Resulting morphologies on quenching of titanium aluminide alloys

Abstract

The effect of alloying elements (aluminium, silicon, niobium, and zirconium) on the mechanism and morphology of the allotropic transformation Ti-β(bcc) → Ti-α(hcp), occurring during the quenching of binary, ternary, and quaternary titanium aluminide alloys, has been studied. The alloys investigated were (at.-%) Ti–16Al, Ti–16Al–1Si, Ti–16Al–3·5Si, Ti–14Al–1Si–1Nb, Ti–14Al–1Si–1Zr, Ti–22Al, Ti–22Al–1Si, Ti–22Al–3·5Si, Ti–20Al–1Si–1Nb, and Ti–20Al–1Si–1Zr. The allotropic transformation in these alloys presented a very narrow temperature range for the formation of all possible α morphologies resulting from quenching. The different morphologies of α phase observed have been correlated with the competing mechanisms of β decomposition. The morphological observations indicated that small variations in thermodynamic and kinetic conditions in the β phase might account for changes in the mechanisms of formation and growth of the α phase. Additionally, the effect of the alloying elements on the ordering reaction α → α₂ occurring during quenching has been investigated. Silicon addition promoted the formation of columnar α₂ domains during quenching.