Microsc. Microanal., 23, 360–365, 2017 doi:10.1017/S1431927617000174
© MICROSCOPY SOCIETY OF AMERICA 2017
Core-Shell Structure of Intermediate Precipitates in a Nb-Based Z-Phase Strengthened 12% Cr Steel
Masoud Rashidi,* Hans-Olof Andrén, and Fang Liu Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
Abstract: In creep resistant Z-phase strengthened 12% Cr steels, MX (M = Nb, Ta, or V, and X = C and/or N) to Z-phase (CrMN,M = Ta, Nb, or V) transformation plays an important role in achieving a fine distribution of Z-phase precipitates for creep strengthening. Atom probe tomography was employed to investigate the phase transformation in a Nb-based Z-phase strengthened trial steel. Using iso-concentration surfaces with different concentration values, and subtracting the matrix contribution enabled us to reveal the core-shell structure of the transient precipitates between MX and Z-phase. It was shown that Z-phase forms by diffusion of Cr into NbN upon ageing, and Z-phase has a composition corresponding to Cr1+xNb1−xN with x = 0.08.
Key words: Atom probe tomography, phase transformation, 9–12% Cr steels, Z-phase, local magnification, precipitates
INTRODUCTION
Martensitic 9–12% Cr steels with good creep and corrosion resistance are widely used in components of steam power plants such as pipes, turbine housings, and turbine rotors. Increasing the steam temperature and pressure in such power plants has led to an improved efficiency, reduced fuel consumption, and lower emission of climate-affecting CO2 during the last decades. Consequently, better steels have been developed for such applications and now operate at up to 600°C (Mayer & Masuyama, 2008). Martensitic 9–12% Cr steels possess smaller thermal expansion and higher thermal conductivity compared with austenitic steels and nickel-base alloys. Thus they can also offer a good performance to meet the required flexibility of steam power plants that are used in combination with renewable energy sources, such as wind and solar power plants (Abe, 2015). Z-phase strengthened 12% Cr steels (Danielsen & Hald,
2009; Liu et al., 2016a, 2016b) have been developed aiming to provide long-term creep strength for an increased steam temperature of 650°C. Precipitation hardening is the most important strengthening mechanism to achieve proper creep resistance in these alloys, so a fundamental understanding of the precipitation behavior is required for appropriate alloy development. Experimental results and thermodynamic modeling
show that Z-phase (CrMN, M = Ta, Nb, or V) is the most thermodynamically stable nitride in 9–12% Cr steels in the temperature range of interest. However, the nucleation of Z-phase precipitates is difficult (Agamennone et al., 2006; Danielsen & Hald, 2007; Fors & Wahnström, 2011). Exten- sive work by researchers from the Technical University of Denmark, using transmission electron microscopy (TEM), has led to an understanding of the formation of Z-phase; it
*Corresponding author.
masoud.rashidi@chalmers.se Received June 30, 2016; accepted January 16, 2017
occurs by a transformation of MX (M = Nb, Ta, or V, and X = C and/or N) to Z-phase (Cipolla et al., 2010; Danielsen et al., 2012). However, energy-dispersive X-ray spectroscopy (EDS) in TEM is of limited accuracy in the composition measurement of such small precipitates that contain light elements such as N and C (Williams & Carter, 2009). Atom probe tomography (APT), capable of detecting all
elements with equal sensitivity, provides the possibility of analyzing carbides and nitrides in 9–12% Cr steels in more detail. However, these precipitates usually have a higher evaporation field compared with the steel matrix. Hence, they will be imaged on the detector with a higher local magnification than the surrounding matrix. Thus, there will be an area of overlap where ions from both the precipitate and the matrix will be detected. This makes it difficult to obtain an accurate chemical composition and morphology of these small precipitates. The aim of this work is to study the formation of
Z-phase by using APT, and to provide an accurate compo- sition of the precipitates during the transformation fromMX to Z-phase. Moreover, a method was developed to accurately measure the composition of small precipitates that suffer from local magnification artifacts using APT. We will show that Z-phase precipitates are formed by diffusion of Cr to MX precipitates and that the composition of the Z-phase corresponds to Cr1+xM1− xN.
MATERIALS ANDMETHODS
A Z-phase strengthened trial steel “Z–Nb” was produced as an 80 kg ingot by vacuum induction melting. The ingot was then hot rolled into 20mm thick plate. The plate was austenitized at 1,150°C and quenched by air-cooling to room temperature. The steel was then aged for 24, 1,005, and 3,000 h at 650°C. The details on the alloy design and the effects of different alloying elements for Z-phase
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