Minor additions of Sn in a bulk glass-forming Fe-based system
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C.T. Liu and X.Z. Wang Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996 (Received 30 May 2006; accepted 24 August 2006)
Minor additions of Sn in the bulk glass-forming Fe61−xSnxY2Zr8Co5Cr2Mo7B15 (x ⳱ 0% to 2%) system were studied in detail. It was found that combinations of Y and Sn can scavenge oxygen out of the undercooled liquids to form innocuous oxides, thus stabilizing the liquids. Besides this beneficial scavenging effect, Sn additions in the present Fe-based alloys also showed complex alloying effects on glass formation, which can be divided into three stages. At stage I (x 艋 0.5%), the microalloyed compositions associate with the same eutectic as that of the base alloy. The glass-forming ability (GFA) of the resulting alloys is determined primarily by their liquidus temperature and similar to that of the base alloy. At stage II (0.85% 艋 x 艋 1.15%), glass-matrix composite structures start to form because the alloy compositions are adjusted into a new “deeper” eutectic system. At stage III (x > 1.15%), however, alloy compositions shift to another new eutectic system, and the GFA is dramatically decreased due to the strong formation of primary phase ␣–Fe. Homogeneous glass-matrix composites with a diameter of 7 mm in the alloy containing 1.0–1.15% Sn were successfully produced.
I. INTRODUCTION
Limited experimental data have shown that a minor alloying addition (usually 1.5%)
Alloys in this category showed multiple melting peaks like those in the last category did. However, their eutectic temperature and liquidus temperature are all greater. Based on the XRD pattern in Fig. 3, it is known that these alloys were adjusted to another eutectic system with a higher eutectic temperature. These alloys are also offeutectic, and the primary competing phase is changed to ␣–Fe. Therefore, the first endothermic peak on the melting curves of these alloys is still related to the melting of the eutectic whose composition, however, is different from that of the alloys in Category II. The second melting peak at the high temperature is due to the melting of ␣–Fe. The TTT diagrams for different phase competition in these alloys are similar to those shown in Fig. 6, but they all would shift to the left side (i.e., short-time side) due to the rapid formation of both the eutectic and the ␣–Fe phase. In this case, the quenching rate Rcasting from our casting method is even slower than Rcom. Consequently, the overall GFA of these alloys is dramatically decreased, and composite structures cannot form because the eutectic structures in these alloys are so stable that our cooling capacity (i.e., Rcasting) is not fast enough to avoid their formation. IV. CONCLUSIONS
Partial substitution of Fe with various contents of Sn in the Fe61−xSnxY2Zr8Co5Cr2Mo7B15 (x ⳱ 0% to 2%) alloy system has been conducted, and glass formation of the Sn-containing alloys has been studied. Sn additions in these alloys showed not only a beneficial scavenging effect of oxygen but also a complex alloying phen
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