Liquid-liquid phase separation and remixing in the Cu-Co system
- PDF / 266,178 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 65 Downloads / 193 Views
I. INTRODUCTION
MOST metal elements are completely miscible in the liquid state. However, some systems where the elements are characterized by large positive enthalpy of mixing exhibit, below a critical temperature, a separation of the melt in two liquid phases. The critical temperature can be lower than the liquidus temperature: the miscibility gap is then metastable and occurs in the undercooled melt. This is the case for some Cu-based alloys, such as Cu-Co. The liquid-liquid separation in this system has been studied for the first time by Nakagawa,[1] who measured the change of magnetic susceptibility as a function of temperature and composition; he also examined the microstructure of quenched specimens. He found a miscibility gap nearly symmetrical about the equiatomic composition and a minimum difference of 90 K between liquidus and critical temperatures. In order to avoid contact of the sample with a solid crucible, thus obtaining high undercooling with respect to the liquidus, Elder et al.[2] investigated the liquid miscibility gap using electromagnetic levitation and monitoring the temperature during cooling by means of a pyrometer. Measuring the compositions of the phases after the solidification in different samples, they determined the position of the binodal line in the phase diagram. Munitz et al.[3,4,5] used electromagnetic levitation, splat cooling, and electron beam surface melting to study the effects of undercooling and cooling rate on the solidification of Cu-Co alloys at different compositions. They reported demixing temperatures below the peritectic for alloys at the edges of the miscibility gap. Yamauchi et al.[6] measured the demixing temperature S. CURIOTTO, Doctoral Student, is with the Materials Research Department, Risø National Laboratory, Frederiksborgvej 399, DK-4000 Roskilde, Denmark; the Niels Bohr Institute, Nanoscience Centre, University of Copenhagen, Copenhagen, Denmark; and the Dipartimento di Chimica IFM, Centro di Eccellenza NIS, Universita` di Torino, Via P. Giuria 9, 10125 Torino, Italy. Contact e-mail: [email protected] N.H. PRYDS, Senior Researcher, formerly with the Materials Research Department, Risø National Laboratory, is with the Fuel Cell and Solid State Chemistry Department, Risø National Laboratory, Frederiksborgvej 399, DK-4000 Roskilde, Denmark. E. JOHNSON, Professor, is with the Niels Bohr Institute, Nanoscience Centre, University of Copenhagen. L. BATTEZZATI, Professor, is with the Dipartimento di Chimica IFM, Centro di Eccellenza NIS, Universita` di Torino. Manuscript submitted February 7, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
of some Cu-Co alloys by inserting a thermocouple directly in the melt. The cooling rates obtained were of the order of 300 K/min; high undercooling below the liquidus temperature were achieved. They also found that the degree of undercooling increased with the number of heating and cooling cycles. Another group[7] investigated the Cu-Co system, monitoring by a pyrometer the change in temperature as a function of time duri
Data Loading...
