Structure and decomposition of A13-type phase in rapidly solidified high-carbon Cr-Si iron alloy
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I.
INTRODUCTION
R A P I D L Y solidified high-carbon iron alloys reveal some nonequilibrium phases. Up to now, nonequilibrium phases of austenite (y), e, X, and amorphous phases have been observed in iron-base alloys by rapid solidification. The e phaseCll is a solid solution o f a hexagonal close-packed structure (A3-type structure), and the X phase 12,31 has the a-Mn type structure (A12-type structure). The authors t4j have reported that a new phase o f A13-type structure, termed tO phase, is retained in addition to y and M3C phases in a Cr-Mo high-carbon iron alloy. In the present work, a Cr-Si high-carbon iron alloy was employed to investigate the A13-type phase in detail in comparison with other nonequilibrium phases. Decomposition behavior o f the phase was also investigated. II.
EXPERIMENTAL P R O C E D U R E
A high-carbon iron alloy containing chromium and silicon was rapidly solidified into a ribbon by means o f a single roller method. The peripheral speed o f the roller was 34 m / s . In order to obtain a higher cooling rate compared with that o f the previous work,14] the nozzle in the single roller method was 0.5 mm in diameter. The thickness and width o f the ribbon were 2 0 / x m and 1.5 to 2.0 mm, respectively. The cooling rate o f the ribbon was estimated to be 4 . 1 0 5 K/s by a measurement o f secondary dendrite arm spacings o f a rapidly solidified high-carbon iron alloy, tSj The chemical composition o f the ribbon was 3.0 wt pct C, 4.2 wt pct Si, and 11.3 wt pct Cr. The ribbon was heat-treated in a vacuum for 3.6 ks at temperatures up to 1273 K and then air-cooled. A Vickers hardness test was carried out on the longitudinal section o f the ribbon with a 50 g load to measure the change o f hardness after heat treatment. Differential thermal analysis and electrical resistivity measurement were also carfled out at rates o f 0.3 and 0.1 K / s , respectively. Structure H. ERA, Associate Professor, and K. KISHITAKE, Professor, Faculty of Engineering, and P . LI, Graduate Student, are with the Department of Materials Science and Technology, Kyushu Institute of Technology, Tobata, Kita-Kyushu, 8 0 4 , Japan. Manuscript submitted December 1 0 , 1991. METALLURGICAL TRANSACTIONS A
o f the ribbon was investigated by an X-ray diffraction using Co-K~ o r Fe-K~ radiation, and the ribbon for X-ray diffraction was roughly ground to powder. M i c r o structure o f the ribbon was observed by a transmission electron microscope equipped with an energy dispersion X-ray spectrometer (EDX). The specimen f o r electron microscope observation was prepared by an ion-milling apparatus. III.
RESULTS A N D DISCUSSION
A . Structure of to Phase Figure 1 shows a transmission electron micrograph o f the rapidly solidified ribbon. The microstructure is composed o f equiaxed grains. Figures 2(a) and (b) are e l e c tron diffraction patterns showing fourfold and threefold symmetries, respectively. The symmetries indicate that the phase has a cubic structure. It is noted that (310) and (431) diffraction spots have a very stro
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