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MICROSTRUCTURAL STUDY OF DEVITRIFIED AMORPHOUS ALLOYS LAWRENCE VASSAMILLET, ROBERT MARINGER, ROBERT CARBONARA AND JAMES MCCALL Battelle-Columbus, 505 King Avenue, Columbus, Ohio, USA

ABSTRACT Several amorphous alloys were subjected to thermal processing subsequent to their rapid solidification. These alloys have been characterized utilizing metallographic (optical, TEM, and SEM) and x-ray methods. Also, mechanical property evaluations have been made. The alloys exhibit extremely fine grain size; some have a high degree of preferred orientation and second-phase precipitates in the submicron size range. Most have good modulus, high yield and ultimate strength, high reduction in area but limited percent elongation.

INTRODUCTION In the as-produced condition, rapidly solidified amorphous alloys normally have good mechanical properties. However, in the amorphous condition, they are always thermally unstable and thermal treatment of amorphous alloys usually results in the conversion of the rather ductile properties of the amorphous state to extreme brittleness in the crystallized state. Nevertheless, during specially designed heat treatments of selected Fe- and Ni-base rapidly solidified alloys, we obtained some preliminary evidence that encourages us to attempt to obtain materials with enhanced properties by thermally processing rapidly solidified alloys. It was hoped that thermodynamically stable materials with good properties could be produced using this approach. Although the research is currently still in a rather exploratory stage, the results appear to be encouraging. Some of the microstructural effects observed have been found to be quite unique and of significance. The present paper will describe the microstructural and mechanical property results we have obtained. The materials that will be described in this paper are alloys which are rather well-known for their ability to become fully amorphous. One is a nickel based alloy consisting of 63Ni, 12Cr, 4Fe, 13B and 8Si. It is used commercially as a brazing alloy. The other is an iron-based alloy consisting of 82Fe12B-6Si. (Compositions are given in atomic percents.) EXPERIMENTAL The materials were produced in strip form by direct casting on a rapidly rotating, chilled drum. The casting speeds were about 17 m/s. Strips about 2.5 cm wide by about 50 Um thick were produced. Cooling rates of approximately 106*C/sec were achieved, which resulted in fully amorphous structures. The amorphousness was verified by differential thermal analysis (DTA) and x-ray diffraction. Amorphousness also was inferred from the high bend ductility of the strips. Heating of the amorphous strips was done for various times at 8159C in hydrogen bubbled through a water bath. A low partial pressure of oxygen is created, which results in the oxidation of the metalloids. The heat treatment obviously transformed the initial metastable glassy structures into crystalline

microstructures.

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Fig. 1. Fe8 2 B1 2 Si 6 alloy (originally amorphous) that has been annealed in wet H2 for 0.5 hours