Phase transition of sigma-CrFe under fast electron irradiation
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Phase transition of sigma-CrFe under fast electron irradiation T. Nagase1, 2, S. Anada1, K. Kobayashi1, H. Yasuda1,2, and H. Mori1 1 Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, 7-1, Mihogaoka, Ibaraki, Osaka 567-0047, Japan. 2 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1, Yamada-oka, Suita, Osaka 565-0871, Japan. ABSTRACT Preparation of a sigma-CrFe single-phase specimen was achieved by arc melting of pure Fe and Cr, cold rolling, and subsequent annealing at 973 K or 1073 K in vacuum. Cold rolling before annealing is effective for the annealing-induced formation of sigma-CrFe from the bcc solid-solution phase. The phase stability and the structural change from sigma-CrFe to a bcc solid-solution phase under fast electron irradiation were investigated by in situ transmission electron microscope (TEM) observation in the temperature range between 22 K and 473 K by using an ultra-high voltage electron microscope (UHVEM). The phase transition of sigma-CrFe by fast electron irradiation was found to occur at a particular temperature. INTRODUCTION Stainless steels are among the most important construction materials in a wide variety of industrial sectors such as civil, chemical, petrochemical, nuclear power plant, and the heavy manufacturing sectors. The precipitation of sigma-CrFe, which is often observed in various types of stainless steels, is one of the main reasons for the deterioration of many useful properties, such as mechanical properties, corrosion resistance, and weldability. Understanding the phase transitions between sigma-CrFe and the corresponding bcc solid solution is of significant interest to industry and engineering. In the present study, a method for preparing a sigma-CrFe singlephase specimen was established, and the phase stability of sigma-CrFe against fast electron irradiation was investigated by high-voltage electron microscopy (HVEM) [1].
EXPERIMENT A CrFe alloy with a composition of Cr48.5Fe51.5 (at%) was prepared because of the wide temperature range over which sigma-CrFe appears in the phase diagram [2]. A master ingot of Cr48.5Fe51.5 alloy was first prepared by an arc-melting technique from the mixture of pure Cr (99.99 wt% purity, Mitsuwa Pure Chemical Co. Ltd.) and Fe (99.99 wt% purity, carbon impurity ≦40 ppm, Toho Zinc Co. Ltd.). Half of the ingot was cold rolled. The ingot specimens with and without cold rolling were annealed at 873 K–1073 K for 3.6 × 104 s in a vacuum quartz tube with Ti as the oxygen getter. Samples prepared in the present study are summarized in Table 1. The constituent phases were analyzed by X-ray diffraction patterns and TEM observation. Thin films for TEM observation and MeV electron irradiation were prepared by an ion-thinning technique.
Table 1. Samples preepared in thee present stu udy. Abbreviatted sample expreessions As-melted UN-873 K UN-973 K UN-1073 K As-rolled R-873 K R-973 K R-1073 K
Cold rolling No No No No Yes Yes Yes Yes
An nnealing No Yes Yes Yes No Yes Yes
