Phasestability during continuous heating/cooling of Ti₃AI–(Nb,V,Mo) titanium aluminide alloys

Abstract

The elevated temperature phase transitions occurring during the continuous heating and cooling of two Ti₃AI– (Nb, V:Mo) titanium aluminides, Ti–25Al–11Nb and Ti–26Al–10Nb–3V–1Mo (at.-%) were investigated using optical microscopy, calorimetric differential thermal analysis, and in situ high temperature X -ray diffraction. Both alloys initially consisted of α₂ + β + O with continuous heating resulting in the sequential reaction: orthorhombic O → α₂, dissolution/disordering of α₂, and disordering of B2. Differences in the initial microstructures of the alloys, i.e. blocky and platelike primary α₂ (α₂^P) colonies in a trahsformed β [Widmansüitten secondary α₂^s+ (β/B2)_t] matrix in Ti–25Al–11Nb, and Widmanstatten α₂^P + (β/B2)_t in Ti–26Al–10Nb–3V–1Mo, were found to affect α₂ dissolution. Dissolution, eventually followed by disordering, of the two morphologically distinct ordered α₂ microconstituents in Ti–25Al–11Nb occurred over two distinct temperature regions, while a single dissolution reaction of Widmanstätten α₂ in Ti–26Al–10Nb–3V–1Mo was observed. Similar reversible transformations were observed on cooling from the β phase field, the quantities of primary and secondary α₂ being rate dependent, decreasing cooling rate increasing the volume fraction of primary α₂ while decreasing the volume fraction of secondary Widmanstätten α₂.

MST/2014