The effect of various transition metals on glass formation in ternary La-TM-Al (TM = Co, Ni, Cu) alloys
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The optimum glass formers in ternary La-TM-Al (TM 5 Co, Ni, Cu) alloys were pinpointed at alloys La69Co17Al14, La66Ni19Al15, and La66Cu20Al14, exhibiting critical sizes for full glass formation of 16, 12, and 5 mm, respectively. Cobalt is found to be the most favorable element for glass formation in La-based alloys. The optimum alloys in La-TM-Al show close composition but significantly different glass-forming ability (GFA). The mechanism of distinct effect of TM elements remains unclear, even discussed based on current GFA-related criteria and indicators. I. INTRODUCTION
As the first bulk metallic glass (BMG)-forming alloy was reported in the La-Ni-Al system,1 La-based BMGs have attracted considerable attention from both scientific and technological perspectives because of their excellent glass-forming ability (GFA) and superior physical properties as compared to their crystalline counterparts.2–9 For example, alloys La65Al14(Cu5/6Ag1/6)11Ni5Co56 and (La0.7Ce0.3)65Al10Co257 exhibit super-high GFA with the critical size (Dc) of 1 inch; La60Al20Ni10Co5Cu5 BMG shows good thermoplasticity and imprintability at low temperatures and, therefore, facilitates its potential applications such as a micro- and nano-imprintable material.9 Up to date, La-based BMGs have been developed in many systems such as La-Cu-Al,2,10 La-Ni-Al,1,2,11 La-Co-Al,5,7 La-(Cu,Ni,Co)-Al,2–4 (La,Ce)-(Cu,Ni,Co)-Al,7,8,12 and La(Cu,Ni,Co)-Ag-Al.6 In all of them, the transition metals (TM) Co, Ni, and Cu are essential components and play a crucial role in the glass formation of La-based BMGs. However, the effect of various TM elements on glass formation unfortunately remains unclear. In this article, we searched the best glass formers in La-TM-Al systems to illustrate the effect of these elements on the glass formation. The optimum alloys in La-TM-Al show close composition but significantly different GFA. Co is found to be the most effective element favoring the glass formation. These findings will be helpful to understand the mechanism of glass formation and develop the higher order multi-component BMGs in La-based and other TM-containing alloys. II. EXPERIMENTAL
Raw materials (La, Co, Ni, Cu, and Al of more than 99% purity) were used for arc melting under an argon a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2011.39 992
J. Mater. Res., Vol. 26, No. 8, Apr 28, 2011
http://journals.cambridge.org
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atmosphere on a water-cooled copper plate. Each ingot was flipped and remelted several times to ensure composition uniformity. The master alloy remelted in a tilting copper hearth was cast into the copper moulds with cylindrical cavities of diameters 4–16 mm and length 60 mm. The as-cast rods were then sectioned longitudinally, grinded, polished, etched, and finally observed by an Olympus PME 3 optical microscope. The degree of the amorphicity of the as-cast rods was also examined by x-ray diffraction (XRD) in a Bruker D8 diffractometer with Cu Ka radiation (k 5 0.154056 nm). The corresponding
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