Elsevier

Micron

Volume 37, Issue 5, July 2006, Pages 492-502
Micron

EELS study of niobium carbo-nitride nano-precipitates in ferrite

https://doi.org/10.1016/j.micron.2005.10.009Get rights and content

Abstract

Micro-alloying steels allow higher strength to be achieved, with lower carbon contents, without a loss in toughness, weldability or formability through the generation of a fine ferrite grain size with additional strengthening being provided by the fine scale precipitation of complex carbo-nitride particles. Niobium is reported to be the most efficient micro-alloying element to achieve refinement of the final grain structure. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus transmission electron microscopy (TEM) is required. Model Fe–(Nb,C) and Fe–(Nb,C,N) ferritic alloys have been studied after annealing under isothermal conditions. However the nanometre scale dimensions of the particles makes their detection, structural and chemical characterization delicate. Various imaging techniques have then been employed. Conventional TEM (CTEM) and high resolution TEM (HRTEM) were used to characterise the morphology, nature and repartition of precipitates. Volume fractions and a statistical approach to particle size distributions of precipitates have been investigated by energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) imaging. Great attention was paid to the chemical analysis of precipitates; their composition has been quantified by electron energy loss spectroscopy (EELS), on the basis of calibrated ‘jump-ratios’ of C–K and N–K edges over the Nb–M edge, using standards of well-defined compositions. It is shown that a significant addition of nitrogen in the alloy leads to a complex precipitation sequence, with the co-existence of two populations of particles: pure nitrides and homogeneous carbo-nitrides respectively.

Introduction

Computer-assisted metallurgy is a recent trend that is more and more employed by manufacturers to help speed up the development of new kinds of alloys. It consists in modeling the final mechanical properties of a material designed to fulfill the requirements of an industrial application, as a function of the composition and applied thermo-mechanical treatments. To make this approach valid, preliminary studies are required. These studies basically consist in validating and/or fitting the predictions of thermodynamic models via an accurate and representative microstructural characterization. This is particularly needed in the case of processes involving precipitation in alloys. Here, models are generally based on the classical theory for diffusive phase transformation (Kampmann and Wagner, 1991), and treat simultaneously the nucleation, growth and ripening phenomena (Shercliff and Ashby, 1990). The particle size distribution, their number and volume fraction can be calculated (Deschamps and Brechet, 1999, Myhr and Grong, 2000) and from these values the effect of the precipitates on the mechanical properties is predicted (Perez and Deschamps, 2003, Maugis and Goune, 2005). Nevertheless, this theoretical approach needs experimental data to compare with and confirm the models.

In the present work, an extensive experimental study of the precipitation of niobium carbo-nitrides in ferrite is presented. This work refers to micro-alloyed steels, which have received considerable interest over many years because they represent very good candidates for a wide range of industrial applications. In this respect, a small addition of niobium to steel is of special interest, since it is well known to yield significant improvements in mechanical properties (Palmiere et al., 1996, DeArdo, 1998, Charleux et al., 2001). Because of its strong affinity for carbon and nitrogen, niobium forms a fine dispersion of niobium carbo-nitride precipitates, generally with a fcc NaCl type structure, in a ferrite (cc structure with a=0.2866 nm). This fine precipitation inhibits austenite grain coarsening during heating (Fossaert et al., 1995), and, in certain cases, suppresses austenite recrystallisation prior to the γ→α transformation through the strain-induced precipitation of NbC (DeArdo, 1998). One of the most difficult aspects of any experimental characterization of the precipitation state is due to the nanometric size of the precipitates that are formed. Hence, transmission electron microscopy work has generally to be undertaken, but further difficulties arise: (i) in thin foils, the magnetic nature of the iron matrix makes any systematic analysis very difficult; (ii) even on extraction replicas, the classical use of carbon as a supporting film makes it almost impossible to quantify the chemistry of the carbo-nitrides, especially their carbon content.

Several TEM works have already been published on such so-called high-strength low alloys (HSLA) (Hofer et al., 1996, Craven et al., 2000, Rainforth et al., 2002, Wilson and Craven, 2003, Beres et al., 2004). However, and owing to the previously mentioned difficulties, there is still a lack of comprehensive study of the evolution of the precipitation state as a function of the annealing time and temperature. Moreover, it is hard to find any statistical approach to the size distribution, volume fraction, crystallography and chemistry of the particles.

Such an ambitious approach has been undertaken on two model (Fe–Nb–C–N) alloys, with significantly different nitrogen contents (Courtois, 2005a, Courtois et al., 2005a; Courtois, et al., 2005); a large part of this work has been devoted to quantitative measurements of size distributions, combining all available imaging techniques on both thin foils and replicas (e.g. conventional dark-field TEM, high-angle annular dark-field imaging in the STEM mode, high resolution TEM and energy-filtered TEM). In this paper, however, we will mainly focus on the quantitative analysis of the chemistry of the precipitates by electron energy-loss spectroscopy (EELS), which will be developed in Section 4, after a presentation of experimental methods (Section 2) and a brief survey of imaging in Section 3.

Section snippets

Materials and sample preparation

The chemical composition for the two steels investigated is reported in Table 1.

Both A and B alloys were solution treated at 1250 °C to dissolve the precipitates, and then water quenched. Precipitation was induced through subsequent annealing at temperatures between 650 and 800 °C (performed in salt bath for short annealing times, e.g. 5 and 30 min, or in vacuum—quartz encapsulation—for longer annealing times, e.g. 300 min and 126 h), followed by water quenching. A final polygonal ferritic

Conventional TEM

Conventional TEM can be extensively used to study the general microstructure of the alloys, especially the precipitation state. In the particular case of metallic carbides within ferrite, the occurrence of an orientation relationship (see Section 3.2) between the precipitates and the matrix makes it easy to image the particles in dark field, as shown in Fig. 3.

As previously observed in the system Fe–Nb–C(–N) (Rainforth et al., 2002, Perrard, 2004), these micrographs show that the precipitation

Methodology

Fig. 9 shows a typical EELS spectrum of a 10 nm niobium carbo-nitride precipitate observed on an extraction replica. For each analyzed precipitate, an additional spectrum was acquired on the replica itself about 10 nm away in order to confirm the absence of any significant carbon content within or on the replica (this point will be re-discussed in Section 4.2, devoted to the estimation of the final accuracy in chemical composition measurements).

In order to perform an elemental analysis of the

Precipitation sequence

In this work, it has been shown that a quantitative chemical analysis of nanometric niobium nitrides and carbo-nitrides could be performed with EELS under conditions, which can be summarized as follows:

  • (i)

    Precipitates as small as 3 nm can be successfully extracted from the ferrite matrix and observed on AlOx replicas. However, reliable EELS data could only be obtained on particles larger than 6 nm typically.

  • (ii)

    Although the absence of carbon within the supporting film allows the precipitates to be

Acknowledgements

The authors gratefully acknowledge the CLYME (Consortium Lyonnais de Microscopie Electronique) for the access to the JEOL 2010F and Leo 912 microscopes, and the French Contrat de Programmes de Recherches (CPR) ‘CNRS-Arcelor-Alcan/Pechiney-CEA’ for financial support. Thanks are also due to Patrick Barges (IRSID, Arcelor) for the help in the extraction replicas procedure, Philippe Maugis ((IRSID, Arcelor, and CIRIMAT-ENSIACET, F-Toulouse) for fruitful discussions, Béatrice Vacher (LTDS, ECL,

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