Effect of Er doping on glass-forming ability of Co 50 Cr 15 Mo 14 C 15 B 6 alloy
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Bulk glass formation of the Co–Cr–Mo–C–B–Er alloy system was investigated in this paper. The Co50Cr15Mo14C15B6 (at.%) alloy could be cast into fully glassy rod with a diameter up to 2 mm. By adding 2 at.% Er to this alloy, the critical diameter for glass formation reached 10 mm. The excellent glass formability of the Er-doped alloy was mainly attributed to its relatively large reduced glass transition temperature of 0.61, near-eutectic composition, and the necessity of redistribution of the Er atoms for precipitation of crystalline Co6Mo6C phase in the undercooled liquid on cooling.
I. INTRODUCTION
In general, alloys with compositions close to deep eutectics exhibit the best glass-forming ability (GFA) in a given system because the kinetic process of crystallization in undercooled liquids of the alloys becomes sluggish, and the critical cooling rate for glass formation can be significantly reduced.1 Therefore, the search for the alloy compositions close to the deep eutectics is crucial for development of bulk metallic glasses (BMGs). For some alloys, the GFA could also be remarkably improved by adding a small amount of a certain element. For example, the critical diameters of Cu47Ti34Zr11Ni8, Ce70Al10Cu20, Fe50Cr15Mo14C15B6, and Fe44.3Cr5Co5Mo12.8Mn11.2C15.8B5.9 BMGs increase dramatically with minor addition of Si, Nb, Er and Y, respectively.2–5 However, the mechanism for the magic roles of the doped elements on improvement of GFA still remains unclear. For instance, the compositions of Cu47Ti33Zr11Ni8Si1 and Fe48Cr15Mo14C15B6Er2 are offeutectic, and Johnson et al. reported that the addition of Si to Cu47Ti34Zr11Ni8 alloy has no influence on the thermodynamics (reduced glass transition temperature), crystallization kinetics (activation energy), and intrinsic kinetics (structural relaxation).2,4 Previous Co-based alloys show very limited GFA, and the critical diameter is only 2 mm for the best Co-based glassy former, i.e., Co43Fe20Ta5.5B31.5 (at.%).6 In our study, Co50Cr15Mo14C15B6 BMG with a diameter up to 2 mm was formed by copper mold casting, and the critical diameter for glass formation was increased to 10 mm
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0109 958
J. Mater. Res., Vol. 21, No. 4, Apr 2006
due to the partial replacement of Co by 2 at.% Er in the Co–Cr–Mo–C–B alloy. This paper intends to present the formation of new Co-based BMGs, and the mechanism of the remarkable improvement of GFA by the Er doping is discussed as well. II. EXPERIMENTAL
Ingots with nominal compositions of Co50Cr15Mo14C15B6 and Co48Cr15Mo14C15B6Er2 (at.%) alloys were prepared by arc melting the mixtures of pure elements of Co, Cr, Mo, C, B, and Er in a purified argon atmosphere. From the ingots, ribbon samples were prepared by melt-spinning, and cylindrical rods of 1 to 12 mm in diameter were obtained by a copper mold casting method. The amorphous nature of as-cast rods was examined by a Rigaku D/max2200PC x-ray diffraction (XRD) using Cu K␣ radiation. The as-cast partially
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