Solid-state synthesis of new glassy Co 65 Ti 20 W 15 alloy powders and subsequent densification into a fully dense bulk
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M. Omori and A. Inoue Fracture and Reliability Research Institute, Graduate School of Engineering, Tohoku University, Sendai 980-8577, Japan (Received 16 May 2005; accepted 14 July 2005)
The mechanical alloying method was used to synthesize a single glassy phase of Co65Ti20W15 alloy powders, using a high-energy ball mill. The glass transition temperature of the end-product, which was obtained after 173 ks of milling time, lies at 786 K, whereas the crystallization takes place at 878 K through a single sharp exothermic peak with an enthalpy change of crystallization of −4.37 kJ/mol. The reduced glass transition temperature was found to be 0.51. This glassy alloy powders exhibit a very large supercooled liquid region (92 K) for a ternary metallic system. The spark plasma sintering method was used to consolidate the glassy powders under an argon gas atmosphere at 843 K with a pressure of 19.6–38.2 MPa. The sample that was consolidated within 180 s maintains its chemically homogeneous glassy structure with a relative density of above 99.6%. Neither the supercooled liquid region nor crystallization temperature was affected by such a rapid consolidation procedure. Thus, the thermal stability of the bulk glassy sample is almost identical with the original glassy powders. The Vickers microhardness of the bulk glassy Co65Ti20W15 reveals high values, ranging between 8.69 and 8.83 GPa. The fabricated bulk glassy alloy shows high compressive strength of 2.44 GPa with a Young’s modulus of 176.81 GPa. Neither yielding stress, nor plastic strain could be detected for this glassy alloy, which its elastic strain is 1.33%. I. INTRODUCTION
Metallic glassy alloys, with their unique and desirable properties are one of the most useful advanced engineering materials that have been prepared by mechanical alloying (MA) method (see for example Refs. 1–4). The world-wide interest in metallic glassy materials, which represent the ultimate state of solid metastability has been sustained to a great degree by the clear benefits seen in using them in a number of application areas5,6 that make them pioneer materials for several industrial applications.7 Due to their excellent soft magnetic and mechanical properties, metallic glassy Fe- and Co-based alloys have attracted many researchers from different materials science schools.8–18 A large amount of spherical metallic glassy powders can be obtained via the inert gas atomization technique. The atomized powders can be then consolidated into bulk metallic glassy (BMG) buttons or rods with a density of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0344 J. Mater. Res., Vol. 20, No. 10, Oct 2005
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above 99% using hot-pressing, hot isostatic pressing, hot extrusion, and, recently, plasma-activated sintering methods (see for example Ref. 10). Nearly net-shape BMG can be also obtained by densification of the pulverized ribbons metallic glassy powder.11,13,15–17 The powders can be then consolida
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