the Advanced Steel Processing and Products Research Center, Department of Metallurgical and Materials Engineering, Colorado School of Mines, Golden, CO 80401. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor of H.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICALAND MATERIALSTRANSACTIONS A
relationship (OR), i.e.,
(111)~-ooopJ/(1 lO)a'ir~ [ l ]-O]e_copper//[1]"1]~ron
[OR1]
They found the same rod axis as Hornbogen and Glenn/s~ Based on their data and the construction of hard-sphere atomic models, they concluded that the rodlike shape was a result of a very large anisotropy in interracial energy not of strain energy minimization.V0] More recently, Dahmen et aLV2] explored the formation of needlelike precipitates in fcc-bcc systems based on the "invariant line hypothesis." For the formation of fcc e-copper in bcc a-iron, they verified the Kurdjumov-Sachs OR. Needle or rod directions were predicted to be (557)~_~o~ and (656)~-i.... and these directions were accurately confirmed. They concluded that the two important factors in determining this morphology are strain minimization and efficient strain accommodation. The use of field-ion microscopy (FIM) and atom-probeFIM (AP-FIM) in the early 1970s complemented TEM studies and provided t
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