Ti(Al,Si) 3 compound formation in non-equilibrated Al-Ti-Si alloy

Abstract

The enhanced grain refinement obtained by adding small amounts of Si to Al-Ti alloys has been reported elsewhere. It was proposed that this enhanced grain refinement results from the constitutional effect that Si has on the Al-Ti system and/or from the increased nucleation rate of the peritectic compound when Si is dissolved in TiAl₃ (kinetic effect). It was shown that the addition of Si extends the peritectic reaction field to lower Ti concentrations, thereby increasing the amount of secondary Al crystals formed by the peritectic transformation of TiAl₃. Grain refinement is also strongly dependent on the nucleation rate of the primary TiAl₃ crystals. It has been shown, using classical nucleation theory, that the nucleation rate increases exponentially with the square of the nucleation entropy and that the nucleation entropy increases with the number of alloying components concentrating in the nucleating phase. Thus the incorporation of even a small amount of Si into the TiAl₃ phase should increase the nucleation rate considerably and the grain refinement will increase correspondingly. The presence of the ternary compound TiAl_2.4Si_0.6 with the TiAl₃ crystal structure (body centred tetragonal bct) in the Al-Ti-Si system has been reported by Ramon and Schubert and the Si-enhanced grain refinement in Al-Ti alloys was shown to be in agreement with the higher nucleation entropy and rate for the ternary system.