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Naota Sugiyama, Rieko Ito, and Nobuhiro Matsushita Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan (Received 24 February 2009; accepted 19 June 2009)

A Cu- and Ni-free Zr-based metallic glass with high glass-forming ability was found in the Zr-Al-Co ternary system. The eutectic Zr56Al16Co28 alloy could be cast into glassy cylindrical rods with diameters up to 18 mm. The glassy alloy exhibited high tensile fracture strength of 1830 MPa and low Young’s modulus of 83 GPa in conjunction with better corrosion resistance compared with the glassy Zr57Nb5Al10Ni12.6Cu15.4 in a simulated body fluid. Hydrothermal-electrochemical treatment in the aqueous 5M-NaOH solution resulted in the formation of amorphous sodium cobaltate layer on the surface of glassy Zr56Al16Co28 alloy. Hydroxyapatite was spontaneously formed on the surface of the alloy, indicating bioactivity after surface modification. The discovery of a Cu- and Ni-free Zr-based metallic glass with a critical diameter larger than 1 cm in conjunction with excellent mechanical properties, superior corrosion resistance, and good bioactivity may open up the application field as biomaterials.

I. INTRODUCTION

Bulk metallic glasses (BMGs) have been developed in a number of systems such as Mg-, La-, Zr-, Ti-, Cu-, Ni-, Pd-, Fe-, and Co-based alloys for the last two decades.1,2 Among these BMGs, the Zr-based BMGs are considered to be one of promising engineering materials due to their unique physical and chemical properties.1,2 The Zr-based BMGs were first discovered by Inoue et al.3 in ternary Zr-Al-TM (TM=Co, Ni, Cu) systems. The arc-melted ternary Zr60Al10Cu30, Zr60Al15Ni25, and Zr55Al20Co25 alloy ingots with 10 g contained a glassy phase in the thickness range of 2.8 to 4.2 mm.3 Zhang and Inoue4 also reported the formation of Zr55Al20Co25 BMG rod of 3 mm in diameter by injection casting using quartz nozzle and Cu mold. Yokoyama et al.5 have investigated the glass-forming ability of the ternary Zr-Al-Cu system and found that Zr50Al10Cu40 alloy is located near a ternary eutectic point and its arc-melted ingot of 40 g has a glassy phase. In quaternary and penternary Zr-based alloys, for example, Zr55Al10Ni5Cu30,6 Zr41.2Ti13.8Cu12.5Ni10Be22.5,7 and Zr48Al8Cu36Ag88 have much higher glass-forming ability than that for the ternary alloys, and their fully glassy sample sizes reach several centimeters in diameter, which are large enough to be applied as industrial materials.9 The Zr-based BMGs are considered to be a promising material for orthopedic applications. From the view point of mechanical properties, the Young’s modulus of a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0348 J. Mater. Res., Vol. 24, No. 9, Sep 2009

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the Zr-based BMG is nearly half of conventional stainless steel or Co-Cr-based alloy orthopedic implants, although the yielding stress is two or three times higher. These may be useful for the reducing the co