Elsevier

Acta Materialia

Volume 56, Issue 7, April 2008, Pages 1407-1416
Acta Materialia

Creep response and deformation processes in nanocluster-strengthened ferritic steels

https://doi.org/10.1016/j.actamat.2007.11.038Get rights and content

Abstract

There is increasing demand for oxide-dispersion-strengthened ferritic alloys that possess both high-temperature strength and irradiation resistance. Improvement of the high-temperature properties requires an understanding of the operative deformation mechanisms. In this study, the microstructures and creep properties of the oxide-dispersion-strengthened alloy 14YWT have been evaluated as a function of annealing at 1000 °C for 1 hour up to 32 days. The ultra-fine initial grain size (approx. 100 nm) is stable after the shortest annealing time, and even after subsequent creep at 800 °C. Longer annealing periods lead to anomalous grain growth that is further enhanced following creep. Remarkably, the minimum creep rate is relatively insensitive to this dramatic grain-coarsening. The creep strength is attributed to highly stable, Ti-rich nanoclusters that appear to pin the initial primary grains, and present strong obstacles to dislocation motion in the large, anomalously grown grains.

Introduction

Recently developed oxide-dispersion-strengthened (ODS) steels are of interest because of their excellent creep strength at around 800 °C [1], [2], [3]. In addition, ferritic ODS steels are considered as candidate structural materials for fission and fusion power plants because ferritic steels exhibit good resistance to irradiation-induced swelling [1], [4], [5], [6].

Though there have been many studies of ODS steels, the mechanism of dispersion-strengthening in ODS steels has not been fully clarified. The alloy designated 14YWT ODS steel, which is made by a mechanical alloying process and was developed at Oak Ridge National Laboratory (ORNL), displays remarkable creep resistance when compared with conventional ODS steels [2]. The 14YWT steel has a fine-grain size, typically 50–200 nm, that is stable even after annealing at 1000 °C for several hours. The fine grains contain many Ti-rich nanoscale oxide clusters based on 3D atom-probe results [7], [8], and these clusters have been proposed as the reason for the creep resistance of the 14YWT steel [2], [7]. However, there is presently no detailed study of the deformation processes of the 14YWT steel. Moreover, the relationship between the microstructure, including the nanoclusters, and the creep behavior has not been explored.

In this study, the microstructure of the 14YWT steel was characterized in the annealed state, as well as after creep deformation, using a combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques, with particular focus on the Ti-rich nanoclusters and grain structure evolution. In addition, the dislocation structures present before and after deformation have also been characterized to provide insight into the relationship between the microstructure and creep response.

Section snippets

Experimental procedure

The 14YWT steel was both fabricated and crept at ORNL. The 14YWT samples were mechanically alloyed from alloy and Y2O3 oxide powders [9]. The mechanically alloyed material was consolidated by extrusion [9]. The extruded material was cut into cylinders of 3.5 mm diameter and 5 mm height, and then annealed and crept at the conditions summarized in Table 1. Annealing for various times at 1000 °C was carried out in evacuated quartz tubes containing a piece of Cr metal to minimize Cr evaporation from

Creep curves

Fig. 1a shows creep curves for the 1 h (both transverse and longitudinal), 25 h, 5-day, and 11-day annealed/crept samples. The primary creep strains appear to vary significantly between samples. The sample annealed for the shortest time (1 h) clearly exhibits the lowest creep strain up to 490 h. Beyond this, there is no consistent tendency between the annealing time and the primary creep strains. For instance, the sample annealed for 5 days shows the largest primary creep strain, while that

Conclusions

  • 1.

    The 14YWT alloy annealed at 1000 °C for 1 h maintains a stable grain structure, even following creep at 800 °C. Remarkably, in spite of the very fine-grain size, the alloy exhibits excellent creep resistance. This suggests that there is no significant grain-boundary sliding even for the very fine-grain size (approx. 100 nm) structure.

  • 2.

    Fine grains are observed in both the 1 h as-annealed and 1 h annealed/crept samples. Moreover, there is no significant difference in dislocation structures in the fine

Acknowledgments

This project has been supported by the Oak Ridge National Laboratory through a Grant from the Department of Energy, Office of Basic Energy Sciences. T.H. gratefully acknowledges the support of the Japan Society for Promotion of Science (JSPS) Postdoctoral Fellowships for Research Abroad. The authors also acknowledge helpful technical discussions with Dr. C.T. Liu, Dr. Jim Bentley, Dr. Mike Miller and Professor Martin Heilmaier.

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