Mechanically induced solid-state reaction for synthesizing glassy Co 75 Ti 25 soft magnet alloy powders with a wide supe
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A. Inoue Institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan (Received 29 January 2002; accepted 1 July 2002)
A single phase of glassy Co75Ti25 alloy powders was synthesized by high-energy ball milling the elemental powders at room temperature, using the mechanical alloying method. The final product of the glassy alloy, which is obtained after ball milling for 86 ks, exhibits soft magnetic properties with polarization and coercivity values of 0.67 T and 2.98 kA/m, respectively. This binary glassy alloy, in which its glass transition temperature (Tg) lies at a rather high temperature (833 K), transforms into face-centered-cubic Co3Ti (ordered phase) at 889 K through a single sharp exothermic reaction with an enthalpy change of crystallization (⌬Hx) of −2.35 kJ/mol. The supercooled liquid region before crystallization ⌬Tx of the synthesized glassy powders shows an extraordinary high value (56 K) for a metallic binary system. The reduced glass transition temperature [ratio between Tg and liquidus temperatures, Tl (Tg/Tl)] was 0.56. We also demonstrated postannealing experiments of the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction). Its heat formation was measured directly and found to be −0.56 kJ/mol. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glassy phase.
I. INTRODUCTION
Over the past three decades, mechanically alloying (MA),1 using the ball-milling and/or rod-milling2 techniques has been employed for preparing several advanced engineering materials at room temperature.3–5 Among these useful materials, metallic glassy alloys, with their unique short-range atomic order, have attracted many MA scientists6–9 due to the desirable properties that make them pioneering materials for several industrial applications.10–13 The worldwide 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 the use of them in a number of application areas. Since the pioneering investigation of Koch et al.,1 numerous numbers of amorphous alloys and metallic glasses were fabricated by the MA method.14–23 Accordingly, the term MA is becoming increasingly common in both metal science and glass science. a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 9, Sep 2002
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In recent years, metallic glassy soft magnetic materials (see for example Refs. 24–27) have received much attention due to their unique properties and their promising applications. These materials offer the opportunity to decrease transformer core losses. In particular, a large elastic flexibility guarantees excellent insensitivity with r
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