The Liquidus Surface of the Al-Ti-B System at the Aluminum Corner

Abstract

In order to obtain the information on the grain refinement of Al cast grain, the liquidus surface of the Al-Ti-B system at the aluminum corner was investigated by a chemical composition analysis of the supernatant solutions and by the composition and structure analysis of the sediments seperated in Al-Ti-B melts by the gravity segregation.The results are summarized as follows.(1) Both the liquidus curve and the peritectic-limit of TiAl3 are little affected by the addition of boron to the Al-Ti system.(2) Based on the result of the micro analysis using IMA, it is concluded that boron is excluded into the liquid phase on the formation of the TiAl3 phase. It can therefore be concluded that the liquidus surface of the TiAl3 phase constitutes an eutectic line with that of TiB2 phase.(3) The solubility product of the TiB2 phase matches closely with those reported in the literature for the range of Ti:B ratio higher than that of the TiB2 phase.(4) Based on the assumption that the distribution coefficients of titanium and boron in aluminum are not different from those for the binary alloy, a simple calculation shows that the intersection line of Al and TiB2 liquidus surfaces forms the peritectic reaction.(5) The Ti:B ratio of TiB2 particles in aluminum-base alloys is about 2.07 by weight and is smaller than the stoichiometric ratio (=2.22). These particles contain several weight per cent of aluminum.(6) Accordng to the present information about the phase diagram, Al cast grains seem to nucleate directly on the TiB2 particles in the composition range determined by the following unequality: 0<total Ti content −2.07×total B content<0.15 (wt%).