Microsc. Microanal. 23, 340–349, 2017 doi:10.1017/S1431927616012629
© MICROSCOPY SOCIETY OF AMERICA 2017
Atom Probe Tomographic Characterization of Nanoscale Cu-Rich Precipitates in 17-4 Precipitate Hardened Stainless Steel Tempered at Different Temperatures
Zemin Wang,1,2 Xulei Fang,1 Hui Li,1 and Wenqing Liu1,*
1Key Laboratory for Microstructures, Shanghai University, Shanghai 200444, P. R. China 2School of Materials Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, P. R. China
Abstract: The formation of copper-rich precipitates of 17-4 precipitate hardened stainless steel has been investigated, after tempering at 350–570°C for 4 h, by atom probe tomography (APT). The results reveal that the clusters, enriched only with Cu, were observed after tempering at 420°C. Segregation of Ni, Mn to the Cu-rich clusters took place at 450°C, contributing to the increased hardening. After tempering at 510°C, Ni and Mn were rejected from Cu-rich precipitates and accumulated at the precipitate/matrix interfaces. Al and Si were present and uniformly distributed in the precipitates that were <1.5nm in radius, but Ni, Mn, Al, and Si were enriched at the interfaces of larger precipitates/matrix. The proxigram profiles of the Cu-rich precipitates formed at 570°C indicated that Ni, Mn, Al, and Si segregated to the precipitate/matrix interfaces to form a Ni(Fe, Mn, Si, Al) shell, which significantly reduced the interfacial energy as the precipitates grew into an elongated shape. In addition, the number density of Cu-rich precipitates was increased with the temperature elevated from 350 up to 450°C and subsequently decreased at higher temperatures. Also, the composition of the matrix and the precipitates were measured and found to vary with temperature.
Key words: 17-4 PH stainless steel, atom probe tomography, tempering, Cu-rich precipitate, hardening
INTRODUCTION
17-4 precipitation hardened stainless steel (17-4 PH SS) is a widely used structural material in nuclear and aero- nautics industries, due its high strength, combined with an acceptable level of ductility and corrosion resistance in harsh conditions, such as a leading screw in light water reactors, missile fittings, and jet engine parts (Rack & Kalish, 1974; Viswanathan et al., 1989; Danoix & Auger, 2000; Hsiao et al., 2002; Wang et al., 2005). Over the past several decades, a variety of studies on 17-4 PH SS have been performed (Viswanathan et al., 1988; Hsiao et al., 2002; Wang et al., 2006). It has been established that nanoscale Cu-rich precipitates (CRPs), uniformly dispersed in the martensitic matrix, contribute to the precipitation hardening of the steel. In a conventional transmission electron microscopic
(TEM) study of 17-4 PH SS, Rack and Kalish (1974) reported that the strengthening was initially attributed to coherent Cu-rich clusters, which gradually transformed to face-centered cubic (FCC) CRPs upon further aging. Viswanathan et al. (1989) discovered that FCC CRPs appeared after tempering above 470°C and that reversed aus- tenite formation that occurred above 580°C . Employing atom
*Corresponding author.
wqliu@staff.shu.edu.cn Received June 29, 2016; accepted December 5, 2016
probe field ion microscopy, Murayama measured the chemical composition of fine spherical CRPs formed at 580°C, which were enriched in Fe, Ni, and Cr. Meanwhile, G-phase precipitates containing Ni, Si, andMnwere possibly formed near the Cu precipitates after aging at 400°C for 5,000 h (Murayama et al., 1999), which is similar to previous reports about the precipitation in Fe–Cu and Fe–Cu–X alloys (Othen et al., 1991; Osamura et al., 1994., Maruyama et al., 1999). However, different results on the morphological structure of Cu precipitates in steels, with various composi- tions, have been reported. Kolli & Seidman (2008) reported, in an atom probe tomographic (APT) study, that a core/shell structure of the CRPs was formed in the over-aged condition of a Fe-2.09 Cu-2.83 Ni-0.68 Al (all in wt%) alloy, with a Cu- rich core and a shell containing Ni and Al. Recently, Jiao et al. (2015) and Wang et al. (2015) reported that composite precipitates, consisting of Ni–Al-rich and Cu-rich phases side by side, were formed during extended aging of Fe-1.5 Cu-5.0 Ni-2.0 Al-3.0 Mn (all in wt%) alloyed steel and Fe- 0.95 Cu-3.13 Ni-1.09 Al-1.87 Mn (all in wt%) alloyed steel. It seems that the presence of Cu, Ni, Al, and Mn in steels have significant effects on the precipitation mechanism, and thus the final precipitate morphology, as well as the resultant precipitation hardening effect of CRPs. Only a limited number of APT studies addressing
the formation of CRPs in 17-4 PH SS have been made. In this paper, APT is utilized to characterize the features of
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