Fe-based bulk metallic glasses with diameter thickness larger than one centimeter
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Gary J. Shiflet Department of Materials Science and Engineering, University of Virginia, Charlottesville, Virginia 22904-4745 (Received 15 December 2003; accepted 17 February 2004)
Fe–Cr–Mo–(Y,Ln)–C–B bulk metallic glasses (Ln are lanthanides) with maximum diameter thicknesses reaching 12 mm have been obtained by casting. The high glass formability is attained despite a low reduced glass transition temperature of 0.58. The inclusion of Y/Ln is motivated by the idea that elements with large atomic sizes can destabilize the competing crystalline phase, enabling the amorphous phase to be formed. It is found that the role of Y/Ln as a fluxing agent is relatively small in terms of glass formability enhancement. The obtained bulk metallic glasses are non-ferromagnetic and exhibit high elastic moduli of approximately 180–200 GPa and microhardness of approximately 13 GPa.
Iron-based bulk metallic glasses have been reported to exhibit high yield strengths two to three times those of high-strength steels and elastic moduli comparable to those of super-austenitic steel alloys.1,2 These bulk metallic glasses alloys are beginning to gain recognition as a new class of structural materials with certain superior properties, such as strength. In particular, bulk amorphous Fe–Mn–Cr–Mo–C–B alloys, which can be cast into 4 mm-diameter rod-shaped samples, are called nonferromagnetic amorphous steel alloys.2 To date, the most formable Fe-based bulk metallic glasses are reported to form 5 to 6-mm-diameter samples.3,4 Because of the ability to process them is limited, the potential for Fe-based bulk metallic glasses as structural materials has yet to be realized. In this paper we report a significant increase in the formability of Fe–Cr–Mo–C–B bulk metallic glasses when alloyed with few atomic volume fractions of Y and Ln (lanthanides). The improvement on glass formability upon adding Y/Ln is realized. The maximum attainable diameter of glassy Fe50Cr15Mo14C15B6 samples is only 1.5 mm, but is increased to 9–12 mm with only 2 at.% Y/Ln addition. Although the study focused mainly on the Er-containing alloys, additions of other Ln such as Dy, Yb, and Gd produce essentially similar results. Preliminary measurements of mechanical and magnetic properties are also reported. While it has recently been
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0176 1320
J. Mater. Res., Vol. 19, No. 5, May 2004
shown that the glass formability of Fe–Co–Mo–Zr–B alloys can be improved by adding 2 at.% Y,4 the idea of adding Y/Ln to Fe–Mn–Cr–Mo–C–B alloys has been independently pursued by our group. The key results presented herein were reported in a program review conducted by DARPA in February 2003; a patent disclosure on our alloys was filed by the University of Virginia Patent Foundation in April 2003. The utilization of Y/Ln to enhance the glass formability is motivated by the idea that if the atomic-level stress due to large atom solutes has become too large for the crystalline state to remain stable, the
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