Impurity distribution In commercial Al-Ti and Al-Ti-B hardeners

Abstract

The concept of nucleation entropy has been developed in previous papers and applied to interpret the markedly enhanced grain refinement of Al by Ti when small amounts of Si (Refs. 2 and 3) and Be (Ref. 4) are added. In the theory, the nucleation entropy and the corresponding rate of nucleation of the peritectic compound increase if the third element concentrates in the peritectic compound. In the case of Si additions, incorporation of Si into the binary TiAl₃ phase is required at a level which exceeds the concentration in the liquid alloy. The presence in an annealed and equilibrated Al-Ti-Si alloy of the compound TiAl_2.4Si_0.6, which has the body centred tetragonal (bct) structure of TiAl₃, has been reported by Ramon and Schubert. In a more recent communication, Youdelis and Yang have shown that the bct Ti(Al, Si)₃ compound (ranging in composition from TiAl_1.83Si_0.04 to TiAl_2.38Si_0.19)forms in a solidified non-equilibrated Al-Ti-Si alloy. The above conclusion that Si is incorporated into the TiAl₃ phase in substantial amounts was for ternary Al alloys containing high levels of Ti and Si, e.g. in the solidified non-equilibrated ternary alloy, the mean composition (wt-%) Al-26.8Ti-11.3Si was reported. The Al alloys for which optimum grain refinement results were obtained contained approximately 0.2% of both Ti and Si. Thus, evidence that Si concentrates in the TiAl₃ phase when the mean Si level in the alloy is 0.2% or lower would give further support to the nucleation entropy theory for grain refinement.