The effect of liquid phase separation on the microstructure of rapidly solidified titanium-Rare earth alloys

Abstract

Titanium alloys containing small additions (1.0 at.% or less) of lanthanum, cerium or neodymium have been rapidly solidified using laser surface melting and melt spinning. The microstructures in the asrapidly-solidified condition consist of a moderately refined dispersion of second-phase particles (about 20–200 nm in diameter) in an α′-Ti matrix and exhibit a bimodal size distribution. The particles have been identified as the metallic rare earth, i.e. lanthanum (f.c.c.), cerium (f.c.c.) or neodymium (h.c.p.). The mechanism of formation of the microstructure is thought to involve liquid phase separation and the attendant monotectic reaction, such that the larger particles result from the pro-monotectic reaction and the smaller particles from the monotectic reaction itself. On subsequent aging at 700 °C for 10 h, these particles are internally oxidized to form the rare earth sesquioxide (RE 2O 3). A number of small rare earth oxide particles are also formed during annealing, and it is assumed that these are formed by the decomposition of the slightly supersaturated matrix. The behavior of these titanium-rare earth alloys is compared and contrasted with that of systems in which relatively concentrated supersaturations of the rare earth elements may be effected by rapid solidification processing, e.g. TiEr.