Experimental and Theoretical Measurements of the Evolution of Embryos Before and During the Nucleation Stage
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NTRODUCTION
COPPER alloy sheets are used widely for electrical terminals in automotive applications. The Cu-Ni-P alloy is one of the typical precipitation hardening type alloys, which are designed to exhibit both high strength and high electrical conductivity.[1,2] Therefore, the characterization of strengthening phases is essential to understand these properties of the alloy. Some researchers have already confirmed the presence of equilibrium nickel phosphides, formed after aging at relatively high temperatures, using transmission electron microscopy (TEM) and selected area electron diffraction pattern analysis.[1,3–5] Atom probe tomography (APT) is a powerful technique with sufficiently high spatial resolution in three dimensions to resolve the position and chemical identity of individual atoms.[6,7] It has been applied successfully to study the earliest stages of decomposition in many alloy systems. Recently, Aruga et al.[8–10] quantitatively characterized solute clustering during aging at 523 K (250 °C) in Cu-Ni-P alloys by means of three-dimensional atom probe (3DAP) and revealed that the clusters play a key role in the stress relaxation performance and age hardening behavior in Cu alloys. However, our understanding of early-stage precipitation is limited, and there is a particular lack of quantitative information regarding the morphology, chemistry, and size-evolution of small, early stage precipitates. In this investigation, early precipitation of a Cu-Ni-P alloy during aging after solution treatment YASUHIRO ARUGA, Senior Researcher, and HIDENORI NAKO, Researcher, are with the Materials Research Laboratory, Kobe Steel, Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan. Contact e-mail: [email protected] Manuscript submitted July 14, 2011. Article published online October 19, 2011 1102—VOLUME 43A, APRIL 2012
has been explored. This research aims to characterize the early-stage precipitation in terms of size and chemical composition, and to suggest a link between the ratio of solute concentration in the features and the nucleation of precipitates.
II.
EXPERIMENTAL PROCEDURE
An ingot of Cu-0.41Ni-0.11P (mass pct) alloy was manufactured using an air furnace, homogenized at 1223 K (950 °C) for 0.5 h, and hot rolled immediately down to 15 mm thickness, followed by cold rolling down to 0.80 mm thickness. The material was fully recrystallized and solution treated at 923 K (650 °C) for 20 seconds. The average grain size for the alloy was about 20 lm after water quenching. The alloy sheets were given an aging treatment for 100 ks at 523 K and 623 K (250 °C and 350 °C) in an air furnace. The alloys were aged in envelopes made of Cu foil filled with charcoal powder to prevent from oxidation. TEM and an energy dispersive X-ray spectroscopy (EDS) were performed using a Hitachi HF-2200 (Hitachi, Ltd., Tokyo, Japan) at an operating voltage of 200 kV. The specimens for TEM were prepared by both thin foil and carbon extraction replica techniques. The foils were prepared using discs, 3 mm in diameter, which were
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