LIQUIDUS TEMPERATURES AND SOLIDIFICATION BEHAVIOR IN THE COPPER-NIOBIUM SYSTEM

The copper-niobium phase diagram has been under active debate; thus, a corroboratory experimental study is needed. In this investigation, th e melts of Cu-Nb alloys at compositions ranging from 5 to 86 wt% Nb we re processed in different environments and solidified at relatively lo w cooling rates of 50-75 degrees C/s to determine liquidus temperature s and to study solidification behavior. For all samples processed unde r very clean conditions, only Nb dendrites in a Cu matrix were observe d; while in the presence of oxy gen impurities, the alloys containing 5-35 wt% Nb exhibited microstructure of Nb-rich spheroids and Nb dendr ites in the Cu matrix. The results obtained from clean conditions are in fair agreement with the Cu-Nb phase diagram having an S-shaped, nea r-horizontal appearance of the liquidus. The formation of Nb-rich drop lets at slow cooling rates is discussed in terms of a stable liquid mi scibility gap induced by oxygen. (C) 1998 Acta Metallurgica Inc.