Influence of Al content on cast microstructures of Ti-Al intermetallic compounds

Abstract

Cast microstructures of Ti-(35-58) at% Al alloys have been studied using optical microscope, scanning electron microscope and transmission electron microscope. The critical compositions of transition from primary beta to alpha phase and from a to gamma phase were determined to be about 49.5 at% Al and 55.5 at% Al respectively. The peritectic compositions of a, and gamma(p),were measured to be about 47 and 54 at% Al respectively. A minimum dendrite cell size was observed at 46.5 at% Al, which is believed to arise primarily from the variation behavior of the partition coefficient and liquidus slope with Al content. The mode of solid state transformation beta-->alpha during solidification at a given cooling rate varied from the massive transformation to the precipitation of Widmanstatten cl plates and of a grains with Al content. The mode of transformation alpha-->gamma, on the other hand, varied from the precipitation to the massive transformation with Al content. This variation of transformation modes at a given cooling rate was explained in terms of the variation of the driving force and mobility with Al content. (C) 1999 Elsevier Science Ltd. All rights reserved.