Microstructure of Cu-Co alloys solidified at various supercoolings
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I.
INTRODUCTION
BULK supercooling can result in diverse solidification modes by the formation of alternate phases,[1] refinement of grain size, partitionless (massive) solidification[2,3] (in which the solid has the same solute concentration as the parent liquid), and/or formation of metallic glasses.[4] However, an important aspect of melt supercooling, which has not yet been studied extensively, is the formation of metastable liquid phase separation in some alloys, such as Cu-Co or CuFe systems. Whether melt separation will occur depends not only on the alloy composition and the level of supercooling but also on the solution thermodynamics of the system. For example, systems such as Cu-Co and Cu-Fe exhibit a definite thermodynamic tendency for liquid immiscibility upon supercooling, as evidenced by nearly flat liquidus curves and positive deviation of their activities from a Raoult’s Law.[5] Indeed, it has recently been shown that supercooling of these alloys beyond a certain limit results in separation of the melt into two liquids: one liquid is Cu rich (L2) and the other is Co or Fe rich (L1).[6–10] Our previous work on the microstructure of supercooled Cu-Co alloys also showed the existence of a metastable Cu phase containing 13 to 20 wt pct Co.[6] However, there is no other information on the miscibility gap or the formation of the metastable ε-Cu phase in other Cu-Co alloys. There are several ways to achieve high bulk supercoolings in a melt, such as emulsification techniques,[11] melting bulk alloys in fused silica crucibles under a cover of slag glass,[12] and electromagnetic levitation.[6,7,10] Electromagnetic levitation technique, in which bulk alloys may be cyclically melted, supercooled and subsequently solidified, has the advantage of reducing contamination and heterogeneous nucleation, as well as enhancing melt homogeneity. The major goal of the present work was to provide additional details on the effects of supercooling on the mi-
crostructure of Cu-Co alloys using the electromagnetic levitation technique.
II.
EXPERIMENTAL PROCEDURES
High-purity copper (99.98 pct) and cobalt (99.99 pct) were used to prepare Cu-Co alloys containing up to 80 wt pct Co. The alloys were then processed using electromagnetic (EM) levitation and splat cooling. The experimental procedure for EM levitation, described elsewhere in detail,[6,7] involved levitation of approximately 1.5-g samples in an inert gas atmosphere followed by freezing of the melt from a superheated or supercooled state in a quenching medium placed directly below the levitation coil. Two different quenching procedures were used: (1) The supercooled liquid was dropped on a copper chill plate, and (2) The alloys were splat quenched in a hammer and anvil apparatus. For the latter, the falling specimen activated a copper piston, which was hurled against a stationary copper plate as the specimen entered the gap between the two. Thin discs of less than 1-mm thickness were produced with cooling rates estimated around 104 K/s.[13] After solidification, the
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