Elsevier

Materials Science and Engineering: A

Volume 534, 1 February 2012, Pages 142-146
Materials Science and Engineering: A

X-ray evaluation of dislocation density in ODS-Eurofer steel

https://doi.org/10.1016/j.msea.2011.11.051Get rights and content

Abstract

The dislocation density of ferritic–martensitic oxide dispersion strengthened ODS-Eurofer steel was evaluated by using the modified Williamson–Hall method (peak broadening analysis). Measurements were performed in several metallurgical conditions (ferritic and martensitic structures). The monochromatic X-ray radiation was provided by a synchrotron source. The results match qualitatively with those provided by Vickers microhardness measurements and metallographic inspection using transmission electron microscopy.

Graphical abstract

Dislocation density (ρ) as a function of annealing temperature in 80% cold rolled ODS-Eurofer. The result of Vickers microhardness is also included in the figure.

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Highlights

► ODS-ferritic–martensitic steels. ► Thermomechanical processing. ► Martensitic transformation. ► X-ray diffraction. ► Dislocation density.

Introduction

The dislocation density (ρ) is a key microstructural parameter since it is strongly related to mechanical properties of metals and alloys. The dislocation density in metals can be significantly changed during plastic straining, thermal annealing or when they undergo phase transformations such as the martensitic transformation found in Fe–C steels. In deformed metals, the stored energy provided by dislocations is the driving force for important solid-state reactions like recovery and recrystallization [1]. Furthermore, the quantitative evaluation of the dislocation density in metals is also very important in the development of theories of plastic deformation [2].

Among the several experimental techniques to evaluate the dislocation density in metals, direct and indirect methods are usually reported including Vickers hardness testing, X-ray diffraction line profile analysis, evaluation of etch pits, transmission electron microscopy (TEM), flow stress, magnetic (coercive field) and electrical resistivity measurements [3]. TEM and X-ray diffraction line profile analysis are the most recommended techniques to quantify the dislocation density, while; for instance, coercive field and resistivity are strongly affected by factors as precipitates and solutes. The choice of the method to evaluate the dislocation density by using X-ray or TEM depends on the magnitude of dislocation density and the nature of the dislocation array. For instance, in severely deformed metals where very high dislocation densities are found in a complex substructure, TEM is less advisable since counting of individual dislocations by using quantitative metallographic methods becomes quite difficult. This difficult can be overcome by using X-ray diffraction. On the other hand, in less deformed metals or in creep-tested specimens, the TEM becomes more reliable than X-ray diffraction since the elastic distortion caused by dislocations in the lattice and consequent peak broadening is very small.

Literature reports many methods to quantify the dislocation density from data provided by X-ray diffraction experiments including the modified Williamson–Hall (W–H) and Warren–Averbach (W–A) methods [4], [5], [6]. Both methods are based on the broadening analysis of the X-ray diffraction peaks. Despite the second one (W–A) is a more sophisticated and powerful method, W–H analysis is less cumbersome and powerful enough for analyzing cubic material results at the level of accuracy we are currently inspecting our material. In order to extract the sample contribution breadth, the instrumental breadth should be removed from the measured peak [7]. Another factor that must be considered is that the radiation produced in X-ray tubes is not strictly monochromatic, e.g., Kα2 contribution should be removed from the Kα1,2 doublet, by using Rachinger correction. Instead, synchrotron X-ray diffraction can be used as an alternative technique. High brilliance, tunability, and a strictly monochromatic beam are some of the characteristics of this technique. Another important advantage over the conventional laboratory X-ray equipments is that the configuration of parallel beam optics reduces the instrumental contribution to peak broadening.

In the present work, the dislocation density of 80% cold-rolled ODS-Eurofer steel was evaluated. ODS-Eurofer, an oxide dispersion strengthened reduced-activated ferritic–martensitic steel (ODS-RAFM), is a promising candidate for structural applications in future nuclear fusion reactors [8], [9], [10]. The thermally stable oxide particles (Y2O3 – about 10 nm in size) allow increasing the operating temperature [11], [12], [13]. As a result, ODS-Eurofer steel has superior creep resistance at high temperatures because nanosized particles are very effective obstacles to moving dislocations and grain boundaries [14], [15], [16], [17]. Particle pinning exerts a considerable effect on both recovery and recrystallization processes. As a result, dislocation annihilation and dislocation rearrangement are hindered [16]. Meanwhile, the dislocations generated to compatibilize mismatches between martensite laths are of a different origin and the nature of the arrays should be investigated. A first approach will be the evaluation of dislocation densities as a contribution for further investigation. In order to evaluate the dislocation density, X-ray diffraction using a high-resolution synchrotron radiation source was performed in ODS-Eurofer steel. The modified Williamson–Hall method was applied to evaluate the dislocation density of representative specimens annealed within a wide range of temperatures. Isothermal annealing treatments were performed in both ferrite and austenite phase fields. Besides high-resolution synchrotron X-ray diffraction, TEM and Vickers hardness testing were used to characterize the microstructure.

Section snippets

Experimental

The nominal composition of the investigated steel was 9Cr–1W–0.08Ta–0.2V–0.07C–0.4Mn–0.3Y2O3 (wt.%). The steel in the tempered condition (annealed at 750 °C for 2 h) was cold rolled to 80% thickness reduction in multiple passes. Samples were annealed in vacuum from 300 °C up to 1350 °C for 1 h followed by air cooling. Vickers microhardness testing was performed in cold rolled and annealed conditions using a load of 200 g. For Vickers microhardness testing, the ND × RD plane (where ND is the normal

Microstructure

The softening behavior of 80% cold rolled ODS-Eurofer steel was shown elsewhere [18] (see Fig. 1 in Ref. [18]). For samples annealed below 800 °C the softening experienced by the material is quite small, about 7%. Results concerning the annealing behavior of ODS-Eurofer steel within the ferritic phase field were reported in a previous work [19]. The interaction between Y2O3 particles, grain boundaries and dislocations is responsible for preventing more pronounced softening. Samples annealed

Summary and conclusions

The dislocation density of 80% cold-rolled ODS-Eurofer steel annealed within a wide range of temperatures (300–1350 °C) has been evaluated by X-ray diffraction using the modified Williamson–Hall method. Based on Vickers microhardness values, X-ray profile analysis and TEM observation the following conclusions can be drawn:

  • 1.

    The values of Vickers microhardness and the dislocation density obtained by the modified Williamson–Hall plot revealed similar trends, i.e., the changes in hardness due to

Acknowledgments

Authors are grateful to FAPESP (Grants 07/56436-0 and 08/54064-1) for the financial support. Authors are also acknowledged to Dr. R. Lindau (KIT, Karlsruhe) for supplying the samples for this investigation. The kind assistance of Dr. M. Klimenkov (KIT, Karlsruhe) in TEM is also acknowledged. This work is partially supported by Brazilian Synchrotron Light Source Laboratory (LNLS) under Research Proposal Nr. 9761 (2010). Discussions with Prof. T. Ungár were very enlightening.

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