JOURNAL OF MATERIALS SCIENCE 33 (1998) 3639 — 3649 Liquid separation in Cu–Co and Cu–Co–Fe alloys solidified at high cooling rates A. MUNITZ Nuclear Research Center—Negev, PO Box 9001, Beer-Sheva, Israel R. ABBASCHIAN Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA The impact of cooling rates on the microstructure of Cu—Co and Cu—Fe—Co alloys was investigated by scanning electron microscopy. The high cooling rates entailed in the electron beam surface melting of the alloys result in bulk supercooling of at least 150 K, which in turn causes three microstructural effects: (i) melt separation into two liquids, namely copper poor, L1, and copper rich, L2; (ii) microstructural refinement; (iii) enhanced solute trapping of Cu in the Fe- or Co-rich phases. No evidence of metastable liquid separation was found for Cu—50 wt % Co. There are indications that similar dynamic supercooling exists in copper-quenched or arc-melted samples near the splat contact.  1998 Kluwer Academic Publishers 1. Introduction Rapid solidification processing (RSP) might affect several alloy properties such as microstructural refinement [1, 2], extension of solid solubilities [3] or formation of non-equilibrium phases [1, 4]. This, in turn, may improve the strength, increase plasticity, extend fatigue life, enhance stress corrosion resistance and affect some other related properties of alloys [1]. The extension of solid solubility of alloys exhibiting metastable miscibility gap, such as Cu—Co alloys, has recently attracted much interest because of the ap- pearance of giant magnetoresistance (GMR) effects [5—8], namely a large drop in the electrical resistance, R, in response to an applied magnetic field. For example, thermal annealing of supersaturated of Cu  Co  solid solution at 440 °C reveals an increased magnetoresistance of up to 11% at room temperature. It is believed that the heat treatment caused solid-state spinodal decomposition, which is responsible for the changes in magnetoresistance. It has been argued that high cooling rates might result in a considerable dynamic bulk supercooling even in thick (larger than 100 m) specimens [9, 10]. For example, 282 K supercooling was achieved for Al—6.9 wt % Mn solidified at cooling rates of 310 Ks [9]. Thermal calculations [11] have shown that the amount of supercooling that one may obtain when an aluminium layer 250 m thickness is brought into good thermal contact with a massive copper substrate, initially at ambient temperature, is a function of the distance from the copper chill. For example, at a distance of 5 m, one may obtain a de- crease as high as 250 K relative to the pouring temper- ature, while the temperature decrease is only about 130 K at a distance of 50 m. Therefore, solidification under high cooling rates, such as effected by electron beam surface melting or splat quenching of layers of thickness less than 1 mm on a copper plate, might also involve solidification under supercooling. The latter can result in diverse solidification modes such as par- titionless (massive) solidification [12, 13] (in which the solid has the same concentration as the parent liquid) and/or melt separation [10, 14—16]. It has been recently shown that supercooling of Cu—Co or Cu—Fe alloys beyond a certain limit results in separation of the melt into two liquids; one is Cu rich (L2), and the other is, respectively, Co or Fe rich (L1) [14—18]. Moreover, our previous work on the microstructure of supercooled Cu—Co alloys showed the existence of a metastable Cu phase containing 13—20 wt % Co [10]. Large cooling rates of the order of 10 Ks during electron beam surface melting was also shown to cause bulk supercooling levels of about 150 K, in turn causing melt separation. Similar melt separation was also observed in Cu—Fe alloys [16]. The present work was aimed at obtaining better understanding of the impact of high cooling rates on the microstructure of Cu—Co and Cu—Fe—Co systems. 2. Experimental procedures High-purity (99.98%) copper, high-purity (99.99%) iron, and high-purity (99.99%) cobalt were used to prepare Cu—Co alloys containing up to 80 wt % Co and ternary Cu—Fe—Co alloys of various composi- tions, as listed in Table I. Specimens of the desired compositions were arc melted using a non-consum- able tungsten electrode, followed by electron beam 0022—2461  1998 Kluwer Academic Publishers 3639