Microstructure evolution of rare earth rich phase for rapidly-solidified TiAl based alloys

Abstract

Much attention has been paid to rapid solidification (RS) of titanium alloys these years. As potential structure material used at elevated temperatures, TiAl based alloys have been given much emphasis in the last two decades. There are two main ways of improving the poor ductility at room temperatures (RT) of TiAl based alloys. They are alloying technique and RS technique. In recent years, rare earth elements have been added to titanium alloys so as to enhance the properties at elevated temperatures through the forming of dispersed second phase particles. Rare earth rich phases are either rare earth oxides (RE2O3) or some intermetallics formed by rare earth elements and other metals such as AlCe [1–4]. In the experiment, rare earth mixture was added to TiAl based alloys. The purpose of adding rare earth elements to TiAl based alloys is obtaining fine dispersed second phase microstructure through RS techniques in order to improve ductility (RT) and mechanical propertied at elevated temperatures. Main constituent of the rare earth mixture is cerium and lanthanum. Table I is chemical analysis result of the alloy used in the experiment. Induction skull melting (ISM) was used in obtaining as-cast TiAl-RE alloy ingot. Through published papers [5, 6] we know that oxygen content in the alloy obtained by ISM can be easily controlled at the scope of 0.04–0.07 wt%. In the experiment, specimens used in RS technology were cut from the as-cast ingot. Two RS techniques employed in the experiment are melt spinning (MS) technique and hammer-and-anvil (HA) technique. The MS device used is 5 T Melt Spinner which was made by Marco Corp. of U.S.A. Ribbons of 50–80 μm thick and 30– 50 mm long were obtained. The cooling rate of MS technique for TiAl based alloys is estimated at the scope of 102–104 T/s. In the HA process, TiAl based alloy cubic of 2 mm × 2 mm × 2 mm was put on the copper anvil, after being arc-melted, the hammer was driven by high-pressure gas and hit the anvil with very high speed, finally, a thin foil (30–100 μm thickness) was obtained. The cooling rates can reach as high as 107 T/s [7]. Gas pressure of protective argon in the furnace is 0.03–0.05 Mpa. Though scanning electron microscopy (SEM) and transmission electron microscopy (TEM) microstructures of as-cast specimens and RS specimens were observed.