Activities of boron in the binary Fe-B, Co-B, and Cu-B melts
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I.
INTRODUCTION
BORON is
one of the important metalloids which is contained in many of the transition metal-metalloid type amorphous materials. Thermodynamic properties of boron in some binary alloys have been reported, I'-4] but they are restricted to narrow composition and temperature ranges and are not always consistent with each other. Also, measured properties are mostly those in solid alloys, and to the knowledge of the authors, no direct measurement has been made of the activity of boron in a melt. Since amorphous materials are generally produced through the liquid state, the activities of boron in liquid alloys are important when reactions between boron and the ambient atmosphere are likely to occur. In this study, the activities of boron in the binary Fe-B, Co-B, and Cu-B melts have been directly determined by applying the e n d measurement. Some modifications to the Fe-B and Co-B binary phase diagrams are also presented. , II.
EXPERIMENTAL
The fundamental arrangement of the cell used in this study was
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CoB sat. ll00
11150
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1200 1250 Temperature. ~
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1300
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1350
Fig. 7--Relationship between measured emf and temperature for Co-B melt. Closedsymbolsdenotethosepoints obtainedfor the mere purpose of determining the general shapes of the liquidus lines.
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11 50 1200 Temperature. =C
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Fig. 8--Relationship between measured emf and temperature for Cu-B melt. increased with increasing temperature; whereas for the Fe-B and Co-B melt, each relationship was linear only at higher temperatures and the measured emf',s changed only slightly with the change in temperature as long as the linear relationship held. Also, for the latter two cases, each straight line exhibited an inflection at a specified temperature and below this temperature, the emf started to vary along a curve. The inflection indicates that the temperature of the alloy has reached its liquidus temperature. In the actual operation, however, supercooling occurred and, on most occasions, no inflection was observed when the emf was measured with intermittently decreasing temperature. Matsuno t7] has reported considerably large supercooling of about 100 ~ in the case of liquid Fe-B alloys. Liquid Co-B alloys in this study were also considered to exhibit supercooling. Therefore, for the purpose of determining the positions of inflections, those liquid alloys which appeared to have been cooled down below the liquidus temperature were once solidified and the e m f ' s were measured with intermittently in708--VOLUME
20B, O C T O B E R 1989
creasing temperature. Also, some experiments were conducted by using alloys with different boron contents merely to obtain the general shapes of the liquidus lines. By this procedure, positions of liquidus lines on the temperature-emf diagram could be determined, as shown in Figures 6 and 7. It should be mentioned here, however, that the solidification of the alloys wa
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